Ppimeta 50Hz? hum

[RoosterX]

So i built my Pimeta and it works ok, not extremely nice sound but quite ok, now there is only one thing that i find quite disturbing - whenever i power on my HP-amp i can hear this low hum, the hum is all the time with volume low and high and always with the same volume.
I believe the hum to be mains outlet related, would fitting a low resistance on the outputs help(20-100ohm?)
Could this be that my 'wall-mart' 24v DC PSU is of really bad quality, i am using Tread without the caps that are parallell with the diode-bridge(that also are not fitted).

BTW: when i listen to music and if the music isn't extremely low volume i don't hear the hum(like the beginning of many Dire Straits songs.)

HP are Sennheiser HD595(50ohm)

[n_maher]

I'm guessing it's a ground loop and not mains hum. Can you post some pictures of the build for us to look at?

[RoosterX]

I am not shure what pictures you want, of the backside of pcb maybe?

Anyway here is a picture of component side.


Todo list: Solder off buffers and move to the bottom of PCB, fuse to between Tread and Pimeta, then maybe install the q1 and q2's.

[n_maher]

Are those plastic end panels?

And that picture is a fine place to start, no need for the underside of the pcb.

[RoosterX]

It is all plastic enclosure, now when i think of it i understand what you mean by ground loop, you mean like the sound you get when the pot is not grounded?
If this is the case i had that problem(pot now grounded), and it does not sound like that, it is much lower volume.

[n_maher]

Yup, sure seems to me that you're still chasing the ground problem. Do you have the rca jacks tied directly together (can't see in the picture)? You could also try running the ground wire from the pot to the RCA's to see if that helps.

I just can't see how you'd be getting AC line noise unless you have the amp parked over a trafo. Try using the amp in various locations around your house and see if there's any difference. If you're right next to a computer monitor that might explain things as well.

[RoosterX]

The RCA GND's are soldered together at end cap.

Now i have moved my buffers LMH6321 to bottom of PCB.

I tried to connect pot-gnd to RCA-gnd, no difference.

The hum is at a constant very very low volume, my GF told me it was almost like hearing a TV test signal(1000Hz), and i probably must agree.

[Pars]

Originally Posted by RoosterX

almost like hearing a TV test signal(1000Hz)

So this is a higher frequency sound, and not 50 or 100Hz hum? Hmmm...

[RoosterX]

Originally Posted by Pars

So this is a higher frequency sound, and not 50 or 100Hz hum? Hmmm...

I just listened to it again, and i take my statement back, it is not as high as 1kHz, it is more like 50/100 or something.

I dont know what i was thinking :/

[RoosterX]

I tested a Tone genereator to see if i found something that sounds quite like the tone i hear, it isn't easy especially since the tone i hear is at such a low volume.

I believe it to be around 50 to 100Hz triangular.. the 100Hz maybe sound most alike.

Anyone have any idea?