Re: Question on audio amplifiers for a change...

From: Stefan Heinzmann (stefan_heinzmann_at_yahoo.com)
Date: 07/11/04 

Date: Sun, 11 Jul 2004 15:15:47 +0200

Paul Burridge wrote:
> On Sat, 10 Jul 2004 19:37:25 GMT, "Kevin Aylward"
> <salesEXTRACT@anasoft.co.uk> wrote:
>
>
>>This cap one is pretty much irrelevant. No ones uses a diffstage because
>>of this somewhat dubious issue.
>
>
> The Doyen of audio amp design, Douglas Self, disagrees with you, Kev.

Where does he say that he uses differential input stages in order to
save capacitors? He does say in chapters 3 and 6 that capacitors can be
a source of distortion, but that doesn't prevent him from using them
anyway. May I refer you to the schematics of his 50W amplifier in
chapter 6. This amp, of which he says that it has the potential to be
Blameless (in his terminology), nevertheless uses a differential input
stage *and* a capacitor to block DC at the input.

If you care to read his argument on capacitor distortion, you will find
that a condition for its occurence is when the capacitor is used to
block DC with a significant load behind it. His measurements for
demonstrating the effect were done with a 680 Ohm load. In his amplifier
the load of the DC blocking capacitor is 10k Ohm.

It can't be the cost of the capacitor, either, that is the problem. A
10µF electrolytic is cheap indeed. Omitting it at the amplifier's input
would remove any protection against DC voltages being applied to the
amp. The consequence of this could be a smoked loudspeaker or a smoked
amp or both. You would have to prevent this through other means which
are invariably more expensive than the omitted cap.

Having said that I have to add that I find the usage of just one
polarized cap at the input somewhat sloppy. It will not offer sufficient
protection if significant DC is applied at the wrong polarity, because
an ordinary electrolytic will start to leak appreciably beyond about 2V
reverse bias. This is unlikely enough not to be a real problem, I admit,
but why not do it right and use a nonpolarized cap (or two normal ones
in series)? 

-- 
Cheers
Stefan

Relevant Pages

  • Re: Need help on Peavey Standard 130 watt Power Module - Peavey Standard.zip (0/1)
    ... will allow you to use a CD player as your signal generator. ... going to be doing amp repair on a regular basis, ... The "death cap" you mention is just an RF dumper; ... old 8 ohm 50 watt speaker for a load. ...
    (alt.guitar.amps)
  • Re: Amplifier stability
    ... capacitor is not a realistic reactive load. ... Note:- In order to reduce spam, any email without 'Brian Gaff' ... What's the best way to check if a power amp is likely to be stable ...
    (uk.rec.audio)
  • Re: 100nF load stability issue
    ... > will give a peak in the sine wave response with a cap only at the output. ... The amp is stable with 0.22 uF or more at the ... But if I want to get rid of oscillations with 0.1 uF as a load, ...
    (rec.audio.tubes)
  • Re: Best tap for NFB?
    ... amp oscillates at LF, especially when there isn't any load connected ... wants to oscillate at 0.5Hz. ... I use a 50k pot in series with a double gang tuning cap from an old ...
    (rec.audio.tubes)
  • Re: Distortion In LM386 Amplifier
    ... battery, no success, I have yet to try the capacitor, but I don't ... Yeah, replaced that cap, sound is undistorted at normal listening ... Another thing I've noticed with this amp, and the iPod amp I built for ...
    (sci.electronics.basics)