• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Apparently the THX 789 is a noisy amp. I call BS!

fieldcar

Addicted to Fun and Learning
Forum Donor
Joined
Sep 27, 2019
Messages
645
Likes
954
Location
Milwaukee, Wisconsin, USA
So a youtuber named Joshua valour has repeatedly said that the THX 789's biggest flaw is audible hum/noise on very sensitive IEM's like his Empire ears Zeus. I'm calling BS, and he's making some sort of noob mistake like running his RCA's close to 120V mains or his source material is crappier than he is ever willing to admit. Many recordings have a very audible noise floor.

See here:

I can tell you that I get a bit of hum (not noise) from my interconnects when they are closer to my power cables, but if I keep them clear of power cables and run my 789 at high gain at 95% volume out of my topping D50S on my most sensitive IEM (Thieaudio Legacy L3, spectacular IEM's BTW), I can hear maybe the softest ~2-5dB of noise. So, I can only hear noise and hiss while nothing is played that would smoke any IEM on the planet and distort the crap out of my 4XX's if music was actually played.

Thanks for letting me vent. It's been bothering me. So far, I'm loving the very slight upgrade from the JDS Atom (I can't really tell the difference), but I love the sturdy build and knobs, so no regrets from me.
 

JohnYang1997

Master Contributor
Technical Expert
Manufacturer
Joined
Dec 28, 2018
Messages
7,074
Likes
17,160
Location
China
It's possible. The reason is because of the SMPS.
SMPS has high leakage current. It's very nasty stuff. The reason Amir's measurements don't have issue is because the ground pin of XLR connection gives the return path for the leakage current.
When not using XLR connection, the mains hum can be very prominent. In practice, there must be an earth pin to sink the leakage current properly. If not there can be audible hum.
 
OP
fieldcar

fieldcar

Addicted to Fun and Learning
Forum Donor
Joined
Sep 27, 2019
Messages
645
Likes
954
Location
Milwaukee, Wisconsin, USA
It's possible. The reason is because of the SMPS.
SMPS has high leakage current. It's very nasty stuff. The reason Amir's measurements don't have issue is because the ground pin of XLR connection gives the return path for the leakage current.
When not using XLR connection, the mains hum can be very prominent. In practice, there must be an earth pin to sink the leakage current properly. If not there can be audible hum.

Thanks for the input John. I always look forward to your replies.

Are you talking about the power brick being junk? Then you're 100% right. It's certainly not going to compare to the output of my P50 that's for sure. Isn't most SMPS noise and transients greater than 100Mhz though? I know, garbage in is garbage out to the ear, and adding low pass filters everywhere pre and post amplification stage is just a bandage.

Like I said though. The audible noise from the mains when its near a power cord is far far far outside of the listening range, so I'm still not ready to toss my THX amp out the window... until the A50S comes out of course ;)
 

JohnYang1997

Master Contributor
Technical Expert
Manufacturer
Joined
Dec 28, 2018
Messages
7,074
Likes
17,160
Location
China
Thanks for the input John. I always look forward to your replies.

Are you talking about the power brick being junk? Then you're 100% right. It's certainly not going to compare to the output of my P50 that's for sure. Isn't most SMPS noise and transients greater than 100Mhz though? I know, garbage in is garbage out to the ear, and adding low pass filters everywhere pre and post amplification stage is just a bandage.

Like I said though. The audible noise from the mains when its near a power cord is far far far outside of the listening range, so I'm still not ready to toss my THX amp out the window... until the A50S comes out of course ;)
It's the leakage current. It's the 50hz mains signal coupled through capacitance into the secondary side. I believe that adi2 thread someone has explained it very clearly. Majority of SMPS suffer from this. A50 and A50s included. Though some are better some are worse. Good ones are medical grade smps with lower leakage current. But as long it's ac-dc it's very high and will couple through the circuit.
However, it's not a issue at all when using balanced outputs. Even without the earth pin.
It's not high frequency noise which can be easily filtered out. It's the 50/60hz and harmonics. It will look ugly on the FFT graph.
 
Last edited:
OP
fieldcar

fieldcar

Addicted to Fun and Learning
Forum Donor
Joined
Sep 27, 2019
Messages
645
Likes
954
Location
Milwaukee, Wisconsin, USA
I definitely see what you mean about the supply. Even without load, its noisy. I don't have a current probe, but ~50 to 100mVpp is certainly going to be audible at some point and I assume it gets even worse under load with some tasty Vripple. So if the amp is driven harder would the SMPS noise become more pronounced? I don't really see harmonics in the audible domain, but again, not under load. Thanks again for responding. It's spiked my interest in the topic.


Drop THX AAA 789 power supply
DS1Z_QuickPrint1.png
DS1Z_Q~2 (2).PNg
NewFile2.png
 

JohnYang1997

Master Contributor
Technical Expert
Manufacturer
Joined
Dec 28, 2018
Messages
7,074
Likes
17,160
Location
China
I definitely see what you mean about the supply. Even without load, its noisy. I don't have a current probe, but ~50 to 100mVpp is certainly going to be audible at some point and I assume it gets even worse under load with some tasty Vripple. So if the amp is driven harder would the SMPS noise become more pronounced? I don't really see harmonics in the audible domain, but again, not under load. Thanks again for responding. It's spiked my interest in the topic.


Drop THX AAA 789 power supply
View attachment 63980View attachment 63981View attachment 63982
Fine. I'll post some measurements of 789...
 

flipflop

Addicted to Fun and Learning
Joined
Feb 22, 2018
Messages
853
Likes
1,129
Why would you even use IEMs with the THX 789? If you're going to blow $300 on an amp, you should be using a comfy pair of over-ear headphones with it.
 

Jimbob54

Master Contributor
Forum Donor
Joined
Oct 25, 2019
Messages
9,351
Likes
12,119
There's a 3.5mm output on the 789 specifically to use for portable earphones....

I think its the same output as the 6.35mm though? Not lower impedance or power? So convenience
 
OP
fieldcar

fieldcar

Addicted to Fun and Learning
Forum Donor
Joined
Sep 27, 2019
Messages
645
Likes
954
Location
Milwaukee, Wisconsin, USA
Why would you even use IEMs with the THX 789? If you're going to blow $300 on an amp, you should be using a comfy pair of over-ear headphones with it.
Don't worry, I'm usually listening on my HE4XX and occasionally on my 58X's (I really dont like that clamping force). When I'm not at my computer, I'm almost always using the Meizu Hifi Pro or ES100 with my IEM's.
 

Veri

Master Contributor
Joined
Feb 6, 2018
Messages
8,946
Likes
11,079
I think its the same output as the 6.35mm though? Not lower impedance or power? So convenience
Convenience yes but I meant, it's there to use it, why would one assume it's not "meant" for IEMs?
 

JohnYang1997

Master Contributor
Technical Expert
Manufacturer
Joined
Dec 28, 2018
Messages
7,074
Likes
17,160
Location
China
That's really intriguing and odd that this wasn't investigated before.
There are many other things like this which were just assumed away. They are small-ish. But some can be very very difficult or impossible to overcome.
 

Veri

Master Contributor
Joined
Feb 6, 2018
Messages
8,946
Likes
11,079
Daft question. I leave an xlr to 4.4mm adapter plugged in regardless. Would this stop the noise whether phones are connected or not?
If I understand correctly the XLR plugged in/not plugged in pertains to the inputs on the back that can provide a ground return path. Not the 4-pin XLR on the front.
 

Jimbob54

Master Contributor
Forum Donor
Joined
Oct 25, 2019
Messages
9,351
Likes
12,119
If I understand correctly the XLR plugged in/not plugged in pertains to the inputs on the back that can provide a ground return path. Not the 4-pin XLR on the front.
Ahhhh! I have rca and xlr input both connected. So golden
 

JohnYang1997

Master Contributor
Technical Expert
Manufacturer
Joined
Dec 28, 2018
Messages
7,074
Likes
17,160
Location
China
Daft question. I leave an xlr to 4.4mm adapter plugged in regardless. Would this stop the noise whether phones are connected or not?
You need to connect the XLR inputs on the back. And it's surprisingly the only way to solve this issue on this device.

This issue affects almost ALL amplifier devices that has SMPS including Neurochrome, Benchmark, THX etc. There's only one device so far on this site that behaves surprisingly good which is Geshelli Archel 2 of -140dB ps noise with 2V out. None-SMPS devices can achieve -150dB but -140dB is really impressive.
 

Jimbob54

Master Contributor
Forum Donor
Joined
Oct 25, 2019
Messages
9,351
Likes
12,119
You need to connect the XLR inputs on the back. And it's surprisingly the only way to solve this issue on this device.

This issue affects almost ALL amplifier devices that has SMPS including Neurochrome, Benchmark, THX etc. There's only one device so far on this site that behaves surprisingly good which is Geshelli Archel 2 of -140dB ps noise with 2V out. None-SMPS devices can achieve -150dB but -140dB is really impressive.

Thanks, Veri just set me straight. I have xlr and rca always connected.
 
OP
fieldcar

fieldcar

Addicted to Fun and Learning
Forum Donor
Joined
Sep 27, 2019
Messages
645
Likes
954
Location
Milwaukee, Wisconsin, USA
Those graphs are fantastic!

I just made a grounding cable and gave it a shot. From what I see with my DMM the XLR pin #3 on both channels, the chassis (screws, not the auto power switch), and all RCA sleeve's are tied together as they should be. Now, the interesting part. I audibly get more noise when grounding. I dont hear a 60Hz buzz/hum, only a faint ~500-600Hz sine wave/harmonic when grounded. Again, the gain is so insanely high when this is audible on my stack that I would destroy my ears and IEMS if I played anything through the DAC. If my method of testing for hum and noise floor if flawed, let me know.

The switch mode PS brick is still just a generic throw away. I'm in the continental US with Romex wiring instead of BX/armored cable and I do have everything plugged into a pure sine ups, but I wouldn't expect that to be doing anything special. Maybe I can consider myself lucky, since your graphs indicate a hum of something very audibly noticeable. No wonder you're not much of a fan. I should also mention that I've got a DROP labeled amp instead of the massdrop unit.

What do you think is going on? Different power brick? Design revisions? Unit variation?

Does anyone else notice a 50/60Hz buzz/harmonic on their 789? 887? SP200?


Again, its so kind of you to be looking into this. Thanks again!


Here is a pic of the brick.
1589739588977.png
 
Top Bottom