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Old 26th October 2011, 07:24 AM   #21
sofaspud is offline sofaspud  United States
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Thanks, I'll definitely follow these leads (the link works). Reviewing some of the composite amp papers I have, I haven't found anything similar to what this schematic shows. Maybe I expected it to jump out and bite me, so I'll have another go at it. The original article does have a table showing several alternate values for the filter, but I'm unsure about what does what eg how is the R2 value determined? The math was probably too deep for a RE article; it could be too deep for me. I figured I could sub some "tweak values" and analyze the o'scope results.
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Old 26th October 2011, 09:55 AM   #22
danielwritesbac is offline danielwritesbac  United States
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It seems that MyRef projects have a little op-amp for pre, an LM3886 for output and negative feedback from the pre all the way to the output. Maybe I read it wrong, but it looks like you'd want to check it out. It looks like Howland Current Pump.
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Old 13th February 2018, 09:11 PM   #23
FauxFrench is offline FauxFrench  France
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Sorry for my ignorance but I just discovered the concept of "composite amplifiers".
For audio power amplifiers the idea, in short, seems to be the use of a high-quality OP-AMP for control of a less good power amplifier such that the overall performance is dominated by the high-quality OP-AMP?
I found an AD711/LM1875 design on the net.

Is it correct that the power amplifier needs a high bandwidth to keep the overall bandwidth reasonably high?

Is it possible to use a class D power amplifier or is the bandwidth to low to keep the "outer loop" reasonably fast? Does the inherent 300KHz-600KHz ripple perturb the outer regulation loop?

Thanks to anyone who provides a reply after so many years of hibernation.

Last edited by FauxFrench; 13th February 2018 at 09:13 PM.
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Old 14th February 2018, 05:08 PM   #24
tomchr is offline tomchr  Canada
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I've been making composite amps for quite a few years now. See my Modulus-series of amplifiers.

The basic idea is to perform error correction on a power amp by using the loop gain of a precision opamp. The topology has been used for decades to get DC precision on high-speed opamps. At the bottom of my Modulus-86 page, you'll find a list of references that you'll probably find insightful.

Composite amps are relatively straightforward to get to work if the output stage has much higher bandwidth than the control opamp. For audio applications, the output stages are pretty low bandwidth compared to the control opamp, so stability becomes a serious challenge. It can be done, though, as I show with the Modulus-86 and beyond.

Tom
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Old 14th February 2018, 07:59 PM   #25
FauxFrench is offline FauxFrench  France
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Tom, thanks a lot for taking time to reply.

I was already aware of Neurochrome/Modulus and your amazing THD results. When you wrote ...uses an LME49710 precision audio op-amp to perform error control on an LM3886 power amplifier… my precision power supply background left me a hint. The net brought the term “composite amplifier” and I wanted my experience with bandwidth of multiple loops confirmed for amplifiers.

I appreciate people who manage to work their way into perfectionism and, nevertheless, offer products most can afford. Your results are impressive. The Modulus-series is a reference.
But, for me it will be interesting to see what a single high-quality OP-AMP can do for a more mediocre power IC. Just to get some experience with the potential of the concept.

Mange tak.
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Old 14th February 2018, 08:11 PM   #26
DPH is offline DPH  United States
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Probably best to start a new thread and let this one die.

As far as using it on something like a TPA3118 or so, well, if you look at the overarching topology of the Hypex modules, then we're not too far off. It's going to be pretty tough to get much extra loop gain into the outer loop, even deep into conditional stability and will require pretty high order filters to pull the loop gain out of the outer loop over a narrow bandwidth if you don't want to waste your time.

IMO, this may be a case where an error correcting amplifier will work much better to decouple the respective bandwidths. (acknowledging that they're tied together AND that I have essentially zero experience doing so)
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Old 14th February 2018, 09:02 PM   #27
FauxFrench is offline FauxFrench  France
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Thanks a lot, DPH.

I have never seen a straight forward composite amplifier with a class D power stage so I guessed there was a reason.
I may try with class A/B.
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Old 14th February 2018, 09:31 PM   #28
00940 is online now 00940  Belgium
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Following DPH's suggestion to look into error correction, you might be interested in this: High loop Gain Composite Op Amp Circuits
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Old 15th February 2018, 12:55 AM   #29
DPH is offline DPH  United States
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Quote:
Originally Posted by FauxFrench View Post
Thanks a lot, DPH.

I have never seen a straight forward composite amplifier with a class D power stage so I guessed there was a reason.
I may try with class A/B.
Start perhaps with a headphone amplifier! Can get pretty fast output buffers/amps that give you some bandwidth to start!
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Old 15th February 2018, 01:53 AM   #30
tomchr is offline tomchr  Canada
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+1 for a headphone amp. Get it running in the simulator first. Do beware that the high-speed buffers are excellent at picking up FM radio and analog TV stations.

Tom
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