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IVX 26th March 2005 08:50 PM

Voltage source decoupling cap
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I'm sure that voltage source decoupling .1-1uF cap is quite enough, and film cap type it's good choice. However, i have advice coming from some high end people to use 470uF/16 nonpolar Black Gate. Someone can explain why? I can't find any reason so far.. BS?

Mr Evil 26th March 2005 08:59 PM

Its purpose is to keep the Vbe multiplier's impedance low at high frequencies, but as long as the transistor is suitable, the impedance will already be low to beyond the audio band, so only a smallish capacitor should be needed.

The only possible reason I can think of to go higher is if the Vbe multiplier is not perfectly stable into a particular value of capacitance, but still 470uF seems very high.

anatech 27th March 2005 12:43 AM

I agree with Mr. Evil. You are further ahead with polerized capacitors. These will never see a reversed polarity unless the rest has gone completely south. So part of the advise given is BS at best. I have had good luck with film caps around 1uF.
Best thing to do is try it and see. The caps are cheap in this range.

IVX 27th March 2005 06:28 AM

BTW, those 470uF BG around $20. High end people = magic people, they talk - we heard it, and no any tech comments. Well, if noone said that 470uF nonpolar BG will be worse vs 1uF polyester Wima, then i'll take BG, but i've still feeling about BS:-)

djk 27th March 2005 07:03 AM

Nelson Pass used 47F in many of his Threshold designs.

Post #31
Point of interest: The Vbe multiplier affects amplifier sonics, not so much of itself as it's a DC voltage source, but via its bypassing capacitor, the size and quality of which is very important, particularly in the ubiquitous Class AB amplifiers most of us use.

Too small a cap will cost strong, tight bass. The wrong cap type will confer a 'grainy' sound on the amp, as the top and bottom halves of the output stage are out of step with one another. You want something with good frequency response and very low dielectric absorption, less than 0.05.

Essentially, all caps in an audio amplifier will have huge effect on the sonics. Use the best you can find everywhere and LISTEN HARD! Even the feedback shunt electro has strong influence on the sonics. Differences in amps all too often can be traced to capacitor choices.



Hugh R. Dean

anatech 27th March 2005 05:07 PM

Hi Hugh,
I will agree with that. But a bipolar electrolytic cap will have higher ESR for no advantage. It will never see a reversed polarity. I do like the sound of a nice big film cap.

djk 28th March 2005 12:15 AM

That was a cut-and-paste from an old hard-to find thread, not real time (post#31).

IVX 28th March 2005 07:53 AM

BTW, I had measured ESR for the nonpolar BG 470uF/16v at 1A square wave current ~20mOhm

anatech 29th March 2005 01:18 AM

Try them and post the results. Empirical evidence is everthing at the end of the day.

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