[seanohue]

So I just finished my millett. I have the right and left bias at 12.5V on each side (25V from TREAD). Tubes light, and so I know I have it almost correct. I plugged my cable up to my ipod and plugged the cord into the RCA inputs of the amp. I can faintly here the music, its really grainy though, but audible. DC offset is about 1mV (the lowest my meter will read). My grounding scheme is an isolated DC jack, metal RCAs and the Neutrik jack NMJ6HCD2 in the metal hammond 1455N case. Any ideas?

 

[n_maher]

Could you please check the value of your output resistors? The symptoms you describe pretty well point to having 22K resistors where 22 ohm resistors belong.

Nate

 

[seanohue]

If R4 is the output resistor, then yes, I have a 22.1K resistor in the spot. Thanks nate

 

[n_maher]

Quote:

Originally Posted by seanohue If R4 is the output resistor, then yes, I have a 22.1K resistor in the spot. Thanks nate

That'd be the one my friend. For now, unless you happen to have 22 ohm resistors kicking around, you can just install a jumper at each of the R4 positions and your amp should be good to go! Just leave the jumpers long enough so they are easy to remove later - it's a good idea to have the 22 ohm value in there if you say, want to use diamond buffers or something.

[edit]And don't feel bad, I did this on the first two MH's that I built

.

Nate

 

[seanohue]

Well, the problem has persisted, its just louder. I think it's a grounding problem because I had this same problem on my CMoy. The music and vocals are both really distorted. They sound like really bad quality MP3s (kind of sounds like being underwater) and sometimes delayed. My RCA jacks are attached to the holes I drilled in the back panel, then I have the grounding wires from both left and right connected in the same pad on the PCB. The neutrik is board mounted and plastic so I think that be ok. Also, I thought I had the pot grounded (alps blue) but touching the shaft stops some of the feedback. I have a wire wrapped around the screw and have that wire going to the ALPs pot ground pad on the board.

 

[seanohue]

Ah here's somethin I just figured out: I took a wire from the metal alps body and touched it to the RCA jack. Perfect sound. So obviously, its a grounding problem. Will this go away since the metal panels will connect the RCA jacks back into the loop?

 

[n_maher]

Quote:

Originally Posted by seanohue Ah here's somethin I just figured out: I took a wire from the metal alps body and touched it to the RCA jack. Perfect sound. So obviously, its a grounding problem. Will this go away since the metal panels will connect the RCA jacks back into the loop?

Without seeing the actual setup that you're trying to describe it's hard to say. Tying the alps ground to the RCA ground is certainly not going to hurt anything as technically you've already done that by connecting the shaft to the pad on the board (via the wire). But sometimes, if you've got a ground loop problem, having a more direct connection can help. I generally also have a wire that directly ties the two RCA jacks together at the jacks in addition to having their grounds connected at the board. Overkill, perhaps, but I don't think it can hurt. Also, it will probably make a difference if the panels are loose vs. everything all connected up.

Just one last question, is your DC jack isolated?

Nate

 

[seanohue]

Yes, the DC jack is isolated (its plastic). Unfortunately my camera is dead so I'm charging the batteries now. I'll have pics tomorrow.

 

[amb]

Sounds like your RCA input jacks aren't really connected to ground. Use your DMM (in resistance mode) and check for continuity between the RCA jacks' ground and the ground on the pcb.

 

[seanohue]

Ok I finished casing it and the problem is still there.
Here's the input wiring
And the pot

And I just fixed it by running a wire from the screw on the alps to one of the input grounds on the RCA. I don't know what was wrong, but its fixed now. Thanks guys