Regulate the psu of a power opamp with a similar power opamp? - diyAudio
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Old 16th October 2004, 07:58 PM   #1
Franz G is offline Franz G  Switzerland
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Default Regulate the psu of a power opamp with a similar power opamp?

Regulators are opamps, as I learned up to now.

So: why not regulate the psu of an poweropamp with absolutely the same opamp?

I think about a LM3875, 3886 or whatever amp, with an psu, regulated with exactely the same chip, therefore the same behaviour.

But: I am not an EE and therefore, it would take me many hours studiyng a new topic.

Maybe, out here in the world, someone could pick up this idea and show some draft of an possible regulation circuit, using a power opamp?

Maybe, this could start the next step, in gainclone evolution?

Franz

P.S.
Maybe, in the different datasheets of the poweropamps, there is a suggestion for an regulator? But I dont remember...
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Old 16th October 2004, 08:08 PM   #2
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It can be done. The biggest issue is typically cost. Also, there is much "talk" about having a supply that can deliver more than the amp wants.

There are a number of regulators that use opamps with a transistor at the output for current gain. Just Google it.

If you go to the Analog Devices or TI web sites and look at IA app notes you will see opamps used as regulators for strain gauges.

Try it, see what happens.
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Old 16th October 2004, 08:29 PM   #3
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Default Re: Regulate the psu of a power opamp with a similar power opamp?

Quote:
Originally posted by Franz G
So: why not regulate the psu of an poweropamp with absolutely the same opamp?
While you're at it, I had a similiar idea:

So: why not buffer the input of an poweropamp with absolutely the same opamp? (or LM1875)

I saw a thread about setting up a LM3875? with a OVERALL gain of one.
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Old 16th October 2004, 08:58 PM   #4
Franz G is offline Franz G  Switzerland
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Quote:
So: why not buffer the input of an poweropamp with absolutely the same opamp? (or LM1875)

I saw a thread about setting up a LM3875? with a OVERALL gain of one.
I think, this is not the same: LM3875 are not designed to use with unity gain. I have an amp setup like this, and it is not a good amp. Too much feedback!

No, the regulator should work exactly like the powerstage. The same gain, the same currents.

Franz
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Old 16th October 2004, 09:25 PM   #5
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Some excellent reference material on using op-amps as regulators can be found HERE
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Old 16th October 2004, 09:37 PM   #6
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Hi Franz,

You can't use the LM chips to make a regulated PSU, because they are only stable at gains of >10.
They were made for audio.
The good news is, of course you can do it with the OPAs.
OPA541 would be the only choice, as the 548 and 549 are limited to +/-30V max. PSU voltage.
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Old 16th October 2004, 11:58 PM   #7
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Hi Franz,

Quote:
Originally posted by Franz G
I think, this is not the same: LM3875 are not designed to use with unity gain. I have an amp setup like this, and it is not a good amp. Too much feedback!
I realise that the LM3875 is not stable with a gain <10, but the thread I was referring to, was using a LM3875 as a unity gain preamp (a buffer). They did this by using a 1:10 voltage divider on the input and then a gain of 10. So the OVERALL gain was 1.

I can't find the original thread but I did find a similiar one.
Unity Gain Operation (or low gain)

More information on this topic:
GC with LM3875 as a voltage regulator also
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Old 17th October 2004, 07:35 PM   #8
Franz G is offline Franz G  Switzerland
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You know, guys, I am living, because I can dream.

Somewhere at the internet, I read an information about an DNM Stereo Amp, using two additional redundant power stages, same gain, same load (I know, this is another problem, as loudspeakers are a complex and not a linear load) as the real power stages, just to regulate the psu.

O.K., it was an advertisement.

But why not follow this idea? With power chips, this is not to much luxurious, to build redundant power stages. And the little bit heat of the needed output resistors: for me this is not a problem, if it is sounding good.

Maybe, someone could pick up this idea? When not, just give me some time...

Franz
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Old 17th October 2004, 07:38 PM   #9
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Where's Phred when you really need him.............?

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Old 17th October 2004, 08:09 PM   #10
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Default Re: Regulate the psu of a power opamp with a similar power opamp?

Quote:
Originally posted by Franz G
Regulators are opamps, as I learned up to now.

So: why not regulate the psu of an poweropamp with absolutely the same opamp?

I think about a LM3875, 3886 or whatever amp, with an psu, regulated with exactely the same chip, therefore the same behaviour.

But: I am not an EE and therefore, it would take me many hours studiyng a new topic.
Take a look at the chart depicting the frequency and phase response of the LM3875 -- compare it to the AD825 -- the '3875 just isn't the same league in terms of performance.

I think that you would find a simple regulator circuit based upon a TL431 and a pass transistor (look at Elso's Kwak-clock power supply) to be much better than trying to bootstrap an LM3875 or LM3886 to provide the same functions. Simpler, less expensive, less real estate consumed and a demonstrated body of knowledge.

But I still want someone to demonstrate why an LM3875 or LM3886 amplifier should be regulated. You still burn watts which could be added to your musical enjoyment.
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