Review and Measurements of Massdrop THX AAA 789 Amp

[AndrewMason]

While we're on this topic, I need to connect my incoming D50 to this amp. I'm hoping these cables will work okay?

https://www.amazon.com/Monoprice-Co...4822955&sr=8-7&keywords=rca+to+xlr+male+cable

Good find! The drawing for those looks correct. Maybe share some experience and photos (disassemble at the xlr) when you receive? And nice 1.5 or 3 ft length options too!

 

[AndrewMason]

Another alternative: https://www.amazon.co.uk/dp/B07B3GC...colid=W25DX4NAX94B&psc=0&ref_=lv_ov_lig_dp_it
They do have 30cm/50cm/75cm/1m variants.

The components and cable seem high quality! Can you see in the write up a description of how its xlr pin 1 is wired? I couldn't see at first glance.

 

[Veri]

My amp is working fine with SE outputs. Yesterday made a XLR silver cable for my HD650s. Whenever I increase the volume in any of the gain modes the overcurrent protection is activated and the red led is lit. I have not used the ground pin in the XLR male plug just pin 1 left positive pin 2 left negative pin 3 right positive and pin 4 right negative.

Mine also just arrived (the amp). Finally using my HD650 with XLR connection for no other reason than "because" . I also had the red light and some clipping/buzzing sounds constantly first 10 minutes. Disconnected all cables (using simple RCA to the amp for now), removed HD650 connectors from the sides of the headphone, removed earpads and all lint/hairs, carefully reconnected each component and it's running without a hitch now. =)

 

[Veri]

@AndrewMason would you be against connecting other devices via 3.5mm>RCA through something like this? Not sure if for some reason doing that would be inherently stupid

 

[AndrewMason]

@AndrewMason would you be against connecting other devices via 3.5mm>RCA through something like this? Not sure if for some reason doing that would be inherently stupid

The aluminum braid may be dodgy. Aluminum oxidizes very easily and so is difficult to terminate without great care and the right metallurgy. Hence how alum house wiring was phased out... too many people not terminating it properly.
Look for something with copper braid instead.

 

[Veri]

The aluminum braid may be dodgy. Aluminum oxidizes very easily and so is difficult to terminate without great care and the right metallurgy. Hence how alum house wiring was phased out... too many people not terminating it properly.
Look for something with copper braid instead.

Got it, thanks!

Now that I've got you here, I noticed that inserting RCA is easy but removing requires quite a bit of pulling strength. Any recommended way to make this process more supple? I'm going to break the RCA connectors sooner or later lol.

Btw, LOVE this amp. thank you so much for bringing this to the market

 

[wadec22]

added some photos of my M10 paired with it. I think they look pretty good together.

https://www.audiosciencereview.com/...msl-m10-initial-impressions.5553/#post-126953

 

[AndrewMason]

Got it, thanks!

Now that I've got you here, I noticed that inserting RCA is easy but removing requires quite a bit of pulling strength. Any recommended way to make this process more supple? I'm going to break the RCA connectors sooner or later lol.

Btw, LOVE this amp. thank you so much for bringing this to the market

I find that problem sometimes too. Maybe someone else here has a tip to share?

 

[Veri]

I find that problem sometimes too. Maybe someone else here has a tip to share?

I think I came across like kind of oil/grease YEARS ago when I had the same problem; to keep interconnects slide on and off with ease but I'm not sure of the implications nor if it's snake oil. @amirm do you have problems with sliding on/off RCA interconnects when testing unbalanced?

 

[jackenhack]

Any other DACs that members are using with the THX 789? I just purchased the Topping D50, which I figure I'll eventually use for a coffee table setup in my living room along with hopefully an A50 amp. Looking at the SMSL DACs currently. They seem reasonably priced.

Hi! I'm currently using the Topping D50 with a THX 789 amp. This combo works perfectly. I first used the Khadas DAC but tested the D50. With my new Audeze LCD-2C, the D50 sounds better than the Khadas board. The midrange (voice) was very piercing with the Khadas DAC. I started to wonder why everyone said that the Audeze LCD-2C was dark. After switching to the Topping D50, it now sounds much more mellow, and I don't need to use equalization at all. Wonder if it's an impedance match problem?
There is protection built-in on the THX amp. When playing back a corrupted DSD file using the Khadas board, the protection kicks in. Probably a DC-offset error, but I haven't had time to measure. This error doesn't happen with the Topping. The front led turns red when the protection kicks in.

 

[Veri]

Hi! I'm currently using the Topping D50 with a THX 789 amp. This combo works perfectly. I first used the Khadas DAC but tested the D50. With my new Audeze LCD-2C, the D50 sounds better than the Khadas board. The midrange (voice) was very piercing with the Khadas DAC. I started to wonder why everyone said that the Audeze LCD-2C was dark. After switching to the Topping D50, it now sounds much more mellow, and I don't need to use equalization at all. Wonder if it's an impedance match problem?

Well that's weird

 

[jackenhack]

Mine also just arrived (the amp). Finally using my HD650 with XLR connection for no other reason than "because" . I also had the red light and some clipping/buzzing sounds constantly first 10 minutes. Disconnected all cables (using simple RCA to the amp for now), removed HD650 connectors from the sides of the headphone, removed earpads and all lint/hairs, carefully reconnected each component and it's running without a hitch now. =)

Hey, now that you mentioned it, I had the same problem with my HD-650 when I first connected up my THX amp! (unbalanced.) Switched the cables around and fiddled a bit, and it all started to work. I thought it was strange but put it down to faulty connections on my cables. Hmm.

 

[rajapruk]

It would be very funny if it ends up to be shown that there is some kind of burn-in taking place in the amp, haha!
I had some small buzz/crack into the headphone in the beginning when turning the volume-knob without any input connected at all. This disappeared when connecting xlr input to a balanced dac.

 

[jackenhack]

Well that's weird

Well, it's easy to check, because the Khadas board has a schematic. There are some 806 Ω resistors on the output, so that shouldn't really be a problem. I don't know the input impedance of the THX amp though.

 

[Veri]

Hey, now that you mentioned it, I had the same problem with my HD-650 when I first connected up my THX amp! (unbalanced.) Switched the cables around and fiddled a bit, and it all started to work. I thought it was strange but put it down to faulty connections on my cables. Hmm.

It would be very funny if it ends up to be shown that there is some kind of burn-in taking place in the amp, haha!
I had some small buzz/crack into the headphone in the beginning when turning the volume-knob without any input connected at all. This disappeared when connecting xlr input to a balanced dac.

There definitely is something strange when you just get it fresh from the box apparently ... I am using SE>BAL and so is jackenhack right? Maybe the problem lies there but right now it's no longer acting up.

 

[jackenhack]

It would be very funny if it ends up to be shown that there is some kind of burn in taking place in the amp, haha!
I had some small buzz/crack into the headphone in the beginning when turning the volume-knob without any input connected. This disappeared when connecting xlr input to a balanced dac.

Haha! Where turning this objective forum into a subjective hocus pocus forum, aren't we! Haha! I hesitated to even mention the problem with my Audeze LCD-2C headphones because it's subjective, but it's straightforward to reproduce.

 

[jackenhack]

I checked the output impedance on the Khadas boards schematic, and it has 806 Ω resistors on the output, so if the input impedance is x10 on the THX amp, it shouldn't matter.

 

[rajapruk]

It would be very funny if it ends up to be shown that there is some kind of burn-in taking place in the amp, haha!
I had some small buzz/crack into the headphone in the beginning when turning the volume-knob without any input connected at all. This disappeared when connecting xlr input to a balanced dac.

This happened for both my new units. I got the feeling that my fingers touching the knob produced a ground loop or something.

 

[jackenhack]

This happened for both my units. I got the feeling that my finger touching the knob produced a ground loop or something.

According to the patents from THX, they use feed-forward, but they also talk about varying the power supply rails, but I don't think they do that on the headphone amp, only on stuff like the Benchmark speaker amplifier. Wonder if it's the feed-forward part that acts up? Total speculation on my part. I had intermittent noise in one of the channels and after a while, the sound was really low. But just like magic, it disappeared after a couple of minutes.

 

[amirm]

It would be very funny if it ends up to be shown that there is some kind of burn-in taking place in the amp, haha!

Hmmm. When I first powered up my loaner unit I too got strange results with one channel being much worse than the other:

When I remeasured, the problem went away and never came back. I did find a bad set of cables though which I replaced. My own unit is still in the box so I will check to see if there is anything going on here.