How do I properly bias the Chop Shop (Barbershop)?

[Idlebeach]

Howdy!

Just curious if anyone who has built or knows anything about this can help me figure out how to bias this correctly. I realize some of it will be to taste but ideally I could get it where it's supposed to be to start.

Do I need to turn the sag all the way one way or the other first? And what voltage should I be shooting for? I feel like I have seen (somewhere on the internet) people say to shoot for 6.66v (maybe that's a joke?) and it seems like most other pedals you'd go for 4.5v.

I'm not in front of my stuff anymore but I assume I need to turn the sag so I get the max voltage, probably bias it over 4.5v so that way there is a better usable range? I hadn't really thought about sag/starve knobs and how to deal with bias before now so maybe I have it all wrong.

Thanks!

 

[Harry Klippton]

I believe someone covered it in this thread:

Chop Shop (Barbershop)

One more build that had been sitting in the queue for far too long and I finally wrapped it up tonight: the Chop Shop. Sounds great! I tend to like discrete overdrives more than opamp-based ones, and this one confirms it. Went with another Gørva enclosure, but decided to use a bare one to sort...

forum.pedalpcb.com

 

[bowanderror]

I biased mine at 6V with the Sag knob at it's loudest (I believe that's fully CCW on the Chop Shop). Gillaume from Fairfield talks about biasing in this FSB thread: "The Bias should indeed be measured when the SAG is essentially 0 ohms, loudest. And yes 2/3 supply is around 6V at the drain."

If you don't have trimpots in yours, you can either install different J201s to try and get closer to that bias value or do some math to get the ideal values for R5 & R9. For this approach, you'll want to install the stock resistor values for R5 & R9 and then use the voltage law (V= I*R) to calculate back out the correct resistance for each:

1. Measure the voltage drop across old resistor - Measure voltage on side connected to the Sag pot and subtract the voltage on the side connected to JFET drain. This is your voltage drop across that resistor (Vdrop)
   1. So if one side of the 8.2kOhm resistor is 9.2V, and the other is 4.2V, then Vdrop = 5V
2. Calculate current (I) through old resistor - If Vdrop = I*R, and we know R (value of resistor in ohms), solve for I since you know R
   1. Vdrop/R = I - so: 5V/8200ohms = 0.00061A = 0.61mA
3. Calculate Vdrop that would give us 6V drain bias - We want a drain bias of 6V, so you want a Voltage drop of whatever our incoming voltage is minus the amount it takes to get to 6V: ~9.XX - 6 = 3.xx.
   1. Our incoming voltage (from Sag pot) is 9.2V, and we want a voltage drop of 3.2V: 9.2V-6V = 3.2V
4. Calculate new resistor value - To get the resistor value that will give us 6V drain bias, take the voltage drop and divide it by the current (I) that we calculated earlier
   1. Vdrop/I = R: 3.2V/0.00061A = 5250ohm = 5.25kOhms for R5 to get drain bias of 6V.