SRPP input gain staging Q

[duck_arse]

....  An SRPP circuit cannot operate linearly without a load with low enough impedance to take the difference current and a BC549A has a minimum hfe of 110. which would give a load of 363 K minimum.  This could only absorb a difference of 24.7 µA to go from 0 VDC to 9 VDC.  You need a lower-gain transistor or a lower emitter resistor than 3300 ohms to keep this circuit linear.  Don't forget, 110 is the lowest hfe for a BC549A.  If you have a BC549B or BC549C, the lowest hfe is 200 or 420 respectively.  Even a BC549A can have an hfe of up to 220 which would double the input impedance.

Thanks again for this - in the absence of a low(er) gain bjt in easy grabbing distance, I just flipped the BC549C around (which should reduce it's hfe by around 80-90% iirc?) which 100% worked.

I changed the gain from an additional input stage to a variable resistor on the emitter follower

Well done but I don't see any Emitter follower with variable gain..

I, too, had a reply with question all typed up last night .... but then I scrolled back, just to make sure ....

perhaps a note on the schem to indicate the strange mode of the emitter follower collector connected.


carn the bloods.

[mdcmdcmdc]

Fair point!

[mdcmdcmdc]

Also just want to acknowledge that there's an (undoubtedly useless) ferrite bead in there as a nod to spaceman from whom I borrowed the (now-deleted) gain stage.

[mdcmdcmdc]

Did a vero layout and boxed it up... a couple of small tweaks and it sounds pretty good.

First, I reduced the value of R24 to 1K.

20K for the gain pot isn't really necessary; A10K has a better feel and gets you from a clean(ish) boost to a pretty saturated overdrive. There's not much additional crunch in the back half of the 20K pot, but putting R24 on a switch with maybe a 5K would give you a low gain and high gain mode which might be nice? Maybe you can find an A15K pot?

Anyway, here's a verified layout. I usually put my power filtering on a separate daughter board.

[amptramp]

One of the easy things about taking an SRPP stage from linear to distortion is that you can add a control to do just that.  Have a 1 megohm audio taper pot in series with a reasonably large capacitor like 0.1 µF and place that between ground and the SRPP output which is the emitter/source of the upper transistor.  That lets you vary the load without affecting DC bias and gives you a distortion control.  Low resistance = linear and high resistance = distorted.  Then you could use a high-impedance emitter follower for the output.