Noisy sex

[RW]

Hey Bottleheads,

I have a small issue with some noise, hopefully someone could help. Here are the symptoms, and some things I've tried.
-all testing was done with out anything hooked up to the rca jacks
-I tried plugging into a line conditioner, and multiple circuits in my apartment
-tried adding 100 ohm resistors in C1 & C2, to C3
-I didn't hear anything with my less sensitive headphones (he500 and Q701) but the buzz/hum is audible in my more sensitive headphones (denon d5k, Focal Classic, B&W p7)
-with the volume all the way off/down there is a hum, constant in both channels
-when I raise the volume pot upto and over 75% a second hum is superimposed over the first hum, if I listen carefully, I can hear that there is definitely 2 different tones of noise. Not sure if it's both from the same source or not.
Any help would be much appreciated, thanks
-r

[fullheadofnothing]

Plug in a source. Any open input will pick up noise.  Also how are the outputs wired ?

[RW]

Still hear it with a source plugged in. That's why I disconnected it, to see if it was the issue.
As for output, I'm using headphones out of the TRS jack. I also have the C4s and impedance switches. You can hear the noise thru any combination of the switches, and using the volume control.

[fullheadofnothing]

Set the impedance switches for lo and unbalanced. This will give the best signal-to-noise regardless of headphone impedance. You can also install 120Ω resistors in series with the headphone output, which will most likely solve your problem.

It sounds like this is very low level stuff. Does it actually interfere with the music?

[RW]

@Full
If I use sensitive headphones it definitely is audible over music. The 120 ohm resistors; do I need one for L,R, and ground? And should I remove the other resistors that are on the headphone jack? Do I need to add resistors to the speaker terminals also? Will this increase the output impedance?

@grainger
Will a single rca cable to from one input to the other be good enough, or should I just connect a jumper under the plate? What should I look at after I short them?

Thanks

[Paul Birkeland]

Before rushing out to modify this amp, I would start by reheating all the solder joints.  This is free, fairly quick, and may drop the noise floor of your amplifier.

[RW]

@PB
I'll reflow and report back

@grainger
Sounds good. I'll post back tomorrow after I resolder

[RW]

Well, reflowing joints ended up being a little more difficult than I thought. The sex is 50% apart, I'm just going to redo the entire kit with extra caution and report back.

[RW]

Hey Bottleheads,

So, it's been quite some time since I posted in this thread last. I rewired the entire kit. I reflowed every joint. I double and triple checked everything. Needless to say, it didn't help at all. I still have a hum in the kit. I don't know what else to look for. I invested a lot of time and money, and I haven't made any progress. I'm at the point where I may just shelf this kit and cut my losses. If anyone has any input that may lead to a solution, I would greatly appreciate it. I listed a few things that Ive tried below
-I tried different tubes
-I tried multiple circuits in my apartment
-I tried plugging into an isolation transformer
-I removed the wiring for the speakers
-I tried a different attenuator
-different components of the same value.

[Zimmer64]

sorry to hear your issues. the sex should be dead quiet (mine is), so there should be a solution. can you post pictures of the underside? the guys here in the forum often spot something that could potentially help solve the problem. also please describe the noise that your hear. Is it low fequency 60 or 120 Hz like or high pitch? Also post the voltage readings as per check out process.

Good luck and don't despair!

Michael

[Doc B.]

I am seeing that you did not try the suggestion of 120 ohms resistors in series. Also, what output impedance configuration are you using?

Try rewiring the output transformer to the 4 ohm output configuration and using the 120 ohm resistors in series with the headphone jack.

[RW]

@Zimmer I will get pics up. It's going to take me a day or two to get the voltages. I didn't note them a second time. The hum is low. 60hz, and it's it's constant

@Doc I haven't installed the resistors. Do I need them just for the L&R or should I use one for the ground as well? Will 1/4 watt do, or do I need bigger? I have the impedance switches installed. The tone is constant through all of the positions, except when I put the switches on HI the tone gets a tad louder.

Thanks
-r

[Doc B.]

The 120R resistors just go in the L&R hot legs.

[mcandmar]

Its very hard to tell what levels of hum you are hearing, but it does sound like there is something wrong. 4 & 8 ohm impedance has a lower noise floor, but if your not hearing any difference that implys the noise isn't coming through the audio path and being amplified, it really does sound like just a bad connection somewhere or a grounding issue.  As suggested, post some pics of the wiring in case we can spot something that doesn't look right.  Don't give up, it will be something simple..

Also try the 120r resistors on the headphone socket as Doc suggested, any 1/4w will be fine.

[RW]

My 120 ohm resistors won't be here for a few days, but I did find four 60ohm resistors. I took two put them in series, hooked them up to the L&R (in series), and the hum is still present I will try again when the 120s come in, but my hopes aren't high. Is there anything else I should start looking into? Any help would be much appreciated.

@Mc
The noise changes if I switch back n forth between high and low on the impedance switches. Anything specific I should take photos or should I just post a bunch at different angles. There's a lot to photograph under the hood