What is wrong?

[MikeB]

Originally posted by lumanauw

But when the setting is in oscilating mode, even when I put the bias pot at minimal, the output is not distorted.

Is that oscilating amps needs less VBE bias than proper one?

Hi lumanauw !
This is the typical sideeffect of oscillation, crossconduction is increasing
the biasing as the bjts are closing slower than opening...
This means one bjt is still conducting while the other is already starting
to conduct.

Mike

[tlf9999]

Originally posted by MikeB
This means one bjt is still conducting while the other is already starting
to conduct.

Mike

I thought that is A in Class AB, .

[dtrif]

Originally posted by lumanauw

When I finished modification (I eliminate the oscilation fully,

Hello lumanauw.
In what way did you eliminate oscillation? Is it possible that eliminating oscillation you did something that as a "side effect" it gave better (subjective) performance? Perhaps better performance is related to the "cure" for oscillations and not oscillations themselves? Perhaps also "curing" oscillations in a different way would not give you better sound?
Just a thought...

[amplifierguru]

Hi Lumanauw,

Trouble with an amplifier that oscillates even mildly, it's a warning that something very much more severe maybe just around the corner - a temperature increase, different load, spike from the fridge!

You cannot leave an amplifier in such a state as it will surely bite you.

Cheers,
greg

[dtrif]

Of course in my previous message instead
of "better" you should read "worse". Sorry for the mess...

[forr]

Miles Prower

"Doug Self published a long polemic against B. Ollsen and the quasi-comp topology. Self had all the numbers to back up his claims against Ollsen. However, I've built them both ways, and Ollsen was right: quasi-comp sounds better AFAIC."

This assertion ("sounds better"... so repetitivly seen) is typical of what I call an inobjective comment. It is absolutely false that it sounds better, it is absolutely true you prefer the sound of Ollsen's amplifer (a huge lot of difference !) I may even prefer it too. Why ? Probably that in most cases, distorsion is prefered, it sounds as more detailed, nicer to our ears.
What is very perturbing is that if the numerrous electronic stages prior to the final power amp generate the same distorsion as the Ollsen amp (or any other claimed to have been subjectively designed) , the final result would probably judged as dirty.
I am not aware of IC's désigned for good sound and not having very good specs and a lot of feedback. And there usually are some IC's at the recording stage. Do they make so much harm to the perceived sound ?

~~~~~~~~~ Forr

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[lumanauw]

Hi, Dtrif,

The oscilation is "set" to occur only at full power with square wave. At moderate level, it doesn't oscilate, otherwise how can the power amp stand even 1 minute?

There are some things that actually wrong, but usually DIYer's don't realize. Do you know that plain triple darlington or CFP oscilates at full power? It only shows up when you test the amp with sufficient dummy load and scope. The oscilation maybe small, but it is oscilation. With small/moderate level, this oscilation will not show up.

You can look at the cure, and how this "cure" can be made "adjustable" by searching here about CFP oscilation topic and answers by Nelson Pass.

Hi, Amplifierguru,

Yes, you are right. My opinion also that any oscilation, mild or severe should be not there. What is confusing me, why my friend's ear prefers the oscilating one? Hence this thread

[mauropenasa]

Hi Lumanauw,
An amplifier is always a compensated oscillator.
I have experimented often the things that you say. I believe that is too much attention to the THD and a few to other dynamicses of the amp.
A amp has to be before all a good interface for the speakers. his THD has a more low weight on the final sound of what much believe.

Because " sounds better " a little compensated amp?

Because the classical compensations alter the DF, the output phase response and other parameters.

I agre totally the Graham's DF theories (and first cicle analysis...).

If you uses the "alternatives" techniques of compensation, in accord with the " inverse driven " test, the acoustic performances increases (like no compensate amp).
I have experimented the feed-forward comp. to eliminate the Cdom effects (in my nested-loop opamp, my_ref), with optimize results...

Ciao

Mauro

[forr]

Mauropenasa
diyAudio Member


>An amplifier is always a compensated oscillator.

An oscillator is an amplifier made voluntarily oscillating.
Egg and chicken dilemna ?

A famous electronician's Murphy's law is :
an amplifier always oscillates when one does not want it.
an oscillator never oscillates when one wants it.

~~~~~~~~ Forr

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[mauropenasa]

...an amplifier always oscillates when one does not want it.
an oscillator never oscillates when one wants it.

Hi Forr,

I share it

Ciao

Mauro