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Old 11th October 2012, 01:26 AM   #41
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Quote:
Originally Posted by hemgjord View Post
Sorry, couldn't resist.
The capacitance value needed for desired ripple rejection in a given circuit is not really open for debate.
As for sound quality, everything is open...

/Olof
Good point, but I assume these values will be exceeded anyway...

--------------------

Good film caps are pricey, and using all film for the PS definitely raises the cost quite a bit, PS caps and coupling caps are probably 25% of the total parts cost of my amp, but the differences in ESR are an order of magnitude or so and this is clearly audible. Also, some high frequency harshness is gone by getting rid of the electrolytic. Unfortunately, electrolytics poison the sound of an amplifier, especially in the signal path and final stage of the PS.

Last edited by Davec113; 11th October 2012 at 01:37 AM.
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Old 11th October 2012, 07:39 AM   #42
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Originally Posted by Davec113 View Post
Unfortunately, electrolytics poison the sound of an amplifier, especially in the signal path and final stage of the PS.
Electrolytics particularly poison the sound as cathode bypasses. I spent a whole afternoon clipping and unclipping about a dozen caps as cathode bypasses. The electrolytics were horrible, bypassed or not. Even the film caps added some fog to the sound. I now use filament bias to eliminate caps altogether in the cathode circuit. And I also couple stages with interstage transformers to avoid coupling caps. Basically I'd always take iron over a cap, even a teflon one and the rest are worse. There's a whole boutique capacitor culture out there and a lot of money is spent, but where there's the option of not using a cap at all, I think that's the route to take. Otherwise film caps everywhere in the PSU where you can't avoid them.
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Old 12th October 2012, 01:13 AM   #43
Rush is offline Rush  United States
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You tube guys are funny.
Some of us use JFETs, MOSFETs and SITs and have .4 Farad power supplies. Think about an all film supply? Naw!
I have been through the gamut with tubes in all stages, preamps, power amps, triodes, tetrodes, pentodes, choke loaded, transformer coupled and right now I am enjoying Nelson Pass' F5 and F5T, F6, Salas Simplistic Phono, DCB1, etc.
Back to by passing electrolytic power supply caps with film.

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Old 12th October 2012, 08:02 AM   #44
disco is offline disco  Netherlands
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Quote:
Originally Posted by andyjevans View Post
Electrolytics particularly poison the sound as cathode bypasses.
Do you describe the effect of degenerative feedback? Take in account that more is happening with a circuit when the cathode is bypassed or not. Think of stage output impedance, reacting to next stage input capacitance for starters.
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Old 12th October 2012, 09:05 AM   #45
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Yes of course, but with filament bias the cathode resistor is usually somewhere between 5 and 50 ohms. That's all.
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Old 12th October 2012, 10:21 AM   #46
disco is offline disco  Netherlands
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Are you saying that because of the low resistor value the change in output impedance is modest? That would be true I guess.
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Old 12th October 2012, 02:03 PM   #47
rongon is offline rongon  United States
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It's funny how these conversations can wander...

The original question was about power supply capacitors, where electrolytics are often unavoidable, especially in power amps that draw too much current for most regulated supplies.

I agree that you can hear an electrolytic cap used as a bypass on a cathode resistor. I've tested that for myself, and I'll usually resort to either leaving the cathode resistor unbypassed (and accepting the changes in performance), or using an LED or two to make the bias voltage.

But in a power amp main power supply, I'll often need something like +470V at 200mA, and let's say that's in a Dyna ST70 chassis (cramped). There's nowhere near enough room to fit a couple of 47uF 630V Solen caps, motor run PPIO caps, or something like that. So high-voltage electrolytics will need to be used.

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Old 12th October 2012, 02:14 PM   #48
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Mount them on the outside cage, Three Mile Island style.
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Old 12th October 2012, 02:26 PM   #49
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IMO, Wavebourn offers wise advice to always think about current flow. You need to think about the design in terms of both LF and RF. Zero impedance is not always a good thing, as it's easy to get ringing against circuit inductance. If you're trying to kill off RF, you have to do it at the source, which is usually the rectifiers.

Getting a very low impedance across a wide band is difficult. It can't be done by bypassing electrolytics with the usual values of film caps. Using motor run or similar large films will do it. The math is a PITA and making decent measurements on the bench is more difficult than one might think, but it can be proven with the right gear. Plots from a wide band LCR meter show it, but such things are a bit outside my budget. The nice Quadtech (IET) or Agilent ones are over $10k.

IMO again, if bypassing electrolytics has an effect, the correct design solution is really to bypass someplace else, either at the rectifiers to kill RF, or at the circuit where you're not fighting with the inductance of the wiring from the main caps. The correct return point for the bypass is also important, and it may not be where you think!
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Old 12th October 2012, 02:33 PM   #50
20to20 is offline 20to20  United States
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Quote:
Originally Posted by andyjevans View Post
Yes of course, but with filament bias the cathode resistor is usually somewhere between 5 and 50 ohms. That's all.
Surely you realize that the filament is a resistor equal to whatever the other resistor's value is reduced by, to meet the biasing needs.
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