Review and Measurements of Massdrop THX AAA 789 Amp

[Headphonaholic]

Monoprice has posted some measurements in a ZIP file at the bottom of the product page if you're curious

I'm no expert, but it looks reasonable at least, right?

Oh nice! I wish there was some explanations with the measurements... maybe Amir can take a look for us It looks good for sure but context and explanations would be helpful for those of us that are still learning how to read measurements.

 

[Veri]

What is intriguing about the monoprice dac: analog inputs get converted to digital with an ADC, so that EQ/surround etc, sound processing is available.

I really wonder how that will impact final performance... seems kind of strange to me that said processing could not simply be done in analog domain, rather than going input>adc>processing>dac... :|

 

[badboygolf16v]

A well implemented ADC will be transparent (and AKM have an excellent reputation for ADC chips.) It's preferable to do this kind of processing in DSP: cheaper, more flexible and potentially better.

 

[Veri]

A well implemented ADC will be transparent (and AKM have an excellent reputation for ADC chips.) It's preferable to do this kind of processing in DSP: cheaper, more flexible and potentially better.

Well that's a relief

 

[rajapruk]

A well implemented ADC will be transparent

What do you base this on? I am not so sure about that.

 

[derp1n]

What do you base this on? I am not so sure about that.

...are you joking?

 

[rajapruk]

...are you joking?

No. Did you think it was funny?

Analog in vs digital in on my DBX 4820 was a subjective difference (and analog levels was right). And the 4820 is well implemented as far as I know. Thats why I am not sure about it. I try to avoid unnecessary AD or DA conversions.

 

[pos]

I can only modify my balance for half the impedance or about 24 ohms. That was enough to trigger the protection circuit though:
View attachment 16961

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.

Amir, I am in a middle of a discussion with Andrew Mason on that subject here: https://www.massdrop.com/buy/massdrop-x-thx-aaa-789-linear-amplifier/talk/2250875

There is something I don't understand with the current limit: it is supposed to be around 0.9Apeak, 0.63Arms
That means that the 16 ohms 6W for 1% THD specification for the balanced output is on the brink of protection with 0.61Arms.
Yet it is part of the specification, so it should be achievable.

But here in your measurement it looks like the current protection kicked-in at around 0.48Arms.
Theoretically with a 24 ohm load the 0.63Arms limit should barely be reached with the full 15V output this amp is capable of, ie 9.3W.

You mentioned repeating this test several times, with different reading. Is it possible that the measurement reached that 9.3W marking in the first run, and then got power limited in subsequent runs?
What is the measurement step here?
Any clue on what could be causing the difference between the specs and this measurement?

 

[amirm]

You mentioned repeating this test several times, with different reading. Is it possible that the measurement reached that 9.3W marking in the first run, and then got power limited in subsequent runs?

I think that was the case as I had high clipping but in the second run stopped short of this. This is from memory which is not 100% reliable.

Also, protection circuits work on multiple factors with current being just one. Over temp, DC output, etc. will all cause the protection circuit to kick in. Andrew mentions this when he uses the acronym "SOA" (Safe Operating Area). Here is an example of SOA (from some other random transistor):

So as you see, the shape is complex.

My analyzer switches levels suddenly as it steps through the levels. That sudden pulse may cause the protection circuit to kick in where it would not otherwise.

I may be able to cobble together a 16 ohm load which is the minimum spec. Do you want me to do that?

 

[amirm]

If you will measure this device more for some reason, I would like to know how much power it can output without going into protection mode in 8, 12, and 16 ohm loads.
The manual states that it can do 8-12ohm with reduced power.

Andrew in the above thread is warning against 12 ohm load. Can you please quote the manual where it says it is OK to do that?

Since this is a loaner unit that cannot be replaced right now, I do not want to subject it to extreme loads unless it is specifically allowed.

 

[rajapruk]

The manual says 12ohm lowest, so skip doing 8 ohm.
Then Andrew says 8-12ohm here:
https://www.massdrop.com/buy/massdrop-x-thx-aaa-789-linear-amplifier/talk/2249304
I confused that with the manual.

 

[rajapruk]

Manual. See chapter ”permissible headphone impedance.

 

[amirm]

Manual. See chapter ”permissible headphone impedance.

But it comes with a serious warning:

4 volt peak to peak? How is a user supposed to know the output voltage?

Even though I can check for that, I just can't take the chance of it being damaged. So I can't go below 16 ohms.

 

[pos]

I may be able to cobble together a 16 ohm load which is the minimum spec. Do you want me to do that?

That would be great!

 

[pos]

But it comes with a serious warning:

View attachment 17342

4 volt peak to peak? How is a user supposed to know the output voltage?

Even though I can check for that, I just can't take the chance of it being damaged. So I can't go below 16 ohms.

That is a typo, according to Adrew Mason:

Hi Pospos. The manual should say "... can tolerate loads between 12 and 8 Ohms if the amplifier is driven to no higher than 4Vpk across the load. Usage outside of these parameters may void the warranty. Do not use the 789 to drive loudspeakers."

 

[pos]

By the way, I think he meant 4Vrms here, ie 0.5A for a 8 ohm load.

 

[amirm]

Understood. I am going to get explicit permission from the owner for testing at other loads.

 

[pos]

6W at 16 ohms balanced would already be great, and this is explicitly specified as producing <1% THD

 

[amirm]

OK, I have permission for 16 ohm testing. I will do that and post the results later.

 

[rajapruk]

Very much appreciated!