CCS at plate? Unknown schematic part

[karsten21]

Dear clever folks,

I often see this schematic part used in music equippment like compressors, EQ, preamps ...Sometimes 12AX7, sometimes 6N1P or other similar tubes.

It looks for me like a CCS but I do not realy know how does this works and what is the benefit of using this. Does somebody can explain me this part please?

Thanks a lot!

Best regards
Karsten

[SY]

It's an emitter follower, with the output fed back to the grid.

[DF96]

It is basically anode follower type feedback, but with a sand follower to avoid loading the anode circuit too much. Input impedance 5k (seems rather low!), voltage gain 4.7ish inverting, fairly low output impedance.

[karsten21]

Hey,

Thanks for the hints. So the Sand ( Transistors ) are only to get an low impedance output and less load to the Anode.
Input impedance is quite low; I saw this too. But the circuit is allways driven by OP-Amps.
The feedback reduce the Gain and ( unfortunately ) the harmonic distortion ? For that I wonder how this "Sand-box" is using tubes if the special sound ( harmonics ) is suppressed.

( Sorry for by bad english.. )

Regards
Karsten

[DF96]

So this circuit comes from an FX box? If people want distortion, then omitting the sand and loading the anode down would give plenty of curvature. As it is, the circuit is more likely to be a fashion statement. Unless overdriven it will have fairly low distortion so you should not be able to tell the difference between this and an inverting op-amp.

[karsten21]

Hey DF96,

This schematic part came from TL-Audio and similar from SWR Bass Amps. As I saw the feedback resistor, I thought similar like you: More or less a fake of tube tone. Especial that all other important parts are "Sand". Compressors are always build using these old LM13600 OTA´s. Pre-Amps are simple TL072 and so on...

Nevertheless, I try to build it and measure arround..

Thanks a lot for your interest and tips!

Regards
Karsten