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#1
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DELTA - Cap Analysis; Cap Biasing
In preparation for replacing or in some cases removing the electrolytics in my Soundcraft Delta, I've been cataloging those in the signal path. I've made offset measurements on both sides of many caps and found some with zero offset on either side. Good candidates for removal, I'd guess. (I did throw a 0.5 uf cap across the scope probe to filter the AC noise component, which was obscuring the small DC offsets.) I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) CAP BIASING - where electrolytic caps must be used, this seems like an interesting idea to try, but I wonder how best to do this. How much bias V do we need? If I grab, say, the 17V positive rail through dropping resistors to provide a couple of volts, won't there be increased crosstalk by some signal getting back into the pos. rail and into other bias injection points? (Not to mention the op amps themselves.) Should I use a little regulator, say a 7803 (if there is such a beast, or a 7805 if not) local to each cap "pair" I'd want to bias? By pair I mean two electros replacing an original single cap, with the + sides tied back-to-back and the bias injected there. Would the output stages of such regulators impact the signal fidelity? Thanks again for the help and experiences in this area. Thanks to those who have responded to these Delta threads in the past, especially Mr. Williams, Mr. Dorsey, and Mr. Sank (sorry if I missed anybody). Frank Stearns Mobile Audio -- |
#2
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On Fri, 22 Apr 2005 19:45:23 -0000, in rec.audio.pro Frank Stearns
wrote: snip Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? snip. Its quite interesting to look at the LF phase response, when simulated in LT spice, using voltage followers instead of all the gain stages. Just make 6 or so opamps with the usual AC coupling between stages and analyse the o/p wierd , probably says a lot about the "sound" of a console Thanks again for the help and experiences in this area. Thanks to those who have responded to these Delta threads in the past, especially Mr. Williams, Mr. Dorsey, and Mr. Sank (sorry if I missed anybody). Frank Stearns Mobile Audio martin After the first death, there is no other. (Dylan Thomas) |
#3
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Frank Stearns wrote:
I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? Yes. Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) For the most part, it's a matter of the low frequency corner vs. price and space. What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? No, they are probably going into a thing with a high input impedance. Remember the low end corner is set by the input impedance and the series capacitance. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) Because the input impedance on that amp is VERY low to keep the noise down. CAP BIASING - where electrolytic caps must be used, this seems like an interesting idea to try, but I wonder how best to do this. How much bias V do we need? If I grab, say, the 17V positive rail through dropping resistors to provide a couple of volts, won't there be increased crosstalk by some signal getting back into the pos. rail and into other bias injection points? (Not to mention the op amps themselves.) Enough that it never goes through the zero-crossing region ever. Should I use a little regulator, say a 7803 (if there is such a beast, or a 7805 if not) local to each cap "pair" I'd want to bias? By pair I mean two electros replacing an original single cap, with the + sides tied back-to-back and the bias injected there. Would the output stages of such regulators impact the signal fidelity? No, you can use resistors since you have a rail that is clean. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#4
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Hi,
you mentioned you are looking at the DC bias with a scope. Good. I hope it is a dual trace scope. I have a suggestion. Set up the two channels of the scope by attaching the two probes to the same signal and adjust the gain and position of the scope channels so the two traces overlap. Use AC coupling on both channels of the scope. After you have done that, now connect the two probes to the cap in question, one probe on each side of the cap. Feed a tone through your circuit. You can now easily compare the signal in and out of the cap. At low audio frequencies you will probably see some phase shift. You can also see if the waveform is distorted by the cap. If you have a distortion analyzer or spectrum analyzer (RTA) (and a good generator) you can do it that way too. If you want to make these design changes, I suggest you try to measure the problem first so you can see how much improvement you made. After making the measurement you may decide you don't need to make the changes. Let us know what you find. Regarding your question about the cap values. The cap value needed is also a function of the impedance of the circuit. At 60 Ohms you need about 270 uF for -3 dB at 10 Hz. At 6,000 Ohms you only need 2.7 uF for -3 dB at 10 Hz. Mark |
#5
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"Frank Stearns" I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? ** No - not from just a few mV. Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) ** By using a large value, the AC signal voltage across the cap is kept small even at 20 Hz and full levels. What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? ** The smaller value cap has lower DC leakage current - leakage would create noise when the fader is moved. Plus, the impedance of the circuit is higher here. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) ** The summing inputs are "virtual earth" points - so no signal voltage appears at that point. This means that ( ideally) no signal crosstalk can find its way from one channel to another via the summing resistors. A small value cap at that point would allow low frequency crosstalk - so a big one is used instead. CAP BIASING - where electrolytic caps must be used, this seems like an interesting idea to try, but I wonder how best to do this. ** Forget it - to messy and troublesome. se bi-polar electos as these are *designed* to work with no bias and not distort. ............... Phil |
#6
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Frank Stearns wrote:
In preparation for replacing or in some cases removing the electrolytics in my Soundcraft Delta, I've been cataloging those in the signal path. I've made offset measurements on both sides of many caps and found some with zero offset on either side. Good candidates for removal, I'd guess. (I did throw a 0.5 uf cap across the scope probe to filter the AC noise component, which was obscuring the small DC offsets.) Interesting. It seems like this information would be pretty easy to confirm from the schematics. I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? In theory yes, but at this point actual performance is the most important thing. If you're so worried about distortion, why haven't you measured it? If you've got a PC with an audio interface, clever use of a free software package called the Audio Rightmark will give you a pretty complete report on noise, distortion, and frequency response in about a minute. Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) The size of the cap is only half the story - how big are the resistors that load the cap down? Tubed equipment usually has coupling caps on the order of 0.1 uF to 0.5 uF, but the load impedances are typically higher. What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? You're missing 50% of the relevant information, which is the impedance that sets the time constant of an RC circuit, along with the size of the cap. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) I guess that this circuit loads the caps down with lower impedances than the other circuits. CAP BIASING - where electrolytic caps must be used, this seems like an interesting idea to try, but I wonder how best to do this. How much bias V do we need? Ideally, the bias should be larger than the peak signal voltage. If I grab, say, the 17V positive rail through dropping resistors to provide a couple of volts, won't there be increased crosstalk by some signal getting back into the pos. rail and into other bias injection points? (Not to mention the op amps themselves.) The bias source should be heavily bypassed to ground to ensure that this doesn't happen. Using large biasing resistors would help. Should I use a little regulator, say a 7803 (if there is such a beast, or a 7805 if not) local to each cap "pair" I'd want to bias? By pair I mean two electros replacing an original single cap, with the + sides tied back-to-back and the bias injected there. Would the output stages of such regulators impact the signal fidelity? If you understand the purpose of bias, you will probably want to set the biasing voltage as high as practically possible. Obviously, you don't want to bias the caps beyond their working voltage. You don't want to bias the caps to the point where their leakage currents upset the operating points of the various circuits. Thanks again for the help and experiences in this area. Thanks to those who have responded to these Delta threads in the past, especially Mr. Williams, Mr. Dorsey, and Mr. Sank (sorry if I missed anybody). I generally don't respond to recap and reopamp posts because they are basically founded on the idea that some poorly-educated, inexperienced guy who doesn't even know how capacitors are sized is going to improve on a design by a well-experienced engineer who most likely does. ;-(. My ego is plenty inflated but not so inflated that I totally disrespect the engineering talents of the staff of the better-known, widely respected firms like Soundcraft. |
#7
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Scott Dorsey wrote:
Frank Stearns wrote: I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? Yes. Not really. With only a few mv DC across the cap, distortion is not an issue. What matters is the ac level across it. This depends on the resistance loading the cap. Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) For the most part, it's a matter of the low frequency corner vs. price and space. And cost possibly. What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? No, they are probably going into a thing with a high input impedance. Remember the low end corner is set by the input impedance and the series capacitance. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) Because the input impedance on that amp is VERY low to keep the noise down. Actually the summing amp is essentially zero input resistance. The cap size is chosen based on the series resister value feeding the virtual Ov point, otherwise it would be several farads! CAP BIASING - where electrolytic caps must be used, this seems like an interesting idea to try, but I wonder how best to do this. How much bias V do we need? If I grab, say, the 17V positive rail through dropping resistors to provide a couple of volts, won't there be increased crosstalk by some signal getting back into the pos. The main issue here is noise form the power supply feeding into the input. rail and into other bias injection points? (Not to mention the op amps themselves.) Enough that it never goes through the zero-crossing region ever. What are you trying to say here? It dosnt make sense. Should I use a little regulator, say a 7803 (if there is such a beast, or a 7805 if not) local to each cap "pair" I'd want to bias? This is making a mountain out of a mole hill. If one really needs to have an unbiased electrolyte coupling cap, use a bi-polar one, which is two back to back ones. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#8
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martin griffith wrote:
On Fri, 22 Apr 2005 19:45:23 -0000, in rec.audio.pro Frank Stearns wrote: snip Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? snip. Its quite interesting to look at the LF phase response, when simulated in LT spice, using voltage followers instead of all the gain stages. Just make 6 or so opamps with the usual AC coupling between stages and analyse the o/p wierd , probably says a lot about the "sound" of a console Unlikely. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#9
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On Sat, 23 Apr 2005 17:07:47 GMT, "Kevin Aylward"
wrote: Enough that it never goes through the zero-crossing region ever. What are you trying to say here? It dosnt make sense. If the absolute voltage across the capacitor is reversed, or actually even too low, the capacitor is nonlinear. This is making a mountain out of a mole hill. If one really needs to have an unbiased electrolyte coupling cap, use a bi-polar one, which is two back to back ones. Yes, two unbiased ones. Chris Hornbeck "This has been an account for those who don't keep them" J-LG, _Tout Va Bien_ 1972 |
#10
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Kevin Aylward wrote:
rail and into other bias injection points? (Not to mention the op amps themselves.) Enough that it never goes through the zero-crossing region ever. What are you trying to say here? It dosnt make sense. The problems with electrolytic linearity issues all have to do with cases when there is very little voltage across the cap. So the recommended design practice is to make sure there is sufficient DC bias across the cap so that at any reasonable signal level, the cap always has some voltage across it for any part of the waveform. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#11
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Phil Allison wrote:
"Frank Stearns" I'm curious about two things. First, in a few cases where offset is present (a few mV) the voltage is opposite the labeled polarity on the cap. Obviously the cap hasn't blown apart from the small reversal, but does the distortion go up? ** No - not from just a few mV. Second item is why certain cap values were chosen by the designer. Most of the interstage couplers are 47 uf/25 V -- perhaps a large enough value so that after going through a number of them the LF roll off is still below 20 Hz. (Though if stuck using these, perhaps I should bump them up to 100 uf for better LF?) ** By using a large value, the AC signal voltage across the cap is kept small even at 20 Hz and full levels. What I don't quite get, though, are the 2.2 uf/50V parts on the fader returns and outputs of most other rotary level controls. Is the idea to band-limit the LF so that the control action is clean(er) across a wide range of dirt in the part and aging? Or am I missing something? ** The smaller value cap has lower DC leakage current - leakage would create noise when the fader is moved. In principle sure, in practise, irrelevant. The capacitor value at these points are not chosen for leakage considerations. Capacitors are usually chosen to be the smallest consistent with the desired frequency response, and possible current noise for the mic input. Plus, the impedance of the circuit is higher here. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) ** The summing inputs are "virtual earth" points - so no signal voltage appears at that point. This means that ( ideally) no signal crosstalk can find its way from one channel to another via the summing resistors. A small value cap at that point would allow low frequency crosstalk - so a big one is used instead. This is not why a summer junction is used. Typically the op-amps drive the summer with a very low output resistance such that one signal feeding the other through its summing resisters will have its x-talk attenuated by this ideally zero output resistance, i.e. say 1/10k. The virtual earth summer is used to prevent *level* interaction between channels. Consider two channels sourced through say, 10k resisters with the load 10k. If one channel was now switched off by opening the resister, the gain of the other channel would change significantly due to the open channel not being in parallel with the load. The virtual summer keeps the resistance that each channel sees indepandant of the other channels state for gain reasons. This has nothing to do with x-talk. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#12
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Scott Dorsey wrote:
Kevin Aylward wrote: rail and into other bias injection points? (Not to mention the op amps themselves.) Enough that it never goes through the zero-crossing region ever. What are you trying to say here? It dosnt make sense. The problems with electrolytic linearity issues all have to do with cases when there is very little voltage across the cap. So the recommended design practice is to make sure there is sufficient DC bias across the cap so that at any reasonable signal level, the cap always has some voltage across it for any part of the waveform. --scott Ok, I understand what you are actually saying now with the term "zero crossing" in this context. This is a little unconventional way of talking about it. However, when there is litle signal accross a capacitor, it won't generate any significant distortion, so its simply not an issue. For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#13
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Some pig arrogant pommy nut case: Phil Allison wrote: ** The smaller value cap has lower DC leakage current - leakage would create noise when the fader is moved. In principle sure, in practise, irrelevant. The capacitor value at these points are not chosen for leakage considerations. ** Arrant bull****. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) ** The summing inputs are "virtual earth" points - so no signal voltage appears at that point. This means that ( ideally) no signal crosstalk can find its way from one channel to another via the summing resistors. A small value cap at that point would allow low frequency crosstalk - so a big one is used instead. This is not why a summer junction is used. ** Blatant straw man. The issue is the reason for the high cap value. Typically the op-amps drive the summer with a very low output resistance such that one signal feeding the other through its summing resisters will have its x-talk attenuated by this ideally zero output resistance, i.e. say 1/10k. ** Wrong. In a great many desks, Soundcraft and others, group and main buss summing resistors are driven from the wipers of the "pan" controls. .............. Phil |
#14
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Phil Allison wrote:
Some pig arrogant pommy nut case: Phil Allison wrote: ** The smaller value cap has lower DC leakage current - leakage would create noise when the fader is moved. In principle sure, in practise, irrelevant. The capacitor value at these points are not chosen for leakage considerations. ** Arrant bull****. Here we go again... The dc leakage at the typical mv of offset between stages is completely insignificant. The capacitor is just there to ensure that as close to zero is on the input of each stage, so that dc offsets don't multiply up. In the ideal case, the caps dont do anything. Next, the input to the L/R mix summing amps (and aux bus summers) is 1000 uf/6.3V. Why the huge value here? Why not 100 or 470? (Not saying they should be that way, just curious as to why.) ** The summing inputs are "virtual earth" points - so no signal voltage appears at that point. This means that ( ideally) no signal crosstalk can find its way from one channel to another via the summing resistors. A small value cap at that point would allow low frequency crosstalk - so a big one is used instead. This is not why a summer junction is used. ** Blatant straw man. I am correcting your erroneous assertion. The issue is the reason for the high cap value. The fundamental reason for the high capacitor value is ensure that the gain settings of each channel do not interact with other. It is not there for x-talk reduction, although it might well do that as well. Typically the op-amps drive the summer with a very low output resistance such that one signal feeding the other through its summing resisters will have its x-talk attenuated by this ideally zero output resistance, i.e. say 1/10k. ** Wrong. In a great many desks, Soundcraft and others, group and main buss summing resistors are driven from the wipers of the "pan" controls. Oh dear...those cheaper ones...well, I'll give you some points for that one... Nothing changes mate. The fundamental reason, for the virtual earth summer, as I correctly stated, is to prevent selection of one signal effecting the level of another signal. Try adding voltages any other way, and you'll get the point, but then probably not. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#15
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Some pig arrogant, ranting INSANE pommy arsehole: Phil Allison: ** The summing inputs are "virtual earth" points - so no signal voltage appears at that point. This means that ( ideally) no signal crosstalk can find its way from one channel to another via the summing resistors. A small value cap at that point would allow low frequency crosstalk - so a big one is used instead. This is not why a summer junction is used. ** Blatant straw man. I am correcting your erroneous assertion. ** No such assertion was ever posted by me. You are not sane, Kev. The issue is the reason for the high cap value. The fundamental reason for the high capacitor value is ensure that the gain settings of each channel do not interact with other. ** More arrant bull****. It is not there for x-talk reduction, although it might well do that as well. * Yet another total self contradiction. You are not sane, Kev. Typically the op-amps drive the summer with a very low output resistance such that one signal feeding the other through its summing resisters will have its x-talk attenuated by this ideally zero output resistance, i.e. say 1/10k. ** Wrong. In a great many desks, Soundcraft and others, group and main buss summing resistors are driven from the wipers of the "pan" controls. Oh dear... ** Is your missus dead yet ? those cheaper ones...well, I'll give you some points for that one... ** You are not sane, Kev. Nothing changes mate. The fundamental reason, for the virtual earth summer, as I correctly stated, is to prevent selection of one signal effecting the level of another signal. ** Same worthless old straw man. You are not sane, Kev. Try adding voltages any other way, and you'll get the point, but then probably not. ** You are not sane, Kev. **** the hell off. .................. Phil |
#16
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Kevin Aylward wrote:
For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Kevin, have you tried replacing that cap with something relevantly non-electrolytic and listened? Kevin Aylward Kind regards Peter Larsen -- ******************************************* * My site is at: http://www.muyiovatki.dk * ******************************************* |
#17
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Replace the 47 uf caps with Panasonic FM 220 uf/25 v. This removes low
end rolloffs and phase shift. If you're wiggy about reverse bias, use slow bipolar caps and bypass with a Wima MKP-2 .01 film cap. The fader amp sees a 100k impedance, that's why the 2.2 uf value. Replace with a Wima MKS-2 3.3 uf with a .01 MKP-2 across it. Use a Black Gate N or NG for the 1000 uf sum cap. Bypass with good films like MIT or InfiniCaps. Opamps with higher gain bandwidth like 5532/OPA2134 etc. require a redesign of the line input stage for stability. LT1358's will allow direct coupling due to low offsets. Replace the solo sum amp input cap to a 470 uf with a film bypass. Replace the 22k feedback resistor with a 10 k for unity gain. Replace the feedback cap with a 150 pf silver mica for stability. Reverse bias is common in these mixers, the EQ section will shift bias from positive to negative by simply rotating the frequency pot. I found it more important to have a quality bypass around these caps, it negates the slow response and allows the details through that normally would be converted to heat through the electrolytic capacitor. BTW, this info is offered as a help to the community, not to start any trouble with you folks... So don't blow a gasket, it's only audio ! Jim Williams Audio Upgrades |
#18
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Frank,
if you decide to make all these changes, please make some before and after distortion and frequency response measurements and share the results with us. thanks Mark |
#19
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Kevin Aylward wrote:
For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Sure, and if the amp is well-designed, it has DC offset on the cap. Not a hard thing to do (since the output should have zero quiescent current and the input stage is easy to design to run with a higher voltage on the base or grid). --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#20
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"Mark" writes:
if you decide to make all these changes, please make some before and after distortion and frequency response measurements and share the results with us. I will indeed. Some excellent suggestions have been made on these various Delta threads and especially this one; the least I can do is report the results -- what worked; what didn't. Step one has been bringing the thing back to a starting point. Simply replacing all the worn/noisy switches, failing 1/4" normalling phone jacks, and the mother and monitor ribbon cables has made an obvious difference already. I have new power supplies awaiting mounting on a rack panel. I have at least two shorted caps; several others I suspect of losing value, as right now the bottom on this console just isn't what it ought to be. Unfortunately I don't have access to an AP, nor can I plunk down US$10K to get one. A friend has a Neutrik Minilyzer, but am not sure that it will have the resolution to do precise measurements but will investigate that, as well as PC-based analyzer options. I can still do some crude tests with the generator and scope. I'd like to have Jim Williams do the whole console, but alas funds are a bit tight and more importantly, I wouldn't learn nearly as much as I have by wrapping myself in the schematics and looking around with the scope. The basic design of each Delta module seems pretty good (and it *is* a modular console), but as is nearly always case these systems are designed to a price point -- at least with parts. Nothing wrong with that of course; meeting a stated design target on budget (and producing a good value for the market) is good engineering. But if in the process of replacing parts that need it anyway I can move that equivalent price point a little (or a lot), and not screw up the design, I should have a very sweet sounding console, and without getting into huge piles of cash. I hope. Thanks again to all for your input. More later. Frank Stearns Mobile Audio -- |
#22
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Scott Dorsey wrote:
Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Sure, and if the amp is well-designed, it has DC offset on the cap. Not at all. There is no practical reason to run a DC offset on the cap, and the majority of well designed professional power amps, imo, don't do this. Do you have any in mind that do? In principle, I am happier using a bipolar, but at the end of the day, its matters little. Not a hard thing to do (since the output should have zero quiescent current and the input stage is easy to design to run with a higher voltage on the base or grid). --scott I know of no standard power amps that this. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#23
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Peter Larsen wrote:
Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Kevin, have you tried replacing that cap with something relevantly non-electrolytic and listened? If the distortion is 0.001%, I see no reason to comparison listen to them at all, and I don't. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#24
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Frank Stearns wrote:
I have at least two shorted caps; several others I suspect of losing value, as right now the bottom on this console just isn't what it ought to be. Unfortunately I don't have access to an AP, nor can I plunk down US$10K to get one. A friend has a Neutrik Minilyzer, but am not sure that it will have the resolution to do precise measurements but will investigate that, as well as PC-based analyzer options. I can still do some crude tests with the generator and scope. You can do some incredibly powerful tests with the generator and scope. The handiest one is just running a 1 KHz square wave through the thing with the controls flat. If the top of the squarewave slopes down, you have a low frequency response problem. If it slopes up, you have a high frequency response problem. If the leading edge rings, you have an impulse problem of some sort. It is a very quick and easy way to get a good notion of the average health of the system. And if you have a problem, you can use the scope and go from stage to stage until you find the offender. But if in the process of replacing parts that need it anyway I can move that equivalent price point a little (or a lot), and not screw up the design, I should have a very sweet sounding console, and without getting into huge piles of cash. I hope. Replace all the electrolytics first.... switches and pots might be fixable by cleaning and a little Cailube, but until you get the leaky caps out you don't know if there's a real problem or not. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#25
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Kevin Aylward wrote:
Scott Dorsey wrote: Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Sure, and if the amp is well-designed, it has DC offset on the cap. Not at all. There is no practical reason to run a DC offset on the cap, and the majority of well designed professional power amps, imo, don't do this. Do you have any in mind that do? All the ones I can think of that use an overall feedback path do, which includes everything from the old Citation II to the modern Adcom 555. If there _wasn't_ any DC offset on it, you wouldn't need a cap there at all, and you could save a few pennies. In principle, I am happier using a bipolar, but at the end of the day, its matters little. Not a hard thing to do (since the output should have zero quiescent current and the input stage is easy to design to run with a higher voltage on the base or grid). I know of no standard power amps that this. Most of them do it naturally. The input stage is a differential pair with the input bases held at a reasonable bias point for the input stage to be in class A. So there's some voltage on the base to begin with. If the input stage is running off of split rails you can even do some tinkering with the collector resistors so that the base voltage is anywhere you want it to be with respect to ground. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#26
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Kevin Aylward wrote:
Peter Larsen wrote: Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Kevin, have you tried replacing that cap with something relevantly non-electrolytic and listened? If the distortion is 0.001%, I see no reason to comparison listen to them at all, and I don't. I can give you two amps with similarly low high-level distortion figures, which sound audibly different in a very obvious way, and which have very different looking outputs on a scope given a 1 KC square wave input. This is why THD is no longer as useful as it used to be. It's still a handy tool for comparing circuits of similar topology in similar situations, but it's not very general-purpose. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#27
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Sounds like someone has blown a gasket. If you react so negativly to
freely offered information, I suspect you may have a heart attack if anyone dared charge you for anything. I don't think you own a Delta console so I wonder why you are even here... BTW, I don't need to advertise for work, I have enough already, feel free to not ever ask me for anything. Go take a walk in the garden, find a girlfriend, go kill a cat and switch to de-caf! One of these should work for you. Jim Williams Audio Upgrades |
#28
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Have you actually _listened_ to said preamp?
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#29
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wrote in message ups.com... Sounds like someone has blown a gasket. If you react so negativly to freely offered information, I suspect you may have a heart attack if anyone dared charge you for anything. I don't think you own a Delta console so I wonder why you are even here... To spoil the party for a potentially large number of people around the world who silently follow this thread with keen interest in the subject - which happens to be related to rather popular Soundcraft Delta consoles and not to some frustrations of personal nature - the people who stand a chance of actually benefitting from your generous sharing of what could be considered proprietary information. That's how it looks from where I stand. Predrag |
#30
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wrote:
Sounds like someone has blown a gasket. If you react so negativly to freely offered information, I suspect you may have a heart attack if anyone dared charge you for anything. I have an aversion to people ripping of the general layman public with snakeoilsmanship. I suppose you also sell those $200 mains cables that add dynamic transparency to the overall sonic enjoyment. I don't think you own a Delta console so I wonder why you are even here... Why on earth would I want to own one? Kevin Aylward http://www.anasoft.co.uk/EE/index.html http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#31
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wrote:
Have you actually _listened_ to said preamp? Why? It will sound identical to any other preamp with 1/10 the spec, and 1/100 the price. Kevin Aylward http://www.anasoft.co.uk/EE/index.html http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#32
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Scott Dorsey wrote:
Kevin Aylward wrote: Peter Larsen wrote: Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Kevin, have you tried replacing that cap with something relevantly non-electrolytic and listened? If the distortion is 0.001%, I see no reason to comparison listen to them at all, and I don't. I can give you two amps with similarly low high-level distortion figures, which sound audibly different in a very obvious way, and which have very different looking outputs on a scope given a 1 KC square wave input. Complete and utter nonsense. Well, not unless one has a BW of 1Khz and the other 20Khz. Assuming the amps are competently designed, i.e. have similar stability phase margins etc. Nothing but mumbo jumbo I see here. This is why THD is no longer as useful as it used to be. Oh dear...here we go again... Nonsense. It's still a handy tool for comparing circuits of similar topology in similar situations, but it's not very general-purpose. --scott If the thd/imd is below say, 0.01%, and say power BW of 50Khz, and the amp is competently designed, i.e. no spurious dynamic biasing issues, stable etc, then they all sound the same. Period. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#33
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Scott Dorsey wrote:
Kevin Aylward wrote: Scott Dorsey wrote: Kevin Aylward wrote: For example, its not uncommon to have a single electrolytic cap in the feedback network of a power amp putting out +/-70V. I can assure you that such a cap typically allows for the amp to have 0.001% thd. Sure, and if the amp is well-designed, it has DC offset on the cap. Not at all. There is no practical reason to run a DC offset on the cap, and the majority of well designed professional power amps, imo, don't do this. Do you have any in mind that do? All the ones I can think of that use an overall feedback path do, which includes everything from the old Citation II to the modern Adcom 555. Weeeel, this is indeed news to me. I don't belive you untill I see a circuit. I can probably dig out 1000's that don't do what you claim. If there _wasn't_ any DC offset on it, you wouldn't need a cap there at all, and you could save a few pennies. In principle, I am happier using a bipolar, but at the end of the day, its matters little. Not a hard thing to do (since the output should have zero quiescent current and the input stage is easy to design to run with a higher voltage on the base or grid). I know of no standard power amps that this. Most of them do it naturally. Not at all. The input stage is a differential pair Well, you got that bit right. with the input bases held at a reasonable bias point for the input stage to be in class A. So there's some voltage on the base to begin with. No. Essentially, all split rail power amps that I have ever seen, have the inputs connected to ground, and I have seen and designed quite a few indeed. If the input stage is running off of split rails you can even do some tinkering with the collector resistors so that the base voltage is anywhere you want it to be with respect to ground. --scott Oh dear...you want to mess with the collector resisters to have an offset on the input? Ahmmm... and just how many amplifiers do you say you have actually designed? hint: http://www.anasoft.co.uk/EE/index.html Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#34
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On Mon, 25 Apr 2005 01:20:54 -0000, Frank Stearns
wrote: "Mark" writes: if you decide to make all these changes, please make some before and after distortion and frequency response measurements and share the results with us. I will indeed. Some excellent suggestions have been made on these various ... be. Unfortunately I don't have access to an AP, nor can I plunk down US$10K to get one. A friend has a Neutrik Minilyzer, but am not sure that it will have the resolution to do precise measurements but will investigate that, as well as PC-based analyzer options. I can still do some crude tests with the generator and scope. Try this: http://rightmark.org/ Frank Stearns Mobile Audio ----- http://mindspring.com/~benbradley |
#35
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Kevin Aylward wrote:
Oh dear...you want to mess with the collector resisters to have an offset on the input? Ahmmm... and just how many amplifiers do you say you have actually designed? Sure, you can have all the offset on the input that you want! After all, there's a DC blocking cap! (Note that by input I was referring to the negative-going input on the differential pair that is used for feedback, but of course if you offset one side of the differential pair, you offset both of them). hint: http://www.anasoft.co.uk/EE/index.html Sorry, I tried to look at the "very low distortion amp" circuits, but the resolution was very poor and they were pretty much unreadable, so I can't judge anything either way. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#36
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"Predrag Trpkov" writes:
wrote in message oups.com... Sounds like someone has blown a gasket. If you react so negativly to freely offered information, I suspect you may have a heart attack if anyone dared charge you for anything. I don't think you own a Delta console so I wonder why you are even here... To spoil the party for a potentially large number of people around the world who silently follow this thread with keen interest in the subject - which happens to be related to rather popular Soundcraft Delta consoles and not to some frustrations of personal nature - the people who stand a chance of actually benefitting from your generous sharing of what could be considered proprietary information. That's how it looks from where I stand. Indeed, Mr. Williams has been very generous with his time and help. Years ago I was in the "if the numbers don't show it, it's not real" camp. Then I got into a very good room with decent monitoring and a whole new world opened up. Differences brought on by changing parts that spec'd the "same" were no longer subtle or ambiguous, they were consistently (and sometime painfully) obvious. Numbers can tell the story -- problem is that after all these years we *still* don't commonly use enough descriptive numbers to tell the whole story. Moreover, marketing departments would have a hard time trying to put a good spin on these numbers and not confuse the average user. In the late 1940s engineers fought over whether AM radio would benefit by extending the bandwidth from 5 KHz to 15 KHz. The advocates were considered silly and ill-informed -- why, anyone knew that a 15KHz bandwidth sounded terrible so why waste all that money upgrading the systems? If I remember the story correctly, Avery Fisher set up a *live* orchestra behind an acoustically transparent (but visually opaque) curtain that in turn had tunable acoustic elements behind it. The live audience would hear either a 5KHz or 15KHz acoustical bandwidth. A nice ABX test. Of course, the full-bandwidth sound won easily. Follow-on research revealed that the original electrical systems had terrible distortion in the high bands, something that in the day many opposing engineers had not considered. It was outside their current orthodoxy, so they ignored or belittled the notion of 15KHz bandwidth. (We saw similar attitudes among some engineers early on with CDs. CDs measured "perfect" therefore were perfect, until someone described the notion of jitter -- with numbers -- and how it could affect the perceived sound.) And so it is with the modifications that Mr. Williams and others do -- there's nothing voodoo about them. (And many design engineers probably know exactly the same tricks but are limited by management to a specific design/build budget.) One of my contentions is that some of these mods really show their true worth at the very *end* of a signal chain -- the consumer's playback system. The cleaner you start back at the very beginning, the more "room" you have to accumulate various inevitable distortions before bumping into the corresponding thresholds of human perception. And this accumulation seems to be of a logrithmic nature, which reinforces the notion of keeping that signal as pure as you can for as long as you can before those accumulative step-ups start getting large. Happy recording. Frank Stearns Mobile Audio -- |
#37
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"Kevin Aylward" writes:
If the thd/imd is below say, 0.01%, and say power BW of 50Khz, and the amp is competently designed, i.e. no spurious dynamic biasing issues, stable etc, then they all sound the same. Period. Kevin - Thank you for your observations, but please -- could you describe your room where you're making these peceptual observations -- LEDE? RFZ? Hybrid? Other? Any acoustical treatment at all? Indeed things often *do* sound much the same in a typical room, even with high-end monitors. But as I found out moving into a properly-treated room suddenly many new details were revealed, and the ease of hearing differences (regardless of the item or project) went up dramatically. Such rooms also help solidly eliminate imagined differences. Frank Stearns Mobile Audio -- |
#38
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Frank Stearns wrote:
"Predrag Trpkov" writes: wrote in message ups.com... Sounds like someone has blown a gasket. If you react so negativly to freely offered information, I suspect you may have a heart attack if anyone dared charge you for anything. I don't think you own a Delta console so I wonder why you are even here... To spoil the party for a potentially large number of people around the world who silently follow this thread with keen interest in the subject - which happens to be related to rather popular Soundcraft Delta consoles and not to some frustrations of personal nature - the people who stand a chance of actually benefitting from your generous sharing of what could be considered proprietary information. That's how it looks from where I stand. Indeed, Mr. Williams has been very generous with his time and help. Yes, but like anyone who posts, its always for a purpose. He expects a return. No one does anything that they know will be net detriment to themselves. Its basic evolution. http://www.anasoft.co.uk/replicators/index.html I am not the bad guy here. The idea of trying to sell people gear at outrageous prices on the basis of specs that are proven to be values is criminal in my books. Years ago I was in the "if the numbers don't show it, it's not real" camp. Then I got into a very good room with decent monitoring and a whole new world opened up. Differences brought on by changing parts that spec'd the "same" were no longer subtle or ambiguous, they were consistently (and sometime painfully) obvious. One needs to know what one means by specs the same. Sure, one might be able to detect 1% second harmonic distortion from 1% 3rd harmonic distortion, but try that at the 0.01% level with speakers at 1%, no chance. The problem with specs is that often the spec are wrong or misleading. The specs tell the story if they are correct and fully understood. Numbers can tell the story -- problem is that after all these years we *still* don't commonly use enough descriptive numbers to tell the whole story. Moreover, marketing departments would have a hard time trying to put a good spin on these numbers and not confuse the average user. In the late 1940s engineers fought over whether AM radio would benefit by extending the bandwidth from 5 KHz to 15 KHz. The advocates were considered silly and ill-informed -- why, anyone knew that a 15KHz bandwidth sounded terrible so why waste all that money upgrading the systems? This is a poor argument as we know a lot more now then we do then. Extensive tests have now been done that quantifies much that was guessed at at that time. If I remember the story correctly, Avery Fisher set up a *live* orchestra behind an acoustically transparent (but visually opaque) curtain that in turn had tunable acoustic elements behind it. The live audience would hear either a 5KHz or 15KHz acoustical bandwidth. A nice ABX test. Of course, the full-bandwidth sound won easily. Follow-on research revealed that the original electrical systems had terrible distortion in the high bands, something that in the day many opposing engineers had not considered. It was outside their current orthodoxy, so they ignored or belittled the notion of 15KHz bandwidth. I doubt this. The considered it, and ignored it. (We saw similar attitudes among some engineers early on with CDs. CDs measured "perfect" therefore were perfect, until someone described the notion of jitter -- with numbers -- and how it could affect the perceived sound.) They were never measured perfect. The limitations were always known. We had a lot of hype from companies trying to sell product despite the shortcomings of that equipment. And so it is with the modifications that Mr. Williams and others do -- there's nothing voodoo about them. (And many design engineers probably know exactly the same tricks but are limited by management to a specific design/build budget.) Selling a two channel mic amp for $1500 is snake oil. There is zero chance that its value for money. One of my contentions is that some of these mods really show their true worth at the very *end* of a signal chain -- the consumer's playback system. The cleaner you start back at the very beginning, the more "room" you have to accumulate various inevitable distortions before bumping into the corresponding thresholds of human perception. And this accumulation seems to be of a logrithmic nature, which reinforces the notion of keeping that signal as pure as you can for as long as you can before those accumulative step-ups start getting large. There is a sensible limit. Anything better is worthless. You cant get any better then the weakest link in the chain. This is the speakers. To get distortion lower then 1% costs you an arm and a leg. To get speakers flat to 1db, costs the other arm and leg. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#39
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Frank Stearns wrote:
"Kevin Aylward" writes: If the thd/imd is below say, 0.01%, and say power BW of 50Khz, and the amp is competently designed, i.e. no spurious dynamic biasing issues, stable etc, then they all sound the same. Period. Kevin - Thank you for your observations, Not just mine, but reputable and repeatable experiments by many researches. but please -- could you describe your room where you're making these peceptual observations -- LEDE? RFZ? Hybrid? Other? Any acoustical treatment at all? Name any room you like. It matters squat. Its a dead issue. Indeed things often *do* sound much the same in a typical room, even with high-end monitors. But as I found out moving into a properly-treated room suddenly many new details were revealed, and the ease of hearing differences (regardless of the item or project) went up dramatically. Such rooms also help solidly eliminate imagined differences. This has all been thrashed 30 odd years ago. Loads of AB tests etc. It was all the rage. Those golden ear ears boys all claiming differences that proved as worthless as astrology. Why this sort of nonsense is still persisting today is pretty much incredible. Just considering a 1% distortion of a speaker for 1% verses 1.01% should tell one something about how dubious to be able to detect 0.01% differances. High end audio is a scam to make money. Its that simple. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
#40
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Kevin Aylward wrote:
Just considering a 1% distortion of a speaker for 1% verses 1.01% should tell one something about how dubious to be able to detect 0.01% differances. If the 1% is all second harmonic distortion and the 0.1% is high order non-harmonic trash, it's not dubious at all. High end audio is a scam to make money. Its that simple. Much of it is, but what does that have to do with anything? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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