Ibanez VM3LC 3band bass EQ layout

[j.frad]

Hi there!
I dimounted my Ibanez EDC700 ergodyne bass and happily discovered that the 3 band EQ with parametric mids was not covered in the usual black plastic and is made with very common parts...
This eq is probably the most interesting part in this bass I think and I noted all the information I needed to clone it before selling the bass...
My question is, since it is not my layout but the one designed by Ibanez's engineers, will I get in trouble by posting this here?
thanks

[j.frad]

Anyway I can still email the layout to anyone interested...
It's all on one picture, the circuit with all the components on it, not to scale and I probably should include the picture of what to etch, an a few notes are in french but nothing you won't understand...

[markusw]

Hi j.frad,

personally I don't think you'll get into troubles for posting the stuff as long as you do not claim it's your layout.

Anyway, I would be happy if you could send me the informations. You've got pm btw 

Thanks a lot in advance!

Markus

[j.frad]

markus, layout e-mailed!

[j.frad]

Ok here are the files:
http://untrucavecdlagerbe.free.fr/ibanez/

the jpg file is the copper side of the circuit, what you need to etch (and put to scale too I guess)

the pdf file is the reverse with the components drawn onto it, and a few notes abour the pots, some note are in french, sorry didn't think about it then, but it's not that hard to understand... mostly notes about how the cap values should be read.

The mids frequency pot is wired real funny, it's a dual pot... Just figure my bass had a double knob for mids and mid freq (you know two knobs on top of one another) , so that knob there had one single pot AND a dual pot in it! 9 lugs!

anyway, hope this is useful to someone! I know there is no schematic, just a layout, it's easier to clone the circuit but hard to read for an experimented electronician who would understand the good and bad points about the layout just by looking at the schem...

[lowstar]

 
cool, thanks !
nice to see some bassmen on the board
cheers,
lowstar

[markusw]

markus, layout e-mailed!

cool! thanks a lot!

markus

[j.frad]

Ok!
George Giblet drew a schematic for this circuit and pointed out some wierd things!
According to his calculation a resistor value might need to be changed, as well as the way the mid-frequency pot schould be wired.
All the files are in a zip file below, experiment and tell use what works better!

http://untrucavecdlagerbe.free.fr/ibanez/Ibanez EDC700 V1.3.zip

[j.frad]

little bug there, make sure you copy the whole line
"http://untrucavecdlagerbe.free.fr/ibanez/Ibanez EDC700 V1.3.zip"
into you browser to get the file, or go to:
http://untrucavecdlagerbe.free.fr/ibanez/

[markusw]

I came up with exactly the same schem (although less nicely drawn) but I couldn't get the mid freq pot "work" in my sims unless I added the connection at the second mid freq pot as suggested by George. from my sims it seems that two lin pots would give a more useful freq range (~88 to 720 Hz) than two rev.log pots (88-230 Hz). just my 2c.
EDIT: please forget about the freq response...this is crap!. however, the two linear pots seem to give a much smoother response than the two rev logs. also freq goes up with pot travel.


btw, thanks a lot for the missing connection George I was already quite desperate because I couldn't find a bug.

one Q: is this mid freq thingy a gyrator or is it a another filter section? any hints would be highly appreciated

Markus

[George Giblet]

> one Q: is this mid freq thingy a gyrator or is it a another filter section?

It's basically a basically a bandpass filter inside the usual boost/cut feedback arrangement - similar to the state variable filter based circuits in principle.

The filter is a little tricky to understand.  It starts with a bridge-T network in the feedback loop (U2B + Freq pot etc.). That gives you a '1 + bandpass' response. The clean '1' part is subtracted off: the filtered signal is mixed in via R17, the pre-filtered signal at pin 5 U2B is passed to the opposite polarity of the mixing circuit through R18-R20 and is effectively subtracted off.  More complicated than usual but economizes on opamps.

[markusw]

It's basically a basically a bandpass filter inside the usual boost/cut feedback arrangement - similar to the state variable filter based circuits in principle.

Thanks a lot for the clarification! I'll dissect it to see how it works

May I ask one more Q: if I add a pickup to the sim (simulated by a 4.5H inductor followed by a 12k resistor in series and a 150p to ground) which I suppose should be OK as an approximation, I get a pretty interesting freq response after that transistor-opamp input buffer.




green: at the blend pot (simulated by a 200k to gnd)
blue at the out of U1A

This pronounced peak at around 5 kHz is still there at the output of the circuit.


With 20k as a "blend pot" I get this (although20k is for active pups):



Also, with the 20k the overall response at the output of the circuit looks more reasonable.

What is this transistor-opamp-filter stage supposed to do? Is this all just a Spice artifact?

Thanks for your help in advance.

Markus

EDIT: I now replaced the "ideal log" mid pot in the sims with a "piher log" pot and now the freq response is perfectly smooth, even better than with the lin pot. Never completely trust a simulation....... at least if you're not 100% sure what you're doing

however, this does not solve my other Q...

[George Giblet]

It is easier to see what is going on by just driving a voltage into that stage and looking at the output (no need for the pickup stuff).

You will find it is a shelving type treble booster.  The treble boost is cancelled out by a treble cut shelf right at the output of the whole circuit.  It's called pre-empphasis and de-emphasis - same idea is used on Chorus and Flanger pedals.

[markusw]

It is easier to see what is going on by just driving a voltage into that stage and looking at the output (no need for the pickup stuff).

You will find it is a shelving type treble booster.  The treble boost is cancelled out by a treble cut shelf right at the output of the whole circuit.  It's called pre-empphasis and de-emphasis - same idea is used on Chorus and Flanger pedals.

Thanks a lot! I found that I had C11 at 6.8n instead of 68n. Now everything is fine, even with the pickup simulation.

Markus

[kriwil]

anyone successed make this clone

[j.frad]

well i haven't built it yet but might do it soon, i'll tell you if i do so!