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Cheap as Chips OPA1688 Low-THD Muscle Amp
Cheap as Chips OPA1688 Low-THD Muscle Amp
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Old 13th July 2018, 12:37 PM   #11
badboygolf16v is offline badboygolf16v
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Another interesting project X!
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Old 13th July 2018, 01:50 PM   #12
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Thank you JPS64 for a nicely done layout! Looks superb. Can't believe how small the WSON8 packages are, they are about the size of a pair of 0805 resistors. Nicely packed in there for a headphone amp that is stable putting 8vpp into an 8ohm load with low distortion!

Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-3d-render-view1-png
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File Type: png OPA1688-Muscle-Amp-3d-render-view1.png (405.7 KB, 775 views)
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Last edited by xrk971; 13th July 2018 at 01:52 PM.
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Old 13th July 2018, 02:20 PM   #13
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Another view...

Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-3d-render-view2-png

Here is draft schematic:
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-draft-schematic-v1-png

Btw, the amp can be all DC-coupled as the input/output coupling caps are optional. Just replace with a wire jumper. OP1688 is specified to have less than 1mV dc offset in spec sheet. You just need to make sure your PSU is reliable, or have a means to detect DC imbalance and shutdown. Or, if you happen to own HE6 planars, you don't care, as they can take 6wrms.
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File Type: png OPA1688-Muscle-Amp-3d-render-view2.png (564.2 KB, 2215 views)
File Type: png OPA1688-Muscle-Amp-draft-schematic-v1.png (272.1 KB, 1213 views)
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Last edited by xrk971; 13th July 2018 at 02:29 PM.
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Old 13th July 2018, 07:46 PM   #14
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
We are looking at providing an integral dual rail DC-DC PSU so that 12v Wallwart can be used.

http://www.ti.com/lit/df/tidreq7/tidreq7.pdf

BOM is about $14 ea and we need two dual rail DC-DC PSU's to handle max 1.2amp at peak power and gives better stereo separation.
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Old 14th July 2018, 07:01 PM   #15
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Here is an interesting surprise, I replaced the LME47720, the supposedly "king of the low THD" opamps in TI's arsenal, with the lowly OPA1688 (which is also serving as the buffer).

Here is the schematic:
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-8-parallel-all1688-fft-schematic-v4-png

If we give the power supply rails lots of caps so that the impedance is infinitessimally low, here is what we can get for 8Vpp into 8ohms:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.006e+00 1.000e+00 -0.05° 0.00°
2 2.000e+03 1.442e-09 3.599e-10 -0.08° -0.04°
3 3.000e+03 2.163e-09 5.399e-10 0.07° 0.12°
4 4.000e+03 2.883e-09 7.199e-10 0.19° 0.23°
5 5.000e+03 3.604e-09 8.998e-10 0.29° 0.33°
6 6.000e+03 4.325e-09 1.080e-09 0.38° 0.42°
7 7.000e+03 5.046e-09 1.260e-09 0.47° 0.51°
8 8.000e+03 5.766e-09 1.439e-09 0.55° 0.60°
9 9.000e+03 6.487e-09 1.619e-09 0.63° 0.68°
Total Harmonic Distortion: 0.000000%(0.000000%)
I think that's PPB (part per billion) HD components.

The graph is unusual too (I have never simulated anything that has zero features before):
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-8-parallel-all1688-fft-8vpp-8ohm-png

So I took it to 28Vpp into 32ohms, that's 2.5wrms nd enough juice to drive the most inefficient planar headphones, here are the FFT components:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.402e+01 1.000e+00 -0.05° 0.00°
2 2.000e+03 8.877e-10 6.332e-11 -0.14° -0.10°
3 3.000e+03 1.331e-09 9.497e-11 -0.33° -0.29°
4 4.000e+03 1.775e-09 1.266e-10 -0.50° -0.45°
5 5.000e+03 2.219e-09 1.583e-10 -0.65° -0.61°
6 6.000e+03 2.662e-09 1.899e-10 -0.81° -0.76°
7 7.000e+03 3.106e-09 2.215e-10 -0.96° -0.91°
8 8.000e+03 3.550e-09 2.532e-10 -1.10° -1.06°
9 9.000e+03 3.993e-09 2.848e-10 -1.25° -1.20°
Total Harmonic Distortion: 0.000000%(0.000000%)
Here is the time series:
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-8-parallel-all1688-time-series-24vpp-32ohm-png

Here is the FFT:
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-8-parallel-all1688-fft-28vpp-32ohm-png

Now, that's a "Muscle Amp".
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Last edited by xrk971; 14th July 2018 at 07:30 PM.
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Old 14th July 2018, 07:39 PM   #16
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
These simulations are assuming perfect power supply of course with limitless current and zero impedance. If we add series resistance of the power supply as 12mOhm (ESR of OSCON cap), 8vpp into 50ohms has a bit more distortion, but still very low. So it looks like having lots of low ESR caps on the rails here will help.

8Vpp into 50ohms:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.006e+00 1.000e+00 -0.04° 0.00°
2 2.000e+03 1.784e-07 4.454e-08 -178.41° -178.36°
3 3.000e+03 2.205e-09 5.504e-10 -1.72° -1.67°
4 4.000e+03 6.898e-08 1.722e-08 -175.87° -175.83°
5 5.000e+03 3.674e-09 9.171e-10 -0.80° -0.76°
6 6.000e+03 4.180e-08 1.044e-08 -173.21° -173.17°
7 7.000e+03 5.142e-09 1.284e-09 -0.33° -0.28°
8 8.000e+03 2.836e-08 7.080e-09 -170.04° -170.00°
9 9.000e+03 6.611e-09 1.650e-09 -0.00° 0.04°
Total Harmonic Distortion: 0.000005%(0.000000%)
Here is 28 Vpp into 50ohms (or 2wrms) - distortion looks almost identical:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.402e+01 1.000e+00 -0.04° 0.00°
2 2.000e+03 6.241e-07 4.451e-08 -178.41° -178.36°
3 3.000e+03 8.300e-09 5.920e-10 -0.35° -0.31°
4 4.000e+03 2.407e-07 1.717e-08 -175.86° -175.81°
5 5.000e+03 1.383e-08 9.866e-10 -0.13° -0.08°
6 6.000e+03 1.452e-07 1.035e-08 -173.14° -173.10°
7 7.000e+03 1.936e-08 1.381e-09 0.00° 0.05°
8 8.000e+03 9.774e-08 6.972e-09 -169.83° -169.78°
9 9.000e+03 2.489e-08 1.776e-09 0.10° 0.14°
Total Harmonic Distortion: 0.000005%(0.000000%)
And here is series resistance of 0.15ohms for the power supplies and 28Vpp into 50ohms:
Quote:
[Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.402e+01 1.000e+00 -0.04° 0.00°
2 2.000e+03 7.864e-06 5.609e-07 -178.42° -178.37°
3 3.000e+03 8.375e-09 5.974e-10 -5.85° -5.80°
4 4.000e+03 3.135e-06 2.236e-07 -176.02° -175.97°
5 5.000e+03 1.392e-08 9.926e-10 -3.43° -3.39°
6 6.000e+03 2.004e-06 1.430e-07 -173.77° -173.73°
7 7.000e+03 1.946e-08 1.388e-09 -2.37° -2.32°
8 8.000e+03 1.473e-06 1.051e-07 -171.53° -171.49°
9 9.000e+03 2.501e-08 1.784e-09 -1.75° -1.70°
Total Harmonic Distortion: 0.000063%(0.000000%)
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Last edited by xrk971; 14th July 2018 at 07:56 PM.
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Old 14th July 2018, 08:03 PM   #17
lineup is offline lineup  Sweden
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"... here is what we can get for 8Vpp into 8ohms"

This is impressive.
In fact this is not only a headphones amp.
It is a hifi power amp for a few Watts into 8 Ohm.
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Old 14th July 2018, 09:01 PM   #18
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Hi Lineup,
Thanks! It’s only a simulation and of course there will be limitations due to the resistors used, the layout, the PSU impedance, etc. so in practice will never be PPB. But nice to see a theoretical PPB amp that’s so simple to make.
Cheers,
X
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Old 15th July 2018, 03:58 AM   #19
stellarelephant is offline stellarelephant  United States
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Wow! Subscribed.
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Old 15th July 2018, 10:41 PM   #20
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Latest development with integrated Dc-DC PSU with low noise LDO’s and lots of low ESR OSCON caps for the amps.
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File Type: jpeg 5E24D06E-7530-4A3C-B2CF-05EFF05F9983.jpeg (488.1 KB, 2308 views)
File Type: jpeg 7AFBE4B1-E399-4917-94A3-F22AD6C746E4.jpeg (431.7 KB, 128 views)
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