Ibanez CS9 Stereo Chorus - how can I kill the delayed signal?

[peaking]

Hello,

I'm attempting to get my Ibanez CS9 to produce a pitch vibrato effect, which I believe can sometimes be achieved by removing the delayed signal from a Chorus' mix. I did this on a Small Clone simply by lifting one end of a resistor (thanks to Colin's thread at expanon for that).

However I'm struggling to get the same result here. After much staring at the schematic all I have achieved by probing the circuit is various scary pops and removing all the affected signal from the mix, which as stated won't do because I want to keep the modulated signal.

As a general rule I don't like to join a forum just to ask a question, but the aforementioned small clone has since been put out of action by my uninformed prodding and I would like to avoid making that mistake again!

Thanks in advance,

Cal

[Mark Hammer]

Well, let's clear this up, one step at a time.

First, it is not the delay signal you wish to kill, but the clean signal.  Killing the delay signal is what takes place when you put this pedal into bypass mode.

One of the characteristics of delay-based, and many modulated effects (chorus, flanger, phaser), is that the effect produced depends on combining an affected and unaffected signal. One of the advantages this provides to manufacturers is that if you simply cancel the processed signal, the "effect" is eliminated.  It is not "bypassed" in the sense of the circuitry being removed from the signal path.  It is merely "cancelled".

The advantage this provides from a circuitry pont of view is that it only take one FET, and whatever components are required to turn the FET on and off, to "cancel" the effect,  The schematic for the CS9 can be seen here: http://www.freeinfosociety.com/electronics/schemview.php?id=2292  Look at the upper right hnad corner of the drawing for Q105.  That's your "bypass".  When that FET is turned on, it provides a low-resistance path for the delay signal to reach the mixing stage at IC101.  When it is turned off, it presents a resistance high enough that it may as well not even reach the mixing stage.  With essentially zero delay signal being mixed in, the pedal sounds like it has been bypassed.  It hasn't, but it sounds like it.

So why didn't Ibanez include a vibrato option?  Well, for one thing, you have to ask yourself what the "opposite" of cancelled is.  The footpedal has two options, engage or bypess/cancel.  As is, bypass turns the FET off, and engage turn the FET on.  To have vibrato, you would need to turn the FET on (to enable the delay signal) and turn an additional pair of (missing) FETs off, to cancel the dry/clean signal, and of course reverse that action when shifting from effect to bypass.  You could produce vibrato by installing a little toggle to simply lift the end of R127 and R140 that each connect to their respective op-amp.  While that would work, when you step on the footswitch to go back to bypass/cancel mode, you would have no output whatsoever since both your wet AND dry signals are cancelled/blocked.

Is there a way around this?  Yes.  Look further down on the right hand side and you'll see the traditional flip-flop circuit formed by Q107 and Q106 (note that the collectors of each transistor seem to have disappeared in either the photocopying or scanning, and should connect up to R151 and R152).  Only one of the two complementary outputs of the flip-flop is used to turn the one FET on and off.  If a person inserted identical FETs between C117 and R127, and between C121 and R140, the same way an FET is inserted between C116 and R126, plus the assorted supporting components (diodes, resistors, caps), then the pedal could be made to alternate between wet-only (vibrato), and dry only.  To use it in chorus mode, one would need to install a toggle to feed the suggested additional FETs with an "on" voltage, rather than the output of the flip-flop so that they were always on, regardless of the state of the footswitch.

My guess is that, based on the nature of your question, this is a bit too involved for you at the present.  It IS feasible, however, and perhaps a year down the road well within your capabilities.

Alternatively, if you rarely or never use the second pedal output, just lift one end of R140 to kill the dry signal, and that output will now be your vibrato-only output.  The problem still remains of there being NOTHING at that output when you hit the footswitch to cancel the effect, but at least there will be a vibrato available to listen to, even if it isn't suitable for gigging.