Adding a balance control to the Crack

[Chris Travis]

Hello! I recently got a Bottlehead Crack and have been slowly doing some upgrades to it. I upgraded the original potentiometer to an Alps Blue 100k pot and it has been going fine. After listening to this for a while, I think that I nee to add a balance control. I think my hearing might a litle skewed and would like to have a physical balance control. Does anyone have any advice on how I would go about this? I am very new to the DIY audio scene and I don't want to make a mistake on my system.

For further context, I have installed the full Speedball upgrade and I am planning on adding 2 Audyn 100uf 400v Caps later after getting some mounting solution.

Secondly, there is a noticable hum at 60HZ when I go past about 50% on the volume control. Is this normal?

Thank you!

 

[Paul Birkeland]

Is the hum there when you have a source connected?

Adding a balance pot requires drilling some extra holes and adding a balance pot behind your volume pot. It may work better to measure the reduction in dB that you need between channels, then we can calculate padding resistors to build this into the amp.

Of course, you could also build a Crack-a-two-a, which has a balance pot (and a spot for it) in the stock circuit.

-PB

 

[Chris Travis]

The hum is there when I have a source connected.

Getting a Crack-a-two-a would be awesome, but id like to keep getting better at soldering and planning before I jump to that. I bought the Crack to kinda being a test case for these things.

Drilling a hole should be fine, I have the right bits to drill through aluminum. Is there a suggested balance pot for this?

 

[Paul Birkeland]

Can you post some build photos? Was the hum there with the stock pot?

 

[Chris Travis]

The hum was there with the pot. But it got a lot better after the speedball upgrade.

Here are some build pictures.

 

[Paul Birkeland]

I'd be sure you have good electrical continuity to the chassis plate.

The Alps pot you have installed is meant to be mounted to a PC board. You can buy a bunch of different ones on eBay. Typically if you run wires to those pins, the pins will tend to snap off.

 

[Chris Travis]

Thank you for the advice on the blue pot pcb. I ordered a couple off ebay and will install when they come in.

I think that there is good continuity with the chassis plate, but ill double check. I'll try to sand the chassis under that bolt to make sure there is good contact for good measure.

Is there any advice regarding what balance pot I should buy and how to wire it? Does it specifically need to be a balance pot or can it just be another alps blue velvet 100k?

 

[Paul Birkeland]

You'd add RV-R, which would be a 100K dual linear stereo pot, and RR. The value of RR can be scaled to change how the balance pot operates. You do not need R1/R16/S1. The decks of each level of the balance pot need to be wired the opposite way to get the appropriate effect.

 

[Chris Travis]

I'd be sure you have good electrical continuity to the chassis plate.

The Alps pot you have installed is meant to be mounted to a PC board. You can buy a bunch of different ones on eBay. Typically if you run wires to those pins, the pins will tend to snap off.

I went back and tightened down the grounding bolt as much as I could and it fixed the hum issue almost entirely. It is still there at about 90% and above, but I doubt I could ever push this trhing that high seeing as I hover around 30%-60 for most sources. Thank you for the guidance there.