A very interesting circuit concept - I hadn't heard the term
"augmentation" before. (It looks to have something in common - and
completely turned upside down - with the bootstrapping technique used
to increase the input resistance of emitter follower stages)
But, looking at the circuit of the preamp in Figure 6, I'm wondering
if, instead of what is virtually two independent input stages, with
their own separate transformers, you considered a differential
push-pull feedback as a single entity. Have a centre tapped
secondary in the bases (with the centre tap to AC ground), but with
the primary connected between the two emitters, ie directly across the
loop input.
The two input transistors now operate in true differential mode,
rather than as mostly-independent amplifier stages, and with only one
feedback transformer, complexity and wire losses are reduced. This
will give even more decoupling of the loop from ground.
Andy
www.g4jnt.com
On 16 December 2011 14:35, Chris Trask <[email protected]> wrote:
>>
>> This one from Chris has an input stage similar to the design
>> I did those years ago, but with added feedback and more.
>> The common base push-pull running open-loop, if memory serves,
>> gave about 0.5 ohms input impedance running the devices at
>> 100mA (on heat sinks !)
>>
>
> Using the augmentation gives you a similar input impedance at far less
> current. It gives you the opportunity to operate the transistor at it's best
> NF.
>
> Chris
>
>
>
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