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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 12th July 2018, 03:30 AM   #1
xrk971 is online now xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Default Cheap as Chips OPA1688 Low-THD Muscle Amp

I normally make discrete SE Class A amps, but I have found that the OPA1688 sounds quite nice. This project started out in this thread, where I explored using a single OPA1688 with an output cap coupler to reduce the complexities of active PSU rail shut-off to prevent DC imbalance on the headphone outputs. I hand-etched a simple single OPA1688 and gave it a non-polar output cap bank and it sounds very nice. Of course, there is global feedback and it is not Class A. But it there is something nice about the simplicity and low power consumption for 9v battery use. So it got me to thinking of how to boost the output power by paralleling the OPA1688. These are capable of 75mA current drive and can handle rails up to +/-18v. Here is the single OPA1688 amp that I built:
Click the image to open in full size.

Click the image to open in full size.

The OPA1688, is not your typical LTP input stage opamp as the measurements show that it has a dominant H2 (second harmonic character with monotonically decreasing higher orders). Here is 2vpp into 47ohms:
Click the image to open in full size.

Internally, the OAP1688's are a 3-stage amp design and capable of remarkable performance. The circuit uses the much-liked LME49720 as the voltage amp stage with the OPA1688 as a power buffer and the global feedback loop is from the output of the summed buffers back to the voltage amp. Load balancing 1R resistors are used at the output of the buffers and 220R summation resistors are used at the inputs to the buffers. With 6 buffers, this will require three dual OPA1688IDR IC's. With SOIC8 package and full use of SMT parts where possible, the total footprint can be quire small. A total of 6 x OPA1366's will be needed with a single dual channel LME49720 for the voltage amp stage. The cost of the 7 chips is about $10 and will produce an amp capable of about 1.3wrms into 50ohms and maybe 3.5wrms into 16ohms. All at exceedingly low THD levels. I modeled it LTSpice with the following schematic:
Click the image to open in full size.

Here is the predicted FFT for 2vpp into 50ohms:
Click the image to open in full size.

The Fourier components and THD are way down there in the noise.
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.001e+00 1.000e+00 -0.03 0.00
2 2.000e+03 7.524e-09 7.513e-09 -24.92 -24.89
3 3.000e+03 8.023e-08 8.012e-08 -89.87 -89.84
4 4.000e+03 1.141e-09 1.139e-09 -40.43 -40.40
5 5.000e+03 9.318e-10 9.305e-10 -8.94 -8.90
6 6.000e+03 1.251e-09 1.249e-09 -18.57 -18.54
7 7.000e+03 1.290e-09 1.289e-09 -0.13 -0.10
8 8.000e+03 1.568e-09 1.566e-09 -10.49 -10.46
9 9.000e+03 1.659e-09 1.656e-09 -0.22 -0.19
Total Harmonic Distortion: 0.000008%(0.000000%)
Here is the predicted frequency response and phase for the above case:
Click the image to open in full size.

Here is the FFT for 8vpp into 50ohms:
Click the image to open in full size.

And the corresponding Fourier components and THD:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.006e+00 1.000e+00 -0.03 0.00
2 2.000e+03 9.238e-08 2.306e-08 -8.09 -8.06
3 3.000e+03 4.755e-06 1.187e-06 -90.26 -90.22
4 4.000e+03 7.334e-09 1.831e-09 -24.03 -24.00
5 5.000e+03 1.277e-07 3.189e-08 -89.48 -89.45
6 6.000e+03 3.201e-09 7.992e-10 -29.19 -29.16
7 7.000e+03 4.595e-09 1.147e-09 -54.64 -54.61
8 8.000e+03 3.582e-09 8.941e-10 -17.89 -17.86
9 9.000e+03 3.472e-09 8.668e-10 -1.11 -1.08
Total Harmonic Distortion: 0.000119%(0.000000%)
So now, just need to build a prototype and test it.

Update July 13, 2018: 3d render of 8 OPA1366 Muscle HPA stable driving 8vpp into 8ohms with low distortion - another superb layout by JPS64:
Cheap as Chips OPA1688 Low-THD Muscle Amp-opa1688-muscle-amp-3d-render-view2-png
Attached Images
File Type: png Muscle-amp-sch.png (47.8 KB, 628 views)
File Type: png Muscle-amp-2vpp-50ohms-FFT.png (18.0 KB, 616 views)
File Type: png Muscle-amp-2vpp-50ohms-phase.png (22.6 KB, 593 views)
File Type: png Muscle-amp-8vpp-50ohms-FFT.png (18.9 KB, 589 views)
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Last edited by xrk971; 13th July 2018 at 01:32 PM.
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Old 12th July 2018, 07:43 AM   #2
lineup is offline lineup  Sweden
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How much current does the circuit draw, per channel?
It is important as you mention 9V battery.

My guess is like 13 mA per channel. Gives like 25 mA total.
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Last edited by lineup; 12th July 2018 at 07:47 AM.
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Old 12th July 2018, 08:16 AM   #3
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Old 12th July 2018, 11:25 AM   #4
xrk971 is online now xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
This will be a desktop amp, but in any case, it is Class AB and the OPA1688 is capable of driving 75mA max each and has a quiescent current of 1.6mA ea. Really designed for portable applications. At max draw under music the amp can provide 450mA of current swing. At idle it is 14 x 1.6mA (if I use the 1688 for the voltage gain stage as well) so that’s 22.4mA idle. Even in portable use this is not too bad as the Pocket Class A amp draws 120mA and works for hours on 600mAhr batteries. So at idle and not playing music, it’s good for 26hrs. Music on depends on type of music and how loud one plays. Probably still well over 12hrs per charge of the dual 9v Li-ions.
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Old 12th July 2018, 11:41 AM   #5
Dibya is offline Dibya
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8vpp on 50ohms load . Oh gosh who need that much of power to drive a headphone.
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Old 12th July 2018, 12:42 PM   #6
xrk971 is online now xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Quote:
Originally Posted by Dibya View Post
8vpp on 50ohms load . Oh gosh who need that much of power to drive a headphone.
Some headphones such as HiFiman HE-6 that are only 83.5dB sensitive needs lots of power.

Headphones & portable audio - HIFIMAN.com
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Old 12th July 2018, 06:44 PM   #7
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You might be interested in some of Douglas Self's experiments in parallel 5532 opamps. He's published some impressive results for something so simple.

Here's a link with measurements, though you have to buy the article from Elektor:

The 5532 OpAmplifier, part 1 | Elektor Magazine

Self covers this in the Small Signal book, IIRC.
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Old 12th July 2018, 08:50 PM   #8
xrk971 is online now xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Thanks for that link Sodacose. No, I did not know about the Doug Self parallel opamp boards. I think the OPA1688 is a higher current opamp and will need quite a few less to achieve similar results.
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Old 13th July 2018, 12:15 AM   #9
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hi XRk971 what's you plan on the decoupling cap on the supply rail?
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Old 13th July 2018, 01:02 AM   #10
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Normally I go with 100uF 25v and 10uF X7R and 0.1uF X7R at each opamp IC at each rail. But with this many amps so close together and actually working in unison, and now possibly 8 opamps output, will be using a pair of 1000uF 25v per 4 IC’s (8 opamps) and will be using the WSON8 package with a power pad on the bottom for heatsink connection and each amp side having its own power pins, will have to let the amps share a common 1000uF for compactness. Each power supply pin has its own 0.1uF X7R cap though.

So for 4 IC’s (8 opamps) there are 2x 1000uF 25v, and qnty. 16 x 0.1uF 100v X7R.
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Last edited by xrk971; 13th July 2018 at 01:04 AM.
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