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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 22nd May 2019, 03:42 AM   #211
yourownbuzz is offline yourownbuzz  Australia
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didn't realize this made its way to a production board - must've missed it. Thought you were still going through development on hand etched PCBs.
Looks good!
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Old 22nd May 2019, 11:16 AM   #212
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Yes, I must have forgotten to mention that I got the boards fab’d. It’s a pretty slick little amp that may actually handle small speakers well too.
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Old 23rd May 2019, 05:33 AM   #213
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
I soldered one set of OPA1622's for a 2x parallel amp just to test and make sure it works before installing the remaining chips. Works great - at just 2x, it is more than enough for my 96dB OB-1 headphones. Using a linear 25w trafo and cap multiplier to get +/18v rails. Build uses Susumu and Dale metal thin film resistors on all key locations. JPS64's excellent layout is perfect, the amp fired up the first time, no issues. I was actually surprised it worked given that my soldering of the VSON10's was a bit sketchy. Those things really self-align surprisingly well via surface tension.

Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-opa1622-build-01-jpg

Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-opa1622-build-02-jpg

Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-opa1622-build-03-jpg
Attached Images
File Type: jpg Muscle-amp-opa1622-Build-01.jpg (649.3 KB, 553 views)
File Type: jpg Muscle-amp-opa1622-Build-02.jpg (497.0 KB, 345 views)
File Type: jpg Muscle-amp-opa1622-Build-03.jpg (523.2 KB, 344 views)
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Last edited by xrk971; 23rd May 2019 at 05:44 AM.
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Old 25th May 2019, 03:46 PM   #214
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
I did not realize that the OPA1622 is the industry’s highest performing opamp?!

That’s quite a claim to be the top in terms of low distortion and high power.

https://www.prnewswire.com/news-rele...300192153.html
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Old 28th May 2019, 11:25 AM   #215
Mark Tillotson is online now Mark Tillotson
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I don't understand the 220 ohm "summation" resistors you mention. There is no summation, there is no need for resistors at the input of a follower (they add a tiny bit of noise, that's all).


I think the Zobel network should be more like 22n/47R, otherwise its roll-off is too close to the audio band and affect the frequency response.
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Old 8th June 2019, 10:03 PM   #216
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
All 8 OPA1622's are now in place. Need to sort out a rather powerful low noise +/-18v power supply capable of supplying 8x150mA=1.2A. I will use a 25VA 15v trafo and cap mulitiplier:
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-opa1622-build-8-parallel-04-jpg

Closeup:
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-opa1622-build-8-parallel-closeup-05-jpg
Attached Images
File Type: jpg Muscle-amp-opa1622-Build-8-parallel-04.jpg (572.7 KB, 220 views)
File Type: jpg Muscle-amp-opa1622-Build-8-parallel-closeup-05.jpg (485.2 KB, 358 views)
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Old 9th June 2019, 03:12 PM   #217
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
Default Measurements of Muscle Amp

The amp is powered by an Antek 25VA 15v transformer and a Prasi/MJ cap multiplier (Darlington topology) with 2200uF of bulk capacitance. I setup the amp to drive a 47ohm 2w resistor using a 2ppm 1khz oscillator. Voltage across dummy load is fed across the differential inputs of the Focusrite 2i4. I replaced the 100nF film cap in the Zobel with 22nF AVX MKT 100v film cap.

Setup:
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-test-setup-jpg

Here is measured distortion profile for 1.0Vpp into 47ohms (0.00061% THD):
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-1vpp-47ohms-fft-ss-jpg

2.0Vpp into 47ohms (0.00082% THD):
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-2vpp-47ohms-fft-ss-jpg

and 4.0Vpp into 47ohms (0.0017% THD):
Cheap as Chips OPA1688 Low-THD Muscle Amp-muscle-amp-4vpp-47ohms-fft-ss-jpg

Settings for the FFT can be seen in the inset setup page.

Listening to the amp now with test tracks. Sounds very nice - neutral, clean and very quiet. Fluke 101 shows 0.0mV in mVAC. Output DC offset is -9mV on both channels. There is no scratchy noises when the volume pot is adjusted - smooth.

One final check, I connected it to a 4ohm speaker (dual PA130-8 XKi) and it sounds great, Qnty 8 of the OPA1622's in parallel should be able to drive a 4ohm speaker load since one can drive 32ohms.
Attached Images
File Type: jpg Muscle-Amp-test-setup.jpg (416.1 KB, 184 views)
File Type: jpg Muscle-Amp-1vpp-47ohms-FFT-ss.jpg (186.1 KB, 492 views)
File Type: jpg Muscle-Amp-2vpp-47ohms-FFT-ss.jpg (184.2 KB, 435 views)
File Type: jpg Muscle-Amp-4vpp-47ohms-FFT-ss.jpg (180.1 KB, 434 views)
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Last edited by xrk971; 9th June 2019 at 03:30 PM.
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Old 10th June 2019, 12:19 PM   #218
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
More closeup shots of the amp during testing.
Attached Images
File Type: jpeg A2507C2E-2393-4473-8D57-C427E02282BB.jpeg (955.2 KB, 55 views)
File Type: jpeg C1C18CA1-713A-4D46-88CF-C538BD805C4A.jpeg (900.0 KB, 58 views)
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Old 13th June 2019, 04:49 PM   #219
xrk971 is offline xrk971  United States
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Cheap as Chips OPA1688 Low-THD Muscle Amp
I have spent a couple of nights listening to this amp now and like to enough to box it up. Will be looking for a suitable case now. This amp is a keeper. Very versatile as can be used as HPa, preamp, or small speaker amp.
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Old 16th June 2019, 07:07 PM   #220
spookydd is offline spookydd  France
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By the way, are you doing the pcb layout with eagle?
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