Thanks a heap for that!Hello, here I have traced Ibanez onboard preamp with variable mids
I've traced that board recently, but only from photos, like yourself.
Apart from unknown values your trace looks spot on.
Some part values were very unclear from the pics, and some I couldn't read at all.
How confident are you of the values?:
- 4n7 treble cap
[Later varimids use 6n8.]
- 51k resistors from the mids
[51k only gives about 6dB where as Ibanez always quoted +/-15 for varimid]
- 5k1 at the output
[quite hefty treble cut]
- JFET vs BJT at the input
The back and white graph, from about 1994, the one in your dropbox, show the mids at +/-15dB.
The exact 15dB on this graph always made me suspicious it was an "artists impresssion" and used
to promote the new varimid which was released around 1994. Treble plot frequency is a little higher,
perhaps matching the 4n7 value you have.
There's this graph which agrees with the graphs in the 2007 and 2010 bass catalogs:
This graph also has +/- 15dB mids. I think the later varimids reasonably match the treble and bass on this graphs.
To get +/-15dB on the mids the 51k needs to be 12k or 15k but as I remember something didn't look right about the result. Since the new circuit matched the graphs, I also notice there was a possible design oversight in the vm old version around the differential amp.
I struggled with these issues for some time and gave-up until better pics or info comes about.
Anyone got one of these?
Here's the circuit of the newer varimid circuit from and Ergodyne EDC700 (maybe 1999).
Now I will point out.
- The pre-emphasis (C3 & R4, R5, R6) do not match the de-emphasis (R21, RV6, R22, C11)
- Also, I'm very suspicious these value may vary on different models because I saw different resistors on a board from an Ibanez BTB bass.
Don't compare these values with the circuit you traced because that one has *no* pre-emphasis. So the de-emphasis will only be a shelving treble cut.