Help with simple Baxandall tube eq?

[skidmorebay]

I am thinking about building a Baxandall-style stereo tube EQ based on this simple circuit that I found in the book "Valve and Transistor Audio Amplifiers."
Could somebody help me to figure out how I could modify the circuit to incorporate switchable EQ points at:
74, 84, 98, 116, 131, 166, 230, and 361 hz in the bass, and
1.6, 1.8, 2.1, 2.5, 3.4, 4.8, 7.1, 10, 14, and 18k in the treble (basically mimicking the Dangerous BAX eq frequencies for the most part)?

Wouldn't this just be a matter of putting a number of different capacitors on a rotary switch at the correct point in the circuit? (My first eq project....)

thanks!

 

[ruffrecords]

That is a rather unusual Baxandall circuit in that the both capacitor and fixed resistor values on either side of each pot are the same. Does the article also give response curves?

To answer your question, basically yes, just change the values of C3, C4, C5 and C6.

Cheers

Ian

 

[skidmorebay]

Thanks for the reply.
No, the article doesn't give response curves. The schematic was apparently adapted from Baxandall's original article in "Wireless World," October, 1952. I think it is meant to be a tone control circuit for a home "hi-fi" system of the day. It is said to offer lower distortion than a passive tone control with make up gain.

Do you know of a resource that would help me determine the values of the capacitors for the EQ points I desire?

Also, wouldn't high-voltage DC be present at the output? From that point on the schematic it seems like an output capacitor is needed, if I understand correctly.

I was attracted to this because of it's simplicity. I thought it would be a good first point-to-point wiring project for me to take on.

 

[ruffrecords]

Try this for design equations:

http://sound.westhost.com/dwopa2.htm

Remember, Google is your friend.

The output does need a dc blocking capacitor and it needs to be fedd into a high impedance input.

Cheers

Ian

 

[skidmorebay]

Perfect! The answers are all right there.
I have learned a ton in the last day of research... Small Signal Audio Design by Douglas Self has lots of information on the circuit and it's variants.

 

[ricardo]

Use the pot & resistor values as in your original diagram but put the caps as in Rod Elliot's fig12.

Rod's circuit gives a shelving response .. which is what you want if you are going to swap capacitors.

The original Baxandall configuration gives a sliding boost which changes the frequency where the boost/cut starts as you twiddle the knob.

I suggest you make an LTspice model of Rod's circuit with your pot & resistor values.  Use this to choose your caps.  You'll find you don't need so many positions.  Much more accurate than using design equations.

 

[abbey road d enfer]

That is a rather unusual Baxandall circuit in that the both capacitor and fixed resistor values on either side of each pot are the same.

The topology where the values are different (generally in a 1:10 to 1:20 ratio) isa James EQ, which is the passive topology often mistaken for a Baxandall.
The Baxandall is NFB-based, and as such is symmetrical when the design calls for symmetrical boost/cut values.

 

[abbey road d enfer]

The output does need a dc blocking capacitor

Yes.

and it needs to be fedd into a high impedance input.

This circuit probably wants to see about 100k; if it was a James (passive) it would want to see at least 1 Meg.