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Building a massively parallel op amp power amplifier
Building a massively parallel op amp power amplifier
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Old 22nd October 2017, 11:13 PM   #31
xrk971 is online now xrk971  United States
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Building a massively parallel op amp power amplifier
Any thread that says massively parallel should at least recognize this amp.

http://www.firstwatt.com/pdf/art_beast.pdf

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Old 23rd October 2017, 08:25 AM   #32
Idiosyncrasy is offline Idiosyncrasy  Netherlands
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Building a massively parallel op amp power amplifier
Quote:
Originally Posted by DPH View Post
OP, given the design and differential input, would it make sense to drop the voltage further and go bridged?
I've considered the option earlier on, but for my specific case the added output power is unnecessary. Of course, you'd have to double the amount of op amps in the output stage on each board, because each amp into the bridged load sees only half the output impedance (e.g., 3Ω instead of 6Ω). So you end up with four times as many op amps per channel.

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Originally Posted by flyingfishtw View Post
Good news everyone, if you haven't buy op amp, you should consider LME49860 for 2 reasons,
1. it's the same as LME49720 just with better voltage tolerance, 22V
2. I think it's end of product life, so on LME49860MA/NOPB - TEXAS INSTRUMENTS - AUDIO AMPLIFIER, CLASS AB, SOIC-8 | e络盟 台湾 it cost 1.5 dollar, half price compare to LME49720
Wow, that's a competitive price for that IC. The LME49860 is usually more expensive here than the LME49720 (for example, at Mouser and Farnell). If someone would want to get more power out of such a paralleled op amp output stage without going bridged, this seems like a good way to do it.

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Originally Posted by flyingfishtw View Post
Heat is your enemy, quiescent current is around 10mA each, @JA 145C/W. Just the idle temperature will be 42*0.01A*145 = 60 degree, and you have to add signal power dissipate as well, I don't know how to calculate that.
I think you can roughly estimate it like this: suppose you have a 20Vp output signal (around 14Vrms) and you have enough output devices to limit the current each channel sources and sinks to ~20mA, the added dissipation per IC would be 2 (op amp channels per chip) * 14Vrms * 20mA = 480mW added to your 420mW of quiescent dissipation. So without extra cooling measures those ICs are way beyond the specified operating temperature range.

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Originally Posted by xrk971 View Post
Any thread that says massively parallel should at least recognize this amp.
http://www.firstwatt.com/pdf/art_beast.pdf
Yeah, that's just brutal. In one of the other threads someone posted another example of an older amp using a massively parallelled output stage using discrete components.
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Old 23rd October 2017, 10:08 PM   #33
PRR is offline PRR  United States
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You took 14Vrms, the _load_ voltage, and accounted it as _chip_ heat.

20mA peak sinewave is like 7mA RMS, under 300mW added. Most of that goes to load.

I don't think it is a real issue.

To check, build one chip with scaled load and beat on it.
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Old 29th October 2017, 06:26 PM   #34
Idiosyncrasy is offline Idiosyncrasy  Netherlands
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Building a massively parallel op amp power amplifier
Quote:
Originally Posted by PRR View Post
20mA peak sinewave is like 7mA RMS, under 300mW added. Most of that goes to load.
I don't think it is a real issue.
Thanks for setting me straight here! I had a feeling it might be too pessimistic.

So, most of the heat generated will be the result of the relatively high quiescent current of the LME49720. After crunching the numbers some more, it seems like this IC wasnít the best pick for a small signal op amp to use in the output stage, especially given how quickly these SOIC packages heat up if I want to maintain some headroom on the supply rails. All in all, something like the OPA1602 might be a better pick here. Its specifications are also impressive across the board, but its Iq is only 2.6mA per amplifier channel (about half of the LME49720).

Next to that, Iím going to add an option to wrap the output stage in the feedback of the buffer op amp after the optional volume control, like this:
Building a massively parallel op amp power amplifier-20171029-tuned-balanced-input-output-stage-feedback-loop-png

If I canít get it to be sufficiently stable, I can always revert to using local feedback only. In the simulator it looks okay though.
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Old 7th November 2017, 07:42 PM   #35
Idiosyncrasy is offline Idiosyncrasy  Netherlands
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Building a massively parallel op amp power amplifier
Quote:
Originally Posted by Idiosyncrasy View Post
Ö In the simulator it looks okay though.
Iím not sure what I was thinking when I put that feedback loop together, but it was a mess. Anyway, a few hours of tweaking later, the simulated small signal response looks much better. Turns out a little gain goes a long way when it comes to stability (which needed some attention because the paralleled emitter followers are slower than the buffer op amp). Iíve added a modest voltage divider before the output stage so that the output level stays well within the realms of what the op amps are able to handle. It now looks like this (simplified a bit for clarityís sake):
Building a massively parallel op amp power amplifier-20171107-feedback-loop-stability-improvements-png

The noise at the output has gone up a little because of R2, part of the voltage divider. Its thermal noise dominates the other noise sources here but is of course still negligible. The simulator is currently reporting THD+N numbers that I donít expect to be even remotely attainable in the real world given my circuit board design experience.

Additionally, the PCB layout is done and Iíve ordered a couple of boards. Hopefully, theyíll arrive within a few weeks so that I can start building the amp and further optimize the component values. Canít wait!
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File Type: png 20171107 Feedback loop stability improvements.png (29.7 KB, 684 views)
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Old 7th November 2017, 08:43 PM   #36
cbdb is offline cbdb  Canada
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Originally Posted by xrk971 View Post
With parallel discrete output transistors we need to spend time matching them. I wonder if the same is true for parallel opamps? Btw, parallel output devices can never sound as resolving or coherent as a single output device (or opamps). The analogy is the sound of a choir vs a soloist. It's just the limitations of being able to match perfectly.
Data or pure speculation? This dosnt make sense. With the opamps the feedback "matches" them, and even if it didn't, if all outputs are in phase the currents sum together perfectly. If one opamp spits out. Io1=99(f(x))ma and the next one Io2=101(f(x))ma (where (f(x)) is the music signal, what's the total current? 200(f(x))ma. No "choir" going on.
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Old 25th November 2017, 01:44 PM   #37
Idiosyncrasy is offline Idiosyncrasy  Netherlands
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Building a massively parallel op amp power amplifier
Smile Time for some updates

Time for some updates! A few days ago the PCBs finally arrived:
Building a massively parallel op amp power amplifier-20171121-blank-pcbs-op-amp-power-amplifier-jpg

Pitch Black.

Because most components are surface mount type, I went with ENIG plating so that the pads are nice and smooth. Additionally, 2oz copper on top and bottom helps to minimize track and plane impedance.

I started by soldering and testing some of the op amp arrays:
Building a massively parallel op amp power amplifier-20171121-populating-op-amp-array-jpg

Hereís some more eye candy with the instrumentation amplifier parts populated:
Building a massively parallel op amp power amplifier-20171122-adding-instrumentation-amplifier-jpg

And right now, Iíve got two finished channels in front of me:
Building a massively parallel op amp power amplifier-20171124-channels-built-jpg

Now on to the important stuff. 1. Does it actually work? (After some debugging, removing a stray metal strand and touching up some solder joints, it does. ) And 2. How does it sound?

Both channels perform well beyond my expectations. Iíve already tried them with my regular headphones (HD650) as well as some sensitive in-ears. With the inputs shorted, theyíre completely silent. Even with the in-ears thereís not a trace of noise or hum whatsoever.

As for the sound, one word that comes to mind is brutal. It sounds just brilliant with jazz and classical, but the authority with which it reproduces bass-heavy tracks while maintaining perfect separation is just stunning. This is definitely going to be my main amp once Iíve got it neatly packaged.

Right now, the differential inputs are AC-coupled. The Nichicon bipolar caps can be bypassed with some jumpers. However, I didnít hear or measure any difference when doing so. DC offset is just 0.7mV on channel 1 and 0.4mV on channel 2. The difference between the two can be traced back to offset voltages at the instrumentation amplifier inputs.

Before starting this build, I was a bit worried about keeping temperatures in check. Especially the quiescent current from the LME49720 could prove troublesome here. Switching to the similarly specced OPA1602 seems to have worked out just fine. After some 20 minutes, both boards are about 30įC above ambient at the warmest spot. Of course, this is still with the amp sitting on my desk, out in the open. I reckon that under heavy load, in a closed case, temperatures wonít exceed 70-75įC.

If I do decide to actively cool the boards in the future, they can support a 90mm fan. Also, thereís some exposed copper on the bottom layer which can be thermally connected to the bottom panel of a metal case. For now, it doesn't seem necessary though.

As the photo's show, I may have went a little overboard with the buffer caps at approximately 0.11F per channel, but I reckon it wonít hurt. The Nichicon UHW1E562MHD capacitors used here have an ESR of only 10mΩ.

Iíve been using the QA401 analyzer to measure the performance of both channels as I was building them up. Unfortunately (or fortunately, depending how you want to look at it), the distortion is below the measurement floor of the analyzer itself (about -115dB; see attachment at end of post). Because of this, Iím not able to discern whether putting the paralleled output op amps in the feedback loop of U48 (an OPA1611 buffer) improves performance. For now, Iíve just configured the IC between the instrumentation amplifier and the output stage as a unity gain buffer. I havenít done any intermodulation distortion measurements yet because they're slightly less straightforward to set up with this analyzer, but I expect to run into the same limitations with those given the op amps used here (OPA1602 in output stage, OPA1611 everywhere else).

All in all, Iíll probably have to build a distortion magnifier if I want to characterize the distortion performance of this amplifier more accurately. Iím leaning towards the one designed by Bob Cordell. His Linear Audio article on the topic mentions a kit that looks good.
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Old 26th November 2017, 04:14 PM   #38
stellarelephant is offline stellarelephant  United States
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Congrats. Trying to count...is that 40 op amps per channel? Massively parallel indeed! Would you say that timing is precise and the sound is focused? Wondering if the "choir" effect is possible or not.

Interesting that you can't hear any difference when bypassing those Muse bipolars. I have tried them for coupling and preferred Silmic, KZ, and Panny FR. But if BP sound the same to you as a wire, well, I am realizing that must mean I prefer coloration over accuracy. Yikes.
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Old 27th November 2017, 08:34 AM   #39
Idiosyncrasy is offline Idiosyncrasy  Netherlands
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Building a massively parallel op amp power amplifier
Quote:
Originally Posted by stellarelephant View Post
Congrats. Trying to count...is that 40 op amps per channel?
That's spot on; there are 40 dual-channel op amps per board.

Quote:
Would you say that timing is precise and the sound is focused? Wondering if the "choir" effect is possible or not.
I'm not sure where xrk's observation earlier in this thread stems from w.r.t. such a choir effect, but there's nothing like it here. Maybe if you managed to introduce large phase shifts varying across the output devices?

Quote:
Interesting that you can't hear any difference when bypassing those Muse bipolars. I have tried them for coupling and preferred Silmic, KZ, and Panny FR. But if BP sound the same to you as a wire, well, I am realizing that must mean I prefer coloration over accuracy. Yikes.
Haha, that doesn't have to be the case. I've oversized the input caps to minimize distortion and low-frequency roll-off. If you're using smaller value capacitors, you might also simply have an audible high-pass filter. (Have a look over here for some measurements of the Muse bipolars.)
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Old 29th November 2017, 05:42 PM   #40
stellarelephant is offline stellarelephant  United States
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I've seen that thread before. Very impressive how those BPs measure with such low distortion. I shoot for a -3dB point of 2 Hz so I doubt I am hearing rolloff. They just all have a different sound that I don't think is reflected so much in THD levels.

Last edited by stellarelephant; 29th November 2017 at 05:44 PM.
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