Adding a mid control to the Boss FA-1 Preamp schematic...

[MakerDP]

In the schematic there's some voltage division after the last "preamp" stage. That could be replaced with a "T filter" to control mid frequencies for similar losses. So the mid control would be it's own entity instead of trying to kludge it into the other tone stack design. The James stack values would need to be modified to give a small mid bump when set flat so that the added T filter wouldn't sound like only additional mid cut after the stack. I think it could be done with a small amount more signal loss than the stock circuit.

Thanks for the helpful reply Chuck. Yes, I considered adding a bridged-T mid control but I didn't know where I would put it. However, after I saw that Carvin mid control I figured it should be rather trivial by simply replacing a single resistor with a pot - something easily done on the pcb I order. The only thing that has my head scratching is in the Carvin design, the wiper goes to ground where there is normally just a junction but that junction is continued on to the NFB loop of that last op-amp stage.

If I did go with the bridged-T I don't think signal loss would be much of a concern. There is plenty of gain available in this circuit I think.

[Dave H]

I really hate to disagree with all the experts here but this IS a James circuit. Want proof? Look at pages 48-50 in this article.

That article shows the James is a passive tone control circuit. The Baxendall is a negative feedback tone control as was the first one in your OP.

Baxandall.pdf

[Enzo]

I am only skimming this thread, but whether passive or active, are not the two approaches similar in the circuit of the passive components? And so regardless of the name on it, would not implementing a mids control be done in similar fashion vis a vis the controls?

[Dave H]

but whether passive or active, are not the two approaches similar in the circuit of the passive components? And so regardless of the name on it, would not implementing a mids control be done in similar fashion vis a vis the controls?

Similar but different You'd have to be careful with the component values and how the bands are summed. The Baxendall is a virtual ground feedback circuit and uses linear pots and is symmetrical wrt capacitor values. The James uses log pots and needs different capacitors.

[Enzo]

DEtails details... But the basic form of the circuit? I think if I read correctly you already determined turning a resistor into a pot effectively did what you desire?

[J M Fahey]

There are many differences between them.

1) Baxandall is active.

Pots are linear, because when set to "5" one half is the NFB resistor, connected to a gain stage output, and the other half must also be same value , connected to signal input, and stage gain is 1.

Not only both pot halves must be symmetrical (same value) but also caps on both sides, and any auxiliary resistor on each side must also be symmetrical, again so the tone control is flat on 5.
Symmetry must be maintained on treble and bass controls.

Difference is that Bass control is shorted at mid/high frequencies by symmetrical high value capacitors, so it varies gain on Bass only, and works as a Bass control.

Treble control is similar, but uses small value caps in series with each end, so it varies gain only on high frequencies and works as a Treble control.

2) the circuit posted by Loudthud adds a third linear pot, same value as Bass and Treble, partially bypssed with a largish capacitor but of course smaller than the value used for Bass, and has series capacitors (or just 1 in series with wiper) , small but larger than used for Treble control.

The combination varies volume in the Mid range , itīs a compromise but works acceptably well.
The full set of formulas is availble on the Linear Applications Handbook.

Almost forgot: wipers are joined by mixing resistors (at least 1) and are fed to the -IN of the gain stage, which may be the -IN of an Op Amp, a tube grid, a bipolar transistor base or a Fet/Mosfet gate.

3) the James control is passive, and works even without any gain stage involved, of course it introduces important loss.

It uses two Logarithmic "volume pots", one bypassed by large caps so it works on Bass frequencies, the other fed from small capacitors, so it works on high frequencies, sadly there is NO practical way to add a Mid control.

The article posted shows the first version, then it evolved into using standard Log pots so on "5" signal at wipers is 10% of input , so -20dB down, since in such a Log pot resistance on 5 is 10% of full value, it also requires that impedance ratio between topside and bottomside caps is also 10:1, so typically the top cap (whether itīs Bass or Treble) is 10% the value of the bottom one.

So on James again flat is on 5, but attenuation is 20 dB, so unless you have excess gain, it will need a 10X (+20dB) gain stage inserted..

"Boost" is actually achieved by attenuating less, and cut by attenuating even more.

4) not sure about the simulation tested on TSC, since it can only simulate James , not Baxandall, and of course with James unsymmetrical values and Log pots, not Baxandall with Lin pots and symmetrical values.

[pdf64]

As I mentioned, when I fiddled around with the R3 value in the TSC James template, it didn't seem to affect the mid range per se, ie I couldn't re-create the report in post #1, that TSC was used to identify that a 50k pot would be a good option.

[J M Fahey]

You might tweak the passive James tone control in TSC, but that wonīt apply to the Boss pedal which uses a different, active circuit.

Here we are stuck with the F1, since the OP has already bought an aftermarket PCB for it.

Itīs tone control is quite the classic Baxandall, Loudthudīt too, with a twist, but basic same structure.

I suggest the OP builds it as per the Boss schematic, then *adds* a 50k midrange pot (half the 100k Loudthud value), scales everything else also to half impedance, meaning 3k6>1k8 ; .005>.01 ; .02>.047 ; he will end out with something very close to the Lt on, with a midrange control working around 1kHz.

He the may shift it up or down, to 2kHz or 500Hz by halving/doubling caps and pick the one he likes best.

He may even add a rotary 3PDT switch and choose any of 3 frequency values , will end up with a very versatile tone control.

[Chuck H]

I worked on a possible solution this morning.

You can do this. The Boss TS already has a good mid bump so there's no need to redesign one into it. This added bridged T filter has a loss of 1.5dB when set flat and a mid cut capability of 18dB averaging around 650Hz (the center shifts from 500Hz at low cut settings to 800Hz at high cut) Which is an appropriate center for a guitar amp. The dual ganged pot circuit prevents the HP filter artifact of the bridged T. The second gang is wired inverse. That is, it acts as a high cut circuit as the mids are increased. Use a 10% pot if you can possibly find it (Bournes perhaps). A little complicated to built right on the pot. There are ways around that though.

[Chuck H]

One concern I have is that while you site the BOSS preamp has having plenty of gain, I fear you are misinterpreting gain with clipping distortion. They are often related but are exclusive properties. We don't have a schematic of the Champ 25 to look at. Not to mention a certainty of how you plan to connect the dots. If you plan to feed a typical guitar amp phase inverter tube with the BOSS preamp I don't see how you'll have enough voltage gain to drive the amp to full power.

EDIT: I found the schematic and it looks like you'll be fine with the BOSS preamp.

http://ampwares.com/schematics/champ_25_se.pdf