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Tear down of Massdrop THX AAA 789 Headphone Amp

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I have one of these that will not power back on after being left on overnight. In my search I found a few who have had issues with the power on circuit. Is it possible to bypass this circuit to power it directly?

Maybe you didn't read the post correctly. I can't turn it off if it won't power on to begin with. But thanks for the comment.
 

hapnermw

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I may be wrong, but the trio of op amps at the bottom left of the pic look to be summing the differential input to single ended before the volume pot (which looks to be of the 2-channel variety); after the pot, the signal looks like it’s getting converted back to differential again at the “THX” section with the quad output buffers driving the balanced phone jack. For the purists here, no, not “fully balanced” all the way through, then. Not that it really matters, given the wonderful measurements on Amir’s test bench.
THX AAA-888 ref design is specified as 'dual mono'. THXAAA-788 ref design is specified as 'stereo amplifier with power supply'. It appears that the Mass Drop 789 uses the THXAAA-788 design. It's possible that the Monoprice 887 may use the 888 design.
https://www.thx.com/mobile/aaa/
 

JohnYang1997

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THX AAA-888 ref design is specified as 'dual mono'. THXAAA-788 ref design is specified as 'stereo amplifier with power supply'. It appears that the Mass Drop 789 uses the THXAAA-788 design. It's possible that the Monoprice 887 may use the 888 design.
https://www.thx.com/mobile/aaa/
788 has nothing to do with 789. 888 887 and 789 are very similar in design.
 
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THX AAA-888 ref design is specified as 'dual mono'. THXAAA-788 ref design is specified as 'stereo amplifier with power supply'. It appears that the Mass Drop 789 uses the THXAAA-788 design. It's possible that the Monoprice 887 may use the 888 design.
https://www.thx.com/mobile/aaa/
It's very likely that the Monoprice 788 uses a THX-788 module, while the Drop + THX 789 and Monoprice THX 887 likely use 2 THX-888 modules each (just going off the balanced output numbers).
 

JohnYang1997

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It's very likely that the Monoprice 788 uses a THX-788 module, while the Drop + THX 789 and Monoprice THX 887 likely use 2 THX-888 modules each (just going off the balanced output numbers).
789 888 887 are basically the same but not exactly. 789 uses opa1602 and 888 uses 1612. 887 likely 888. 788 tho completely different.
 
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789 888 887 are basically the same but not exactly. 789 uses opa1602 and 888 uses 1612. 887 likely 888. 788 tho completely different.
That would definitely explain the incredibly minor 2dB advantage for the Monoprice 887 - same THX-888 module/circuit, slightly better parts in one particular spot of the module/circuit. I wonder if Drop has done or plans to do a silent revision to their 789 updating their 1602s to 1612s, especially given the $50 price hike.

Anyone have an HPA4 they want to tear open and post pictures of? Now I really want to see what that has...
 

Veri

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That would definitely explain the incredibly minor 2dB advantage for the Monoprice 887 - same THX-888 module/circuit, slightly better parts in one particular spot of the module/circuit. I wonder if Drop has done or plans to do a silent revision to their 789 updating their 1602s to 1612s, especially given the $50 price hike.
Doubt it :)
 

JohnYang1997

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That would definitely explain the incredibly minor 2dB advantage for the Monoprice 887 - same THX-888 module/circuit, slightly better parts in one particular spot of the module/circuit. I wonder if Drop has done or plans to do a silent revision to their 789 updating their 1602s to 1612s, especially given the $50 price hike.

Anyone have an HPA4 they want to tear open and post pictures of? Now I really want to see what that has...
opa1612.
 

Veri

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Doubt on the circuit design similarity, or doubt on drop doing a silent revision? I think there's a chance they did. Small, but present.
On the silent revision side. I would be doubtful but who knows without looking at the hardware, of course.
 
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