preamp

[Gus]

This thread
http://www.diystompboxes.com/smfforum/index.php?topic=101979.0
got me thinking about an NPN 9VDC preamp

I simmed the following idea.  Not built but I controlled the gains of each stage and bootstrapped two stages the input and the stage after the tone section.  I picked x10 because of the 9VDC supply.  The loading of stage two and stage four should allow the use of 100k linear volume controls.  It was an idea to use higher hfe NPNs instead of JFETs



Please comment good or bad

[WaveshapeIllusions]

It looks prerty good to me. I suppose it's meant to distort a bit? With a gain of ~10 on each stage it'll overdrive pretty well. I suppose with the volume pot and the insertion loss of the tone stack it'll keep it from getting too crazy. Looking at it for bass, I'd scale the gains down to make it cleaner, but I'd say it's perfectly sensible for guitar.

My only disagreement would be to scale down the impedances in the tone stack. For a tube amp, those values make sense, but with the lower output impedance of an emitter follower you can bring the pot values down. That's actually a thing I have against a lot of the JFET conversions too. One can scale the resistances down quite a bit, as it will help with noise.

I'd say I like it. BJTs are quite a bit more consistent. They do distort pretty nicely on their own too. With the gain scaling, I think you'll get a nice smooth OD out of it. BJTs are mostly third harmonic, right? That could be pretty close to what comes out of a push-pull output stage. Was this based on any amp in particular? You linked the Bogner thread, so I'd assume it's related?

Thanks for posting this idea.   BJTs should be a lot simpler to use than JFETs. No worries sbout trimpots to set bias. Just need to test to see how it sounds.

[J0K3RX]

It is interesting and will probably work. Not sure how it will sound but worth a try on the bread board!

[ggedamed]

Thread necromancy: I breadboarded it with BC550Cs and it works. Because of drawer parts availability it ended like this:



Don't expect to use it for its distortion, though. I definitely prefer JFETs and CMOS inverters for that.