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Old 18th February 2004, 02:36 AM   #1
lukio is offline lukio  Indonesia
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Talking Is there anybody built a non feedback amplifier??

Hello ....

I want to build a non feedback (NFB)amplifier now... Is there some body knows the advantage and disadvantage of NFB? And who have circuit of NFB amp???Is there any theory on WEB for NFB amp??

Best regards,,



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Old 18th February 2004, 03:04 AM   #2
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Why would you like to built non feedback amp? I have never seen a non-feedback power amplifier without excessive distorsion and/or excessively high output impedance
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Old 18th February 2004, 04:24 AM   #3
Praise! is offline Praise!  Australia
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Default zero neg feedback

This is my opinion:

By having no gobal feeback, the amp is essentially isolated form the speaker which is non linear so that no alien signal is being feedback to the input stage.

Due to low loop gain at high frequency and compensation which is mandatory in any feedback amp, the feedback loop is not very effective at high frequency to reduce distortion. In fact, the amp produces many higher order harmonics.

I have a few zero FB amp design, all have current feedback complementary feedback pair as output stage operating in Class A
I don't design any class AB amp without gobal FB.

If you are interested to see a few no FB amp design, pop me a email.
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Old 18th February 2004, 07:33 AM   #4
hjelm is offline hjelm  Sweden
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LC Audio has done a NFB amp. I built a previous version (MKII) and it was a very nice AMP. I had to discontinue its use when the preamp gave up and i bought an integrated amp.

There is a schematic at their website for their current version, look for the manual of the amp.
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Old 18th February 2004, 09:42 AM   #5
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Default Re: zero neg feedback

Quote:
Originally posted by Praise!
This is my opinion:

By having no gobal feeback, the amp is essentially isolated form the speaker which is non linear so that no alien signal is being feedback to the input stage.

If you are interested to see a few no FB amp design, pop me a email.
Yes ...you are right...
I have talked about the some ...many times in this forum but nobody care

The first person, that i knew talk about the problem is M. Otala.. ..and he call it interface induced distortion...
But if you build a amp with a very low open loop output impedance...then you can use overall feedback and get even lower distortion...

Congratulations!!

Is very rare in this forum people come with solutions that are not the usual ..."text book engineering solutions"

PS: I would like see some of yours NFB design !!!
PS(2):Your's designs also use feedback (a complementary feedback pair at the output use heavy inside feedback)...they must be called "Overall Feedback Free"
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Old 18th February 2004, 10:21 AM   #6
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wouldn't some of Pass' SE designs qualify as no-feedback amps? people have built a lot of them.
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Old 18th February 2004, 10:41 AM   #7
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Quote:
Originally posted by millwood
wouldn't some of Pass' SE designs qualify as no-feedback amps? people have built a lot of them.
Older Pass designs...in times of Treshold used even written in the front panel... "Overall Feedback Free"

I still have one of this ...the S 350 ..
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Old 18th February 2004, 10:42 AM   #8
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Using NFB around the voltage gain stage with the O/P stage outside the loop can also work well and avoids the tricky business of designing a stable, low distortion, low gain open-loop voltage amplifier.
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Old 18th February 2004, 10:45 AM   #9
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Default Re: zero neg feedback

Quote:
Originally posted by Praise!
This is my opinion:

By having no gobal feeback, the amp is essentially isolated form the speaker which is non linear so that no alien signal is being feedback to the input stage.

I can agree to the second part of the statement (the first part is not true of course, the amp is not isolated, it is fully connected to the speaker).

But having a non-fb amp connected to a non-linear speaker load means that the output signal becomes very dependent on the speaker behaviour. This is made worse by the fact that the amp output impedance is also non-linear. That in itself can be a source of large distortions. So, it is difficult to say what is best and what is worse. What you say is partly true, but only part of the picture. The other part can be much worse.

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Old 18th February 2004, 10:45 AM   #10
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Quote:
Originally posted by Richard C
Using NFB around the voltage gain stage with the O/P stage outside the loop can also work well and avoids the tricky business of designing a stable, low distortion, low gain open-loop voltage amplifier.

Yes..that is what use Densen...but they claim...a non feedback design..

At least in the DM10...
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