Non inverting Shelving EQ

[1wildthing]

Hi there, for a build I'm working on, I need to design an active non-inverting low shelf eq band. I need it to have a pot/trimmer for adjust gain (+7db to -7db) and I would like it to work from 600hz to low. I've tried to look on the web and I found a lot of inverting shelf eq or non inverting circuits with fixed gain. I've also tried to face with equations in order to calculate the right parameters but I didn't succeed.
This is what I would like to end with:

Could you please help me to design this circuit?

 

[ruffrecords]

Take a standard op amp based inverting version. Ground the input. The op amp +ve input is likely connected to 0V. Disconnect it and reconnect to 0V via a 100K resistor. Feed your signal into the +ve input.

Cheers

Ian

 

[1wildthing]

Take a standard op amp based inverting version. Ground the input. The op amp +ve input is likely connected to 0V. Disconnect it and reconnect to 0V via a 100K resistor. Feed your signal into the +ve input.

Thanks Ian. Here is the original circuit


and this is the modified version. Could you please check if I have correctly understood?

 

[JohnRoberts]

My head hurts from just looking at that... The beauty of inverting Baxandall topology is flat frequency response when input network and feedback network are equal.

These days with cheap op amps the typical way to accomplish overall non-inverting is to pass the signal through an additional unity gain inverting stage either before or after the EQ. Since, that Baxandal topology has a non flat input impedance that changes with boost/cut setting, the inverting stage before it can also serve as a buffer.

I recall years ago bench testing a competitor's value mixer that used such a tone control circuit topology being fed from an insert point unbuffered. I was able to measure interaction caused by that input Z change with modest/hi Z (600 ohm) signal sources patched into that insert.

JR

 

[ruffrecords]

That is basically correct. You may or may not need input and output coupling capacitors but what you have drawn is exactly what I meant.

Cheers

Ian

 

[1wildthing]

That is basically correct. You may or may not need input and output coupling capacitors but what you have drawn is exactly what I meant.

Cheers

Ian

Thanks again Ian!

 

[ruffrecords]

My head hurts from just looking at that... The beauty of inverting Baxandall topology is flat frequency response when input network and feedback network are equal.


JR

The feedback network is identical. The result is the same as the original except the gain is 6dB in the flat position. You could always add a 6dB attenuator on the +ve input to make it identical.

Cheers

Ian

 

[57sputnik]

When transposed to the non-inverting configuration how can the circuit cut below unity gain? An inverting Bax does.

I would follow JR's suggestion and buffer the input with an inverting stage then use an inverting Bax EQ stage.

 

[ruffrecords]

When transposed to the non-inverting configuration how can the circuit cut below unity gain? An inverting Bax does.

I would follow JR's suggestion and buffer the input with an inverting stage then use an inverting Bax EQ stage.

Mmmmm....... good point and well spotted. Looks like I suffered a major brain fart. Sorry to the OP; please ignore everything I said.

Edit: Although I am sure there is a non-inverting API EQ that uses a single pot to to achive boost via NFB and cut via a pot divider on the +ve input. I will see if I can find it.

Cheers

Ian

 

[JohnRoberts]

The Baxandal EQ requires inverting topology for symmetrical boost/cut.

JR

 

[57sputnik]

John remind us again about your use of soft-limiting diodes in the LF section. Seems like a good thread for it.

 

[JohnRoberts]

even I get bored talking about my patents... The OP is looking for a simple bass tone control.
===
One thing perhaps worth mentioning when trying to use the Baxandal topology for 3 band EQ is that the mid band will interact with the other two bands. By putting the mid band in the second inverting stage we get to flip the polarity back to correct and reduce interaction.

But the OP is not making a 3 band EQ either.

JR

 

[ruffrecords]

The API 553 EQ is the one I was thinking of. Schematic attached - this has three bands and the OP only needs one of them but the principle still holds.

Cheers

Ian

 

[abbey road d enfer]

Google swinging-input EQ.

 

[1wildthing]

Thanks everyone for the suggestions. The baxandall seems to be an easy go solution indeed. I found a suitable design (using the bass band only):

Do you think that an inverting stage after this circuit could work? May be something like:
Regards

 

[abbey road d enfer]

Do you think that an inverting stage after this circuit could work? May be something like:

Yes. And no.
In principle, an inverter placed after OR before a Baxandall provides the polarity inversion that is needed for respect of absolute polarity.
But the inverter you submit is one that's made for single-rail power, where AC voltages are DC-shifted by about half the rail voltage, when the Baxandall circuit supposes bipolar rails.
You have to go one way or the other, single or dual rail.

 

[1wildthing]

Yes, my fault. I didn’t notice the power rail structure.