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

[ParasKPatel]

What if you use a RCA to XLR cable from your DAC?

 

[fieldcar]

What if you use a RCA to XLR cable from your DAC?

Good question.

I'm not sure it would do much. Single ended XLR usually ties one of the signal wires (either 1 or 2) and 3(GND) together at the RCA sleeve. There is still no dedicated path to earth ground like a balanced DAC with XLR's would provide. I wonder if I'm already grounded by using the P50 (linear power supply) with my D50S, and the extra ground wire that I used yesterday is creating a ground loop. That should have been the first thing I checked. I'll have to check tonight when I'm back home to see if I have continuity between my potentially un-grounded stack and ground, and to try removing my P50.

 

[JohnYang1997]

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.
View attachment 64026

5uV/10uV is not very audible if at all for most cases.

 

[JohnYang1997]

What if you use a RCA to XLR cable from your DAC?

The ground pin on XLR input will be left open hence have noise.

 

[ReaderZ]

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So Amp like JDS atom will have this issue since it does not have XLR in?

Also on the A90, if I want to use RCA(XLR may not be an option) do I need to keep XLR plugged? Yes, no or only when using single ended output?

 

[JohnYang1997]

So Amp like JDS atom will have this issue since it does not have XLR in?

Also on the A90, if I want to use RCA(XLR may not be an option) do I need to keep XLR plugged? Yes, no or only when using single ended output?

bruh....

1, Atom doesn't use SMPS hence no leakage current issue. Thus will not have large power supply noise.
2, THX 789 doesn't have a dedicated ground pin. Hence XLR input is the only way to introduce the return path. SP200 and A90 both have ground pin in the supply, so the issue will be much better.
3, Why do you need a90 when you use only both single ended input and output?
4, The noise may not be so audible.

 

[ReaderZ]

bruh....

1, Atom doesn't use SMPS hence no leakage current issue. Thus will not have large power supply noise.
2, THX 789 doesn't have a dedicated ground pin. Hence XLR input is the only way to introduce the return path. SP200 and A90 both have ground pin in the supply, so the issue will be much better.
3, Why do you need a90 when you use only both single ended input and output?
4, The noise may not be so audible.

Thanks for the reply!

1. Icic, did not look into it in depth.

2. I noticed that, but just want to be sure as I am getting an A90.

3. I will use balanced out, just don't have balanced cable for all my headphones nor do I plan to get all of them balanced cable. Also further done the road maybe splurge on another balanced DAC so it can use XLR.

4. True, but if I am buying a SOTA product, I want SOTA performance out of it. Otherwise, I could just keep using DX7 pro's headphone out or get my friend to send me my old prehead.

 

[JohnYang1997]

Thanks for the reply!

1. Icic, did not look into it in depth.

2. I noticed that, but just want to be sure as I am getting an A90.

3. I will use balanced out, just don't have balanced cable for all my headphones nor do I plan to get all of them balanced cable. Also further done the road maybe splurge on another balanced DAC so it can use XLR.

4. True, but if I am buying a SOTA product, I want SOTA performance out of it. Otherwise, I could just keep using DX7 pro's headphone out or get my friend to send me my old prehead.

So, no issue.

 

[wric01]

Joshua valour is typical non geeky loud mouth. He bs about how bad the Ora graphene headphones were and yet everyone loved the sound and won technology and sound awards .

 

[Lttlwing16]

I'm picking up PC noise through the SE input on my 789. I was gifted a gaming PC and since introducing it has been a PITA for noise. This is heterogenous electrical noise which changes depending on what's going on in the PC, i.e. moving the mouse etc.

Is this a grounding issue with the PC or 789 or both? FWIW I have a the DROP GRACE SDAC balanced output going into the XLR input of the 789. This input does not have the same noise. The SE input of the 789 is connected to my Technics Sl-23 turntable RCA output.

If I power off the troublesome PC the noise goes away. The 789 is not directly connected to the PC accept for being plugged in the same power outlet (two different and separate power strips exhibited the same issue), and both are connected to the same mixer which outputs to my powered Mackie monitors. The 789 is connected to the mixer via RCA, and the PC is connected via Toslink>cheap DAC>RCA to the mixer.