Review and Measurements of Massdrop THX AAA 789 Amp

[mi-fu]

I just compared my THX 789 and Sennheiser HDVA 600 (I believe it has the same amp as HDVD 800, only without the dac?) with HD6xx. I actually kinda prefer the sound of 789. But I don't find the difference that significant anyway.

If I were to listen to the HD6xx + THX 789 at gain 2 and 70% volume

Yes... it should be very, very loud. I only use gain 2 with around 9 o'clock on HD6xx.

 

[ReAlien]

Yes... it should be very, very loud. I only use gain 2 with around 9 o'clock on HD6xx.

Am I right that we don't know if he's using balanced or SE connection to his 6XX? Are you using a balanced one?

 

[mi-fu]

Am I right that we don't know if he's using balanced or SE connection to his 6XX? Are you using a balanced one?

Ahh!! It is true. Thanks for correcting me. I was using balanced input from my DAC to the 789. I was using SE output from the 789 to my 6xx though.

 

[kiat]

Based on the spec of my dac, it is outputting via RCA 1.414 VRMS which is low. I've tried switching over to balanced XLR. Things are much better now.

 

[Jorj]

Just a short update after a bit more listening time on the 789.

For those that are using sensitive IEMs, the 789 has no issue whatsoever with them. I'm guessing the Noble K10 runs around 108-110dB equivalent sensitivity (my Mojo is nearly at its lowest volume with them). I can run the 789 at gain 2 and volume just barely opened up with no detectable imbalance and zero hiss at any volume level. Again, I could gush a bit, but this is a science forum, so let's just say that if you were thinking of this kind of pairing, have no concerns. The 789 does fine against lower loads.

The Sennheiser HD800 also pairs nicely, but we knew they would, as they are 300 ohm, just as the HD650 and HD6xx are. If you are sensitive to the peakiness of the HD800, the 789 does nothing to alleviate that. For me, having a bit of hearing loss in the 6-8 KHz range, the HD800 is a good fit and the 789 drives them as well as I've ever heard. Good stuff.

Do we know yet who designed & manufactured this amazing little black box for Massdrop? I'd buy them a beer.

 

[helloworld]

Do we know yet who designed & manufactured this amazing little black box for Massdrop? I'd buy them a beer.

@AndrewMason

 

[Jorj]

@AndrewMason

I know Andrew was involved with with the project, but did he really design the whole thing for them...? That seems awfully generous of THX, but maybe the relationship is tighter than I know. Maybe we can get CeeTee to give up the SBAF show and come hang out here with the cool kids.

 

[elira]

let's just say that if you were thinking of this kind of pairing, have no concerns. The 789 does fine against lower loads.

Mine has problems of imbalance on Gain I, I guess your unit has a better pot than mine. Those pots are unpredictable and doesn't seem like Massdrop cherry picked them.

 

[ReAlien]

Based on the spec of my dac, it is outputting via RCA 1.414 VRMS which is low. I've tried switching over to balanced XLR. Things are much better now.

In fact, I've meant a cable between your headphones and amp, as there's a big difference in power between SE and balanced connections. This might account for volume knob differences.

 

[kiat]

In fact, I've meant a cable between your headphones and amp, as there's a big difference in power between SE and balanced connections. This might account for volume knob differences.

Currently it's SE connection between amp and headphone

 

[gino1961]

Hi ! first of all i would like to thank you very much for your excellent review. This test has been very enlightening for me. I had a big confusion in my mind caused by reading countless reviews that separated completely sound quality from measurements. For one time we have a unit that have spectacular measurements and equally spectacular sound. As for me it would be very difficult to get one unit living in Italy i will wait for another unit that has also good measurements even if i understand we have here a benchmark difficult to match. Thanks a lot again, gino

 

[Music1969]

Mine has problems of imbalance on Gain I, I guess your unit has a better pot than mine. Those pots are unpredictable and doesn't seem like Massdrop cherry picked them.

Over what range on gain 1?

Do you have problems past 12 o’clock on the volume knob?

Or only under 12 o’clock knob position? Or only under 10 o’clock?

 

[elira]

Over what range on gain 1?

Do you have problems past 12 o’clock on the volume knob?

Or only under 12 o’clock knob position? Or only under 10 o’clock?

Under 9 o'clock, a little over 9 o'clock it's fine. But with some headphones 9 o'clock is louder than I like.

 

[Music1969]

Under 9 o'clock, a little over 9 o'clock it's fine. But with some headphones 9 o'clock is louder than I like.

Cool thanks. Yeh I have the same issue. I use some digital volume control (HQPlayer) which allows me to get to 12 o’clock (with MrSpeakers AEONs).

I know there’s no free lunch with digital vol control but at least I don’t have this imbalance issue.

 

[kiat]

Currently it's SE connection between amp and headphone

I've change input from RCA -> XLR on the THX 789. It's sounds much better running on balance than on RCA. Improved in the dynamics.

 

[March Audio]

. I have just spent over a week with the HD6xx and HD650 side-by-side and the new model HD650 sounds, to my ear, exactly like the HD6xx.

I'm am glad to read this as I was starting to doubt my pair of hd6xx.

I had a pair of 650s for many years, being replaced about 3 years ago by a pair of Oppo PM1s. First cans I found that I thought improved over the 650s.

I recently bought a pair of hd6xx as an alternative to test out my amp design and I was a little disappointed by the sound. It just wasn't how I remember the 650s. They just didn't have the naturalness I remember in the high frequencies. To the point where I was wondering if the 6xx really is the same as the 650.

Your comments just lead me to think it's just my rubbish audio memory

 

[Veri]

I've change input from RCA -> XLR on the THX 789. It's sounds much better running on balance than on RCA. Improved in the dynamics.

The THX runs SE internally. Balanced goes SE to then go balanced again. There is no improvement in Dynamics... only a chance for better noise suppression.

 

[Jorj]

They just didn't have the naturalness I remember in the high frequencies. To the point where I was wondering if the 6xx really is the same as the 650.

Your comments just lead me to think it's just my rubbish audio memory

I'm no golden ear, and am approaching 50 years and it is certain my hearing in those upper frequencies is not as good as it was. There might be differences and I just can't hear it, but the measurements I've seen support the thinking that they are identical except for paint.

 

[gtb75]

FYI, the THX 789 drop is live again. I just pulled the trigger on one - looking forward to it.

 

[catman]

This is a review and detailed measurements of the just Massdrop THX AAA 789 headphone amplifier. It was drop shipped by request of a kind member for testing. It retails for USD $349.99. The unit had a log gestation period with customers having to wait months to receive the first allotment. The second drop had a ship date of February so if you have not gotten in line, you will be waiting a long time for one. Is the wait worth it? Let's find out.

On the industrial design, Massdrop is using their current design language for their larger desktop audio products which in my opinion is a good thing. While the unit does not have a luxury feel to it, it looks serious and not cheap:

View attachment 16938​

Very conveniently, there are three headphone jacks in front: 4-pin XLR for balanced headphones, and both 3.5 mm and 1/4 inch for single ended. As usual, please note that the word "balanced" here simply means more potential power, and has nothing to do with balanced interconnects between audio gear.

Nicely again, we have three gain settings and not he usual two.

On the backside, we have both unbalanced input (and pass through) and balanced. For my measurements, I tested with the balanced input. For listening tests, I used the unbalanced RCA.

The heart of this unit as the name indicates, is a special amplifier from THX with distortion correction logic that is supposed to yield unbelievably low distortion. Let's get into that as I am sure that is the main thing you all are interested in.

Measurements
For consistency with my last couple of headphone amplifier reviews, I set the output at about 5.7 volts:

View attachment 16939

Wow! That is stunning. We are getting SINAD (signal above distortion and noise) of 117 dB. That beats the best DAC we have ever measured (115 dB SINAD). It by far shatters all previous records.

THX/Massdrop advertise much lower numbers though. If you look at the distortion products, they are around -130 dB and that is the number advertised (more or less). Our SINAD is much lower because of noise. The FFT transform artificially lowers the noise allowing us to see those distortion spikes. We can confirm that by measuring signal to noise ratio:
View attachment 16940

The dashboard uses a 600 ohm load. So let's test it with a more realistic 300 Ohm load and see how much power we get:

View attachment 16941

We get 150 milliwatts of power at distortion of 0.0002% or essentially our dashboard SINAD number (right axis). The graph is essentially noise limited until we hit the saturation point.

Let's go to the other extreme at 33 ohm:
View attachment 16942

As expected we get a ton more power at 1.3 watts. Performance remains impeccable with both channels exactly on top of each other producing astonishingly small distortion and noise at that power level. Quite remarkable.

For balanced, I have a single fixture at 50 ohm so let's see how much power we get there relative to unbalanced:

View attachment 16943

There is no penalty in noise and distortion for using balanced. You simply get copious more power at whopping level of 3.5 watts before clipping.

Output impedance is 1.0 ohm which I would normally say is very good but given the above performance, I wish it was half an ohm lower:
View attachment 16944

EDIT: my measurements always show higher than minimum that can be measured. See this post for the best case impedance which is close to zero: https://www.audiosciencereview.com/...-of-massdrop-thx-aaa-789-amp.5001/post-112221


Frequency response is essentially ruler flat to 40 kHz:
View attachment 16945

Last but not least, let's look at channel imbalance as we manually vary the volume from max to min:
View attachment 16946

Not as good as digitally controlled volume controls but very good nevertheless with just 0.2 dB of error right before muting.

Listening Test
For the source, I hooked up my Khadas Tone Board DAC to the unbalanced input of the Massdrop THX AAA 789. And as always started listening with my Sennheiser HD-650. This combination provided incredible level of transparency. I could set the volume level high and hear the slightest amount of background noise in recordings! It was so good that variations in fidelity of the source was more apparent than I had ever experienced.

With loud electronic music, I could easily set the cups to motion on the HD-650 whit no hint of distortion. Power was ample and clean.

With "audiophile" recordings with a lot of headroom, I had plenty of power but not ear piercing. So would be nice to test the balanced output with that. Alas, this unit arrived too soon for me to build the balanced cable for my Sennheisers. I will finish that project and report back. For now, it is really not necessary to have balanced given the clean power you get single ended out of this unit.

I then switched to Hifiman HE-400i. Same experience continued with effortless power pushing the transducer to point of rattling/distortion. It is an interesting sensation akin to having subwoofers in your ears! Of course not recommended for any length of listening.

I plugged in my low impedance IEMs to see if I could detect any hiss but I could not.

Conclusions
It takes a lot for someone like me who likes finding faults to smile ear to ear in testing audio gear. But that is exactly what THX and Massdrop have achieved. With a SIAND that matches the best dynamic range of our hearing system, you are assured that the only distortion you hear is either in the source or your headphones. Not only that, but we also have tons of power to drive any headphone at a price that is hard to believe for this level of performance.

So let me make this short: the Massdrop THX AAA 789 has my strongest recommendation. Buy one and be happy forever.

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As always, any questions, concerns, comments, corrections, etc. are welcome.

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