Quote Originally Posted by Puffin View Post
There are two ways to solve this. I am at work at the moment, but when I get home I will post a picture of where to add some variable resistors to balance the channels.

I have made several of these and in the one I currently use I have two additional pots that act in a way similar to a balance control. I made a diagram of how I wired it up (but will need to find it) or I will have to open up the pre-amp.

IMO the L/Speed is the best pre-amp I have ever heard. Mated with a Pass Labs B1 buffer it is very special for very silly money.

EDIT : here is the diagram. The variable resistors are the square blocks with a cross in (5k cermet trimmers)



Rob.
Hi Rob, apologies if I appear to be pi**ing on your fire here

This could be a good system, but there are more than a few things that can f*ck it up imo. For starters you need the variable resistor to track both channels accurately. Which is the equivalent of what you get in a normal stereo (or hifi) system.

Then you need the LEDs to give out the same brightness for the same current input....

Then you need the LDRs to be matched as well so they give the same resistance with 'X' amount of light input... Both input to output need to be matched & output to ground, I'd suggest that is impossible

On top of that you have the problem that LEDs don't all emit on the same wavelength even if they are from the same batch & LDRs respond in a different way to different wavelengths of light

To me that looks like at least four things that will make channel balance a pain in the ass. You might be able to balance it at one volume level, but turn that volume control pot & I can see it all falling to bits again balance wise

I'd suggest using an ADC with a single potentiometer & like the old ladder DACs setting up each increment to balance..

They certainly wont in that system as there are miles too many variables. In a potentiometer there is only that to blame, but the pictured system includes three more variables which will degrade channel balance in an unpredictable way.

Imho