Charvel active mid boost schematic JE1200

[Ice-9]

I was just doing some repairs on my guitar and decided to take out the Active preamp and draw out the schematic in case anyone wants to look at it.
The guitar is a charvel Model 4 and has the jackson JE1200 mid boost preamp in it and can be broken down into 2 main blocks, the first being a buffer stage with the volume pot at the end feeding a mid boost which looks like its altered using a pot as a mixer . When the pot is at one extreme its taking most of the direct signal from the 1st stage but at the other extreme is taking most of the signal from the active filter stage.
there are a couple of points about the circuit that I would also like to ask questions about if anyone would like to chip in with an answer.

Oh and the IC is a LF442CN



The first stage looks like a standard non inverting op amp stage very much like we see in a lot of effects pedals, one thing I'm not sure about with this stage is R7 (why is this resistor there and going to Vb when C4 and R6 are going to ground) in all the circuits i have seen the inverting input usually goes to ground OR Vb through a R/C network. Not both

the second stage again is a non inverting op amp with i think a highpass filter based around C9/R13 followed by a low pass filter based around R14/C10, I am not sure if VR2 is just a mixer/pan pot that in each extreme takes the signal from either the output of the volume pot or from the output of the second stage , or does it introduce some positive feedback to the input of the second stage ?

[slacker]

Cool looking circuit.

The first stage is similar to the opamp stage in the RAT. You've got sets of boost going on, R7 going to vb gives a gain of 2 for frequencies up to about 7KHz where the filter made out of R5/C5 starts rolling off the response. C4/R6 then give frequencies over about 2.3KHz a gain of 3.2, so you get a peak in the mid frequencies.
You could connect C4/R6 to vb instead of ground it would work the same.

I simulated the circuit and the Mid boost pot does seem to have an effect on the response so it might be adding some positive feedback. How much effect it has seems to depend on its value, the bigger you make it the less effect it has.

Might be worth building to see what it sounds like.

[Ice-9]

It's the preamp that is in my guitar, but i think i will breadboard it to see what difference componet values make in the mid boost and see if i can use this filter in some kind of overdrive/dist pedalif it sounds good.

[EARNEST]

any luck?

[Ice-9]

I breadboarded the preamp using a 4558d opamp instead of rhe one used in the JE1200 , the preamp works fine and the mid control is very much to my liking. I didn't do anymore work to make it a dist/OD effect but a simgle opamp and diode clipping circuit should give good results, or possible a Fet Mu amp section  would give good results.

[Ben N]

Might make a nice switchable boost in front of a Dist+/OD250 type circuit, as a rhythm/lead kind of thing.

[EARNEST]

hm...i am building TS808 now, not sure if i should add similar mod to that pedal, to get EXTRA mid cuts/boosts

[thecustomluthiershop]

hello, I have a problem with a jackson model 6. The mid boost dont work well and I dont know why.
I think that another owner of the guitar made changes in the preamp.
I can facilitate some pics . Im grateful of help in this email address: info@thecustomluthiershop.es
IMG_0963
IMG_0964
Thank you

[Gordo]

It's the preamp that is in my guitar, but i think i will breadboard it to see what difference componet values make in the mid boost and see if i can use this filter in some kind of overdrive/dist pedalif it sounds good.

What'd you end up using for VR3 and R2?

[Ice-9]

A little late to reply but R2 is 220K and VR3 is 50k

[HarryR]

Hi everybody,

i'm new here and first of all i wanna
say hello to everybody here 

I hope someone reads my request,
although this thread seems to be finished.

I found this thread as i was looking for a midbooster circuit
which i wanna build into my futurly being build guitar.

You see i'm in the stage of planing,
the circuit seems to be what i was looking for.

Maybe it's because i'm from germany ..
i don't understand where those two arrows
with "VB" lead to ....

If someone would be so kind to tell me where they have to be connected to 

Greets and thanks in advance

Harry

[Thomeeque]

i don't understand where those two arrows
with "VB" lead to ....

If someone would be so kind to tell me where they have to be connected to 

Hi Harry, most probably to the R3/R4 joint - source of the V_CC/2 reference voltage (analogically to the first stage)..

Good luck, T.

[Ice-9]

i don't understand where those two arrows
with "VB" lead to ....

If someone would be so kind to tell me where they have to be connected to 

Hi Harry, most probably to the R3/R4 joint - source of the V_CC/2 reference voltage (analogically to the first stage)..

Good luck, T.

Yes they go to the junction of R3/R4 and provide a bias voltage to the opamps.

[HarryR]

Hi,

thanks for the really quick answers.

I compared the circuit to others which are similar,
but i can't see a reason for R3 and R12 ..

Why is the inverting input connected to Vb ?
Does it control the rate of amplification ?
What would happen if they would be left ?

My understanding of a midboost is/would be:

Buffering in first stage with no further amplification (only limiting loss by cable capacities),
then iltering mids ín the second stage with a gain of ~ 20-25dB

Mixing via VR3: pur signal from stage 1 means just buffering
full signal from stage 2 means a boost on mid frequencies + buffering lw and high freqs ...

Sorry for those maybe silly questions,
but before i start to build the circuit i wanna understand (and maybe change) what it does ..


Greets Harry


PS.: I built an excellent buffer with OPA 37GP, does anybody know an opamp with "two OPA 37GP"
in one DIP ?

[Thomeeque]

I compared the circuit to others which are similar,
but i can't see a reason for R3 and R12 ..

R7 and R12?

Why is the inverting input connected to Vb ?
Does it control the rate of amplification ?
What would happen if they would be left ?

http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifier

E.g. for the first stage:

R_1@WIKI = R₇ || (X_C4 + R₆)
R_2@WIKI = R₅ || X_C5

T.

[Thomeeque]

 Btw. whole circuit could be beautifully emulated by LTSipce emulator (AC analysis)..

[HarryR]

Man (and women) you are so quick 

Sorry, R7 and R12 ...

I will follow the links, thanks for the answers ..

Any suggestion for the opamp ?


Greets
Harry

[Ice-9]

Oh sorry, I never realized that I didn't label the opamp when I originally drew the schematic. I remember it was a low power consumtion opamp (can't remember the code something like a 2262) but anything like a 4558 or Tl072 will suffice. I will check my guitar when i get home for proper chip identification.

[Ice-9]

I've just looked back at my original post and although i didnt mark the chip on the schematic i have named it in the text of the message


"Oh and the IC is a LF442CN"

[aron]

Cool! I missed this. I have a model 4 - Charvel. I need to check if this is the same preamp. If so, thanks!!!! It will help me with fixing some problems it has.