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Old 11th May 2007, 11:45 PM   #1
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Default Are electrolytics evil?

Does this logic make any sense?

First, differential measurements show me that electrolytic coupling caps introduce some kind of mid or high frequency "hash" into the signal that can't be nulled out. It appears to be a distortion product of some type, nothing simple or linear. Film caps don't do this, and the signal can be nulled down into the noise floor. [I've proven this on the bench to my satisfaction]

With an unregulated power supply you'll see signal modulation on the filter caps, so this same hash is likely created at some level on the power supply lines. [I can see the modulation, but can't see the hash- there's nothing useful to compare against for a measurement. if the hash is there, the source impedance is too low to get rid of it by using bypass caps, film or otherwise]

Amplifiers typically have far better low frequency PSRR than high. If mid or high frequency signals are being generated, they may affect the signal. [PSRR is, I think, well understood and easily measured]

Thus, the last thing in the power supply shouldn't be an electrolytic of any type, but a fast regulator or at least a "clean up" circuit, with a non-electrolytic bypass cap. [Explains some high end practices I previously thought were nuts]

The traditional circuit designer in me has a lot of trouble believing there is anything wrong with traditional PS circuits, much less that they affect the sound, but a lot of people say otherwise. Worse, some say regulators drain the life out of music. No idea what that means in technical terms. What do you think, pro or con? If for nothing more than entertainment purposes, rip this logic to shreds!
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Old 11th May 2007, 11:58 PM   #2
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Okay, before we take this further, why don't you draw up a couple of graphs relating the working frequency of the cap with the amount and type of distortion? That should give a better idea of what's going on. It may well be that distortion is frequency related, and not significant at 50/60Hz, (or 100/120Hz depending on your PSU configuration).
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Old 12th May 2007, 12:19 AM   #3
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pinkmouse- I'm not at all concerned with what happens at the line (mains) frequency, though I do need to put some levels to all this. Anything at line frequency should be covered by PSRR. What I'm concerned about is the audio signal on the caps, and the resulting higher frequency hash getting back into the signal- I'm mostly talking power amps here because I can see the effects.
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Old 12th May 2007, 12:22 AM   #4
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Could you explain in detail your nulling technique and how you managed to measure distortion? I would like to replicate that experiment myself.
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Old 12th May 2007, 12:40 AM   #5
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Sure- get a high gain differential amp with lots of CMRR. I use a 1A7A plug-in for the old Tek boatanchor scopes. It's the best reason to keep them around. Method 1- Feed a signal to your power amp and to one channel of the diff amp. Feed the output of the power amp to a load, and to a 10K or so multi-turn pot. Connect that to the other input of the diff amp. Adjust the pot for best null, while increasing the gain of the diff amp. You'll have trouble if the amp has any phase shift or other oddities, but it's possible to concentrate on specific frequency bands, or use the filters on the diff amp to eliminate some of the LF stuff. Method 2- Feed a signal (I use preamp and CD player) to both amplifier inputs. Connect the diff amp between the amplifier outputs. Load them as you desire, but load them exactly the same. Put the 10K pot on whichever amplifier output has a bit more signal than the other- they'll never be exactly identical at the sensitivity of this measurement. Null the amp and see how good you can do. Now, change one component in one channel, say a coupling cap. Note the difference in null, and see if you can re-null it. When I do this test, the difference between decent film caps doesn't show up, but the difference between films and electrolytics clearly does. You can do the same test without the amp at all. Just drive two caps through two resistors to ground. Measure the difference between the two cap-resistor junctions. Note that any difference in cap values will foul up the null at low frequencies, so don't try to compare a large electrolytic with a small film! Gotta match 'em closely.
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Old 12th May 2007, 03:10 AM   #6
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Default Re: Are electrolytics evil?

Originally posted by Conrad Hoffman

Are electrolytics evil?

Does this logic make any sense?

Yes. Yes they are...

from the 30's-60's,

electrolytics, were not considered a permanet fixture

in any equipment, and a sorce of revenue, when replaced.

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