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

From: Ben Bradley (ben_nospam_bradley_at_mindspring.com)
Date: 07/09/04 

Date: Fri, 09 Jul 2004 17:18:08 -0400

On Fri, 09 Jul 2004 21:18:48 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

>Hi all,
>
>Why does the first stage of all commercial audio amps seem to consist
>of a differential (long-tailed) pair?

   My short answer: it makes it like an op-amp, one base is the signal
input, the other base is feedback from a resistive divider on the
output to set gain, with the lower resistor having big cap in series
to ground so gain at DC is low, keeping the output near zero.
   It's much easier to design this way, using the usual transistors
that have widely varying beta, yet the amplier's gain from unit to
unit varies very little because it's mostly determined by the feedback
resistors.

>AFAIA, the chief characteristic
>of this configuration is just to eliminate 'mush' on an input signal,

   Huh?

>like if it's come from a long line input and has a high noise floor,

   You mean like a balanced input to cancel interference induced in
the cable?

>which clearly isn't the case between your CD/tape-deck/phonograph or
>whatever. So why?
>
>p.

Relevant Pages

  • Re: Opamp preamplifier design question
    ... if you want, with a feedback capacitor, but within the audio ... the accuracy of the programmed gain. ... capacitor to limit the maximum voltage amplitude out of the pre- ... SPDT analog switch in series with the feedback resistor & capacitor ...
    (sci.electronics.basics)
  • Re: Question on audio amplifiers for a change...
    ... the other base is feedback from a resistive divider on the ... >to ground so gain at DC is low, ... >unit varies very little because it's mostly determined by the feedback ...
    (sci.electronics.design)
  • Re: Ping Turner --- a real tube question
    ... >>> First gain stage measurements are about what I expected, ... Or too high a plate resistor ... > feedback loop isn't in but, when I add that, the feedback R (DC ... > drop to 116V with IaQ about .5mA at this gain setting. ...
    (rec.audio.tubes)
  • Re: NFB101 Crisis!
    ... Imagine an inverting amp with an open loop gain of -Ao with a feedback ... resistor from output to input of Rf and an input resistor from signal ...
    (rec.audio.tubes)
  • Re: feedback & stability (long)
    ... Perhaps the same transient response observation method can be used on the ... Feedback widens LF bandwidth to a few Hz. ... transformer is rolling off, this circuit is rising loop gain up, ...
    (rec.audio.tubes)
Quantcast