minimoog vcf: c4-c7 values & transistor hFE

[reve]

Hi Yves et al!

So after a number of successes building modules of Yves', I picked up a pair of Moog filter PCBs from the Bridechamber.

Being American, I have a lot more 3904s on hand than I have BC547s. I figured I'd just match those, use sockets (reversing them, of course) and if it's not optimal I could just pull 'em.

But after trolling old forum posts, I saw Yves said he generally used the (higher hFE) BC547C in non-VCO designs. But as I understand it, the optimal value of c4-c7 was determined by the hFE of the transistors used in the ladder. Presumably my trannies will have a lower forward current gain, but I did note that Yves didn't specify a suffix for the transistors in the parts list.

So: does this matter? Am I being pedantic? Or should I just match some 3904s, solder in 47n caps and not worry about it?

Thanks for reading!

[iopop3]

I built one of these recently. Put off the matching process for so long that I said, screw it, and just used what I had at home. BC547Bs, which I presume are for the same batch, got them loose in a bag from my supplier. Did match the caps though.

Everything seems to work ok, I do get it to self oscillate and it feels pretty stable when checking via scope.

What impact does the matched transistors have, temperature stability?

[reve]

[varice]

[iopop3]

Thanks for your answers, it does sound logical. I was probably thinking too much about VCOs when it comes to matched transistors and temperature stability.

[reve]

[yusynth]