How come my modded Sonic-T keep having "hissing" sound?

[poozebear]

Hi folks,
Need your advise.... Thanks to the excellent instructions on Mike Mardis's website, I have successfully completed the "stealth" mod.

BUT... for some reason, there is this constant "hissing" sound on the speakers. It is really faint (barely audible unless ears next to it) and does not increase with the turning of the pot (I'm using 100K Alps Pot). But, the "hissing" sound does get real loud when I touch the pot. I tried touching the cap and cables... no sudden amplification.

I'm using a single ground cable to the pot. It is wired to both L & R terminals on the pot. The ground from the RCAs then tap from this same point on the pot's terminal.

I have since resoldered the terminals on the PCB and ground cables on the pot. But does not make any difference...

Anyone has similar encounter? What else do I need to check? Thanks in advance...

[poozebear]

Anyone?

[Pano]

Can you post photos and diagram of your wiring?

It is probably a grounding or shielding issue, but it is hard to tell ithout seeing it.

[poozebear]

Here goes.... first time posting pictures.. hopefully it comes out fine... thanks!

Btw, I read in one of the thread that high DC offset is bad. It should not exceed 100mV. How do we measure that? Probe the +ve and -ve of each set of speaker binding post? What can we do if it exceeds 100mV? Thanks!

[rpapps]

Shielded cable for all the low level signal leads would probably help a lot. Make them as short as possible.
Also move the speaker leads well away from the input leads.
Cheers
Rob

[poozebear]

Hi Rob,
Low level signals refer to the input? I will try using copper braids. Does this goes back to supply ground (as in 110/240v earth), the 12V ground (or 0v), or the digital ground that the inputs are going to?

Chances is that I cannot shorten the cable anymore coz I will be re-housing this into a fairly long wooden box...

Do I need to shield the output/speaker cables as well?

Thanks!

[rpapps]

Hi
I assume you mean by "copper braids", the shielding around the cable. Just connect it to the same place as the existing black wires. Shielding the input leads should be sufficient as long as you keep the speaker leads well away from them.
Cheers
Rob

[poozebear]

Okie dokie... thanks! Will post an updated pic and results later...

[Pano]

Rob is right, you need to clean up your wiring. Keep everthing as short as possible and use sheilded cable for the inputs. Keep the speaker wires away from the input wires.

As far as I can see, all the connections are correct.

That's a mighty big power supply you have there!!

[poozebear]

Heh...heh... that's a SMPS I bought off some second-hand shop.. think is a 75W power supply. Comes with both 12v and 5v... I tuned the pot within the SMPS to supply 12.8v. So far still hanging on...

I will try the shielding of the input cables first. But strangely, how does my hand touching the potentiometer's shaft (I have yet to put in a knob) or it's metalic surface amplifies the ringing sound?

Will update once I'm done with the grounding...

Thanks!