Voltage source decoupling cap

IVX · 26th March 2005, 07:50 PM

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, 07:59 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 · 26th March 2005, 11:43 PM

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.
-Chris

IVX · 27th March 2005, 05:28 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, 06:03 AM

Nelson Pass used 47µF 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.

Cheers,

Hugh

__________________
Hugh R. Dean

anatech · 27th March 2005, 04:07 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.
-Chris

djk · 27th March 2005, 11:15 PM

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

IVX · 28th March 2005, 06:53 AM

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

anatech · 29th March 2005, 12:18 AM

Hi IVX,
Try them and post the results. Empirical evidence is everthing at the end of the day.
-Chris

26th March 2005, 07:59 PM	#2
Mr Evil diyAudio Member Join Date: Aug 2004 Location: Behind you	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. __________________ https://mrevil.asvachin.eu/

27th March 2005, 05:28 AM	#4
IVX diyAudio Member Join Date: Jul 2003 Location: Everywhere (Buddhist's context)	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:-) __________________ Best regards, E1.

28th March 2005, 06:53 AM	#8
IVX diyAudio Member Join Date: Jul 2003 Location: Everywhere (Buddhist's context)	BTW, I had measured ESR for the nonpolar BG 470uF/16v at 1A square wave current ~20mOhm __________________ Best regards, E1.

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26th March 2005, 11:43 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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. -Chris

27th March 2005, 06:03 AM	#5
djk diyAudio Member Join Date: Feb 2001 Location: USA	Nelson Pass used 47µF 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. Cheers, Hugh __________________ Hugh R. Dean

27th March 2005, 04:07 PM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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. -Chris

27th March 2005, 11:15 PM	#7
djk diyAudio Member Join Date: Feb 2001 Location: USA	That was a cut-and-paste from an old hard-to find thread, not real time (post#31).

29th March 2005, 12:18 AM	#9
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi IVX, Try them and post the results. Empirical evidence is everthing at the end of the day. -Chris

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