First thanks to pf_fan for his encouragement and pictures, and a special thanks to the nice person on the EMG forum for providing some nice hi-res pictures of an SPC and EXG circuit board.
Now to the nitty gritty
EMG produce a range of active tone controls, but the most popular configuration is that used by david gilmour in his red strat - the DG20.
The most recent version is completely solderless with push on connectors and all the componentry is hidden between the rear of the pot controls and the PCB.
There is an earlier push on version with components mounted on the rear of the PCB but there is also a lot of plated through holes to the other side - damn
BUT - a still earlier version (non push on connectors) is very easy to analyse (sounds so much nicer than reverse engineering
) - it only has a few connections on the rear of the board AND a hi res picture is readily available on the EMG forum
I was able to determine the schematic relatively easily from this picture, however being surface mount, none of the capacitor values (except the 1uf and 150n polarised ones) are visible.
I tried a few different values in Micro Cap 9, but due to the interaction of the parallel branches in the EXG part, I was finding it very difficult.
EMG have kindly provided response curve graphs on their website so it was just a matter of trying to get the same response characteristics by juggling capacitors. Alas MC9 was just not up to the task, so I decided to mount a fresh horse (so to speak).
Enter National Instruments Multisim 10.1.
Some of our senior members may remeber a program called Electronics Workbench (back in the days of 486 computers
) - anything bigger than 4 components took about 20 minutes to run an analysis.
WELL - thank the Lord for Pentium 4 computers, because EWB has grown up and become Multisim 10.1, and it runs analyses instantly.
I set to work and using the AC analysis/grapher function I was able to "fiddle" with the unknown capacitor values until i arrived at almost identical response curves to those on the EMG site for the EXG SPC AND RPC modules.
Stay tuned and these will all be uploaded in due course.
First up though - here is my hand drawn schematic for the buffered input SPC and EXG module.
Someone here may like to redraw it and pretty it up - feel free.
Happy New Year
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