[Resolved] Low humming and RCA sound issues

[Chris Travis]

I recently built my Bottlehead Crack and I plugged my headphones in (DT 1990 Pro) and I can hear a low hum when I turn on the amp and it gets very loud at about 60% volume. I have tried resoldering everything multiple times, but it does not seem to fix the issue.

The second issue I am experiencing is when I plug RCA cables in that come from my PC, it has a lot of static. It gets worse whenever I use my mouse and keyboard. I plugged in a cheap low pass filter and that fixed the issue, but I am wondering if there is some bigger issue at play. Static does not persist if the RCA cable is removed from the amp.

I have added pictures of the components for clarity. Thank you all for your help so far!

 

[Chris Travis]

Here is an overview of the entire build.

 

[Chris Travis]

It looks like in my resoldering I messed up the LED on A3 to the center pin. Looks like I will have to buy some and try again.

 

[Paul Birkeland]

You'll need to add this to deal with the computer noise:
https://forums.bottlehead.com/threa...s-usb-dacs-and-powerline-ethernet-kits.11676/

What are your voltages on terminals 1-10? Having a messed up LED will cause your DC voltages to be wrong, then you would not want to use the amp...

 

[Chris Travis]

Thank you very much for this! I will buy the diodes and report back if that fixes everything.

I went back to some other posts and started to resoldering all of the black connections to ensure everything was working properly. I did that and then rechecked my Resistances and Voltages and they were all correct. Everything was working including both LEDs. But the low humming was still there.

I Looked at some other posts and some were talking about the 9 pin possibly having issues. Unfortunately when I tried to resolder the A3 connection, the entire lead came off of the LED. At that point I stopped everything and haven't used it/ turned it on since. I have ordered a replacement LED to try the connection again.
Again, thank you for all your help!

 

[Chris Travis]

Hello! I just got the LED in today and popped it in. I rechecked all resistances and all Voltages and everything looks good.

Voltages:
1) 84.8
2) 173.3
3) 0
4) 173.3
5) 86.6
6) 0
7) 112.5
8) 0
9) 111.5
10) 0

I am still experiencing a humming noise when the amp is on with or without an input. I tried looking around and it sounds like a ground loop hum. I am not sure if this is absolutely true, but it sounds a lot like my issue. It is noticable when the amp is turned on, but gets substantially worse when the knob is turned about 2/3 of its potential. Anything past that is impossible to listen to. Is there any way to fix this?

 

[Thermioniclife]

Without anything plugged into the input jacks you would not get a ground loop. Have you tried shorting out the inputs by using a shorted pair of rca plugs or use alligator clips to short the pos and ground for both channels?

 

[Chris Travis]

Without anything plugged into the input jacks you would not get a ground loop. Have you tried shorting out the inputs by using a shorted pair of rca plugs or use alligator clips to short the pos and ground for both channels?

This seems to have fixed 95% of the issue. There is still a slight background hum but way better than before. Do you know what is causing this/ how to fix this?

 

[Thermioniclife]

Listen to a 60Hz and an 120Hz tone on your computer and decide which you have, if it's 120 Hz look for poorly soldered filter caps or damaged, if its 60 Hz look for tight grounds and check any shields in the amp. If your 95% there I would just call it a day.

 

[Thermioniclife]

I'm sure PB can give you better advice than me.

 

[Chris Travis]

It sounds like it is 120Hz. I have tried resoldering a couple times, but I will try again. Thank you so much for helping me so far!

 

[Paul Birkeland]

Noise that's present when there isn't anything plugged into the inputs isn't particularly meaningful, especially if it's influenced by the position of the volume pot.

Listening to some test tones is a good idea. A 120Hz noise could be a loose solder joint on one of the 220uF/250V capacitors, which would leave you with DC voltages that look great but a lot of excess power supply ripple. You could also get something resembling 120Hz noise potentially if you don't have an earth reference on your AC powerline, or if there's some kind of loose hardware on the chassis, paint on both sides of the chassis, RCA jack solder lug ring terminals not in the right spot, etc.

(Or if you're using extremely sensitive, low impedance headphones, that can be its own problem)

 

[Chris Travis]

I am using DT 1990 Pro MK1 which has an impedenve of 250 ohms, so I don't think that is the issue. It certainly sounds like 120Hz tone. I tried resoldering the caps, but that did not change anything. I did paint the top of the chassis, but there should be very little if any on the bottom. How much paint would be a problem? I am sorry for all the questions and problems.

When I use the shorted cables, the noise is drastically reduce. By 90-95%.

 

[Paul Birkeland]

If there's no paint on the bottom, then that won't be your issue. What source component are you using? Do you have modern-ish AC mains wiring where you are using the amp?

 

[Chris Travis]

The place was built in 1975 and renovated in 2005, so I assume it has modern AC mains wiring. Currently, I am not using any audio source. The humming happens when nothing is plugged in at all and is reduced signifcantly when I use shorted RCA cables. There is still a slight noise when using the shorted cables, but a lot less.

 

[Doc B]

An open input will often pick up noise. Thankfully that's not the intended way to use the amp. It's best to evaluate the noise level with a good quality source connected to the RCA cables. Sometimes the cables can be the culprit and it can be useful to try a different pair.

 

[Chris Travis]

An open input will often pick up noise. Thankfully that's not the intended way to use the amp. It's best to evaluate the noise level with a good quality source connected to the RCA cables. Sometimes the cables can be the culprit and it can be useful to try a different pair.

Doc, you are the man! I should've thought about that to begin with, but I was certain that I had screwed something up myself so I didn't even think about it. I originally had it plugged in to my pc, but there was a lot electronic noise on that, so PB suggested that I do the dual diode mod to reduce that. Unfortunately those wont get here till Monday, so I was just testing without an input. I tried plugging it in to a laptop and holy smokes, that soundtage is amazing. Thank you all for everything you have done. I am super excited by all of this and I cant wait to start tube rolling (RIP my wallet). Next is to install the Speedball and an Alps Blue Velvet Pot, but I wanna enjoy this incredible sound for a while before going to the next level.

 

[Chris Travis]

P.B. , When I do the two diodes mod two of the resistances change. Is this normal?
Terminal 6 2.5k Ohm
Terminal 10 2.5k Ohm

 

[Chris Travis]

Nevermind, I am closing this issue out. I had an adapter plugged in to the 1/4" socket and it threw off the measurements. Thank you all for everything and I cant wait to start listening.