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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

(http://i90.photobucket.com/albums/k243/lenny1dog/P7160001.jpg)

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 ?

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.

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.

any luck?

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.

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

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

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

Quote from: Ice-9 on August 19, 2009, 03:37:53 PM
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?

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

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

Quote from: HarryR on January 10, 2011, 05:41:13 AM
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.

Quote from: Thomeeque on January 10, 2011, 06:07:13 AM

Quote from: HarryR on January 10, 2011, 05:41:13 AM
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.

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 ?

Quote from: HarryR on January 12, 2011, 11:04:01 AM
I compared the circuit to others which are similar,
but i can't see a reason for R3 and R12 ..

R7 and R12?

Quote from: HarryR on January 12, 2011, 11:04:01 AM
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 (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.

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

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

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.

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"

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.

Quote from: aron on January 12, 2011, 09:24:45 PM
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.

No problem Aaron, I still have my Charvel model 4 which i've had from new, so if you have any questions about the active circuit ask away, It was my first "real guitar" and my dad bought it for me before he passed away, it's just one of those guitars that i would never sell.

> IC is a LF442CN

LF442 is, for all practical purpose, the grand-daddy of the TL072.

TI figured out how to get LF442 performance better and cheaper (smaller die area). TL072 soon became THE most popular BiFET dual opamp. LF442 still exists for some legacy and military applications.

"CN" is commercial grade plastic DIP. (LF442AMH selected Military Hermetric can is still available.... for $23!)

i had a charvel/jackson guitar.. an it has different booster.. i doesnt have IC but tansistor.. sk30a i think.. i took the booster because i dont really like it  :icon_rolleyes: i will yours ice-9

Quote from: PRR on January 13, 2011, 10:22:13 PM
> IC is a LF442CN

LF442 is, for all practical purpose, the grand-daddy of the TL072.

TI figured out how to get LF442 performance better and cheaper (smaller die area). TL072 soon became THE most popular BiFET dual opamp. LF442 still exists for some legacy and military applications.

"CN" is commercial grade plastic DIP. (LF442AMH selected Military Hermetric can is still available.... for $23!)

I believe in the this case the LF442 was used beacause it has very low power needs making it ideal in a guitar where you want a long battery life. I have had a batteries last for over a year in this guitar, and that was in the days when I used to gig 2 or three times a week and also jam and rehearse.
Wow $23 ! If it needs replacing i know what I will use (TL072) Then again i have just checked digikey and they sell LF442CN for $1.60

Hello everybody,

i'm in the mood to discuss  :)

My aim is to get something like the (in)famous E.C.-midbooster,
that's why i was looking for a diy-circuit and why i found this forum ...

I analyzed the "charvel" circuit (thanks for the hint - LTSpice will be one of my favourite
tools in the future !!!).

Now im thinking about what i'm tryin to do and what a midbooster really is.

So, here's what i wanna discuss my friends ...

The frequences on my guitar go from 82Hz up to ~660 Hz
(excluding the harmonics now ..), where is the "mid" of our boosting ?

Slacker wrote
"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."

2,3Khz and 7KHz are far above everything i will find on my strat (again : no harmonics discussed)...

As i first heard about midboosting, i was thinking about boosting  the d- and g- string (frequencies) ..

So, you will ask yourself, "what the hell does this guy" (me) really wanna know !?

Hmm, difficult to answer ... think about me as one who want's to design a midbooster
not knowing what i'm going to boost ...

Is a "midbooster" just a nonlinear amplifier with bandpass characteristics ?
What is the real "gimmick" on E.C.'s midboosting ?
Is a wha-wha doing the same, when leaving it alone in a preferred position  ?

Okay, enough questions for a good discussion i think  ...
and please remember - it's not for the discussion ...
i just wana build something to get a good sounding guitar ..  :)

Greets from
Germany

Harry

Yo,
   I love to build also but check this:  http://cgi.ebay.com/New-FENDER-Eric-Clapton-Mid-Boost-Kit-/300516677603?pt=LH_DefaultDomain_0&hash=item45f83097e3
I installed one in my Am Std Strat. Even without using the boost, the frequency response is expanded from any passive tone control due to the improved design using a ganged pot.
Enjoy,  M.O.

Thanks for the reply familyortiz ...  :)

I will build it myself definitely .. because:

1) i'm building my own guitar including the electronics,
think i'll do it only once in life, so it should fit perfect to my needs
(or name it visions ..)   :)
2) i don't like tbx don't really know why.
3) the plan is :
3 singlecoils , wired through a brianmaylike wiring (3xpu on/off 2x phaseswitch, 1x serial/parallel-switch)
one volume- , one treble- , one basscontrol.
Then comes the midboost, which i can shortcut so it just boosts only (3dB).
the shortcut will be a pushpullpot so i can activate "prepositioned" midboost ..

See it a a classic strat, the switches expand the mixing of the pu's
the basscut expands the tonecontrol
the boost "eliminates" cabelcapacities
the midboost expands you know what ....
but it will sound like a "normal" 5wayswitched strat (maybe tele when bridge/neck activated) if i want it ..

And if i don't like it, no problem, i'll install a usual
5waysw 1vol 2 tone pickguard so only the
guitarhardway (body, neck, trem , tuner) is selfassembled the way i like it


greets Harry

PS.: Anyway, i'd like to know your opinions about "what is the trick with E.C.-midboost" 

I'm reposting the schematic for the Jackson JE1200 as it is one of the many images that went missing from my posts when photobucket changed its rules.


First the original image from the first post.


(https://i.imgur.com/rLSOpHX.jpg)


The second image I have redrawn in an easier format to follow, both are the same.

(https://i.imgur.com/js6uJhm.jpg)

Thank you!

What happened to C4 R6 and R12 ?

Quote from: PRR on August 12, 2018, 01:15:42 PM
What happened to C4 R6 and R12 ?

:icon_redface: Woops. I will re do that

Thanks to PRR noticing I had missed out C4, R6 and R12 from the second schematic, so here is a fixed circuit.

(https://i.imgur.com/2zEeVvU.jpg)