Opamp based power amp

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UltimateX86

Member
Joined 2004
circlotron.audio
#1
Hello,

Starting a Zenquito Jean Marc Plantefeve 100W 4ohm , I wanted to edit it with OPAMP to boost performance, distortion , power supply rejection , thermal stability and load ...

First diagram : http://img.photobucket.com/albums/v637/zippy670/mf_p-amp_170_scaled.jpg

AOP choice : AD844 : 2000v/us !

By web searching I found JCB schematic De la puce à l'oreille - Acoustique et lectroacoustique who also used Lateral Mosfets but supplemented by bipolar power from a current to pull the best of each .

After having chat with CATSIANO , it seems that the Musical Fidelity P170 type diagram induce a phase shift close to the audio band and implies to curb the amp. I found the same thing on DIYAUDIO .

Here is what I was offered CATSIANO (still in draft) :

Using OPAMP bootstrapping and current mirror to ensure thermal stability.

An externally hosted image should be here but it was not working when we last tested it.
 
#2
100W into 4ohms needs an output of 28.3Vpk.
You can't get that with the limited supply rail voltages of normal opamps.

Most opamps range from +-18Vdc to +-22Vdc.
You have to use a "cheat" circuit to get them to operate from +-36Vdc for your 100W target.
 
#3
Its not a problem for many opamps.
I had few of these in hands Operational Amplifier (Op Amp) - Precision Amplifier - OPA445 - TI.com
They sound pretty nice but they warm up at 20mA +-43V PEAK unregulated.
I have 240V in my country home and here in my department its only 220-223V.
Got a million dollar cable too... 🙂
I dont know why tho... Didnt had scope back then too but i doubt it was oscillating.

Those are fairly old too, I bet theres alot better ones out there.
 
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#6
0.000016% THD à 1khz 100W 4ohm

Can you help me to improve this scheme ?
 

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#9
Its deeply impressive as a simulation. I would go as far as to say that it can't approach anything like that level of performance as a real build (if it did it would be better than an LM4562 driving 100k but I wonder how close can it get. The opamp is "special" in that its really a video device... its no pretensions to audio quality.

Interesting 🙂
 
#11
Thanks 🙂 I was curious and actually simmed it myself and can confirm your results. Even at zero bias its outstanding. A quick sim of squarewave response shows excellent behaviour too.

I tried it with an LT1056 FET opamp and it was good, but nothing like the video opamp used.
 
#13
Should be no issues with bias stability if using lateral FET's. The "sound", well thats the big question isn't it. With specs like that it shouldn't have any "sound" as such. Other opamp... well you would need to build and test it thoroughly for stability, and stability into adverse loads etc.
 
#17
UltimateX86 said:
I was thinking to improve the stability, bias, sound, maybe other transistor or OPAMP
Click to expand...

Do the sym at 20kHz and you will see that it just has truck loads of
OLG at lower frequencies with steeply rising distortion at higher frequencies.

Generally this is not a great thing for sound quality. You are better off having
a flatter distortion versus frequency curve, even at the expense of increased low freq distortion.

Also include square wave testing into R and C loads.

How accurate is the opamp model? Very often they are very much more ideal
than a buildup of discrete components.

Perfect - not quite, but still a good effort 🙂

cheers

Terry
 
#20
Do the sym at 20kHz and you will see that it just has truck loads of
OLG at lower frequencies with steeply rising distortion at higher frequencies.

Generally this is not a great thing for sound quality. You are better off having
a flatter distortion versus frequency curve, even at the expense of increased low freq distortion.
Click to expand...

I see no reason to believe this if the "high" high frequency distortion is low enough to inaudible

and maybe not even if the high frequency distortion rises to questionably audible levels

it is IMD of high frequencies that are more likely to be heard - no one directly hears THD 20KHz

for most audio actual harmonic distortion alone at any frequency is heavily masked in the upward direction

I see even more justification for high, sloping with frequency loop gain because IMD/PM/TIM products folding down from multiple high frequencies interacting in the amp nonlinearities are reduced by the feedback excess gain at the lower difference product frequency

of course if you could get really high loop gain over the full audio range that would be good - but simple dominant pole compensation makes it hard to go over 60 dB by 20 kHz
 
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