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Tear down of Massdrop THX AAA 789 Headphone Amp

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.
 

ZeDestructor

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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.
 

ZeDestructor

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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.
 

Evgeniy

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Anyone tried change volume attenuator to stepped, for example EIZZ 25K 24 Step HIFI Audio Potentiometer or other ?
What attenuator need, 50K or 100K ?
 
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YSC

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Sorry to resurrect an old post, any idea what potentiometer it’s using? It looks like pretty tiny and I suspect the sample to sample variation
 

JohnYang1997

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That don’t induce a lot of confidence on variation though
Apart from the scratch sound after some use, it's actually much more consistent in channel balance than 09 type alps.
 

YSC

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Apart from the scratch sound after some use, it's actually much more consistent in channel balance than 09 type alps.
But the scratchy part seems like a degradation of the pot and the long term usage is having limited lifespan?
For this type of performance better make it a last for decades device
 

JohnYang1997

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But the scratchy part seems like a degradation of the pot and the long term usage is having limited lifespan?
For this type of performance better make it a last for decades device
It's an unforseen outcome. They of course didn't predict that.
 
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YSC

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It's an unforseen outcome. They of course didn't predict that.
Right, but it’s always sad that when I looked at tempting products to replace the old ones serviced for 6-8 years they always have some kind of drawbacks holding me from pulling the trigger
 

Tks

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Right, but it’s always sad that when I looked at tempting products to replace the old ones serviced for 6-8 years they always have some kind of drawbacks holding me from pulling the trigger

That is what planned and intrinsic obsolescence looks like. You can't actually find products that are truly built to last mostly by design and economic reality since the Industrial Revolution that has for the first time in human history laid to rest ever demand outpacing supply in terms of production of goods. Perfect products only exist in the imagination, and ones that get close, in echelons of pricing that put the company making them at risk of going defunct.

Upper middle-class stuff I feel is the sweet spot for most things beyond performance (like vanity based, or requirements concerning warranties, or brand pedigree).

The idea of buying something with design of repair-ability in mind is becoming more and more of a relic. That might sound like some old-person talk (boomer talk I should say), but I'm not joking. It's not just literally designs aren't handled this way anymore, but even the judicial system of even allowing things to be repaired is being assaulted by industry lobbyists. Folks like Louis Rossman as a basic business owner is actually trying to fight these pathetic fucking clowns in these industries sending out barely-literate messengers ("lobbyists") reading off scripts to legislators.

So not only is the boomer message of "ain't built like a used to be" a valid anecdote, it's actually sound, and can be demonstrated from points of psychology, historical precedence, economic perspective, and now legislative initiatives.

My advice? Don't sweat it so much, try and bite a bullet with companies that will seemingly be around, and will send you a shipping label to get your product in their repair facilities. (Or in the case with the apex efficient monster of planned obsolescence -Apple- they'll just toss your old phone in the recycling bin, and make you pay half price for a replacement, only after you pay the deductible, on the warranty you purchased at nearly half the price of the phone anyway a few years back, which you are hopefully still covered under).

What this advice wants to say in conclusion. Just get a Monolith 887 if you're this keen on a THX amp of around this cost.
 

Killingbeans

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The biggest shock here is that there is no "THX module" of any sort! No custom or hybrid parts that I can see. Instead, we have a bunch of exclusively Texas Instruments parts. I have what I think is the sum total of it encased in white square, with a set of OPA 1602A op-amps providing the feed forward/low power output, augmented by pair of high power OPA 564 amps to drive larger signals.

Sorry for reviving an old thread, but this has puzzled me lately :)

I'm thinking it might be possible to utilize the design around the LM3886 (?) and thereby jump into the realm of speaker amplification.

But maybe it's not that easy?
 
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