BSIAB Questions

[soggybag]

I had a couple questions about the BSIAB. Referring to the schematic posted at gaussmarkov.net below. What's the reasoning for placing Q3 before the tone section? I've built this and it sounds awesome so obviously it works fine the way it is. Intuitively I'd want to place Q3 after the tone section. Is a lot of the character and distorted sound coming from "over driving" Q3? Or is most of the distortion coming from the two mini-booster sections first the gain reduced by R12 and Q3 just boosting the signal enough to push it through the tone section? In this case why not place Q3 after the tone section?

Why C3A and C3B are really a a single 1.22µ cap. This seems like a very specific value. Seems at 1µ you'd get most everything the guitar put out is this allowing for a little extra bass?

[darron]

the Q3 stage is the last gain stage of three stages. putting the tone section in between will wreck the nature of the distortion for one, and also the last gain section might compress the frequencies out a bit and ruin the work of the tone control.


with the lower gains, the Q3 stage isn't really there as a clean volume boost, but as a loading gain boost, the first micro amp makes a very hot signal. with the feedback on C4 it makes it a slightly bright signal (not completely full range at least). then there is a bass cut passing through C6.

C7 and R10 do something smart. Think that R10 (470K) and the 500K gain pot are actually forming a voltage divider, so only HALF the signal gets passed there, EXCEPT that C7 bypasses the 470K resistor. so by letting the highs bleed through and not drop on the 470K resistor, you're actually only lowering the bass frequencies back a bit. this is a much better trick than a smaller coupling cap which just rolls them out to zero too sharply. by this stage the signal is a bit clipped, but it's very bright now and not so bass dominant so that the next stage isn't saturated too much.

by the time this is feed into the next micro amp stage it's made SUPER hot all over again, and a great deal of the distortion is done in stage two, but not quite enough to give the signal heaps of sustain. the creator may have liked the distortion of a single FET boost at the end more than a microamp? adding three micro amp stages it TOO much! i've tried...

i think it's very important that frequencies and volume are controlled in-between gain stages so that the distortion saturation isn't out of control and making a collapsing distortion like a bad fuzz.


with the 1uf and the 0.22uf, I agree. I don't think in this case there will be much difference between a plain 1uF MKT.... BUT, i believe the old GGG circuit called for a monolithic cap and a small NP one. i've experimented in that position of this circuit with caps from 10uf to 22uf etc. and the all sound the same passing all frequencies, but once you put a small NP MKT across them they really liven up. i like to do things the way the designer liked, just for fun.




i'll be interested to hear what people think. cool circuit!

[Davelectro]

Why C3A and C3B are really a a single 1.22µ cap. This seems like a very specific value. Seems at 1µ you'd get most everything the guitar put out is this allowing for a little extra bass?

C3a, C3b and R15 form an RC filter with a frequency corner near 200Hz. That's the same frequency response you get on the first stage of a classic Marshall (taking the 12ax7 internal resistance into consideration).

[soggybag]

When you are talking about MKT and NP you're suggesting using a Electrolytic and say a ceramic or film cap in parallel?

[darron]

i can't find the old layout i made years ago. i put in a monolithic and a green cap i think.

i think in positions like that if you can keep one of them film it's cool. like MKT is a NP film which works well for me.


maybe Davelectro is on the ball, just to let some even lower freq. through. i didn't find too much difference last time i breadboarded it.

[Davelectro]

Again: the purpose of the filter is to achieve maximum gain only for frequencies above 192Hz, thus preventing muddiness/farty bass.

You won't notice any difference if you ditch that 0.22u cap. The frequency corner will move to 234Hz. No big deal.