Baxandall Analysis

[The Electrician]

In another thread, audioguru posted an example of a Baxandall tone control circuit with bass, midrange and treble controls. Also included was a graph of the response of the three controls at full boost, full cut and flat.

An analysis of the circuit performance does not verify the graphs. The main problem is that the treble performance is not as portrayed.

Here is the circuit and graph plus an analysis of the actual circuit performance. My analysis shows more than just full boost and cut. I have shown the responses with each control varied in 10% steps from one end to the other of the potentiometer range.

The problem with the treble response is clearly seen.

Capacitor C2 is too small; with only 1 μF there is a db or so rolloff at the lowest bass, so I changed it to 10 μF for my analyses.

In the next thread I'll deal with the treble problem.

 

[The Electrician]

Note that the designator R4 has been used for two fixed resistors and also the bass pot. I have changed the bass pot designator to R9.

I've attached 3 images of the treble performance.

The first image shows the treble performance with the component values in the original circuit. There appear to be two corner frequenciies. There's somewhat of a shelf between about 70 and 700 Hz. This is due to the effect of C5 in the midrange control.

There is insufficient treble range except at the very extreme ends of the control. This is due to the high value of the potentiometer R7. Its value should be reduced to 25K ohms. The second image shows the result of doing this. The shelf due to C5 is still present.

The third image shows the result of removing C5 from the circuit. The shelf is gone, but the treble corner frequency is too low.

Continued in the next post.

 

[The Electrician]

Let's put C5 back and continue, but with R7 remaining 25k.

The first image in this post shows the result of reducing C6 from 5 nF to 2 nF. This has raised the treble response in frequency and also reduced the "shelf" characteristic.

The second image shows the response of all three controls with R7=25k and C6=2nF. This is much better than the original circuit response. Notice that there has been some effect on the bass and midrange responses as well just from the two changes to the treble circuit.

There is some interaction among the control responses. The "shelf" characteristic of the treble is still there to some extent. This could be eliminated if two opamps were used, with the bass and treble circuit using one and the midrange using another.

I can't help but wonder if the person who designed the original circuit bothered to verify its performance.

Added in edit:

I thought I'd show the response with just R7 reduced to 25k.


The third image shows the response of all three controls with R7=25k and C6=5nF. The "shelf" is rather pronounced and the midrange max and min are reduced.

 

[audioguru]

Thanks for fixing it, Electrician.
I didn't test it and i did not build it because my sound system is accurate and usually I do not use tone controls.

 

[The Electrician]

Thanks for fixing it, Electrician.
I didn't test it and i did not build it because my sound system is accurate and usually I do not use tone controls.

I guessed you didn't design it; it looks like it came from the web somewhere.

 

[audioguru]

I guessed you didn't design it; it looks like it came from the web somewhere.

Many circuit sites copy that old circuit. Popular Electronics magazine published it a long time ago.
I don't know where it began.

 

[MrAl]

Hi,

Nice job there Mr. Electrician! Very well done and well illustrated.
This is the kind of stuff i like to see on the web, some analysis, some circuit improvement, etc.

 

[MD MUBDIUL HASAN]

Very interesting.

 

[audioguru]

I forgot that The Electrician fixed the tone controls circuit 5 years ago. I never updated my copy of the old schematic because I don't use tone controls.

 

[MD MUBDIUL HASAN]

I forgot that The Electrician fixed the tone controls circuit 5 years ago. I never updated my copy of the old schematic because I don't use tone controls.

Please forward it to me.

 

[alec_t]

It doesn't need forwarding: it's in post #1.