Question about active baxandall

[juan_felt]

Hi!

I have a question regarding this active baxandall.



If I use it instead of the tone control in an effect pedal, like the BSIAB, is the input side of the IC necessary?

What's the IC doing there? Also, if I don't use it, should I terminate it like this? http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

Thanks!

[GibsonGM]

Hi Juan,

The first IC section is a buffer. It is separating the passive components of the Baxandall from the circuitry that comes before it.  Most tone stacks need this as they tend to have a low input impedance, so require something between them and other things to perform well.

Those resistors, pots and caps will load down the 'source' that is feeding the Baxandall, resulting in a loss of high frequencies and signal level.  The buffer prevents this, by sourcing current from the power supply to feed the pots etc. 

If I were to plug this into a BSIAB, I would keep the buffer.  This has a lower input impedance than what is 'expected' by the BSIAB, and would probably load it down without it.   You can of course use TWO TL081's (or TL071s) if you don't have the dual opamp.

[dschwartz]

The bsiab has its own buffer after the gain stages..you could put that eq with 1 opamp after the buffer.
But..the active baxandall has gain, and the signal at this point is pretty hot.. I'd expect the eq to clip if you boost the frequencies..
A workaround is to attenuate the signal before the jfet buffer.. I don't know if the bsiab buffer is supposed to clip, if so, you'll loose that..

[GibsonGM]

<Couldn't find "BSIAB" schematic, I looked at BSIAB II.  No buffer before the tone control>

I'd try it both ways on the II just for grins....if BSIAB 1 has the buffer, then of course omit the opamp one!   

[antonis]

Some BSIABs have buffer and some haven't but they ALL work on single supply so beware of non-inverting input(s) bias.. 

[GibsonGM]

Some BSIABs have buffer and some haven't but they ALL work on single supply so beware of non-inverting input(s) bias.. 

I've never seen one with a buffer. I wish Juan had posted a link to the BSIAB "1"!    Anyway, if there is already a buffer, you don't need the opamp buffer, of course!  But you should use SOME buffer before the tone control circuitry...   

And as Antonis saw with his good eyesight, you may need to add bias voltage to it as the pedal runs on single supply!   So this must be considered for each stage,at the "+" inputs. 

[juan_felt]

Hi everyone!

Thank you so much for your replies.

I thought I  posted the schematic for the Bsiab (oopss...). I'm actually referring to the BSIAB2, with the following schematic:


I'll try both configurations, the info you gave me is really good.

Another question, I want to use this same baxandall in the following schematic for the Mesa Boogie Throttlebox instead of the tone and mids control:

Should I place it after the second gain stage (the one with the gain pot)? In this case, should I leave the buffer stage in the baxandall, and leave out the last buffer stage (the one before the volume pot?) or should I leave this one as well?

Thanks and sorry for not posting the BSIAB2 schematic..

[Kipper4]

[GibsonGM]

Then it is like I thought!   

On BSIAB II, I would keep C9, insert the bax circuit, and bax out goes to R21...

[antonis]

Just a note on Mesa Boogie diagram..

IMHO, Diode D2 (marked with BY?) should be something like 9V1 Zener..
(and no 1N4148 as suggested at right of scheme..)

[juan_felt]

Just a note on Mesa Boogie diagram..

IMHO, Diode D2 (marked with BY?) should something like 9V1 Zener..
(and no 1N4148 as suggested at right of scheme..)

Then it is like I thought!   

On BSIAB II, I would keep C9, insert the bax circuit, and bax out goes to R21...

Thanks for the replies people! As usual, very helpful.

Cheers,

J

[erio tedesco]

Take a look here, maybe will be usefull for you:

http://www.diystompboxes.com/smfforum/index.php?topic=111382.msg1024325#msg1024325