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#1
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Low Frequency Mains Noise
On several preamp PSUs I have built in that last couple of years I have
noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian |
#2
Posted to rec.audio.tubes
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Low Frequency Mains Noise
On Fri, 31 Oct 2008 11:01:16 +0000, Ian Bell
wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. What kind of scope are you using that when set to AC input will show a less than 1 Hz ripple? I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian |
#3
Posted to rec.audio.tubes
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Low Frequency Mains Noise
PeterD wrote:
On Fri, 31 Oct 2008 11:01:16 +0000, Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. What kind of scope are you using that when set to AC input will show a less than 1 Hz ripple? First it is not a ripple - it is not repetitive. Secondly, it is shortish but sudden changes of a few mV so the scope does not really need a response down to 1Hz. Lastly, the scope is a Rapid Electronics 7020A 20MHz dual channel (which is a re-branded Pintek) and its -3dB point is quoted as 20Hz. Cheers Ian |
#4
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Graham |
#5
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Ian Bell wrote: First it is not a ripple - it is not repetitive. Secondly, it is shortish but sudden changes of a few mV so the scope does not really need a response down to 1Hz. Lastly, the scope is a Rapid Electronics 7020A 20MHz dual channel (which is a re-branded Pintek) and its -3dB point is quoted as 20Hz. Junk. Grow up and use solid state. Graham |
#6
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Eeyore wrote:
Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Graham Unhelpful replies like yours I guess. Cheers Ian |
#7
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Low Frequency Mains Noise
In article ,
Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#8
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Low Frequency Mains Noise
On Oct 31, 3:55*pm, Eeyore
wrote: Junk. Grow up and use solid state. Graham Hi RATs! Eyesore poses proudly as a knowledgeable and clever adult. I hope we all have a wonderful Halloween! Happy Ears! Al |
#9
Posted to rec.audio.tubes
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Low Frequency Mains Noise
"Ian Bell" First it is not a ripple - it is not repetitive. Secondly, it is shortish but sudden changes of a few mV so the scope does not really need a response down to 1Hz. Lastly, the scope is a Rapid Electronics 7020A 20MHz dual channel (which is a re-branded Pintek) and its -3dB point is quoted as 20Hz. ** Very unusual for a scope to have that much roll off at 20 Hz, most are 3dB down at 3 to 6 Hz in "AC " mode. In any case, leave the scope input set to " DC " and fit your own isolating cap in series with the probe - a 0.1 uF, 400 volt film type will give a -3dB point at 1.6 Hz, or 0.16 Hz if you use a 10:1 probe. ...... Phil |
#10
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian Because there are thousands of people hooked to the mains and they all turn gear on and off randomly, the mains voltage level changes. The changes are random, and when you rectify the mains after your power tranny, you are creating a dc voltage which tracks the levels of applied ac voltage. Its the very same effect that is used in detecting amplitude changes of an AM RF signal. Because power supplies for amp have caps between B+ and OV and after the diode/magnetic circuit of mains and PT, there are two ways of getting rid of LF signal if its in an output signal and from the B+ rails. One option is make the circuit you have less responsive to LF, with every stage R&C coupling with a cut off at 5Hz, Especially in Phono amps. Another is to regulate the B+ rail after the first C stage, and perhaps at sensitve stages like a phono amp. But I have used several stages of R&C with C = 470uF and R maybe 220 ohms in a preamp and chosen poles for RC couplings that cut out very LF. But if you insist on response to 2Hz, you may need to regulate each stage. There is no point in "lookong at it" on the mains. Assume the mains is a sewer of noise. Simple filters and good design or regs will remove all your noise. If you have LC filters, make sure their resonant F is below 7Hz or lower. And maybe add some series R to damp the LC resonance. With resonance, noise content around the LC Fo will appear in the B+ larger than at the resevoir cap unless you have applied critical damping. This may mean the added series R = 1.4 x XC or XL at Fo; the response won't be peaked at the pole F, ie, Fo. Z100uF = 1,600 ohms at 1 Hz, so with R = 1k, there is little filtering at 1Hz. If you had C = 470uF, ZC = 340 ohms and 4 RC filter sections would give attenuation = approx 1/2 x 1/2 x 1/2 x 1/2 = 1/16. Its still not a huge amount of filtering. But say your amp output has 1mV of LF noise with 1k + 100uF, then with 1k + 470uF the noise should be maybe 0.1 mV, and maybe OK for you. A regulator will do better because they work right down to DC. Patrick Turner. |
#11
Posted to rec.audio.tubes
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Low Frequency Mains Noise
John Byrns wrote:
In article , Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. I did not say I do not want to use regulation. I was looking for confirmation or information on the cause of the problem and its solution. Since there are untold preamp designs with unregulated supplies it came as a surprise to me to see this noise. If it is indeed typical with unregulated supplies that's fine. Cheers Ian |
#12
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Phil Allison wrote:
"Ian Bell" First it is not a ripple - it is not repetitive. Secondly, it is shortish but sudden changes of a few mV so the scope does not really need a response down to 1Hz. Lastly, the scope is a Rapid Electronics 7020A 20MHz dual channel (which is a re-branded Pintek) and its -3dB point is quoted as 20Hz. ** Very unusual for a scope to have that much roll off at 20 Hz, most are 3dB down at 3 to 6 Hz in "AC " mode. That's what the spec says but I suspect it does better than that in practice (see below). In any case, leave the scope input set to " DC " and fit your own isolating cap in series with the probe - a 0.1 uF, 400 volt film type will give a -3dB point at 1.6 Hz, or 0.16 Hz if you use a 10:1 probe. I did that and got identical results which suggests the scope is better than I thought. Cheers Ian |
#13
Posted to rec.audio.tubes
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Low Frequency Mains Noise
flipper wrote:
On Fri, 31 Oct 2008 11:01:16 +0000, Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? From what you describe it sounds like mains noise, as you suspected, and with the thousands upon thousands of things running, coming on and off, or whatever, throughout the grid lord only knows what causes each 'blip'. AC mains are neither 'clean' nor stable, at least not to the degree you're talking about. Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region plus the five stage RC filter I am using is over 120dB down at 50Hz so even a decade or so lower I would have expected its attenuation to be significant. With it out of band and low I'm not sure what you're trying to fix but the common choices are to filter till it's below whatever tolerance you decide upon or regulate. I wreaks havoc with distortion measurements at 100Hz when you are expecting a result below 0.1%. Cheers Ian |
#14
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Patrick Turner wrote:
Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian Because there are thousands of people hooked to the mains and they all turn gear on and off randomly, the mains voltage level changes. The changes are random, and when you rectify the mains after your power tranny, you are creating a dc voltage which tracks the levels of applied ac voltage. Its the very same effect that is used in detecting amplitude changes of an AM RF signal. Because power supplies for amp have caps between B+ and OV and after the diode/magnetic circuit of mains and PT, there are two ways of getting rid of LF signal if its in an output signal and from the B+ rails. One option is make the circuit you have less responsive to LF, with every stage R&C coupling with a cut off at 5Hz, Especially in Phono amps. Another is to regulate the B+ rail after the first C stage, and perhaps at sensitve stages like a phono amp. But I have used several stages of R&C with C = 470uF and R maybe 220 ohms in a preamp and chosen poles for RC couplings that cut out very LF. I am using five stages of 1K and 100uF which is pretty much the same as 220R and 470uF and I still see the LF noise. But if you insist on response to 2Hz, you may need to regulate each stage. To maintain a response flat down to 20Hz over several stages implies most will need to be flat to around 5Hz. There is no point in "lookong at it" on the mains. Assume the mains is a sewer of noise. Simple filters and good design or regs will remove all your noise. I only suggested that as a means of confirming its source, but all the replies I have had so far seem to confirm it is mains borne so I don't now see a need to do that. If you have LC filters, make sure their resonant F is below 7Hz or lower. And maybe add some series R to damp the LC resonance. With resonance, noise content around the LC Fo will appear in the B+ larger than at the resevoir cap unless you have applied critical damping. This may mean the added series R = 1.4 x XC or XL at Fo; the response won't be peaked at the pole F, ie, Fo. Z100uF = 1,600 ohms at 1 Hz, so with R = 1k, there is little filtering at 1Hz. If you had C = 470uF, ZC = 340 ohms and 4 RC filter sections would give attenuation = approx 1/2 x 1/2 x 1/2 x 1/2 = 1/16. Its still not a huge amount of filtering. But say your amp output has 1mV of LF noise with 1k + 100uF, then with 1k + 470uF the noise should be maybe 0.1 mV, and maybe OK for you. A regulator will do better because they work right down to DC. Patrick Turner. Cheers Ian |
#15
Posted to rec.audio.tubes
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Low Frequency Mains Noise
"Ian Bell" Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming it unless it is over 5 kHz. Only " ferroresonant " transformers actually regulate the AC voltage. plus the five stage RC filter I am using is over 120dB down at 50Hz so even a decade or so lower I would have expected its attenuation to be significant. ** It is. Try monitoring the 230 volt AC supply voltage on a DMM and see it jumping around by up to +/ - 5 volts all the time. Imbecile. ...... Phil |
#16
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Ian Bell wrote: Patrick Turner wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian Because there are thousands of people hooked to the mains and they all turn gear on and off randomly, the mains voltage level changes. The changes are random, and when you rectify the mains after your power tranny, you are creating a dc voltage which tracks the levels of applied ac voltage. Its the very same effect that is used in detecting amplitude changes of an AM RF signal. Because power supplies for amp have caps between B+ and OV and after the diode/magnetic circuit of mains and PT, there are two ways of getting rid of LF signal if its in an output signal and from the B+ rails. One option is make the circuit you have less responsive to LF, with every stage R&C coupling with a cut off at 5Hz, Especially in Phono amps. Another is to regulate the B+ rail after the first C stage, and perhaps at sensitve stages like a phono amp. But I have used several stages of R&C with C = 470uF and R maybe 220 ohms in a preamp and chosen poles for RC couplings that cut out very LF. I am using five stages of 1K and 100uF which is pretty much the same as 220R and 470uF and I still see the LF noise. Indeed, but if all you did was to replace the 100uF in your amp with 470uF, and leave the 1k R you have, then you will see a big reduction of LF noise. But if you insist on response to 2Hz, you may need to regulate each stage. To maintain a response flat down to 20Hz over several stages implies most will need to be flat to around 5Hz. Yes, but if the response is -3dB at 5Hz at say 3 consecutive stages then at 1Hz the response is becoming 18dB/Octave. There is no point in "lookong at it" on the mains. Assume the mains is a sewer of noise. Simple filters and good design or regs will remove all your noise. I only suggested that as a means of confirming its source, but all the replies I have had so far seem to confirm it is mains borne so I don't now see a need to do that. Remember that the mains 50Hz or 60Hz signal is a 'carrier' of LF noise information just as audio information is carried by an RF carrier in a radio set, Sometimes slow moving signals in mains are like dc variations as well. Most of this is filtered out by the transformer. Patrick Turner. If you have LC filters, make sure their resonant F is below 7Hz or lower. And maybe add some series R to damp the LC resonance. With resonance, noise content around the LC Fo will appear in the B+ larger than at the resevoir cap unless you have applied critical damping. This may mean the added series R = 1.4 x XC or XL at Fo; the response won't be peaked at the pole F, ie, Fo. Z100uF = 1,600 ohms at 1 Hz, so with R = 1k, there is little filtering at 1Hz. If you had C = 470uF, ZC = 340 ohms and 4 RC filter sections would give attenuation = approx 1/2 x 1/2 x 1/2 x 1/2 = 1/16. Its still not a huge amount of filtering. But say your amp output has 1mV of LF noise with 1k + 100uF, then with 1k + 470uF the noise should be maybe 0.1 mV, and maybe OK for you. A regulator will do better because they work right down to DC. Patrick Turner. Cheers Ian |
#17
Posted to rec.audio.tubes
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Low Frequency Mains Noise
John Byrns wrote: In article , Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. -- Regards, John Byrns Considerable self regulation occurs in mains supplies in populos cities and towns because where you have thousands of people all using something and averaging say 1kW mains input per household 24/7, then if the mains voltage is 240V, the current average is 4 amps and the load must be 60 ohms. Say there are 3,000 housholds sharing your circuit locally and that the wiring between them is low resistance then the load connected to the circuit you are on is 60 / 3,000, ie, a very low load indeed on average, but one that changes its value enough to cause Ian's observed LF voltages on a B+ rail of +/- 30mV. That's about what I see here. After several R&C stages of filtering the rail noise movements are much less than +/-30mV. If you have a CCS type of load supplying dc to a triode anode, then the low Ra of ther triode makes a divider with the CCS, or with a resistance load for dc feed, and the anode LF noise voltage will be much lower than at the rail. But where you have a pentode or cascode pair of triodes then Ra is high, and any noise on the rail appears at the anode. The other source of LF noise in phono and line level stages is where you have a large grid bias R to bias a typical gain triode. Its grid may be cap coupled to a preceding triode gain stage with low Ra. At say 1kHz, the low Ra and low XC shunt the high resistance of the grid biasing R, say 470k or 1M. But as F becomes lower, the XC becomes a high impedance, and effectively the grid is then coupled toa good noise source, ie, the high resistance. Its most important in phono or microphone first stages to have a relatively low grid biasing R, and 47k is typical for MM inputs. The carts are usually direct coupled to a grid, and the standard 47k is so the grid is biased when no cart in connected and the 47 happens to be the most common recommended loading for most MM carts. If you have C&R coupling from a cart to grid, then the bias R will make LF noise which is not shunted by lower Z of the cart because of the rising XC of the coupling C. I have had to use about 9uF from cart to 47k to avoid unwanted LF noise in phono amps. Phono amps have 10 times more gain at LF than at 1kHz, so the slightest LF noise gets through. Its even worse with MC carts which may have 26dB less sensitivity than an MM cart. If there is R&C coupling from an MC cart to j-fet input, the best LF noise performance occurs when the biasing R for the j-fet gate is say 470 ohms to load the cart correctly and to keep LF noise low. MC carts have tyical output reistance of less than 30 ohms, hence their noise overall is very low, so an SNR of over 60dB is achieved despite their low signal voltage level, but with C&R input coupling, C and R values have to be chosen carefully. See samples of phono amp circuits which have good LF noise performance at http://www.turneraudio.com.au/preamp...hono-2005.html http://www.turneraudio.com.au/preamp...ated-2006.html Patrick Turner. |
#18
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Phil Allison wrote:
"Ian Bell" Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming it unless it is over 5 kHz. ** Complete fallacy. Transformers by definition have a zero at zero Hz. They have a pole at some frequency determined by the transformer inductance and the source and load impedance. From that pole down to zero Hz their response falls by at least 20dB/decade. Only " ferroresonant " transformers actually regulate the AC voltage. plus the five stage RC filter I am using is over 120dB down at 50Hz so even a decade or so lower I would have expected its attenuation to be significant. ** It is. Try monitoring the 230 volt AC supply voltage on a DMM and see it jumping around by up to +/ - 5 volts all the time. I shall Imbecile. lol ..... Phil cHEERS iAN |
#19
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Low Frequency Mains Noise
Patrick Turner wrote:
Ian Bell wrote: Patrick Turner wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian Because there are thousands of people hooked to the mains and they all turn gear on and off randomly, the mains voltage level changes. The changes are random, and when you rectify the mains after your power tranny, you are creating a dc voltage which tracks the levels of applied ac voltage. Its the very same effect that is used in detecting amplitude changes of an AM RF signal. Because power supplies for amp have caps between B+ and OV and after the diode/magnetic circuit of mains and PT, there are two ways of getting rid of LF signal if its in an output signal and from the B+ rails. One option is make the circuit you have less responsive to LF, with every stage R&C coupling with a cut off at 5Hz, Especially in Phono amps. Another is to regulate the B+ rail after the first C stage, and perhaps at sensitve stages like a phono amp. But I have used several stages of R&C with C = 470uF and R maybe 220 ohms in a preamp and chosen poles for RC couplings that cut out very LF. I am using five stages of 1K and 100uF which is pretty much the same as 220R and 470uF and I still see the LF noise. Indeed, but if all you did was to replace the 100uF in your amp with 470uF, and leave the 1k R you have, then you will see a big reduction of LF noise. I agree. I just might try that. But if you insist on response to 2Hz, you may need to regulate each stage. To maintain a response flat down to 20Hz over several stages implies most will need to be flat to around 5Hz. Yes, but if the response is -3dB at 5Hz at say 3 consecutive stages then at 1Hz the response is becoming 18dB/Octave. Yes but at present I am working with just one stage and this LF noise makes it impossible to measure distortion of the order of 0.1% or less at 100Hz . Cheers ian There is no point in "lookong at it" on the mains. Assume the mains is a sewer of noise. Simple filters and good design or regs will remove all your noise. I only suggested that as a means of confirming its source, but all the replies I have had so far seem to confirm it is mains borne so I don't now see a need to do that. Remember that the mains 50Hz or 60Hz signal is a 'carrier' of LF noise information just as audio information is carried by an RF carrier in a radio set, Sometimes slow moving signals in mains are like dc variations as well. Most of this is filtered out by the transformer. Patrick Turner. If you have LC filters, make sure their resonant F is below 7Hz or lower. And maybe add some series R to damp the LC resonance. With resonance, noise content around the LC Fo will appear in the B+ larger than at the resevoir cap unless you have applied critical damping. This may mean the added series R = 1.4 x XC or XL at Fo; the response won't be peaked at the pole F, ie, Fo. Z100uF = 1,600 ohms at 1 Hz, so with R = 1k, there is little filtering at 1Hz. If you had C = 470uF, ZC = 340 ohms and 4 RC filter sections would give attenuation = approx 1/2 x 1/2 x 1/2 x 1/2 = 1/16. Its still not a huge amount of filtering. But say your amp output has 1mV of LF noise with 1k + 100uF, then with 1k + 470uF the noise should be maybe 0.1 mV, and maybe OK for you. A regulator will do better because they work right down to DC. Patrick Turner. Cheers Ian |
#20
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Patrick Turner wrote:
John Byrns wrote: In article , Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. -- Regards, John Byrns Considerable self regulation occurs in mains supplies in populos cities and towns because where you have thousands of people all using something and averaging say 1kW mains input per household 24/7, then if the mains voltage is 240V, the current average is 4 amps and the load must be 60 ohms. Say there are 3,000 housholds sharing your circuit locally and that the wiring between them is low resistance then the load connected to the circuit you are on is 60 / 3,000, ie, a very low load indeed on average, but one that changes its value enough to cause Ian's observed LF voltages on a B+ rail of +/- 30mV. That's about what I see here. After several R&C stages of filtering the rail noise movements are much less than +/-30mV. If you have a CCS type of load supplying dc to a triode anode, then the low Ra of ther triode makes a divider with the CCS, or with a resistance load for dc feed, and the anode LF noise voltage will be much lower than at the rail. Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. But where you have a pentode or cascode pair of triodes then Ra is high, and any noise on the rail appears at the anode. The other source of LF noise in phono and line level stages is where you have a large grid bias R to bias a typical gain triode. Its grid may be cap coupled to a preceding triode gain stage with low Ra. At say 1kHz, the low Ra and low XC shunt the high resistance of the grid biasing R, say 470k or 1M. But as F becomes lower, the XC becomes a high impedance, and effectively the grid is then coupled toa good noise source, ie, the high resistance. Its most important in phono or microphone first stages to have a relatively low grid biasing R, and 47k is typical for MM inputs. The carts are usually direct coupled to a grid, and the standard 47k is so the grid is biased when no cart in connected and the 47 happens to be the most common recommended loading for most MM carts. That's not a problem in this case. The mic transformer is connected direct to the first stage grid. Cheers Ian If you have C&R coupling from a cart to grid, then the bias R will make LF noise which is not shunted by lower Z of the cart because of the rising XC of the coupling C. I have had to use about 9uF from cart to 47k to avoid unwanted LF noise in phono amps. Phono amps have 10 times more gain at LF than at 1kHz, so the slightest LF noise gets through. Its even worse with MC carts which may have 26dB less sensitivity than an MM cart. If there is R&C coupling from an MC cart to j-fet input, the best LF noise performance occurs when the biasing R for the j-fet gate is say 470 ohms to load the cart correctly and to keep LF noise low. MC carts have tyical output reistance of less than 30 ohms, hence their noise overall is very low, so an SNR of over 60dB is achieved despite their low signal voltage level, but with C&R input coupling, C and R values have to be chosen carefully. See samples of phono amp circuits which have good LF noise performance at http://www.turneraudio.com.au/preamp...hono-2005.html http://www.turneraudio.com.au/preamp...ated-2006.html Patrick Turner. |
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Low Frequency Mains Noise
"Ian Bell" Phil Allison wrote: Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming in unless it is over 5 kHz. ** Complete fallacy. ** No - it is TOTAL FACT. Transformers by definition have a zero at zero Hz. ** Got nothing to do with *VARIATIONS * occurring in the AC supply voltage !!!!! They have a pole at some frequency determined by the transformer inductance and the source and load impedance. ** Complete drivel - there is no such pole as (un-gapped) transformers have no defined primary inductance value. Core saturation is what limits an AC power transformer's ability to work at lower frequencies than 56/60 Hz. This must be where you made you IDIOT assumption that ordinary transformers remove low frequency variations on the AC supply. Try monitoring the 230 volt AC supply voltage on a DMM and see it jumping around by up to +/ - 5 volts all the time. I shall ** **** off you arrogant, know nothing ****ing IMBECILE. ...... Phil |
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Low Frequency Mains Noise
"flipper" Ian Bell Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region You have a couple of dubious assumptions here with the first being it's a 'separate' signal. It's more likely riding on the AC with the AC acting as, essentially, a carrier. The second is that the AC 'noise' has the same frequency component as your filtered measurement. ** Maybe it is time someone explained what "de-modulation" is to the congenital Ian Bell FOOL. ..... Phil |
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Low Frequency Mains Noise
Ian Bell wrote: Phil Allison wrote: "Ian Bell" Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming it unless it is over 5 kHz. ** Complete fallacy. Transformers by definition have a zero at zero Hz. They have a pole at some frequency determined by the transformer inductance and the source and load impedance. From that pole down to zero Hz their response falls by at least 20dB/decade. But you missed the reason why someone said "Complete insanity". Transformers convey *all* the variations of mains amplitude, and its these amplitude variations which are seen on the rectified signal at the resovoir cap of the dc supply. Hence there are *very* low frenquencies seen at the this resovoir capacitor. Try studying the way AM detectors work in radio sets using a diode and RC network. Low F noise ac waves in mains supplies are sometimes present with the 110V at 60Hz or 240V at 50Hz, but are not a large % of the total F present, and the public supply which has several transformers between the generator turbine and your house does not pass these low level ac waves which are simply part of the noise spectra in mains supplies. Some of this spuriae affects the level of rectified Vdc, but most of the "low speed riple" on the Vdc rails is caused by mains frequency signal amplitude changes. Only " ferroresonant " transformers actually regulate the AC voltage. plus the five stage RC filter I am using is over 120dB down at 50Hz so even a decade or so lower I would have expected its attenuation to be significant. ** It is. Try monitoring the 230 volt AC supply voltage on a DMM and see it jumping around by up to +/ - 5 volts all the time. When I measure the "240Vac" here is usually is stable enough to get a nearly constant reading on a DMM, and certainly on an analog meter with a needle. But it slowly varies between 235Vac on cold winter nights of heavy loadings to 255Vac when load is light. When I look at the rectified Vdc, it shows the expected variations of +/- 30mV. Patrick Turner. |
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Low Frequency Mains Noise
Ian Bell wrote: Patrick Turner wrote: Ian Bell wrote: Patrick Turner wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? Cheers Ian Because there are thousands of people hooked to the mains and they all turn gear on and off randomly, the mains voltage level changes. The changes are random, and when you rectify the mains after your power tranny, you are creating a dc voltage which tracks the levels of applied ac voltage. Its the very same effect that is used in detecting amplitude changes of an AM RF signal. Because power supplies for amp have caps between B+ and OV and after the diode/magnetic circuit of mains and PT, there are two ways of getting rid of LF signal if its in an output signal and from the B+ rails. One option is make the circuit you have less responsive to LF, with every stage R&C coupling with a cut off at 5Hz, Especially in Phono amps. Another is to regulate the B+ rail after the first C stage, and perhaps at sensitve stages like a phono amp. But I have used several stages of R&C with C = 470uF and R maybe 220 ohms in a preamp and chosen poles for RC couplings that cut out very LF. I am using five stages of 1K and 100uF which is pretty much the same as 220R and 470uF and I still see the LF noise. Indeed, but if all you did was to replace the 100uF in your amp with 470uF, and leave the 1k R you have, then you will see a big reduction of LF noise. I agree. I just might try that. But if you insist on response to 2Hz, you may need to regulate each stage. To maintain a response flat down to 20Hz over several stages implies most will need to be flat to around 5Hz. Yes, but if the response is -3dB at 5Hz at say 3 consecutive stages then at 1Hz the response is becoming 18dB/Octave. Yes but at present I am working with just one stage and this LF noise makes it impossible to measure distortion of the order of 0.1% or less at 100Hz . You are not measuring distortion properly. If you wish to view the harmonic products of a 100Hz signal, ie, the 200, 300, 400, 500, 600, 700 Hz content, then you should have a filter at the input to the THD measuring gear with a steep slope of attenuation below 150Hz to exclude the slow speed "jitter" of the LF noise content from the B+ rails. Often you will find that if your mains F = 50Hz, you'll get 100Hz rectifier hum trying to enter your THD test gear wherever possible, and it will beat with your 100Hz test signal from a sig gene, and if the mains caused hum is a very accurate 100.00Hz signal, and your sig gene is say 103Hz, then you'l get a damned confounding 3Hz tone appearing in your test signal after trying so hard to remove the 100Hz to see what the other harmonics are. So I'd never ever try to use a test F close to the mains F x 2, and my low THD oscilator operated at 1kHz. The test gear to detect THD has a bridged T LC notch filter I built and with course and fine adjustment pots for the slight variation of oscillator F and notch depth so that phase accuracy is nearly perfect. The oscillator F stability needs to be excellent, but I achieved that with an Wien Bridge type using a tiny lamp and opamp. THD is 0.002%, OK for all tube circuits.I have a bandpass filter after the after the LCR bridged T notch filter to examine how much THD is in signals below 1Vrms. 0.002% can be seen after being filtered out and amplified up to see it on the CRO. The BPF has a sharp cut off below 1kHz. Mains hum and other LF junk is all well fitered out, and I can see all THD content up to 10kHz. I have yet another filter with a tunable bandpass filter with Q = 50 so i can tune along the band between 1kHz and 1kHz and look at each harmonic above 0.002%.Intermodulation products can also be viewd using this filter. I built all my own THD measuring gear BC, or before I got a computer. If I was now beginning to measure signals now I'd have a SpectraLab program and let the PC analyse it all and show the results onthe PC screen, but I have never got around to buying the program and setting up a spare old PC in the dirty dusty workshed where I spend time building wonderful sounding amps which measure as well as the best tube amps ever made. The old fashioned analog gear I have tells me enough, and I cannot really achieve a better outcome without drastically increasing the NFB which I refuse to do to get THD from 0.05% to 0.001% at normal listening levels. Patrick Turner. Cheers ian There is no point in "lookong at it" on the mains. Assume the mains is a sewer of noise. Simple filters and good design or regs will remove all your noise. I only suggested that as a means of confirming its source, but all the replies I have had so far seem to confirm it is mains borne so I don't now see a need to do that. Remember that the mains 50Hz or 60Hz signal is a 'carrier' of LF noise information just as audio information is carried by an RF carrier in a radio set, Sometimes slow moving signals in mains are like dc variations as well. Most of this is filtered out by the transformer. Patrick Turner. If you have LC filters, make sure their resonant F is below 7Hz or lower. And maybe add some series R to damp the LC resonance. With resonance, noise content around the LC Fo will appear in the B+ larger than at the resevoir cap unless you have applied critical damping. This may mean the added series R = 1.4 x XC or XL at Fo; the response won't be peaked at the pole F, ie, Fo. Z100uF = 1,600 ohms at 1 Hz, so with R = 1k, there is little filtering at 1Hz. If you had C = 470uF, ZC = 340 ohms and 4 RC filter sections would give attenuation = approx 1/2 x 1/2 x 1/2 x 1/2 = 1/16. Its still not a huge amount of filtering. But say your amp output has 1mV of LF noise with 1k + 100uF, then with 1k + 470uF the noise should be maybe 0.1 mV, and maybe OK for you. A regulator will do better because they work right down to DC. Patrick Turner. Cheers Ian |
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Low Frequency Mains Noise
Ian Bell wrote: Patrick Turner wrote: John Byrns wrote: In article , Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. -- Regards, John Byrns Considerable self regulation occurs in mains supplies in populos cities and towns because where you have thousands of people all using something and averaging say 1kW mains input per household 24/7, then if the mains voltage is 240V, the current average is 4 amps and the load must be 60 ohms. Say there are 3,000 housholds sharing your circuit locally and that the wiring between them is low resistance then the load connected to the circuit you are on is 60 / 3,000, ie, a very low load indeed on average, but one that changes its value enough to cause Ian's observed LF voltages on a B+ rail of +/- 30mV. That's about what I see here. After several R&C stages of filtering the rail noise movements are much less than +/-30mV. If you have a CCS type of load supplying dc to a triode anode, then the low Ra of ther triode makes a divider with the CCS, or with a resistance load for dc feed, and the anode LF noise voltage will be much lower than at the rail. Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. Smooth thy B+ rails lest ye suffer noise... 'Tis sunday here, and I've been to church. My church has walls extending from horizon to horizon, and the ceiling is the sky above. The God Of Triodes resides in His Regality of the Firmament, and for me to attend this Church I ride 100km on bicycle in the presence of a few like minded souls and under watchful eye of the God Of Bicycles, or GOB to ye who are not aware of His Presence. But where you have a pentode or cascode pair of triodes then Ra is high, and any noise on the rail appears at the anode. The other source of LF noise in phono and line level stages is where you have a large grid bias R to bias a typical gain triode. Its grid may be cap coupled to a preceding triode gain stage with low Ra. At say 1kHz, the low Ra and low XC shunt the high resistance of the grid biasing R, say 470k or 1M. But as F becomes lower, the XC becomes a high impedance, and effectively the grid is then coupled toa good noise source, ie, the high resistance. Its most important in phono or microphone first stages to have a relatively low grid biasing R, and 47k is typical for MM inputs. The carts are usually direct coupled to a grid, and the standard 47k is so the grid is biased when no cart in connected and the 47 happens to be the most common recommended loading for most MM carts. That's not a problem in this case. The mic transformer is connected direct to the first stage grid. OK. So its a high gain mic amp we have here. I assume the response is level between at least 20Hz to 20kHz. What is the maximum voltage gain at say 1kHz? What is the noise at the amp output with maximum gain and with input grid directly shunted to 0V close to the input? What happens if you temporally connect a spare 1,000uF or more to be in parallel to the last 100uF cap in the filter line up, ie, the filter cap giving the B+ supply to stage 1 of the mic amp? Noise should fall a lot with the extra C added where it'll do the most good. Think big, use enormous C values if you cannot bring yourself to make what might be a very simple shunt regulator in your preamp. Beware using simple zener diode based shunt regs close to mic input stages though. The LF noise of the zener will find its way into signal paths. Patrick Turner. |
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Low Frequency Mains Noise
"Patrick Turner" When I measure the "240Vac" here is usually is stable enough to get a nearly constant reading on a DMM, ** Must be a basic 3.5 digit one ( 2000 count) with only 1 volt resolution when reading 240 volts AC. Any DMM with a larger count allows changes of 0.1 volts to be seen - then the last digit is never steady. But it slowly varies between 235Vac on cold winter nights of heavy loadings to 255Vac when load is light. ** What drivel. It will instantly drop by 7 or 8 volts if you switch a ( 2.4 kW) electric heater on AND jumps up by 6 volts when the ( 2kW) jug turns itself off when it has boiled. When I look at the rectified Vdc, it shows the expected variations of +/- 30mV. ** Complete ********. Any unregulated DC supply FOLLOWS all variations in the AC voltage by the same percentage. Regulated DC supplies REGULATE against supply voltage and load current variations, not just load current. ...... Phil |
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Low Frequency Mains Noise
Phil Allison wrote: "Ian Bell" Phil Allison wrote: Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming in unless it is over 5 kHz. ** Complete fallacy. ** No - it is TOTAL FACT. Transformers by definition have a zero at zero Hz. ** Got nothing to do with *VARIATIONS * occurring in the AC supply voltage !!!!! They have a pole at some frequency determined by the transformer inductance and the source and load impedance. ** Complete drivel - there is no such pole as (un-gapped) transformers have no defined primary inductance value. Core saturation is what limits an AC power transformer's ability to work at lower frequencies than 56/60 Hz. This must be where you made you IDIOT assumption that ordinary transformers remove low frequency variations on the AC supply. Try monitoring the 230 volt AC supply voltage on a DMM and see it jumping around by up to +/ - 5 volts all the time. I shall ** **** off you arrogant, know nothing ****ing IMBECILE. I think he wants to learn though. I doubt he will accept your travel recomendations or be perturbed by your colourful character description. And perhaps, like so many of us, he has a lot to learn because like us he doesn't know everything; maybe all the group can learn from us giving more advice on what he should be trying to see, but for confusinous and wavatious signal vexations. In principle, I agree with you about the perfectly lousy way transformers can pass 1Hz sine waves. Bmax is often 1.2Tesla in any normal PT if designed for industrial use, or maybe 0.8T for good audio amps. Consider a 240V primary winding. The B is proportional to the applied winding voltage and inversely to applied frequency. So at 1Hz, a B = 1.2T would occur when the applied voltage was about 4.8Vrms. Iron isn't very linear though. Certainly, if you tried to apply 240V at 1Hz to a mains tranny, the iron would saturate badly and heavy currents would flow and the fuse would blow. At such low F, the winding inductance has a reactance that is very low, and the winding almost works like a resistance equal to the resistance of the primary winding. I suspect the OP has yet to investigate fully the operation of transformers to see the limits of their operation. I recommend he hasten to his work shed and CRO to teach himself about how trannies work, and armed with books written about the subject, lest he fill his head with bull**** concepts that don't describe what is happening as they are described in the right books. Its electro magnetic understanding that is required, and that is always a difficult for so many... Patrick Turner. ..... Phil |
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Low Frequency Mains Noise
flipper wrote:
On Sat, 01 Nov 2008 10:21:18 +0000, Ian Bell wrote: flipper wrote: On Fri, 31 Oct 2008 11:01:16 +0000, Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Because the noise is so small, about the only place you can see it is on the smoothed output with a scope set to ac input. I suspect this noise is mains borne but I don't know how to see such small low frequency signals on the mains itself. Any ideas what it is, how to look at it on the mains and how to get rid of it? From what you describe it sounds like mains noise, as you suspected, and with the thousands upon thousands of things running, coming on and off, or whatever, throughout the grid lord only knows what causes each 'blip'. AC mains are neither 'clean' nor stable, at least not to the degree you're talking about. Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region You have a couple of dubious assumptions here with the first being it's a 'separate' signal. It's more likely riding on the AC with the AC acting as, essentially, a carrier. Yes, of course, it's amplitude modulation caused by the varying overall load - no wonder it gets through the transformer. The second is that the AC 'noise' has the same frequency component as your filtered measurement. I don't think I was assuming that, only that it contained a component at the frequency I meausured. plus the five stage RC filter I am using is over 120dB down at 50Hz so even a decade or so lower I would have expected its attenuation to be significant. I'm sure it is 'significant', which gives you an idea of just how bad it is on the AC mains itself. It isn't a 10mV blip out there. Which means I could probably see it on the mains side - where's that scope probe? With it out of band and low I'm not sure what you're trying to fix but the common choices are to filter till it's below whatever tolerance you decide upon or regulate. I wreaks havoc with distortion measurements at 100Hz when you are expecting a result below 0.1%. Sounds like maybe a measurement problem because out of band signals shouldn't be in the measurement. Cheers Ian |
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Low Frequency Mains Noise
Patrick Turner wrote:
Ian Bell wrote: Phil Allison wrote: "Ian Bell" Indeed. However, I would have expected the transformer to attenuate noise in the 1Hz region ** Complete insanity. Ordinary power transformers attenuate nothing coming it unless it is over 5 kHz. ** Complete fallacy. Transformers by definition have a zero at zero Hz. They have a pole at some frequency determined by the transformer inductance and the source and load impedance. From that pole down to zero Hz their response falls by at least 20dB/decade. But you missed the reason why someone said "Complete insanity". Transformers convey *all* the variations of mains amplitude, and its these amplitude variations which are seen on the rectified signal at the resovoir cap of the dc supply. Hence there are *very* low frenquencies seen at the this resovoir capacitor. Try studying the way AM detectors work in radio sets using a diode and RC network. I realise that now. I am really disappointed that some members of this group have to take such an arrogant stance in explaining things. All that was necessary was to point out, as flipper did, that it is low frequency amplitude modulation of the ac mains I am seeing. Asinine remarks like 'try studying the way am detectors work' are singularly unhelpful. And apologies in advance for the rant. Cheers Ian |
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Low Frequency Mains Noise
Ian Bell wrote: Since there are untold preamp designs with unregulated supplies Bad preamps you mean ? Graham |
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Low Frequency Mains Noise
Ian Bell wrote: Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. Not a problem with solid state you see. PSRRs are typically in the 120dB region. Graham |
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Low Frequency Mains Noise
Ian Bell wrote: That's what the spec says but I suspect it does better than that in practice (see below). WHY ? Did you have Divine guidance on this point ? Graham |
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Low Frequency Mains Noise
Patrick Turner wrote:
Ian Bell wrote: Patrick Turner wrote: John Byrns wrote: In article , Ian Bell wrote: Eeyore wrote: Ian Bell wrote: On several preamp PSUs I have built in that last couple of years I have noticed the same thing. A very low frequency low level variation in the output voltage. My latest PSU uses a series of five 100uF caps with 1K resistors in between and it still exhibits this problem. The 'noise' seems to be below 1Hz in frequency, fairly random and peaks typically between +- 10mV with occasional excursions to +20 to 30mV. Well what do you expect without regulation ? Unhelpful replies like yours I guess. His reply may have been unhelpful, but it was certainly to the point. I don't get it, if you don't want to use regulation how can you expect to avoid low frequency voltage fluctuations? You should probably count yourself as lucky that you are seeing only "occasional excursions to +20 to 30mV", I would expect several volts, or more, without regulation. -- Regards, John Byrns Considerable self regulation occurs in mains supplies in populos cities and towns because where you have thousands of people all using something and averaging say 1kW mains input per household 24/7, then if the mains voltage is 240V, the current average is 4 amps and the load must be 60 ohms. Say there are 3,000 housholds sharing your circuit locally and that the wiring between them is low resistance then the load connected to the circuit you are on is 60 / 3,000, ie, a very low load indeed on average, but one that changes its value enough to cause Ian's observed LF voltages on a B+ rail of +/- 30mV. That's about what I see here. After several R&C stages of filtering the rail noise movements are much less than +/-30mV. If you have a CCS type of load supplying dc to a triode anode, then the low Ra of ther triode makes a divider with the CCS, or with a resistance load for dc feed, and the anode LF noise voltage will be much lower than at the rail. Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. Smooth thy B+ rails lest ye suffer noise... 'Tis sunday here, and I've been to church. My church has walls extending from horizon to horizon, and the ceiling is the sky above. The God Of Triodes resides in His Regality of the Firmament, and for me to attend this Church I ride 100km on bicycle in the presence of a few like minded souls and under watchful eye of the God Of Bicycles, or GOB to ye who are not aware of His Presence. But where you have a pentode or cascode pair of triodes then Ra is high, and any noise on the rail appears at the anode. The other source of LF noise in phono and line level stages is where you have a large grid bias R to bias a typical gain triode. Its grid may be cap coupled to a preceding triode gain stage with low Ra. At say 1kHz, the low Ra and low XC shunt the high resistance of the grid biasing R, say 470k or 1M. But as F becomes lower, the XC becomes a high impedance, and effectively the grid is then coupled toa good noise source, ie, the high resistance. Its most important in phono or microphone first stages to have a relatively low grid biasing R, and 47k is typical for MM inputs. The carts are usually direct coupled to a grid, and the standard 47k is so the grid is biased when no cart in connected and the 47 happens to be the most common recommended loading for most MM carts. That's not a problem in this case. The mic transformer is connected direct to the first stage grid. OK. So its a high gain mic amp we have here. I assume the response is level between at least 20Hz to 20kHz. What is the maximum voltage gain at say 1kHz? 24dB What is the noise at the amp output with maximum gain and with input grid directly shunted to 0V close to the input? Not possible to measure accurately at the moment as the LF blips whack the meter needle all over the place one you try to see noise below 1mV. That said, looking at it on a scope you can see the broadband noise underneath the LF blips and I would estimate the noise at the output with the input shorted as about 50uV rms. What happens if you temporally connect a spare 1,000uF or more to be in parallel to the last 100uF cap in the filter line up, ie, the filter cap giving the B+ supply to stage 1 of the mic amp? Not tried that yet, I have a spare 470uF or two so I'll try that. Noise should fall a lot with the extra C added where it'll do the most good. Think big, use enormous C values if you cannot bring yourself to make what might be a very simple shunt regulator in your preamp. Yes and no. I was using just a couple of RC stages using 470uF but then I realised the five RC stage of 100Uf each would perform better. I suppose I could go bananas and replace all the 100uF caps with 470uF ones. Beware using simple zener diode based shunt regs close to mic input stages though. The LF noise of the zener will find its way into signal paths. Agreed. I have been looking at the Maida regulator as a means of eliminating the LF noise *prior* to the normal RC string. Cheers Ian Patrick Turner. |
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Low Frequency Mains Noise
Eeyore wrote:
Ian Bell wrote: Since there are untold preamp designs with unregulated supplies Bad preamps you mean ? Graham You tell me. I have the schematics for many broadcast consoles and professional music mixers from the 50s that use unregulated supplies. Cheers Ian |
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Low Frequency Mains Noise
"Eeysore the Congenital LIAR " Ian Bell wrote: Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. Not a problem with solid state you see. PSRRs are typically in the 120dB region. ** Really? On what planet is that ??? TL071 = 86 dB typ NE5532 = 100 dB typ. ..... Phil |
#36
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Eeyore wrote:
Ian Bell wrote: Yes, that's my other problem. The simple CC cascade pair with NFB I am using has a very poor PSRR. Not a problem with solid state you see. PSRRs are typically in the 120dB region. Graham Only if you use op amps. The dc coupled triple typical of the Neve class A preamps is just as bad as its tubed version. Cheers Ian |
#37
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Eeyore wrote:
Ian Bell wrote: That's what the spec says but I suspect it does better than that in practice (see below). WHY ? Did you have Divine guidance on this point ? Graham No, I used my eyes. Cheers Ian |
#38
Posted to rec.audio.tubes
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Low Frequency Mains Noise
"Ian Bell" wrote in message
Eeyore wrote: Ian Bell wrote: Since there are untold preamp designs with unregulated supplies Bad preamps you mean ? Graham You tell me. I have the schematics for many broadcast consoles and professional music mixers from the 50s that use unregulated supplies. Standards for audio were a lot lower in the 50s, if you didn't ever notice when you listened to a lot of recordings from that time. There are some exceptional recordings that still sound good, but in general, it was not a good time for quality sound reproduction. |
#39
Posted to rec.audio.tubes
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Low Frequency Mains Noise
"Ian Bell" wrote in message
I wreaks havoc with distortion measurements at 100Hz when you are expecting a result below 0.1%. Build or buy yourself a high pass filter. In the days of tubes and vinyl, high pass filter in the 400-500 Hz range were commonly used for measurements at 1 KHz and above. |
#40
Posted to rec.audio.tubes
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Low Frequency Mains Noise
Arny Krueger wrote:
"Ian Bell" wrote in message Eeyore wrote: Ian Bell wrote: Since there are untold preamp designs with unregulated supplies Bad preamps you mean ? Graham You tell me. I have the schematics for many broadcast consoles and professional music mixers from the 50s that use unregulated supplies. Standards for audio were a lot lower in the 50s, if you didn't ever notice when you listened to a lot of recordings from that time. There are some exceptional recordings that still sound good, but in general, it was not a good time for quality sound reproduction. I would not say they were a lot lower. The flat bandwidth extended only from 50Hz to 15KHz but elsewhere the specs were close to today's. An RCA broadcast console achieved a 68dB S/N ratio with a -60dBm input signal which implies an equivalent input noise of -128dBm - and that would not be achievable with the LF noise I am seeing right now. Maybe they had better conditioned mains supplies for studios or maybe there was just less crap on the mains in those days. Cheers Ian |
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