T Amp input noise

pixpop

Member
2005-01-20 4:24 pm
I've finished modding up my T amp. It's pretty good actually. I put it all in a small Hammond diecast box, painted bright red. I put a regulated 13.5 volt power supply in there.

Initially, I changed the input caps to be 15uF, but this caused a huge power-on transient. The larger caps didn't seem to help the sound at all, so I took them out and left it with the stock caps.

I added a larger electro in parallel with the on board PSU filter cap. I rewired all the input, power, and output circuitry with better wire & conenctors etc. I kept the stock output caps across the speaker terminals.

I use the stock volume control and power switch, but I'm switching the AC line with the switch, instead of the DC. This all works fine.

The only problem I have is that I hear power supply noise when the volume pot is in the middle third of its travel. It sounds like 120 Hz sawtooth noise. It goes away completely as the volume approaches maximum.

Before you jump on me, I checked that it's not caused by routing the AC line through the volume control switch. I took that off, and it made no difference at all.

It seems to me that the amp input impedance must be high enough that when the pot is in the middle, which is when it has highest resistance to ground and to signal, some radiated noise can be picked up. Yet moving the PSU further from the amp doesn't help. Could be picking up noise from the DC power signals, I guess. Maybe I should shield them.

Any clues about how to trouble shoot this? Anyone noticed anything similar?
 

johnm

Member
2003-09-10 2:48 pm
Hampshire
Hi there!

You're not alone!! Yes I'm experiencing exactly the same problem as you! The buzz isn't too loud, and to be honest can only be heard when my head is near the speaker, but even so it's annoying. Also it seems worse when my finger is touching the volume pot, which is weird as it's insulated with a bakelite knob! Like yours it onyl occurs from about the 9 o'clock position to around 3-4 o'clock one. I've tried all sorts of different earthing arrangements to no avail. I was thinking it might be a faulty ALPS 50k volume pot, bit since reading your post I'm beginning to think the problem might lie elsewhere.

I'm also using unshielded cable, so perhaps this is the problem? Apparently these things put out a great deal of noise so perhaps shielded cable could be the cure? I'll try and get some shielded cable and see if this cures it. Other than that it is sounding great!

Just an observation about your 15uF cap. This is wayyy too big - no wonder you got a large switch-on noise. 2.2uF is as high as you need to go, and this is what I use running into a 22k resistor as per the diagram here:

http://www.michael.mardis.com/sonic/start.html

Great website by the way, and Michael is a really helpful guy!

Let us know how you get on with the hum!

- John
 

pixpop

Member
2005-01-20 4:24 pm
Yes, the noise level is not high. People don't notice it unless I point it out to them.

Given that the noise goes away at either end of the pot travel, I reason that the noise comes from a high impedance source. The source impedance of the signal is low enough to load the noise down to negligible levels.

Your experience with the noise getting worse when you touch the knob supports this as well.. it seems that the noise is being capacitively coupled into the circuit.

I wonder if reducing the input resistors would help. In Michael's drawing, the 20K resistors labelled Rf and Ri.. maybe change them to 5K?

Buffering the input might help. I can't test this at the moment though.
 

johnm

Member
2003-09-10 2:48 pm
Hampshire
Hi.

Afraid I'm still very much a novice when it comes to electrical stuff, but by input resistors do you mean the ones attached to the phono sockets in Michael's diagram? If so mine are 33k which I guess makes the situation even worse? The resistors I've attached to the board are 22k - same as the drawing.

Wonder if getting rid of the 33k at the input would be worthwhile?
 

pixpop

Member
2005-01-20 4:24 pm
On Michael's diagram, he shows the signal from the input RCA going through a capacitor, and then to a pair of 20K resistors. I'm talking about those two resistors.

I don't have the Sonic Impact schematic, but I know it's not like Michael's. For a start, there's an inductor in there on the Sonic Impact.

Anyway, I'm wondering if reducing those two resistors on Michael's schematic would reduce the noise. Certainly, I would expect that if you have the noise with Michael's version, then making the resistors bigger would make it worse.

In my case, I don't think the noise is coming from the Tripath chip, because it has a strong 120 Hz sound. The Tripath switches at a much higher frequency. I think my noise must be picked up from my power supply, or from the environment (e.g., AC line noise).
 

pixpop

Member
2005-01-20 4:24 pm
I'm not going to be able to modify mine for some time. But, just had a thought... try driving it from a preamp. I should be able to try that soon and report. I had been driving it from a computer audio out socket. Maybe the computer output impedance is too high.
 

johnm

Member
2003-09-10 2:48 pm
Hampshire
Hi.

Just to let you know I fixed the hum problem in my amp by removing the 33k resistors attached between the + and - of each phono input. So it must have been an impendence mis-match then?

Hopefully this might work with your amp?

Quiet as a mouse now ;-)

Cheers,

- John

P.S. I only have the DC grounded to the chassis. Signal ground is not attached to chassis in my enclosure...