Compensating multi-stage feedback

[Magnus Danielson]

From: eric <svetengr at earthlink.net>
Subject: Compensating multi-stage feedback
Date: Sat, 01 Jul 2000 00:21:24 -0700

> >Bottom line for *me* is that until I see any groundbreaking article
> >on that subject, I'm either using integrated opamps, or
> >(and with increasing pleasure) discrete circuits that don't use
> >multi stage feedback at all. 
> >>100% agreed. I'd even go one step further: The complete audio path can be
> >>done without negative feedback. (And hence no compensation.) 
> 
> 
> ---Gosh! We'll make audiophiles out of you guys yet...

Ooohhh... your dreaming Eric ;)

> PS--I know the perfect devices for no-feedback
> designs---they're called "vacuum tubes".......

Right. But I also think it is worth mentioning that you can do good
things with JFETs and bipolar as well, as long as you do not overuse
the negative feedback.

Personally I use JFETs and MOSFETs (as freestanding components)
equally often as tubes in audio curcuit, that is never. Not that I
mind any of them really, but it is a habit thing. This is one of the
thing one has to be aware of, much of the way people solve things is
limited by their habit but also the habit of others. I do love to
learn other alternative ways of doing things, since this helps you to
understand what is important and what is not.

We did have a thread recently regarding the usage of JFETs and I
beleive that there where some usefull info in there. Don's buffer was
a good example.

There are however cases where the OP type of operation is really
usefull, regardless of implementational form. It is even being used in
tube amps for long. I recall that the Quad II amp had two tubes wired
up as the diff input pair and two tubes as the output buffers and then
the feedback controls the amplification, while this migth not be a
valid case for OP-amp type it is an example at least that it can be
done.

One of the things that people forget about OP-amp is that the high
internal amplification is being put there for a purpose, namely to
provide an approximate ideal OP. Such an OP-amp would have inifinit
gain such that the output would be set at some value that the voltage
difference between the inputs is 0 V. You can however set up OP-amps
with lower internal amplifications and have them usefull, it is just
that we run in the habbit of assuming the infinit amplification.

Anyway, I have built myself a portable headphone amp (for my AKG 240,
600 ohm) many years ago. It used the 20-transistor op-amp that formed
the basis for an low-noise RIAA amplifier in an Elector article waay
back in time. This op-amp where built out of 10 BC550 and 10 BC560
where 8 BC550s where being used to build an input pair (4 transistors
in parallell for each transistor) and 8 BC560s where being used in
exactly the same way. Then, these op-amps had common inputs and
controled their own half of a push-pull ladder (a BC550 and a BC560)
operating in class A operation. Thus, the NPN/PNP sides provided the
active load on each other. You thus had some cancelation there.
The other 2 trannies where used in a gyrator setup to stabilize the
power. Anyway, this was a rather sweet setup and also a bit
unconventional. I ran it of 4 9V batteries and also built myself a
very simple charger.

It would be interesting as a case of study how the usage of
JFET/MOSFET as well as tubes would look for this application.
Thus, there are some limits on size, weigth while we would like the
actual curcuit to preform well. I do beleive that you could do it out
of tubes, but I am too unoriented on weither you could go
transformator less or not (target is 600 ohm, not 4 or 8).

Cheers,
Magnus