Tear down of Massdrop THX AAA 789 Headphone Amp

[MalinYamato]

With kind permission of the owner, I took apart the Massdrop THX AAA 789 headphone amplifier. If you have not seen it yet, check my my review of Massdrop THX 789.

The internal structure of this unit follows other models from Massdrop. I was impressed with the effort to keep all the components on one side using mostly SMT (surface mount) parts. The bottom of the PCB is only through-hole pins which I am assuming was wave soldered.

Here is lower resolution shot of the PCB with my annotations:

View attachment 17795

Click on this attachment to get 4X higher resolution image.

View attachment 17796

I have 4X larger image yet again. Send me a message if you want that.

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.

With no custom or proprietary parts, the design is open to replication by Asian vendors.

The auto-off and power signals are in the top left part which I have zoomed in and shown beside it. There is an audio signal detector from JRC that allows them to automatically power the unit on and off based on whether there is music signal or not. And that dreaded logic for power on/off switch.

Fair number of relays are on the board. Interesting to see a pair of them by the RCA input signals. Do they shut them off when balanced input is in use?

The input stage is made up of a bunch of OPA 1602A op-amps. Curious to see room for two more relays there but only one populated. Likewise, there is room for two large electrolytic caps in the middle right that are missing. I don't see the need for them give total absence of any power supply noise but maybe you want to add them to make yourself feel better post warranty.

I see no hand modifications or soldering of any sort. This may have been behind the delays, with them turning the PCB multiple times to make sure it is fully automated assembly with no need for manual labor to fix things.

Here the links to various ICs used:
OPA564: http://www.ti.com/lit/ds/symlink/opa564.pdf

View attachment 17797

TL062: http://www.ti.com/lit/ds/symlink/tl062.pdf

View attachment 17798

OPA1602: http://www.ti.com/lit/ds/symlink/opa1604.pdf

View attachment 17799

OPA564: http://www.ti.com/lit/ds/symlink/opa564.pdf

View attachment 17800

JRC NJU7181: https://www.njr.com/semicon/PDF/NJU7181_E.pdf
View attachment 17801

TI LM239: http://www.ti.com/lit/ds/symlink/lm239.pdf
View attachment 17802

Conclusions
The Massdrop THX AAA 789 has a rather high quality and optimized mechanical and electronic design. Nothing jumps out at me to note as problem areas.

The big surprise is lack of any custom parts to implement the THX feed forward amplification logic. Instead, this is THX know-how, and design of a discrete amplifier using standard off-the-shelf ICs and parts. Even though they say they have patent protection, they have left themselves open to reverse engineering by the usual people. The core ideas behind feed forward design are 20+ years old anyway so any patents on the main idea have long expired. Not encouraging anyone to go and copy their design of course. They are not charging hardly any premium here so might as well get the real deal.

I hope the other experts on the forum study the details and chime in with what they figure out. I just performed a cursory look and have made little attempt at fully documenting the unit.

Overall, a thumbs up for Massdrop THX AAA 789 from build quality.

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why so much empty space in the box .... why can't you make it at least 50% smaller and more compact?

 

[Onik]

can anyone tell me roughly how much power I can get for LCD3(110ohms) out from P20? Will it able to power it properly?

 

[Jimster480]

can anyone tell me roughly how much power I can get for LCD3(110ohms) out from P20? Will it able to power it properly?

This isn't the topic for that, there are many other threads or you can start your own. This thread is for the THX AAA 789.

 

[SoNic]

I am late to the "party", but I am interested in this amp.
Looking at the hires pics in the first post and the patent filing posted later, I think that the part circled in red is actually the "magic" that makes AAA have lower distortions.
I see there a few discrete SMD transistors, and those can be fed from the output of the four power OpAmps, and the associated feedback networks, according with the patent diagram. Not really high power transistors, but for this application probably they are enough.
Those discrete transistors are close enough to the headphone jack to make sense of this supposition.

 

[norcalscott]

I am late to the "party", but I am interested in this amp.
Looking at the hires pics in the first post and the patent filing posted later, I think that the part circled in red is actually the "magic" that makes AAA have lower distortions.
I see there a few discrete SMD transistors, and those can be fed from the output of the four power OpAmps, and the associated feedback networks, according with the patent diagram. Not really high power transistors, but for this application probably they are enough.
Those discrete transistors are close enough to the headphone jack to make sense of this supposition.

If you're not using IEM's, have you considered the THX SP200? - See this comparison.

Drop has this for $175.00 (it had been a little lower) which is still a great bargain. I ordered one to replace an old Monoprice headphone amp, it should arrive this week.

 

[SoNic]

I was considering it, but I would prefer to use a true balanced output, from different amps. Less distortions, obtained "the easy way", by cancellation of similar stage distortions instead of feedback mechanisms. IDK, maybe I am completely wrong.

PS: HIFIMAN Deva Pro and AKM K701 headphones.

 

[LeaveItTubeBeaver]

I bought this amp with the dreaded red light issue. I replaced this transistor which keeps it fr going into protect mode, but unfortunately I believe another competent is bad.

The power switch is a momentary type and just completes a circuit. I'm wondering if it failed open. Anybody have any clues? I'm having a hard time finding a relay on the schematic.

Edit: I didn't mention that the amp does work now but won't power off. You have to actually unplug it. When plugged in it automatically turns on, even with the auto power turned off)

 

[Rottmannash]

I bought this amp with the dreaded red light issue. I replaced this transistor which keeps it fr going into protect mode, but unfortunately I believe another competent is bad.

The power switch is a momentary type and just completes a circuit. I'm wondering if it failed open. Anybody have any clues? I'm having a hard time finding a relay on the schematic.

Edit: I didn't mention that the amp does work now but won't power off. You have to actually unplug it. When plugged in it automatically turns on, even with the auto power turned off)

Do you mean it doesn't automatically shut off after so many minutes w/o input signal?

 

[restorer-john]

I bought this amp with the dreaded red light issue. I replaced this transistor which keeps it fr going into protect mode, but unfortunately I believe another competent is bad.

The power switch is a momentary type and just completes a circuit. I'm wondering if it failed open. Anybody have any clues? I'm having a hard time finding a relay on the schematic.

Edit: I didn't mention that the amp does work now but won't power off. You have to actually unplug it. When plugged in it automatically turns on, even with the auto power turned off)

You have the schematic, just post the pdf here and we can help.

 

[LeaveItTubeBeaver]

You have the schematic, just post the pdf here and we can help.

The best info I could find was from another user on this site. I think this is patent info but it's the closest I could find.

The last attachment is a better picture of the whole power circuit

Do you mean it doesn't automatically shut off after so many minutes w/o input signal?

Even with the auto off toggle switches to off, the power button does nothing. A signal is not being sent to shut down. Only way to turn it off is to unplug power

 

[LeaveItTubeBeaver]

 

[LeaveItTubeBeaver]

It has a signal detect IC in it, so it turns on automatically when music is applied and off when the music goes away (after a delay). There's an always-on override. You can see the block diagram of the signal detect IC (and its part number) in the first post.

Tom

Is this part responsible for on/off in general? My amp now works after replacing transistor q4 but it's now stuck powered on. Bypassing auto off and using the power button does nothing and the unit has to be unplugged

 

[LeaveItTubeBeaver]

Despite my better judgement (or lack there of) I went ahead and purchased 2 more defective units from ebay.
One came with a while light always on but no sound, the same transistor was bad as my previous amp, as well as a broken toggle switch for auto off and bypass.
The second one had the same issue as my previous one, always on red light. I replaced the transistor and it powered up, but same issue as my last attempted fix, I actually saw the puff of smoke. After powering it off, it returned to red light when plugged in and wouldn't turn on. Since I have plenty of 2n2222 I replaced Q4 again and it's back to working but not powering off.

At this point, I don't think it's a matter of this transistor being bad, I think it's something further up the line. Maybe there is a part that is throwing too much voltage at it? no idea, but whatever it is, i'm sure it's this transistor that's bad in every red light case.

 

[SoNic]

Despite my better judgement (or lack there of) I went ahead and purchased 2 more defective units from ebay.

So you were the one that up-bid me
The soft power off circuit probably is undersized if this keeps happening.

 

[SoNic]

Since I have plenty of 2n2222 I replaced Q4 again and it's back to working but not powering off.

I have read that that Q4 transistor is actually a BC846-1B. It's even a lower current than 2N2222.
I also bought an unit "as-is", waiting for the transistors. I wonder if that's a bad design and needs a different transistor, capable of taking the current.

 

[drewdawg999]

I just got one of these used, and unfortunately there was some shipping damage which rendered the RCA jacks inoperable. Everything else is fine, so with balanced in and out the amp performs admirably. But I was forced to take the thing apart to check out the RCA jacks, and they're all busted up, plastic parts broken and I believe a couple leads were severed. So I'm all balanced at the moment, and I have to lug my balanced DAC around from bedroom to the big system (oh woe is me). But I intend to fix the thing since I want single ended operation. I desoldered and removed the broken RCAs and already ordered a part of eBay that I hope fits, it's coming all the way from China so we shall see.

 

[drewdawg999]

So I fixed my unit today, part off ebay fit perfectly. Just drop in, soldered 6 pins, and that was it. The thing is built like a tank to take my clumsy abuse, but I love the thing.

I got the job done and I'm pretty stoked about it. As far as I can tell, there's no difference between SE and balanced, such are the miracles of THX. So I can restore order and put the DO100 back in the big rig, not have to lug it around anymore. The old Sanskrit 10th mkII is a great little DAC, perfect for this bedroom system.

 

[RenEH]

Does anyone know what value of the two missing smoothing caps at the power supply input should be in value? I'm assume these were cost cut, but I'm interested if I could add them in just cause.

 

[Jimster480]

Does anyone know what value of the two missing smoothing caps at the power supply input should be in value? I'm assume these were cost cut, but I'm interested if I could add them in just cause.

You mean they aren't in the Amp? Why would you want to add additional things to the Amp?

So I fixed my unit today, part off ebay fit perfectly. Just drop in, soldered 6 pins, and that was it. The thing is built like a tank to take my clumsy abuse, but I love the thing.
View attachment 294269
I got the job done and I'm pretty stoked about it. As far as I can tell, there's no difference between SE and balanced, such are the miracles of THX. So I can restore order and put the DO100 back in the big rig, not have to lug it around anymore. The old Sanskrit 10th mkII is a great little DAC, perfect for this bedroom system.
View attachment 294271
View attachment 294272

Nice setup!

 

[Killingbeans]

I'm assume these were cost cut

It could also be because the switch mode power brick used with the final design turned out to be unhappy with capacitive loading. Who knows...