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#41
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Transformer attenuators
Iain Cherches Complete ASS "
Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ........ Phil |
#42
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Transformer attenuators
"Phil Allison" wrote in message
"Eeysore" For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point. A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. No doubt your intent is to show a very conservative configuration, Phil. You've done that! In fact droops of 1 dB @ 20KHz are still innocious. In fact it is common for power amplifiers with output inductors to size them for 0.5 to 1 dB droop with minimum rated load. |
#43
Posted to rec.audio.tubes
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Transformer attenuators
Phil Allison wrote: "Eeysore" For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point. I did worse than that. I wasn't fully awake. A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. Sounds right. To use my -0.1dB criterion that gives a C of 480pF or about 15 feet of typical cable. Graham |
#44
Posted to rec.audio.tubes
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Transformer attenuators
Eeyore wrote
Ian Iveson wrote: It is not difficult, given short cables, to match a CD player with a valve amp via a potentiometer, because there can be a x100 difference between source impedance and power amp input impedance, thus allowing for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k source and 100k load. In my view a resistor is preferable to an inductor if both work OK. There is NO *MATCHING* involved whatever. It's purely a voltage transfer process. Voltage transfer works just fine with *ANY* practical ratio of impedances where the load impedance is typically ~ = 10 times the source impedance for minimal loading ( 1dB ). Matching refers a situation where 2 equal values of source and load resistance / impedance are used to maximise *power* transfer as opposed to voltage transfer. Maximum power transfer (impedance matching) is not a requirement for good performance in modern equipment and would indeed tend to degrade performance in modern equipment were it practiced. Please use the correct terms. What would you consider to be the correct term, in the context of my post? The way I have used the term "matching" is common. Clearly I have not used it in the sense of matching for optimum power transfer. I have used it in the sense of matching for compatibility, to preserve linearity. Considering I spelled this out in terms of the 10-1 rule of thumb, there is no fear of confusion. Thanks anyway. Ian |
#45
Posted to rec.audio.tubes
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Transformer attenuators
Iain wrote 1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? No. I would be interested to hear from folk who have compared the two sensibly. I am planning to do just that, when time permits. So in most circumstances this is a question about whether a transformer is better than an active preamp such as your own. In which case the issue is distortion, presumably. I am told that the tracking accuracy between channels is superior, althought with most stepped attenuators at betetr than 1% I wonder if the improvement is audible. Stepped attenuators can be trimmed, surely? I don't think it's a question of distortion either,and besides, the Sowter has a THD of 0.03% at +20dBu at 50Hz. But what is it for a 2V input at, say 20Hz, or whatever is the lowest frequency it may encounter? There may be some folk who would prefer transformer coupling in any case, possibly because they object to using capacitors in the signal path, but it's a while since anyone owned up to this apparent fetish, and I forget how they argue their case. One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. Most matchning and input transformers are not designed to have DC on them. Oh. OK. I have a couple here and they aren't gapped. But this seems a bit of a nonsequitur to me anyway, since I hadn't intended to imply that they are used with DC. Only that they have by their nature no DC output, and therefore obviate the need for a DC blocking cap on the following input. That's what got me to wonder about the wisdom of using a transformer to define the lower limit of bandwidth. Signals of lower frequency will be distorted, and the distortion products will be in band. So if you use a transformer as an attenuator, do you ensure that it is not subject to frequencies below its quoted bandwidth? How? Just wondering. cheers, Ian |
#46
Posted to rec.audio.tubes
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Transformer attenuators
Phil Allison wrote: "Eeysore ****ing Pommy **** " For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point - ****HEAD ! A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. Why do you keep re-posting your correction ? Graham |
#47
Posted to rec.audio.tubes
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Transformer attenuators
Arny Krueger wrote: In fact droops of 1 dB @ 20KHz are still innocious. Innocuous my arse ! In fact it is common for power amplifiers with output inductors to size them for 0.5 to 1 dB droop with minimum rated load. Typical output inductor 6uH j0.75 ohms at 20kHz Droop due to the inductor into 8 ohms (more relevant to hi-fi ) ~ 0.04 dB. Graham |
#48
Posted to rec.audio.tubes
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Transformer attenuators
flipper wrote: Eeyore wrote: west wrote: "Eeyore" wrote Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. Why are you always trying to pick fly **** out of pepper? Maybe you should frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to see you post something in his language. Food for thought. "Better" has no definable meaning. That's the question he's asking: for an explanation or definition of why it's supposedly 'better', or by what 'definition one would claim it's 'better', and why one would use it. We do finally seem to have established that point thank you. Some clarity never hurt. E.g. one often hears here that SET's are better than other amplifiers types yet they are demonstrably hugely technically inferior in almost every respect. If this is to determine what 'better' means then better = worse. How about asking a question that has a possible meaningful answer instead ? He did. You just want to play word games rather than deal with the intent of the question. Determining the real intent of the OP's question is hardly an example of word games. For those you'd need to engage Mr Joot's attention. Graham |
#49
Posted to rec.audio.tubes
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Transformer attenuators
Ian Iveson wrote: Eeyore wrote Ian Iveson wrote: It is not difficult, given short cables, to match a CD player with a valve amp via a potentiometer, because there can be a x100 difference between source impedance and power amp input impedance, thus allowing for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k source and 100k load. In my view a resistor is preferable to an inductor if both work OK. There is NO *MATCHING* involved whatever. It's purely a voltage transfer process. Voltage transfer works just fine with *ANY* practical ratio of impedances where the load impedance is typically ~ = 10 times the source impedance for minimal loading ( 1dB ). Matching refers a situation where 2 equal values of source and load resistance / impedance are used to maximise *power* transfer as opposed to voltage transfer. Maximum power transfer (impedance matching) is not a requirement for good performance in modern equipment and would indeed tend to degrade performance in modern equipment were it practiced. Please use the correct terms. What would you consider to be the correct term, in the context of my post? The way I have used the term "matching" is common. And like many things that are 'common' it is completely wrong. Clearly I have not used it in the sense of matching for optimum power transfer. I have used it in the sense of matching for compatibility, to preserve linearity. Considering I spelled this out in terms of the 10-1 rule of thumb, there is no fear of confusion. That's not *matching* though. At least not to my way of thinking. The point being that in a voltage transfer interface there is nothing to match. That's one of the many attractions it has. In the pro-audio world if 'matching' came into the conversation it would invariably be in the context of 'level matching' such as when interconnecting professional and consumer equipment with different operating levels. Impedances would never normally merit a second thought, or even a first one ! Graham |
#50
Posted to rec.audio.tubes
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Transformer attenuators
"Eeysore ****ing Lying Pommy **** From Hell " Genetic, autistic ****wits like Graham Stevenson are not human. Just sub human Zombies with malicious intent. Ought to be baited like any other vermin. ........ Phil |
#51
Posted to rec.audio.tubes
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Transformer attenuators
"Eeysore ****ing Lying Pommy **** From Hell " Genetic, autistic ****wits like Graham Stevenson are not human. Just sub human Zombies with malicious intent. Ought to be baited like any other vermin. ........ Phil |
#52
Posted to rec.audio.tubes
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Transformer attenuators
In article ,
"Ian Iveson" wrote: One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. I don't see why distortion would become any more of a problem below the lowest frequency the transformer the transformer is rated for, assuming the LF signal level is no higher than the signal level the transformer is designed for in its specified range. If the shunt inductance of the transformer is being used in this way to limit the LF response it implies that a finite source impedance is being used to drive the transformer and if you go through the math I think you will find that LF signals take the transformer no closer to saturation than do signals at the transformers rated LF limit. I'm not sure if this argument hold's for actual DC though? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#53
Posted to rec.audio.tubes
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Transformer attenuators
In article i,
"Iain Churches" wrote: "west" wrote in message news:zIL9i.212$ng.61@trnddc07... "Eeyore" wrote in message ... west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Possibly. How much do you consider a cable which is long? I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance in reasonable lengths. I was looking at a typical commercially-built passive controller yesterday. It had cables of 1m50 on both input and output, and a 100k DACT stepped attenuator. What does "1m50" mean? It doesn't make sense any way I parse it. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#54
Posted to rec.audio.tubes
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Transformer attenuators
John Byrns wrote: if you go through the math I think you will find that LF signals take the transformer no closer to saturation than do signals at the transformers rated LF limit. I'm not sure if this argument hold's for actual DC though? Uh ??? Can someone unscramble that ? Graham |
#55
Posted to rec.audio.tubes
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Transformer attenuators
In article ,
Eeyore wrote: John Byrns wrote: if you go through the math I think you will find that LF signals take the transformer no closer to saturation than do signals at the transformers rated LF limit. I'm not sure if this argument hold's for actual DC though? Uh ??? Can someone unscramble that ? Graham, I am surprised that someone of your supposed technical knowledge doesn't get the point immediately! Please keep in mind the part of my post that you clipped, which was that the OP was talking about using a transformer to limit the LF response of a system, and my claim that if that were the case it would assume a finite source impedance driving the transformer. Do the math, think about it a bit, and if you have a normal number of neurons you should be able to figure it out. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#56
Posted to rec.audio.tubes
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Transformer attenuators
John Byrns wrote: What does "1m50" mean? It doesn't make sense any way I parse it. 1.50 metres (59") Ever seen a 2.2 microfarad cap marked 2u2 ? Graham |
#57
Posted to rec.audio.tubes
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Transformer attenuators
John Byrns wrote: In article , Eeyore wrote: John Byrns wrote: if you go through the math I think you will find that LF signals take the transformer no closer to saturation than do signals at the transformers rated LF limit. I'm not sure if this argument hold's for actual DC though? Uh ??? Can someone unscramble that ? Graham, I am surprised that someone of your supposed technical knowledge doesn't get the point immediately! Please keep in mind the part of my post that you clipped, which was that the OP was talking about using a transformer to limit the LF response of a system, and my claim that if that were the case it would assume a finite source impedance driving the transformer. Do the math, think about it a bit, and if you have a normal number of neurons you should be able to figure it out. You assertion still doesn't help the above quoted sentences of yours make any sense though. Graham |
#58
Posted to rec.audio.tubes
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Transformer attenuators
"John Byrns" wrote in message ... What does "1m50" mean? It doesn't make sense any way I parse it. Sorry John. That's how we write one metre and fifty centimetres or 150cms here in Scandinavia. We also use the comma and not the point (full stop/dot) as a decimal marker. That might have confused you even mo-) Iain |
#59
Posted to rec.audio.tubes
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Transformer attenuators
In article ,
Eeyore wrote: John Byrns wrote: What does "1m50" mean? It doesn't make sense any way I parse it. 1.50 metres (59") Ever seen a 2.2 microfarad cap marked 2u2 ? Yes, frequently, and that was exactly my problem, I thought Iain was talking about the cable capacitance and I was trying to decode it as such, and it just didn't make sense in that context. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#60
Posted to rec.audio.tubes
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Transformer attenuators
In article ,
"Iain Churches" wrote: "John Byrns" wrote in message ... What does "1m50" mean? It doesn't make sense any way I parse it. Sorry John. That's how we write one metre and fifty centimetres or 150cms here in Scandinavia. Thanks, I thought you were talking about the cable capacitance where "1m50" doesn't make sense and/or is a little extreme. We also use the comma and not the point (full stop/dot) as a decimal marker. That might have confused you even mo-) No, I am not even aware of comma/point usage in that context. I learned my lesson on that as a small boy reading British radio magazines, or was it German? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#61
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Transformer attenuators
"John Byrns" wrote in message ... In article , "Iain Churches" wrote: "John Byrns" wrote in message ... What does "1m50" mean? It doesn't make sense any way I parse it. Sorry John. That's how we write one metre and fifty centimetres or 150cms here in Scandinavia. Thanks, I thought you were talking about the cable capacitance where "1m50" doesn't make sense and/or is a little extreme. We also use the comma and not the point (full stop/dot) as a decimal marker. That might have confused you even mo-) No, I am not even aware of comma/point usage in that context. I learned my lesson on that as a small boy reading British radio magazines, or was it German? Almost certainly German. Here we use the point as a designator to mark thousands, so 10. 000 is ten thousand, as opposed to 10,000 in the UK (and also in the US?) One has to be *very* carefully when entering bids on e-Bay:-))) Regards Iain Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#62
Posted to rec.audio.tubes
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Transformer attenuators
Most people seem to pick 100k. For a passive control unit ? That would be exceptionally silly, but given the audiophoolery that routinely exists surprises me not one jot. At its -6dB position, a 100k volume control presnts an output resistance of 25k to the world. A relatively modest cable capacitance of ~ 300 pF will cause a -3dB low pass filter effect at ~ 20kHz. Now that'll be audible. Isn't that why volume controls are usually located inside the audio amplifier? The input jack feeds thru a selector switch to the top of the pot. The wiper of the pot feeds (usually via a cap) directly to a tube grid, without the capacitance to ground you'd get with a long patch cord. Or interconnect if you prefer... |
#63
Posted to rec.audio.tubes
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Transformer attenuators
John Byrns wrote: In article , "Iain Churches" wrote: "John Byrns" wrote in message ... What does "1m50" mean? It doesn't make sense any way I parse it. Sorry John. That's how we write one metre and fifty centimetres or 150cms here in Scandinavia. Thanks, I thought you were talking about the cable capacitance where "1m50" doesn't make sense and/or is a little extreme. We also use the comma and not the point (full stop/dot) as a decimal marker. That might have confused you even mo-) No, I am not even aware of comma/point usage in that context. I learned my lesson on that as a small boy reading British radio magazines, or was it German? That would have been German. Brits use the period / full stop / point as the decimal separator like yourselves. I love the French equivalent for 'floating point'. It translates as 'flying comma'. :~) Graham |
#64
Posted to rec.audio.tubes
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Transformer attenuators
robert casey wrote: Most people seem to pick 100k. For a passive control unit ? That would be exceptionally silly, but given the audiophoolery that routinely exists surprises me not one jot. At its -6dB position, a 100k volume control presnts an output resistance of 25k to the world. A relatively modest cable capacitance of ~ 300 pF will cause a -3dB low pass filter effect at ~ 20kHz. Now that'll be audible. Isn't that why volume controls are usually located inside the audio amplifier? The input jack feeds thru a selector switch to the top of the pot. The wiper of the pot feeds (usually via a cap) directly to a tube grid, without the capacitance to ground you'd get with a long patch cord. Or interconnect if you prefer... I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. Graham |
#65
Posted to rec.audio.tubes
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Transformer attenuators
John said:
So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. I don't see why distortion would become any more of a problem below the lowest frequency the transformer the transformer is rated for, assuming the LF signal level is no higher than the signal level the transformer is designed for in its specified range. If the shunt inductance of the transformer is being used in this way to limit the LF response it implies that a finite source impedance is being used to drive the transformer and if you go through the math I think you will find that LF signals take the transformer no closer to saturation than do signals at the transformers rated LF limit. I'm not sure if this argument hold's for actual DC though? What argument? You are asking me if my guess at your maths hold true for DC? Dunno, haven't done the sums yet. Let's see... No. Actually, my guess at your maths doesn't hold true at all, as far as I have calculated so far. I can see a countervailing force...a restorative reaction...vaguely. Voltage falls and frequency falls and you reckon the upshot remains pretty much the same. But I don't think it's enough. Maths on NG is really hard. Take two constants, K = 2 x pi and B = N squared. N is the turns ratio f is the frequency Vs is the source voltage Rs is the source resistance RL is the load resistance Lp is the primary inductance Adopt the following circuit model. Vs is applied to a voltage divider comprising Rs and X(Lp)||B.RL ip is the current through X(Lp), and is directly related to magnetising current. From some algebra I reckon that: ip = Vs / {Rs + K.f.Lp.(1 + Rs / B.RL)} Note I have no primary resistance. Note then that, at DC, f = 0 and so ip = Vs / Rs Your argument could be true if Rs was great enough compared to the rest of the bottom line. But if the transformer is being used to limit bandwidth, then we should know how big it is...ie at -6dB it will be equal to X(Lp)||B.RL For f to be insignificant wrt magnetising current, Rs would need to be an order of magnitude greater than that, I would expect. A bit more maths to do...but so far I don't think you are right. I guess you have an easier solution. What is it? And why, if as seems inevitable your maths has a term for frequency in it, can't you find out whether it "holds for DC" by substituting zero for f in the equation? Incidentally, if I include the fact that Lp increases with ip according to the BH curve for the core material, then the maths becomes impossible, as we are short of an expression for the BH curve and even if we had one the equation would end up a proper dogs dinner. I still can't simulate a non-linear inductor...stuck for either an expression or a way of getting spice to use a look-up table and interpolate. Isn't this already worked out in RDH4 or somewhere? Patrick? cheers, Ian |
#66
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Transformer attenuators
Isn't that why volume controls are usually located inside the audio amplifier? The input jack feeds thru a selector switch to the top of the pot. The wiper of the pot feeds (usually via a cap) directly to a tube grid, without the capacitance to ground you'd get with a long patch cord. Or interconnect if you prefer... I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. Also a good reason, which doesn't conflict with my reason. Sometimes things work out together like that. |
#67
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Transformer attenuators
flipper wrote: Eeyore wrote: I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. Depends on who's convenience you're talking about and it might have been more 'convenient' for the 50's TV viewer to have a 20 foot wired remote. 50's wives would never have stood for such a thing. Graham |
#68
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Transformer attenuators
"Eeyore" wrote in message ... robert casey wrote: Most people seem to pick 100k. For a passive control unit ? That would be exceptionally silly, but given the audiophoolery that routinely exists surprises me not one jot. At its -6dB position, a 100k volume control presnts an output resistance of 25k to the world. A relatively modest cable capacitance of ~ 300 pF will cause a -3dB low pass filter effect at ~ 20kHz. Now that'll be audible. Isn't that why volume controls are usually located inside the audio amplifier? The input jack feeds thru a selector switch to the top of the pot. The wiper of the pot feeds (usually via a cap) directly to a tube grid, without the capacitance to ground you'd get with a long patch cord. Or interconnect if you prefer... I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. In these days of high level sources, no preamp is actually required, so one can mount a pair stepped attenuators in the power amp chassis, and feed the CD player direct. http://www.kolumbus.fi/iain.churches...-30%20SA01.jpg Iain Graham |
#69
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Transformer attenuators
"Eeyore" wrote in message ... Iain Churches wrote: "Eeyore" wrote Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Assuming the source impedance is lowish compared to the pot value, the maximum output impedance / resistance of a potentiometer volume control is 1/4 the pots' value. For a 10k pot , that's 2.5 kohms. If we want to be confident that the pot's resistance and the load capacitance of the output cable's not introducing any colouration we need to choose a 'minus dB' @ say 20kHz that's small enough to be inaudible. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? Doing the sums elsewhere....... Could you do a typical calculation here? It would be useful to know how to calculate a pot and cable combination. Best regards Iain |
#70
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Transformer attenuators
"Iain Churches" Doing the sums elsewhere....... Could you do a typical calculation here? It would be useful to know how to calculate a pot and cable combination. ** It is sooooo simple. The highest output resistance for a pot or step attenuator is when the level is at -6dB. For a 10 kohm pot, when the wiper is set exactly half way - you get 5 kohms to ground and 5 kohms in series with the source. The two 5 kohms are effectively in parallel for a low impedance source & so sum electrically to 2.5 kohms. he formula for capacitive reactance is: Xc = 1 / ( 2.pi.F.C ) Plus, when the resistance and Xc are the same number, attenuation is 3dB. For a simple 6 dB per octave low pass filter you can apply a very good rule of thumb. If the - 3dB frequency is at F, then at F/2 the attenuation is 1dB from flat & at F/4 the attenuation is 0.25dB from flat. Example: Say the cable capacitance is known to be 800pF, Then at 80 kHz Xc = 1 ( 2 x 3.14 x 8exp4 x 8exp-10 ) = 2590 ohms = near enough to 2.5 kohms So, the attenuation will be 3 dB at 80 kHz, worst case. It follows from the rules of thumb that at 40 kHz it will be 1 dB and at 20 kHz it will be 0.25dB. ....... Phil |
#71
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Transformer attenuators
On Fri, 8 Jun 2007 08:21:08 +0300, "Iain Churches"
wrote: Could you do a typical calculation here? It would be useful to know how to calculate a pot and cable combination. Two useful simplifications (subject to correction and embellishment I'd hope): Assume source impedance to be negligibly small and load impedance to be capacitive only. Output resistance is at maximum at 6dB attenuation, where it's 1/4 of the ladder's resistance. For example, a 10K ohm ladder has a maximum output resistance of 2K5 ohms. Calculate a -3db frequency where output resistance equals 1/ 2 pi F C , where C is loading capacitance. This works out as: F-sub-3dB = reciprocal 2 pi (output resistance) (loading C) Divide the 3dB number by four to get a 1/4dB number and assume that that is good enough! (Or fudge from here. Arf.) In practice, if you can drive a 10K ohm ladder, you're golden, as I believe Arny has already said. Not everything can drive 10K ohms well, so the numbers do still matter, but the ability is a useful idealized goal. Much thanks, as always, Chris Hornbeck |
#72
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Transformer attenuators
"Phil Allison" wrote in message ... "Iain Churches" Doing the sums elsewhere....... Could you do a typical calculation here? It would be useful to know how to calculate a pot and cable combination. ** It is sooooo simple. The highest output resistance for a pot or step attenuator is when the level is at -6dB. For a 10 kohm pot, when the wiper is set exactly half way - you get 5 kohms to ground and 5 kohms in series with the source. The two 5 kohms are effectively in parallel for a low impedance source & so sum electrically to 2.5 kohms. he formula for capacitive reactance is: Xc = 1 / ( 2.pi.F.C ) Plus, when the resistance and Xc are the same number, attenuation is 3dB. For a simple 6 dB per octave low pass filter you can apply a very good rule of thumb. If the - 3dB frequency is at F, then at F/2 the attenuation is 1dB from flat & at F/4 the attenuation is 0.25dB from flat. Example: Say the cable capacitance is known to be 800pF, Then at 80 kHz Xc = 1 ( 2 x 3.14 x 8exp4 x 8exp-10 ) = 2590 ohms = near enough to 2.5 kohms So, the attenuation will be 3 dB at 80 kHz, worst case. It follows from the rules of thumb that at 40 kHz it will be 1 dB and at 20 kHz it will be 0.25dB. Phil. It's a lot simpler than I imagined. I have never needed to calculate that kind of thing, even though the formula was familiar. Thanks for taking the time to explain it. Iain |
#73
Posted to rec.audio.tubes
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote in message I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. In these days of high level sources, no preamp is actually required, so one can mount a pair stepped attenuators in the power amp chassis, and feed the CD player direct. http://www.kolumbus.fi/iain.churches...-30%20SA01.jpg For many decades I didn't own a preamp. Everything of mine had line level outs anyway (including my turntable). The current one only qualifies as a preamp since it has a phono input but in reality it's just a source switcher the way I use it. Graham |
#74
Posted to rec.audio.tubes
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? Doing the sums elsewhere....... Could you do a typical calculation here? It would be useful to know how to calculate a pot and cable combination. I messed up last time round. I post a new thread. Graham |
#75
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in
message flipper wrote: Eeyore wrote: I thought volume controls are usually inside the amplifier because it's simply convenient to put them there. Depends on who's convenience you're talking about and it might have been more 'convenient' for the 50's TV viewer to have a 20 foot wired remote. 50's wives would never have stood for such a thing. The first commercial wireless TV remote went on the market in 1955, and sold well. 50's wives arguably had wireless remotes at their disposal. |
#76
Posted to rec.audio.tubes
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Transformer attenuators
"Iain Churches" wrote in message
i.fi In these days of high level sources, no preamp is actually required, so one can mount a pair stepped attenuators in the power amp chassis, and feed the CD player direct. Somebody needs to clue Iain in on the existence of 2-channel stepped attenuators, and the 50-year old technology about how to wire in a stepped or continuously-variable balance control. http://www.kolumbus.fi/iain.churches...-30%20SA01.jpg Butt-ugly amplfier if there ever was one. |
#77
Posted to rec.audio.tubes
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Transformer attenuators
"John Byrns" wrote in message
In article , "Ian Iveson" wrote: One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. I don't see why distortion would become any more of a problem below the lowest frequency the transformer the transformer is rated for, assuming the LF signal level is no higher than the signal level the transformer is designed for in its specified range. Distortion is more of a problem at low frequencies because of core saturation effects: http://www.rane.com/note159.html "Core saturation happens when the magnetic field in the core reaches its maximum possible density, which is what happens when the applied voltage polarity remains the same for too long." Obviously, the applied voltage polarity remains the same for a longer time at lower frequencies than higher frequencies. "Saturation has nothing to do with power delivery: the onset of saturation depends only on the voltage waveform applied to the primary." |
#78
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in
message Arny Krueger wrote: In fact droops of 1 dB @ 20KHz are still innocious. Innocuous my arse ! http://www.pcavtech.com/soundcards/t...LevelMatch.gif Shows that octave-wide dip at 20 KHz can be almost 3 dB without being audible. http://www.pcabx.com/technical/dips_pips_tips/index.htm Has a downloadable sample illustrating a - 3 dB first order butterworth roll-off. In fact it is common for power amplifiers with output inductors to size them for 0.5 to 1 dB droop with minimum rated load. Typical output inductor 6uH j0.75 ohms at 20kHz Droop due to the inductor into 8 ohms (more relevant to hi-fi ) ~ 0.04 dB. I've measured the droop - the amp's own roll-off must be a major contributor. |
#79
Posted to rec.audio.tubes
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Transformer attenuators
In article ,
"Arny Krueger" wrote: "John Byrns" wrote in message In article , "Ian Iveson" wrote: One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. I don't see why distortion would become any more of a problem below the lowest frequency the transformer the transformer is rated for, assuming the LF signal level is no higher than the signal level the transformer is designed for in its specified range. Distortion is more of a problem at low frequencies because of core saturation effects: http://www.rane.com/note159.html "Core saturation happens when the magnetic field in the core reaches its maximum possible density, which is what happens when the applied voltage polarity remains the same for too long." Obviously, the applied voltage polarity remains the same for a longer time at lower frequencies than higher frequencies. "Saturation has nothing to do with power delivery: the onset of saturation depends only on the voltage waveform applied to the primary." That quote should be corrected to state that saturation depends on frequency as well as voltage. The reason distortion (saturation) would not be a problem when a transformer is being used to limit the LF response as the OP was asking about, is because what we are talking about then is a low pass filter where the source resistance driving the transformer primary operates in conjunction with the transformer's primary inductance to roll of the LF response below a specific point. This roll off of the primary voltage at lower frequencies prevents the core from going into saturation (distortion) at lower frequencies as would happen if the primary voltage were held constant as the frequency decreases. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#80
Posted to rec.audio.tubes
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Transformer attenuators
"John Byrns" wrote in message
In article , "Arny Krueger" wrote: "John Byrns" wrote in message In article , "Ian Iveson" wrote: One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. I don't see why distortion would become any more of a problem below the lowest frequency the transformer the transformer is rated for, assuming the LF signal level is no higher than the signal level the transformer is designed for in its specified range. Distortion is more of a problem at low frequencies because of core saturation effects: http://www.rane.com/note159.html "Core saturation happens when the magnetic field in the core reaches its maximum possible density, which is what happens when the applied voltage polarity remains the same for too long." Obviously, the applied voltage polarity remains the same for a longer time at lower frequencies than higher frequencies. "Saturation has nothing to do with power delivery: the onset of saturation depends only on the voltage waveform applied to the primary." That quote should be corrected to state that saturation depends on frequency as well as voltage. Why? the previous quote already said that once! The reason distortion (saturation) would not be a problem when a transformer is being used to limit the LF response as the OP was asking about, It is the frequency and amplitude of the applied voltage that counts. When a transformer is being used as a high pass filter, its primary is receiving the applied voltage. |
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