low THD guitar preamp

[Antonix]

I have to project a tube preamp for acoustic and jazz guitar.
I have studied several schematics from various brand like Fender, Marshall, Peavey etc.

All this brand have circuits with never NFB and THD near 5-8%.

I whant to obtain low distortion by no global NFB but only local NFB.

I have thought something like this:

 

[ALEKSANDR]

Hello!

If you don't mind, I would like to ask you what soft you used to draw the schematics.

As for the answer, I've built a similar circuit but using only one ECC83 tube (I can see you use one and a half).

ALEKS

 

[kevinkr]

Your design looks pretty reasonable, however you need to change that 820 ohm in the first stage to something much larger, even with bootstrapping the input impedance will not be that high.

You probably should also take the output directly off of the cathode in your cathode follower, if you are concerned about rfi or external capacitance interacting with the CF, best to put a 1K resistor in series with the output connection. This will eliminate external load interaction with the bootstrap..

 

[Antonix]

Hello!

If you don't mind, I would like to ask you what soft you used to draw the schematics.

As for the answer, I've built a similar circuit but using only one ECC83 tube (I can see you use one and a half).

ALEKS

I have used Orcad/pspice for schematics.
I have used 2 tubes 12ax7 because I have to add a second input to the first stage with a separate gain control.

Your design looks pretty reasonable, however you need to change that 820 ohm in the first stage to something much larger, even with bootstrapping the input impedance will not be that high.

Yes you have reason, I have to change the 820ohm.

You probably should also take the output directly off of the cathode in your cathode follower, if you are concerned about rfi or external capacitance interacting with the CF, best to put a 1K resistor in series with the output connection. This will eliminate external load interaction with the bootstrap..

[/B]

What you intend to say for directly off of the catode ?

 

[kevinkr]

Yep, directly off the cathode is what I meant, don't neglect the series resistor though, (min 220 ohms, I'd use 1K) it helps with rf immunity and will keep things stable in the presence of large amounts of external capacitance.