Looking for advice building an active BMP tone control as an onboard preamp

[Mr. Lime]

Somewhere I saw a circuit with a BMP tone section in the feedback loop of a op-amp gain stage to archive boost and cut options which I found pretty useful.

For my new tele project I would like to build an onboard preamp with such a tone control as it only needs one tone pot.
The frequency ranges are similar to a James tone stack.

Craig Anderton has designed a preamp called Clarifier which is a active James TS and I would like to replace it with the new BMP TS.
Jack Orman wrote an article about the presence control which is an modified BMP TS where a additional pot is used to control the mids.

I would like to have this additional pot in form of a trimmer too. Can I simply change R1 to a pot? Which value would make sense?
Do I need a buffer infront of the stage or at the output?

How about R4 & R2?
Can I ditch those resistors?

The +9V/-9V power supply is fine for this circuit?

Down in the right corner is my drawing.

I would be glad if someone could give me some suggestions..
Thanks a lot!

[blackieNYC]

Have you seen the geofex onboard preamp/buffer?  6v in two coin cells. Instead of two 9v batteries.  Can it be made to make up for tonestack gain loss though?

[GibsonGM]

Why couldn't this be hacked to run from one 9V? 

[Mr. Lime]

It runs on one battery, but the - pole is not connected to ground.

[GibsonGM]

?  I don't get it, Lime....usually, you would have the opamp set up to take a bias voltage (1/2 supply voltage), so you can use only one battery.   A 2nd battery without the "-" connected sounds like it is not connected at all. 

[Mr. Lime]

I would connect "+9V" with the positive pole and "-9V" with the negative pole which equals a +/- 4.5V power supply.
Using two batteries would give you +/- 9V which is 18V total headroom, or do I get that wrong?

A IC7660 could give you a +/- 9V supply too using just one battery, but it won't last long.

[GibsonGM]

Oh, yes, I see what you mean.   That looks like it will work. You will not have as much headroom as in a "typical" circuit, but you don't need it as you're using this before any effects input, and it's not intended to give very much "drive", anyway. 

[Mr. Lime]

You're right, drive is not it's job.
I'm still wonding if I can simply change R1 to a trim pot?

If there is really no need for an additional buffer, I would use the second stage of a TL072 OP-amp for a piezo preamp.
Any suggestions how to mix both preamps then?

Thanks

[GibsonGM]

Yes, you can make R1 into a trim pot (on your LT spice schematic, right?)

If you are using a dual opamp, you could do what you are asking several ways. 

You can make a basic preamp from one section, and then feed the output to this active tone control, yes.   Although that may be a bit redundant.  This circuit could be redesigned - you can add some gain with one opamp stage, then run it through the AMZ circuit, and into a buffer using the 2nd stage.   

Preamp  >>> AMZ tone control  >> buffer

OR, if you prefer,

Buffer >> AMZ tone control >>  gain stage (to add more level to the output)

Both of the above ideas are a little more simple, regarding how to power the circuit.  I'm not sure which one is better, from the viewpoint of noise floor, impedance issues and so on.

I think simulating this in LT Spice, or just experimenting on the breadboard, is in order!    It is a good idea, and with some time building and listening, you will get what you want

[ashcat_lt]

I would connect "+9V" with the positive pole and "-9V" with the negative pole which equals a +/- 4.5V power supply.
Using two batteries would give you +/- 9V which is 18V total headroom, or do I get that wrong?

A IC7660 could give you a +/- 9V supply too using just one battery, but it won't last long.

The way you've got it drawn actually does imply that it is 18V total:  +9 at the top, -9 at the bottom, and 0V in the middle.  The distance between 9 and -9 is 18.  You should either show it as +4.5 and -4.5, or just draw in the battery itself.  

I don't think you want the top of R5 connected to the output there below C2, that is definitely not going to work out well.

You really only need one of C7 or C1. The 2u2 isn't actually changing the total value much, so could just be removed.

An input cap would be nice just for "playing nice" purposes, though I guess if it's onboard, you can be reasonably sure of what is feeding it.  You might want a  1K or so current-limiting resistor before C3 (on the right hand side of the dot).  A pulldown resistor after C3 would probably be nice, too.

As for blending with a peizo system - assuming that the peizo is going through another active stage with reasonably low out-Z you could probably get away with a simple pot for a mixer, maybe followed by a buffer/make up gain stage, but it would work better with a dual-gang style blend or pan pot.

[Mr. Lime]

Thanks for advice, GibsonGM and ashcat_lt!

The connection of C2 was a drawing mistake and is now removed.
I have a 12k blend pot laying around which should suit the situation.

I would like to keep the single tone/gain stage for the magnetic pickups and the split supply as well. Somehow I like this arrangement.
For the piezo part; there's an input buffer, AMZ's "Stupidly Wonderful Tone Control" and a recovery stage with adjustable gain from 1-11.
Body, Tone and Gain should be trimmers for fine tuning. An On-On-On switch allows to turn off either the piezo or the magnetic circuit.
In middle position both are active. If the piezo part is shunt to ground, the blend pot acts at the second half as a volume pot for the magnetic pickups.

I know it's not the best solution but for a Tele type of guitar it's hard to create a useful control layout with so many features.

I'm not sure about the output stage which is an inverting summing circuit instead of a buffer.
The piezo circuit is fine so far?

I'm sure I forgot something or did a mistake..  

Again thanks for help!


By the way, here you can take a look at the body:

[Mr. Lime]

No opinions on this?

Everything looks fine, even the piezp circuit?

Just would like to get an educated "okay" before I go for it and build the preamp..

Thanks