Re: 'PC Speaker' Tube Amp

On Fri, 9 May 2008 04:26:34 +0100, "Ian Iveson"
<IanIveson.home@[EMAIL PROTECTED]
> wrote:

>John Byrns wrote:
<snip>

>>
>> Flipper explained that it is positive feedback used to 
>> increase the open
>> loop gain of the amplifier.  This is common in phono 
>> preamp stages, and
>> not unheard of in power amps.
>
>OK, sorry flipper. But why, exactly? Is there some general 
>consideration that leads a designer, in this or that 
>situation, to nest a lesser positive loop inside a greater, 
>negative one? I imagine it may be possible to get more gain 
>out of a relatively linear section of the amp, so that can 
>be used to counter the distortion produced by a relatively 
>non-linear section somewhere else in the global loop. Or 
>perhaps I am missing some advantage in the frequency domain?
>

Sorry for the delay but you answered the question just as you asked
it.

It's to get more gain for the subsequent 18dB global NFB, with enough
left over for good sensitivity.

It wouldn't do any good if it were over just one stage because PFB
increases distortion and the NFB would just get you back to where you
started. But it's quite useful with multiple stages where one stage,
(or more) is better than the other, as if the case with the gain
triode vs the PP output stage. It has the effect of 'shifting' some of
that linearity from one to the other so there's a net overall
improvement even if NFB=PFB.

The topology I used is almost identical to that in the Harmon Kardon
A300 and A500 except they used 1/2 a 12AX7 for the gain triode and 1/2
a 12AU7 for the phase splitter. They used a plate boot strapped
version in the A224 and A230, both from a single 12AX7.

I've used both, as well as a variant with SS for the phase splitter,
but chose the (cap coupled) A300 version in this case because it's
less susceptible to bias shift over tube variations (note that the PFB
operates on the bias just as it does signal). I.E. The gain triode
plate can end up 'anywhere' without throwing off the phase splitter
operating point and I wanted to be able to substitute 6JW8s for the
6AW8s, which potentially aggravates the tube to tube bias variation.