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

[MakerDP]

I just bought a Fender Champ25 amp for only $80 and I think I want to gut it, basically reuse the tube output section more-or-less-as-is and build a solid state two-channel preamp into it.

I am considering the Boss FA-1 Preamp as the "clean channel" input and possibly as the "lead channel's" EQ section, post any overdrive circuit I end up using, then possibly a BBE Sonic Stomp circuit at the very end of everything before it hits the tube output section.

My question is this... I want to add a mid control to the tonestack, which is of the James variety. Here is the circuit I will be working with... http://www.8bitsindgenug.net/BossFA_1.png

I found a simple implementation of a mid control added to a James stack in the Carvin Nomad Bel Air here... http://www.carvinmuseum.com/pdf/amps...r%20REV-E1.pdf

In the Carvin, the wiper of the mid control is going to ground, but the resistor I would replace with the pot (R9 from FA-1 schematic) looks to me like it's also part of the feedback loop of the output buffer stage (please correct me if that is not what's actually going on.) Would I replace R9 with my pot, connecting pin 1 to the VR2/C7 junction, the wiper to the IC2pin1/C9 junction and leave pin 3 not connected? Or is there another way this should be done?

(I used the Duncan Tone Stack Calculator to determine a 50kB pot would work well here.)

Thanks for looking.

[loudthud]

The Boss circuit conforms pretty close to the classic Baxandall. Here is a schematic from an old National Semiconductor data sheet for the LF353-N. You can probably find it at TI.com since they bought NS.

[SoulFetish]

The Boss circuit conforms pretty close to the classic Baxandall. Here is a link to a Texas Instruments data sheet for the LF353. The Baxandall type Mid control is on page 13.

LF353N PDF Datasheet - Texas Instruments High-Performance Analog - Datasheets360.com

LT, I got nothing in that link...

[SoulFetish]

I got something here..

lf353.pdf

[MakerDP]

The Boss circuit conforms pretty close to the classic Baxandall. Here is a schematic from an old National Semiconductor data sheet for the LF353-N. You can probably find it at TI.com since they bought NS.

Well the Boss circuit is close to a Bandaxall because the James is very similar to the Bandaxall but the Boss is most definitely a James stack.

That link looks interesting and potentially useful in another situation but I need a solution that will match what I posted since I will be using a premade PCB of the FA-1 circuit.

[roseblood11]

What sense does it make to replace a tube circuit with a cheap 9V powered effects pedal? I would build that Carvin clean channel into the Champ chassis. It's really an interesting circuit, esp. for use with pedals, because the EQ can be set totally flat. (I played a Carvin Vintage 33 as my first amp for 15 years). The FA-1 is nice to boost a tube amp, I use a diy clone at the end of my pedal board for that, but I'd never use it as a preamp. It would need a much better symmetrical power supply (with higher voltage) and a high quality opamp. And even then, I wouldn't understand, why anyone would want to do that...
This is my veroboard layout, maybe you could use it as a starting point for a version with mid control:

[MakerDP]

What sense does it make to replace a tube circuit with a cheap 9V powered effects pedal? I would build that Carvin clean channel into the Champ chassis.

Normally I would agree with you 100% but this is a Fender CHAMP 25. The schematics for the Champ 25SE are all over the web but this version doesn't have the mid-shift, direct out or headphone out. It has a solid state clean channel, a solid state dirt channel and a solid state reverb. The tubes are a single 12AX7 with one triode that acts as a final gain stage to boost the signal up to the level the second triode phase inverter needs to drive the 5881 non-adjustable fixed-bias power tubes. I highly doubt a transformer from an early 90's Fender amp has any extra current than it absolutely needs so there is no way the filament supply will handle two more tubes. Besides, I have that ground covered very nicely with my other creations.

I know I could add a filament transformer and easily make an all-tube design, but this is going to be a "tinkerer's experiment amp." I am going to use this for the FA-1 https://aionelectronics.com/project/...1-fet-amp-pcb/ and their Blues Breaker PCB for the overdrive channel. If I don't like how the FA-1 sounds, I can easily remove it and put something else there. Nice modular design that allows for very simple tweaking.

I did something all-tube with a Fender Super Champ XD that had a dead DSP board (very similar topology as the Champ 25 but the preamp section is DSP modelling into 12AX7/6V6). I gutted it and made a 6U8A preamp into a self-split 6V6 output section. That thing is a tone monster! Wow... pretty sweet amp.

Still hoping someone can answer my original question and not just point me to something else they think would work better...

[pdf64]

What do you want the mid control to do?
I tweaked R3 on the James template of TSC and it didn't have much affect on the mids, more of an overall level adjust with treble roll off.
Have you actually tried the Carvin mid control?
Note that most guitar amps tone shaping controls don't cut/boost against a flat response; rather there's generally a treble peaking effect with controls midway, and mid controls don't actually boost mid, rather just adjust the depth of mid scoop.
If you want mid cut and boost, post #2 shows how to do it.

[MakerDP]

OK. Obviously I just need to fiddle with it to see if I hook it up correctly.

I am totally familiar with how a passive tone stack works. This is not my first trip to the rodeo. It works just as I would expect in the TSC if implemented like the FA-1 schematic.

[J M Fahey]

1) The Boss EQ IS a Baxandall which is an active control, that´s why it is the Op Amp NFB..

2) Thanks Loudthud.

No need to get the full LF353 datasheet because the relevant part is already on Loudthud´s image.
It is basically a Baxandall, tweaked to add a third band.

In the original explanation about that design, it said that they had found a good balance betweenn the three, and that "with difficulty" a fourth band could be added, not recommended.
Doubt it´s in the datasheet, I seem to remember it was in National Semiconductor´s Audio Applications handbook, which is is an Audio Design Bible, of course

3) that is a good EQ, can be set flat, is fully symmetrical, only "problem" is that it´s meant for Hi Fi or PA; as is can only be used for Jazz or Acoustic Guitar; it´s the same as plugging into your Home Audio amplifier; for Guitar use you will notice it has "too much Bass" , "too little treble" ; I suggest you copy the Loudthud posted one but double all EQ capacitors, so frequencies fall more into what´s present on a guitar .

No big deal adding a third potentiometer, 2 caps and 2 resistors in parallel with the existing network.

The EQ range and sound will be similar to what you achieve with the classic Ampeg tone stack, which is a true James type.

4) almost forgot, no need at all to use 500k pot for Treble, 100k is perfect and used by everybody.

5) all pots are linear.

6) amp sound will be "mild" , "flat" , in fact about what a Polytone amp will give you, just with a Tube power section.

Not bad at all if you get your grit from a previous pedal.

[SoulFetish]

1)

No need to get the full LF353 datasheet because the relevant part is already on Loudthud´s image.
It is basically a Baxandall, tweaked to add a third band.

.

But then the wouldn't get all this stuff. Riveting stuff.

[Enzo]

No no no. Don't rivet, solder is still preferred.

[J M Fahey]

I still prefer:



EDIT: sorry , I posted the wrong Rosie

[Chuck H]

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.

[MakerDP]

Well this thread is completely derailed... 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. I don't know... Other than the two resistors between the output line it seems pretty much identical to me. Those are there to shift everything down in frequency. In fact, if you replaced those two resistors with a potentiometer it becomes a useful shift control.

http://www.americanradiohistory.com/...ld-1949-02.pdf