Review and Measurements of Massdrop THX AAA 789 Amp

[xineis]

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.

Amir, as I really don't know much about this stuff, do you need a device with balanced audio outputs to feed the signal into the amplifier? Using a Topping D50 or Grace SDAC, how do you use the balanced inputs of the THX789?

 

[amirm]

Amir, as I really don't know much about this stuff, do you need a device with balanced audio outputs to feed the signal into the amplifier? Using a Topping D50 or Grace SDAC, how do you use the balanced inputs of the THX789?

You would need a DAC with balanced output to use balanced inputs on this. So you can't do that with Topping D50 or SDAC.

 

[xineis]

You would need a DAC with balanced output to use balanced inputs on this. So you can't do that with Topping D50 or SDAC.

And if you don't "play" with the grounding, how much performance do we lose when using the SE inputs (from a D50 or SDAC)?

 

[amirm]

I will pair it up with D50 and report back later.

 

[Schackmannen]

I forget who asked to see a real headphone load versus dummy load so here it is:

View attachment 16962

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.

I know that at least I asked for it so thank you for doing the test It's a shame that it also distorts a lot more when driving headphones, I was hoping that maybe the THX AAA technology would have solved that issue. Hopefully the Benchmark HPA4 will not show the same pattern considering what they showed here.

But i have a beginners question that you've probably answered before but I haven't seen it in that case. How do you measure the distortion when driving headphones (or a dummy load for that matter), do you use some sort of a spliter to drive the headphones while simultaneously leading the amp's output into your analyzer? Also, is it possible to show the dashboard view when driving the headphones so we can see the difference in SINAD compared to a dummy load? Regardless, I very much appreciate you for taking your time and doing all this work for us

 

[Dro]

Review makes me wonder what would have better SINAD: ADI-2 DAC or a Benchmark DAC3 with the THX amp. The big practical issue I see with this amp is that there is no adequate source to actually get to this excellent SINAD unless you are listening via an AP analyzer as source... At least you'll know your amp won't limit your signal quality.

 

[RayDunzl]

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.

Just a thought here:

The headphone acts as a microphone...

How much of the increased distortion measurement at lower frequencies might be caused by ambient noise?

HD650 plugged into ADC of UMC202HD, music softly playing thorugh speakers, headphones acting as microphone...

Ambient noise signal produced by the headphone (red peaks) vs just the electronics (black, with headphone unplugged)

The general frequency response of the HD650 (as a microphone) seems to correspond well to the increased distortion measurement you made, over the range that it showed an increase:

 

[amirm]

But i have a beginners question that you've probably answered before but I haven't seen it in that case. How do you measure the distortion when driving headphones (or a dummy load for that matter), do you use some sort of a spliter to drive the headphones while simultaneously leading the amp's output into your analyzer?

Correct. My dummy load box has a selector with one of the settings just going to a headphone jack instead of resistors. So I plug in a headphone there and I am up and running.

 

[amirm]

Just a thought here:

The headphone acts as a microphone...

How much of the measurement might be caused by ambient noise?

I tested for that. The main contribution is from the driver. If I block the back of the open headphone for example, the distortion gets worse as more energy bounced back into the amp.

For this reason I put the headphone on my headphone measurement dummy head so that conditions remain the same in tests.

 

[amirm]

Just a thought here:

Let me double check this though seeing how you see that in your measurements.....

 

[RayDunzl]

For this reason I put the headphone on my headphone measurement dummy head so that conditions remain the same in tests.

The HD650 is an open back headphone.

I'd think:

Run a couple of sweeps at decreasing levels, back EMF should remain (relatively) about the same, noise would remain about the same in absolute terms, so there would be more (relative) distortion measured, not the same or less, as the sweep level is reduced.

Or I'm all wet, that happens.

I will now put my headphones back on my dummy head, and listen to some Nino Rota, and see what you discover.

 

[RayDunzl]

Oh, I see you mentioned it being open back already,

You'll just have to construct a Cone of Silence for these tests.

 

[Nango]

And if you don't "play" with the grounding, how much performance do we lose when using the SE inputs (from a D50 or SDAC)?

I think it will allways matter (sound-wise, not only volume-wise) whether you apply 2V or (double) 2x 2V to any headphone's driver unit. Those with weird frequency response like HD800 probably more than others. It can't be the same, I think .............

 

[RayDunzl]

It can't be the same, I think ...

What would the mechanism for a difference be?

 

[derp1n]

I think it will allways matter (sound-wise, not only volume-wise) whether you apply 2V or (double) 2x 2V to any headphone's driver unit. Those with weird frequency response like HD800 probably more than others. It can't be the same, I think .............

That makes no sense at all.

 

[amirm]

The Monoprice unit states dual THX AAA 788 amps.

From my quick look, THX 7888 has substantially less power but otherwise the same. Monoprice specs reflect this with max power to be 3 watts in balanced versus I think advertized 6 watts for Massdrop.

 

[Grave]

I might be interested in this, if it actually existed.

Then again, it will not sound any better than my $100 amplifier, so meh.

 

[mi-fu]

From my quick look, THX 7888 has substantially less power but otherwise the same. Monoprice specs reflect this with max power to be 3 watts in balanced versus I think advertized 6 watts for Massdrop.

Assuming both of their performance would be similar in other aspects, do you still think the Massdrop one is a better choice over Monoprice 788, because of the higher max power?

 

[maxxevv]

You would need a DAC with balanced output to use balanced inputs on this. So you can't do that with Topping D50 or SDAC.

There are RCA-3 pin XLR convertors that allows one to do that.

My Matrix HPA-3B actually comes shipped with such adaptors.

 

[xineis]

There are RCA-3 pin XLR convertors that allows one to do that.

My Matrix HPA-3B actually comes shipped with such adaptors.

But how do you convert unbalanced into balanced?