Marshal Jackhammer Posable Mid Boost Mod

[Johnny G]

i have been toying around with the idea of modding my JH-1 for a while now and i figured that the first thing i would try and change is making the frequency contouring controls a little bit more usable. at the moment they only allow you to have a scoop at a frequency of your choise. thats all very well and good but i hate the mid scooped sound and really much prefer going the other way and boosting them

having gotten the JH-1 schem off a very kind person on this forum (im afraid i can't remember names) i started playing around with it in the circuit modelling program i have. after a fair bit of time spent going off in completelly the wrong direction i think ive cracked it but i just wanna make sure that im right and it'll be ok.

this picture is the regular contour and frequency controls.



thesignal is split and goes through the two op-amps. the bottom op-amp has a band pass filter after it which is controlled by the dual ganged frequency pot. the non inverting input of the other opamp is then connected to this bandpass filter meaning that at the certain frequency and increased amount of the signal is fed into it which cuts the gain (im guessing this is some form of differential amplifier)

the amount of scoop is vaied by the contour pot which feeds a variable amount of the input signal into the bandpass opamp.

what ive found is that by removing the 100k resistor in series with contour pot, it is possable to send the output signal into the bandpass opamp and create a variable boost. im guessing theres some funkeh stuff with phase differences here but im not too sure so if somebody can clarify it thatd be cool.

last thing i thought might be useful is to change the values in the bandpass slightlly. the two 2.2k resistors can be made smaller to allow the frequency to be set higher into the treble end of things. also i increased the value of the cap in the lowpass section of the filter to broaden out the boost a bit.



what i really want is just for someone to have a look at this and just tell me whether it would work or whether id be buggering up my jackhammer

many thanks

JohnnyG

[dudeduck]

i know this is a reeaaaallly old thread but did you find anything?