Re: NEWBIE: Designing a Simple Audio Amp

From: Anthony Fremont (spam_at_anywhere.com)
Date: 01/02/05 

Date: Sun, 02 Jan 2005 15:22:19 GMT

"John Popelish" wrote
> Anthony Fremont wrote:
> >
> > "Graham Knott" wrote:
> > > John Popelish wrote:
> > > > Forgot this one (I just bought 10 of these so I should have
> > remembered
> > > > it)
> > > >
http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/TBA820M.pdf
> >
> > > Have a look at
> > > http://homepage.ntlworld.com/g.knott/elect24.htm
> >
> > I have a question about your circuit. R2 and C3; what is their
> > function?
>
> At the high frequency end of the amplifiers capability, the speaker
> tends to look inductive (rising impedance with frequency), which
> raises the open loop gain, just as the phase shift of the amplifier is
> increasing and tending to decrease the stability of the closed loop.
> This network absorbs energy at this part of the spectrum, loading down
> the gain, so that the amplifier will not oscillate at a few hundred
> kilohertz, especially if you disconnect the speaker. At ordinary
> audio frequencies, it consumes very little power.
>
> See Zobel network on this page:
> http://www.rane.com/par-z.html

Thanks for the link, I've never heard that term before. I kinda figured
it was being used to "dump" higher frequencies to ground, but didn't
know if it was an oscillation preventer or just some kind of tilt
compensation (though I would have expected that the amp would have lost
gain as frequency increased anyway, so I didn't understand bypassing the
higher frequencies to ground). I wasn't thinking about the changing
(rising) impedance of the load causing an increase in gain and leading
to potential oscillations.

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