Class B - high feedback

[Stefanoo]

Hi to everybody!

I just finished to read the douglas Self' book -Audio Power Amplifier Design Handbook.

It was my first "Audio Book".
By reding it, i found out that the ideas of Daglas about the circuity topology are are very different from what i have been set about audio equipments.

He considers the feedback as a weapon to use.
He suggests to use as much feedback as you can!
I have been used from the magazines to consider high feedback as a bad thing.

I 've been always thought that class a amp with low feedback is the best choice.

I would like to know your view point about this guys.

Just a thought, i would have rather preferred to read togheter with the technical datas, also sounds impression about the different configurations and not just tdh measuements.


I'm very curious to read your posts.


Best Regards,
Stefano.

[Babowana]

Originally posted by Stefanoo


He considers the feedback as a weapon to use.

My weapon feedforward . . . ???

I have never read his book, at least not yet . . .
I think catching his meaning would be better than debating . . .
I believe there must be good points to learn.

[Stefanoo]

of course, there are a lot of good points to learn and take into account!
What i would like to know is: what the people here on the forum think about his way of design amp.
..about the feedback.....i would expecially like to know the effects on the sound.
how would an amp with wide feedback would sound compare with one at low feedback.
Furthermore i would like to know what do you guys think about class b and class a: the difference on the sound.

I would also like to know .... what's the best audio authour for you guys!



Regards,
Stefano.

[richie00boy]

Doug Self writes a lot of good stuff but doesn't seem to believe in listening to things other than maybe the final developed design. I think you should always listen as well as measure all along the way. I also wonder if he even knows what to listen for.

I have to say most magazines I have seen have been written by clueless/blinded by paradigm people really, perpetuating myth and trying to look good by using scientific concepts, although wrongly most of the time.

I think that ways of doing things is just as important if not more than simply taking feedback as a metric to judge sound quality. Same goes for class-a vs class-b.

I rate John Linsley Hood as an author and an audio engineer, but there are many good and bad people about.

[lineup]

hi

good topic = negative feedback level

my own amateur DIY approach:

Total open loop gain = 100 x closed loop gain
( this is a feedback of 40dB )

I have figured this will give good quality
while avoiding some of the bad drawbacks of very high feedback
such as extensive compensation filtering

why overdo things?
too much cocaine will kill you
too much of anything is bad

Try the sensible way of:
"Just use whatever much you need, and no more!"

as is said by
the old familiar lineup proverb, expression

[Tube_Dude]

Originally posted by Stefanoo

how would an amp with wide feedback would sound compare with one at low feedback.

The one with heavy feedback will sound the same (accurate ), as the input signal source (no more ,no less ).
The other with no feedback , will sound "romantic", self effacing and "nicer" than the source signal...
The choice is your's...

[sreten]

Hi,

In the standard amplifier topology considered by D. Self in his book
and used in the vast majority of commercial amplifiers in existence,
the use of feedback is implicit, it allows the topology.

D. Self gives lots of insights into this topology and how it can be
optimised in objective performance terms, and avoids getting
embroiled in subjective and meaningless debate.

Nevertheless there is plenty of information is his book regarding
optimising subjective sound quality (depending on the principles
you choose) if you can read between the lines of the information
he gives.

However the coverage of amplifier types is deliberately limited.
Subjective performance is not directly considered either, the
point of the book is how to avoid / minimise THD, as its the
most quoted parameter, and the mechanisms to do this.

It is not a good book as an introduction to "the sort of amplifiers
manafacturers foist upon us with dubious claims of superior
performance for reasons that cannot be proved and are usually
simply wrong" or understanding valve amplifier topologies where
low feedback is a given, as high feedback cannot be used, and
the common topologies used do allow no feedback to be used.

The fact is nearly all music you listen to has passed through
high feedback amplifiers many times before it gets to your
system, so high feedback in itself is not implicitly wrong.

/sreten.

[st r]

Usually feedback amps have open loop gain which is very variable with frequency (to achieve stability), and since feedback is the more effective the more gain you have, this means that in certain regions of the spectrum the residual error and all the effects of feedback will be higher than in other regions. Feedback also needs very good circuit layout to avoid feedthrough, parasitic oscillations or (even worse) behaviors at the edge of instability, all frequency-dependent so that sound is affected in unpredictable ways. Moreover, you should dimension things so that in all circumstances you will never go anywhere near to clipping, which is much nastier in fed-back circuits (is this term OK? I have invented it right now).

I am not an expert, but I suspect that if people knew how to use feedback it would not have got its bad fame. As a matter of fact, who knows how many audiophile records have been recorded through tens (100's) of operational amplifier circuits, each with its own feedback, but seriously designed so that professionals could use them reliably.

--st.r.

[traderbam]

"Hi to everybody!"
Hi Stephano.
I suppose you really want to know why feedback makes some circuits sound bad when it ought to make all circuits sound better?

I know but if I tell you I will have to shoot you.

[traderbam]

I am not an expert, but I suspect that if people knew how to use feedback it would not have got its bad fame.

Spoken like an expert.