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Old 4th January 2011, 10:32 PM   #11
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Originally Posted by hartze11 View Post
Can someone clarify the use of the auxiliary cap?
For single ended amplifiers, the last capacitor in the power supply chain becomes part of the audio signal path. Whatever AC signal fluctuates through the output transformer and the power tube must also travel through that capacitor. You want a fair amount of capacitance there to minimize hum, but you also want the "best" audio characteristics possible too. This probably means a low AC impedance, but surely includes other attributes as well (minimal hysteresis? high frequency response?). I'm not an electrical engineer, nor have I studied signal theory. AC is still a lot of black art to me, and I never did care for vectors. I don't fully understand it, but I know there is some science there.

Regardless, the concept is that an aluminum electrolytic will buy you a lot of hum reducing capacitance for not too much money, and an additional oil filled foil type cap will offer the desirable audio characteristics. I say give it a try and see if you can hear a difference. Motor run caps sell for less than $20 shipped.
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Old 4th January 2011, 11:40 PM   #12
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So let me ask this a little differently...

I think what we're dealing with is power supply filtering and ripple reduction.

I found a 50 uF motor-run cap that I installed (yep, $20, oil filled, HUGE). It seems that the Aux Cap is in parallel with C2 (I used a 120 uF or so) giving me a total of 170 uF.

Wouldn't I get the same ripple filtering by installing a larger value C2 ? This would reduce the need for the external motor run cap? Maybe larger values aren't feasible before needing to go to the motor run cap sizes...
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Old 5th January 2011, 12:32 AM   #13
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As Ty stated the key here is that there is some synergism between the two different types of capacitors. The on board electrolytic packs a lot of capacitance in a small package. It like all capacitors is not perfect. Electrolytics have flaws that make it hard to get all of the energy out of the cap quickly and they aren't too good of a bypass at the upper end of the audio spectrum. The polypropelyne motor run cap can dump its energy quickly enough to blow the end off of a small screwdriver (really improves the transients) and it works great across the entire audio range, but one large enough to provide all of the capacitance to run the entire amp would be quite large. Together thay work quite well.
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Old 5th January 2011, 01:18 AM   #14
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OK, I think I'm starting to get it now.... I'll stick with the BUC (big ugly cap).

Thanks guys.
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