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Review and Measurements of Massdrop THX AAA 789 Amp

Sythrix

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Thank you Amir for reviewing this and thank you to the person (@Sythrix?) that drop shipped it to Amir!
No problem. I'm glad it was well worth it to ship it direct. Looks like we've got a real game changer. :)

We know that THX was very very much involved in the total technical design of the Massdrop amp. That is the critical success factor here, I think.
I wonder if the Monoprice amp has the same kind of involvement from THX? I read somewhere that Andrew Mason of THX wrote that Monoprice choosed the DAC-chip themselves. That maybe indicates a little less THX-involvement overall.
But isn't that same Andrew answering questions on the monoprice Question/answer product page of the amp?
Andrew works for THX yes and is answering questions. He has already answered one of mine that I asked a long time ago, trying to clear things up on conflicting information. It seems to me that THX has at least a heavy hand in this, so I would be optimistic about the Monoprice unit. Here's the chain of questions:

"OK, so the 788 isn't a true balanced amplifier... but you're putting two in. AND you're putting in two AKM 4493 DACs.

Are you putting two of these in to make it a balanced output? What is the reasoning behind using two of each of these units if the unit won't be balanced? Furthermore, why put a 4-pin XLR connector on the unit if it won't be balanced? You have a lot of clarifying to do Monoprice and I suggest you get someone who actually knows what they're talking about to answer all these questions, because they are not being answered with any clarity."

"BEST ANSWER: Incorrect. This Monoprice unit is truly balanced in & out. It contains dual THXAAA-788 stereo amplifier modules configured to create a fully balanced bridged stereo output. It also supports TRS unbalanced output.
The XLR-4 is present to permit balanced headphone connection."
Andrew M on Aug 23, 2018


"Incorrect? What information do you have? Do you work for Monoprice? Because the only thing I've seen is a staff response that told someone that it was not a truly balanced output (that response seems to be gone now), which prompted this question."

Also, during questioning over at Massdrop for the THX 789, on of the THX team stated that this DAC/AMP would not be truly balanced.
You on Aug 23, 2018


"I work for THX and have seen the schematic ;)
The Monoprice desktop XLR-4 output IS truly balanced. There was an early miscommunication about how many 788 amplifiers were going to be included in the design. i.e. having balanced output requires more amplifiers than just single-ended output."
Andrew M on Oct 17, 2018


Support Staff also answered the question:

"The gentleman that answered is the AAA tech engineer from THX. Obviously, we stand by his answer as we worked extensively with THX in bringing this product to fruition. "
 
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amirm

amirm

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Thread Starter #122
Monoprice is just a marketing brand. Someone else has to design this and it seems right that THX has done it.

BTW, Benchmark is also out of stock with their THX headphone amplifier. So clearly there is a shortage of these modules from THX.
 
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No problem. I'm glad it was well worth it to ship it direct. Looks like we've got a real game changer. :)




Andrew works for THX yes and is answering questions. He has already answered one of mine that I asked a long time ago, trying to clear things up on conflicting information. It seems to me that THX has at least a heavy hand in this, so I would be optimistic about the Monoprice unit. Here's the chain of questions:

"OK, so the 788 isn't a true balanced amplifier... but you're putting two in. AND you're putting in two AKM 4493 DACs.

Are you putting two of these in to make it a balanced output? What is the reasoning behind using two of each of these units if the unit won't be balanced? Furthermore, why put a 4-pin XLR connector on the unit if it won't be balanced? You have a lot of clarifying to do Monoprice and I suggest you get someone who actually knows what they're talking about to answer all these questions, because they are not being answered with any clarity."

"BEST ANSWER: Incorrect. This Monoprice unit is truly balanced in & out. It contains dual THXAAA-788 stereo amplifier modules configured to create a fully balanced bridged stereo output. It also supports TRS unbalanced output.
The XLR-4 is present to permit balanced headphone connection."
Andrew M on Aug 23, 2018


"Incorrect? What information do you have? Do you work for Monoprice? Because the only thing I've seen is a staff response that told someone that it was not a truly balanced output (that response seems to be gone now), which prompted this question."

Also, during questioning over at Massdrop for the THX 789, on of the THX team stated that this DAC/AMP would not be truly balanced.
You on Aug 23, 2018


"I work for THX and have seen the schematic ;)
The Monoprice desktop XLR-4 output IS truly balanced. There was an early miscommunication about how many 788 amplifiers were going to be included in the design. i.e. having balanced output requires more amplifiers than just single-ended output."
Andrew M on Oct 17, 2018


Support Staff also answered the question:

"The gentleman that answered is the AAA tech engineer from THX. Obviously, we stand by his answer as we worked extensively with THX in bringing this product to fruition. "
That's some great info, thank you for sharing! Knowing that THX has a very heavy influence in the Monoprice product, I may order that when it goes live and drop ship it to Amir. The Massdrop one is obviously an amazing product but I'm curious about the DAC portion of the Monoprice offering. Additionally the Monoprice one has some interesting DAC settings and a really nice looking design (the Massdrop one looks good too). Curious if the volume knob is digital as well?
 

Sythrix

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I'm curious about the DAC portion of the Monoprice offering
So am I. Twin AK4493s powering what what we now know as some of the best tech in amplifiers? All inclusive? If it measures well (even though it doesn't have as much power as the 789) it should serve most people and headphones just fine, barring any quirks.

It would be perfect to sit next to my computer. It seems like they haven't pushed it back again, so I think we'll see true release this time.

Unfortunately, I don't have any more money to spend on such things this year. Still, should be an interesting review when it comes around.
 
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If it measures well (even though it doesn't have as much power as the 789) it should serve most people and headphones just fine, barring any quirks.
I honestly can't imagine anyone needing the amount of power the Massdrop one has. Like the Fostex T50RP is probably my most demanding headphones and you can get them plenty loud in my opinion with an JDS Labs o2.

Unfortunately, I don't have any more money to spend on such things this year. Still, should be an interesting review when it comes around.
And that is why I am going to be selling stuff lol!
 

maverickronin

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I honestly can't imagine anyone needing the amount of power the Massdrop one has. Like the Fostex T50RP is probably my most demanding headphones and you can get them plenty loud in my opinion with an JDS Labs o2.
HiFiMan did just re-release the HE-6...
 

AndrewMason

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HI @amirm
Thank you for taking the time to perform and post the nice review of the 789! We're excited to make this level of performance available for everyone.

I've got one small item to mention: the output impedance we measure is quite a bit lower than the 1.0 Ohms you stated. Attached is the output impedance vs Frequency that we measure using a 1/4" plug.

Perhaps we have a different measurement technique?

We measure single-ended output impedance as follows:
- amplifier on and active. Inputs terminated (e.g. with 600 Ohms, or a short). No input signal.
- configure the AP unbal generator as 1 Vrms with 600 Ohms source impedance, stepped freq sweep 20-20kHz, and backfeed this generator signal into the amplifier output jack.
- use the AP analyzer to monitor the voltage appearing at the amplifier output (make a Kelvin connection right inside the 1/4" plug)
- the lower the amplifier output impedance, the more it will divide down this 1V drive.
- export the V vs Freq data to Excel, then calculate the Zout vs freq from it knowing that you have 1V drive from 600 ohms source.
- we measure 50-70 mOhms for everything up to 1 kHz. I've attached a plot.

We use Kelvin connections right inside the 1/4" plug to eliminate effects of cable impedance which cannot be controlled.

And regarding the Grado and other headphone non-linearities due to EMF, I'm guessing it's mostly the headphones cable impedance that permits the non-linear EMF to create this distortion. A Kelvin connection inside the 1/4" plug would help you separate the variables. This is a good example of why cable impedance should ideally be much lower than driver impedance.

Thanks again for reviewing the 789, and more importantly for all that you do so generously and enthusiastically for this community!
 

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amirm

amirm

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Thread Starter #133
@amirm Have you measured any other audio device that has the distortion below the noise-floor like this? AHB2? Or some very noisy old Behringer maybe?!
I have not measured any audio gear regardless of type that is this good. But I have not yet measured the AHB2 yet.
 

rajapruk

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What do you think, will this be a future classic? It has the potential to be, I guess.

We here in Europe are still waiting for ours to arrive, eagerly! (and really hope the power-button works on our units).
 

derp1n

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amirm

amirm

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Thread Starter #136
It all depends on whether THX can achieve volume production of these modules. If they do, I expect broad adoption as I am sure they won't turn anyone away.

So the risk is that these modules are very difficult to manufacture (e.g. require had tuning) and are being sold at a loss (when all costs are accounted for). And the company pulls the plug on it.

The strategy is weird so far. Usually when you have production problems, you only license/provide components to higher end brands like Benchmark. To run down to the bottom tier of commodity mass market first where volume is everything, is strange and backward to me.
 

Sythrix

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The strategy is weird so far. Usually when you have production problems, you only license/provide components to higher end brands like Benchmark. To run down to the bottom tier of commodity mass market first where volume is everything, is strange and backward to me
THX is new at this? :)
 
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HI @amirm
Thank you for taking the time to perform and post the nice review of the 789! We're excited to make this level of performance available for everyone.

I've got one small item to mention: the output impedance we measure is quite a bit lower than the 1.0 Ohms you stated. Attached is the output impedance vs Frequency that we measure using a 1/4" plug.

Perhaps we have a different measurement technique?

We measure single-ended output impedance as follows:
- amplifier on and active. Inputs terminated (e.g. with 600 Ohms, or a short). No input signal.
- configure the AP unbal generator as 1 Vrms with 600 Ohms source impedance, stepped freq sweep 20-20kHz, and backfeed this generator signal into the amplifier output jack.
- use the AP analyzer to monitor the voltage appearing at the amplifier output (make a Kelvin connection right inside the 1/4" plug)
- the lower the amplifier output impedance, the more it will divide down this 1V drive.
- export the V vs Freq data to Excel, then calculate the Zout vs freq from it knowing that you have 1V drive from 600 ohms source.
- we measure 50-70 mOhms for everything up to 1 kHz. I've attached a plot.

We use Kelvin connections right inside the 1/4" plug to eliminate effects of cable impedance which cannot be controlled.

And regarding the Grado and other headphone non-linearities due to EMF, I'm guessing it's mostly the headphones cable impedance that permits the non-linear EMF to create this distortion. A Kelvin connection inside the 1/4" plug would help you separate the variables. This is a good example of why cable impedance should ideally be much lower than driver impedance.

Thanks again for reviewing the 789, and more importantly for all that you do so generously and enthusiastically for this community!

This is rather interesting for the output impedance! However, for the headphone measurements I wonder what the coil at the headphone is actually seeing. The clean signal from the amp or the distorted signal? I guess one way to find out is to use a headphone with a plug-able cable and a Kelvin connection measurement right at the headphone cup?

This also makes me wonder whether this effect should be smaller with more inefficient headphones where at least any ambient noise pickup should have less of an effect compared to a highly efficient headphone. Same question with regards to an open headphone vs. say a closed back or in-ear model. Can shielding the cables help?

Cheers

Thomas
 
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amirm

amirm

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Thread Starter #139
HI @amirm
Thank you for taking the time to perform and post the nice review of the 789! We're excited to make this level of performance available for everyone.
Hi Andrew. Thanks for the kind words and warm welcome to the forum.

Your measurements are the correct way to do things. The difference between our methods is that I use my standard dummy load setup for measurements which involves more connectors and wiring. As you know, by the time we get down to such small impedances, all of this matters.

In that regard, my measurements always show higher values. I can build a fixture as you have to reduce this (i.e. tap into the connector directly) but I feel that what I measure is more realistic relative to actual headphone setup.

My comment about impedance in the review was that with the same setup I am able to get down to 0.4 ohms with some devices. I went back and remeasured this amp and got down to 0.9 ohm. There may be variability in my setup that causes run to run changes of a few 0.1 ohm. I will need to refine that.

For now, I will accept your measurements to be correct and will post a link to it from my original review.
 
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amirm

amirm

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Thread Starter #140
And regarding the Grado and other headphone non-linearities due to EMF, I'm guessing it's mostly the headphones cable impedance that permits the non-linear EMF to create this distortion. A Kelvin connection inside the 1/4" plug would help you separate the variables. This is a good example of why cable impedance should ideally be much lower than driver impedance.
Funny you say this as I woke up this morning thinking this may be an issue! :)

My setup is using a very short cable to the amp and then splits off to the headphone and AP. So it is kelvin style in that manner.

Do you want to take a shot at measuring this effect yourself and see what you can find out?
 
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