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Review and Measurements of Massdrop THX AAA 789 Amp

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amirm

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Thread Starter #61
It does say on the manual that comes with it that single ended performance is worse.
The question is how much worse.
Here is the dashboard with unbalanced *input*:
Massdrop THX AAA 789 Headphone Amplifier Dashboard Unbalanced RCA Measurement.png


Ground loops become an issue here and I had to play a bit to get this number.

So no, I don't see the issue they are talking about. We are getting over 6.6 volts (single-ended output) at incredibly small amounts of distortion and noise.

FYI I can get the balanced input a bit better too by using Gain 2 setting and playing with volume control. I got it up to SINAD of 117 so essentially the same performance.
 
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Thread Starter #62
What about the imd smpte performance?
Here you go:

Massdrop THX AAA 789 Headphone Amplifier IMD Measurement.png


As noted, the balanced input at highest gain (3) clips at full input voltage of 4 volts. Switching to gain 2 remedies that (green):

Massdrop THX AAA 789 Headphone Amplifier IMD gain 2 Measurement.png
 

mi-fu

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#63
The Monoprice unit states dual THX AAA 788 amps. Same as the Massdrop '789' ? The real question is the implementation of the dual AKM4493 chips. As amirm has shown in many tests, just having a name DAC chip does not mean automatic success. The components upstream and downstream from the chip are just as important. However at the price of $479.00 against $499.00 for a DX7s this is very attractive. Can't wait for a review.
I think Topping choosing a AKM chip for the DX3Pro might give a clue to future Topping products. What could Topping do with the THX chips?

Speaking of Massdrops long lead times, and yes I have waited too, remember the very contested R-2R DAC from back in, what, July? Those folks are still waiting for first shipment! Now that would be a DAC for amirm to test.
I'm very interested in the Monoprice 788 too. It is $130 more than the Massdrop 789, but it has dual AKM4493 dac and EQ functions. And I don't need to wait for another drop and months for delivery. I really hope the Monoprice one will measure well. Then I will just bite it.

The one downside of the Monoprice 788 is the lack of XLR pre-amp output. Otherwise, it will be perfect to run it with studio monitor and i can use the RCA output for another amp.
 
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Thread Starter #65
BTW as a reference, here is the performance of the Audio Precision APx555 in balanced input/output mode:

Audio Precision APx555 Dashboard.png


The noise floor is so low that it almost falls off the bottom of the chart. Distortion spikes are at -148 dB and lower!

SINAD is 6 dB better than Massdrop THX AAA 789. So we have good bit of headroom left to test even higher fidelity audio products. :)

BTW, Jude at head-fi at made comments that he had to resort to 1 million point fft and such to see the distortions of Massdrop 789. The dashboard FFT is with just 32 thousand points so not sure what he was talking about.

Anyway, if you want something better than Massdrop THX 789 it exists: it is called APx555 and costs $28,000. :)
 
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Thread Starter #66
amirm, do you mean there are ground loops with RCA input, but not with "balanced" input?
Correct. There always is. Sometimes it doesn't matter because distortions are much higher than it. Here of course it matters a lot because distortions are so low.

I play with chassis grounding versus device to get the best performance and post it. I used to not do this but then it wouldn't be representative of the best or worst. Figured with what I am doing, you can at least see the best case performance.
 
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Thread Starter #67
BTW, on IMD charts, be careful in how you compare the lines. The level of power output is different at any point on one graph versus another. What is shown is the input to the headphone amplifier, not output. I haven't figured out yet how to have the analyzer show the output power instead....
 
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Thread Starter #68
Amir - based on your experience in measuring amps, what would you say the best performing amp is for $500 or less? Best if has both SE and balanced outputs, but since price/value will probably be very different, what about best SE only, and best SE and balanced combo?
This amplifier is. I have not measured any audio product with this low of distortion.

I will be measuring more high performance units but their cost will be multiples of this one so well above your budget.
 

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#69
Correct. There always is. Sometimes it doesn't matter because distortions are much higher than it. Here of course it matters a lot because distortions are so low.

I play with chassis grounding versus device to get the best performance and post it. I used to not do this but then it wouldn't be representative of the best or worst. Figured with what I am doing, you can at least see the best case performance.
Thanks for the explanation :)

How about the worst case scenario for ground loops? How serious it will affect, say, from the best case performance?
 
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Thread Starter #70
@amirm, can you please test THD vs power level with a lower load on the balanced output (16 ohms, and maybe lower) ?
I would be curious to see how this amp react when the current limit hits.
I can only modify my balance for half the impedance or about 24 ohms. That was enough to trigger the protection circuit though:
Massdrop THX AAA 789 Headphone Amplifier Power at 24 ohm balanced Measurement.png


Here, it was clicking on and off. With a different run it severely power limited instead. So it is a bit unstable.

That said, with 5.6 watts of power your ears will be going into protection mode before the amp does. :)
 
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Thread Starter #71
Thanks for the explanation :)

How about the worst case scenario for ground loops? How serious it will affect, say, from the best case performance?
I can only speak from memory. I have managed to drop the SINAD performance easily by 20 dB or even more.

This whole unbalanced audio architecture is broken solid. There are no standards of how to ground to the chassis for example. And the notion of using the ground as reference is just wrong. This is why I am a big fan of balanced audio inputs.
 

mi-fu

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#72
I can only speak from memory. I have managed to drop the SINAD performance easily by 20 dB or even more.

This whole unbalanced audio architecture is broken solid. There are no standards of how to ground to the chassis for example. And the notion of using the ground as reference is just wrong. This is why I am a big fan of balanced audio inputs.
Got it. Thanks again for the explanation!!

And 20dB is quite serious!!! o_O
 
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Thread Starter #73
Also please test it with dual outputs and see if the quality drops. I haven't been able to detect any drops in quality (or volume) while driving both my Aeon and PM-2 at the same time.
I don't have two dummy loads to use simultaneously unfortunately. There is a lot of power here so I am not surprised it can drive two headphones at the same time without issue.
 

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#74
That said, with 5.6 watts of power your ears will be going into protection mode before the amp does. :)
Thanks!
I would never put that amount of power into a pair of headphones, but it will come handy for my use case, which is 16/32 ohms compression drivers amplification :D

For pure headphone use (ie probably 99.9% of the market of this amplifier), and from the preliminary measurement I have seen, March Audio's headphone amp might be a good contender :)
 
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Thread Starter #75
I forget who asked to see a real headphone load versus dummy load so here it is:

Massdrop THX AAA 789 Headphone Amplifier THD dummy load versus Grado SR60e Headphone Measurement.png


As with my previous tests of Benchmark DAC3 and ADI-2 Pro, there is significant degradation in low frequencies due to reverse EMF current from the headphone driver getting strong.

Note that the above is at low output level (1 volt) and with double the bandwidth of the dashboard. As such, THD+N is much lower even with the dummy load (33 ohm).

On the plus side, the ruler flat response is impressive with dummy load. There is no frequency dependent components.
 
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Thread Starter #77
RCA Input's and outputs.
Basically how you tested the balanced in/out, I want to see the results of the SE in/out.
That's what I post. :)
 

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#78
Speaking of Massdrops long lead times, and yes I have waited too, remember the very contested R-2R DAC from back in, what, July? Those folks are still waiting for first shipment! Now that would be a DAC for amirm to test.
You mean the Airist R2R DAC? There is nothing to get excited about, objectively speaking. SBAF have done some measurements for it and it barely got 10 bits ENOB (10.4 bits to be exact), which converts to a measly 64 dB SINAD.
 
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