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Review and Measurements of Massdrop THX AAA 789 Amp

amirm

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#1
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:

Massdrop THX AAA 789 Headphone Amplifier Review and Measurement.jpg

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:

Massdrop THX AAA 789 Headphone Amplifier Dashboard Measurement.png


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:
Massdrop THX AAA 789 Headphone Amplifier SNR Measurement.png


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:

Massdrop THX AAA 789 Headphone Amplifier Power at 300 ohm Measurement.png


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:
Massdrop THX AAA 789 Headphone Amplifier Power at 33 ohm Measurement.png


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:

Massdrop THX AAA 789 Headphone Amplifier Power at 50 ohm balanced Measurement.png


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:
Massdrop THX AAA 789 Headphone Amplifier Output Impedance Measurement.png


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:
Massdrop THX AAA 789 Headphone Amplifier Frequency Response Measurement.png


Last but not least, let's look at channel imbalance as we manually vary the volume from max to min:
Massdrop THX AAA 789 Headphone Amplifier Channel Imbalance Measurement.png


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! :D 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. :)

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

If you like this review, please consider donating using Patreon (https://www.patreon.com/audiosciencereview), or upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054). It will enable me to pay for gear that is not offered for loan.
 
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Sythrix

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#4
So let me make this short: the Massdrop THX AAA 789 has my strongest recommendation. Buy one and be happy forever. :)
Excellent news and fantastic review. Thanks again for reviewing it!
 

amirm

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#5
My pleasure. I think some new ground has been broken here in transparency and dynamics. Should make for an interesting setup to see if some very low levels of distortion are audible.

Hopefully availability increases a few months from now.
 

Sythrix

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#6
Hopefully availability increases a few months from now.
Typically after a drop or two, they enter normal production and become available for shipping within a few days. I think your review should help garner interest, which might speed things up ;).
 

helloworld

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#7
Unfortunately you don’t own this unit, otherwise I would really like to request if you could change the circuit inside to make the blance in become the blance out (modify the 4 pin blance out) and measure the performance of blance out. I would like to use this amplifier as a preamp for my speakers.
 
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#8
Thanks for the review, and which gain setting was used during the test? Also by any chance could you possibly do a teardown of this amp?
 

jsmiller58

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#9
This Feb, when the units from the second drop are shipped I will be one happy guy! And I think I will get a second one for my office when available!

@amirm - are you getting a sample of the Monoprice 788 to review? I may vector my $$ I would spend on the second 789 to the 788 if it measures well!
 

amirm

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#11
Thanks for the review, and which gain setting was used during the test? Also by any chance could you possibly do a teardown of this amp?
Gain 3 was used. And yes, a teardown is being considered. :)
 

amirm

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#12
@amirm - are you getting a sample of the Monoprice 788 to review? I may vector my $$ I would spend on the second 789 to the 788 if it measures well!
Yes, one has been promised by a member. Looks like it is still being promised for 11/5.
 
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#13
Glad to see actual progress in amplifier technology being made (given how they emphasize on measurement this is not that surprising).
Yes, one has been promised by a member. Looks like it is still being promised for 11/5.
Is this the one?
https://www.monoprice.com/product?p_id=24459
It has AKM 4493, and seeing that it performs well with Topping (no hump), coupled with THX and we might just have the new reference on our hand.
 

helloworld

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#14
@amirm do you have the 4 pin xlr to 3 pin xlr cable? If so I would like to see the blance output performance as I am interested to use this as a volume control for my active speaker.
 

amirm

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#15
@amirm do you have the 4 pin xlr to 3 pin xlr cable? If so I would like to see the blance output performance as I am interested to use this as a volume control for my active speaker.
You mean without loading it up with 50 ohm?
 

helloworld

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#16
You mean without loading it up with 50 ohm?
I am sorry I dont really understand this 50ohm load here. To me I want to get a good headphone amplifier to control the volume of my speaker. This amplifier should have higher dynamic range and thd+n than my Dac. It looks to me this one is perfect. But I don’t really know if I could directly connect the blance out to my speaker’s blance in.
 

amirm

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#18
I am sorry I dont really understand this 50ohm load here. To me I want to get a good headphone amplifier to control the volume of my speaker. This amplifier should have higher dynamic range and thd+n than my Dac. It looks to me this one is perfect. But I don’t really know if I could directly connect the blance out to my speaker’s blance in.
I tested the balanced output with a 50 ohm load already and hence my question. With the higher impedance of your active speaker, it should even have less distortion.

To use it as you want, you would need to build a special cable that splits the 4-pin into two 3-pin XLRs.
 

amirm

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#19
Thanks for the review!
The price 349USD is way too low for this product. I am glad I bought 2 of them :)
You bought two??? No wonder there is a shortage. :D

I agree. For its performance, it has a very reasonable price.
 

rajapruk

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#20
Am I interpreting this correctly:
If measured with lower impedance than 600ohm then the SINAD would go up even higher?
(You wrote noise was restriction, not distortion. And noise was not going up for more power output)

If so, what is SINAD and dynamic range for 300ohm and 32ohm load (with balanced input and output)?
 
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