Variable caps - fake them?

[GCA]

NICE. Fig. 2b is...awesome I wonder why a 12AT7 though,and how to calculate the Fc on it.

Ahhh how to work it in now in 4 bands....

(duffy this rocks)

 

[GCA]

for tubification:

Guitar Pedals: Gyrator Filter Calculator

Anyone have caveats I should know about before I undertake this? Like... where is the output being taken from, off R1?

 

[Roff]

for tubification:

Guitar Pedals: Gyrator Filter Calculator

Anyone have caveats I should know about before I undertake this? Like... where is the output being taken from, off R1?

Off the emitter, or op amp output. You should cap-couple it to the next stage.

 

[GCA]

Off the emitter, or op amp output. You should cap-couple it to the next stage.

So then after the 10k resistor in the schem, right? I hope I'm understanding it correctly that this is essentially meant to be a cap-coupled cathode follower (emitter-follower) with neg local feedback.

That would then make the 4.5v in there off R1 analogous to a bias supply, which varies according to the value of R1, and then R2 varies the amount of feedback.

I will have to experiment and breadboard this up to understand it. Thx!

 

[Roff]

So then after the 10k resistor in the schem, right? I hope I'm understanding it correctly that this is essentially meant to be a cap-coupled cathode follower (emitter-follower) with neg local feedback.

That would then make the 4.5v in there off R1 analogous to a bias supply, which varies according to the value of R1, and then R2 varies the amount of feedback.

I will have to experiment and breadboard this up to understand it. Thx!

Changing the value of R1 will cause little change in the emitter voltage and current unless R1 is huge, because the base current will only be a few microamps.

 

[GCA]

I'm breadboarding it right this second actually. Only turned the computer back on to re-check the schematic since I didn't print it.

 

[GCA]

Ok. I know crap about transistor circuits; I'm guessing too much with it. R2 acted as a volume knob when breadboarded. I have to have done it wrong. There was no frequency change and no filtering whatsoever.

 

[GCA]

Varying R1 and R2 worked. I used 100k pots, and they worked off of each other exactly as per the calculator on the website. Needed to be linear taper pots! Audio taper made very little change in the frequency adjustment after about 2 (on a 1-10 scale).

Beautiful. I'll make this work better with only one pot and a fixed resistor tomorrow.

I didn't use a separate 4.5v on R1. I connected back to what would be B+, the +9v. I took the output from before the 10k resistor with the other end of the jack to the ground. Since it's an 18v active bass I didn't need a preamp, and I went directly from this to the power amp input on a Fender Rumble 100. I got it sweeping frequency nicely.

I guess anybody else looking for an easy continuously variable frequency tone control need look no further than this simple gyrator.