Chop Shop BJT output buffer?

[iamjackslackof]

Hi all,

I have some questions about the role of the 2N2222 BJT at the output of the Chop Shop schematic. It seems to me after some research that it's functioning as an output buffer.

My first question is about how and why it was added. What was the reasoning behind including it in the circuit? I get the idea of using buffers in general, but most similar ODs don't include one, so why did they here? Is it because they can and it's a nice thing to do, or is it because of something about the rest of the circuit that necessitated it?

Second, how is it affecting the sound? I don't have the time to breadboard it and just chop it off (heh) from the end and listen for changes, so does anyone know what audible effect there would be, if any? I would expect none if it's a simple transparent buffer, as it seems to be.

Third, if I wanted to add one to a circuit, what considerations do I need to take to pick part values? Seems like I could just get a common NPN, hook it up to VCC and the signal input and be mostly there. The emitter resistor needs to be considered for sure, and seems to be the A10K volume pot. Not sure what would happen with different/bigger pots. I'm also not sure if C6 or R10 are related/impact the buffer, or how their values would be chosen.

Thanks!

 

[iamjackslackof]

After some more research, I found that BJT provide lower output impedance than JFETs, so maybe that's why it's being used here?

 

[Mcknib]

There's a little bit of info here

Fairfield Circuitry - Barbershop Overdrive

Guillaume Fairfield of Fairfield Circuitry makes some cool pedals, and his circuit designs are refreshingly interesting. The Barbershop ...

revolutiondeux.blogspot.com

Gray bench electronics also does a teardown I haven't watched it so not sure how much detail he goes into he does V1 and 2

https://youtube.com/channel/UCz2RmB7jBh3_TdD3gKjeC-Q

 

[iamjackslackof]

Thanks @Mcknib ! Reviewing both, there's not much coverage of the buffer itself unfortunately, especially at the level of detail I'm hoping for.

 

[Chuck D. Bones]

Q3 is an emitter follower, which has unity gain, high input impedance and low output impedance. It's there to keep the VOL pot and the next device in the chain from loading Q2. If you removed Q3 from the Chop Shop, you'd lose 6dB from the max available volume and Q2's gain would also drop by 6dB. Which may or may not make any difference to you. An output buffer is critical in some circuits, useful in some and superfluous in others. It all depends on the output impedance and sensitivity to loading of the previous stage. Adding one won't hurt (if done properly), but if it's not necessary, it won't help either. I was going to say "trust the pedal designer" but then I remembered that I know better than to trust some of them to do a good circuit design.

 

[iamjackslackof]

Thanks Chuck! This is the type of analysis I was hoping for.