active bax EQ using tubes

Here is a schem I drew yesterday. I was planning on using it in a bass gtr pre-amp for my brother. I haven't yet calculated the filter cap and resistor values. That's why they all say 10k and 10nF.


I got the idea from here. http://headwize.com/projects/showfile.php?file=equal_prj.htm
Here is just their schem.


All i did was replace the opamps with the 2 halves of a 12AX7. Does anyone see anything wrong with my version?

What is the output resistance of the opamps?

What is the output resistance of the tube gain stages?

Some things to think about.

Nothing "wrong" with that, but you have to take into account the impedance of the catode-follower and the impedabnce of the output stage. Being different, the amount of boost and cut will be different (less cut than boost) unless you increase artificially the output impedance of the input stage, by inserting a resistor. The overall efficiency (amount of boost/cut) will be somewhat limited by the ratio of these impedances to the load due to the tone networks; the half-impedance of the complete tone networks altogether should be at least 10 times larger than the output impedance.

About halfway down that Headwize page there is a design variant using triodes.

Thanks for the replies. I kinda figured hat the different output impedances were going to affect my calculations in regards to frequencies. I didn't realize it would also cause asymmetrical boost/cut. Maybe it would be better to use the filter network passively and drive it from the CF and use the second half of the 12AX7 to provide make-up gain?

bigugly said:

Thanks for the replies. I kinda figured hat the different output impedances were going to affect my calculations in regards to frequencies. I didn't realize it would also cause asymmetrical boost/cut.

Click to expand...

I just mentioned this but in fact for your application, is it a big problem?

 Maybe it would be better to use the filter network passively and drive it from the CF and use the second half of the 12AX7 to provide make-up gain?

Click to expand...

That's the old way it was done in HiFi preamps, and it has its flaws also (very difficult to really make it neutral with all controls at mid position). It think the best solution would be to drive both sides of the tone control stack with CF's; you would add a CF after the second triode and drive the output and the tone netweorks from it.
Edit: I should have turned on some more brains before answering: it is not absolutely necessary to have a CF at the output, because this output is under NFB control.

abbey road d enfer said:

Maybe it would be better to use the filter network passively and drive it from the CF and use the second half of the 12AX7 to provide make-up gain?

Click to expand...

like this:

I drew this shortly after your first reply. All the tubes would be 12DW7 using the lower mu section as the cathode followers.

That's still active if I looked well.

Here's a passive B & T section with make-up gain, from an Ampeg (attached).

Regards,

  Peter

sorry clintrubber, I quoted the wrong part of abbey's post. It should have been this.

abbey road d enfer said:

It think the best solution would be to drive both sides of the tone control stack with CF's; you would add a CF after the second triode and drive the output and the tone netweorks from it.
Edit: I should have turned on some more brains before answering: it is not absolutely necessary to have a CF at the output, because this output is under NFB control.

Click to expand...

OK !

Looks like a very promising project, bigugly!