Ross Phaser/vibrato switch odd behavior

[Fp-www.Tonepad.com]

I added a DPDT FOOT switch to my ross phaser to have the vibrato option and an LED that turns on when in vibrato mode. It's cool sounding, but I have the following problem:

When playing on phaser mode and the vibrato mode is switched on (27k resistor lifted), there's a volume drop (which is kind of normal), BUT when switched back to phaser mode, the volume remains lowered (and changing between phaser/vibrato doesn't bring it back up). You have to switch to bypass to return to the original normal level.

This is a very strange problem that I really didn't expect.

Suggestions on what may be causing it and how to solve it?

Fp

[Mark Hammer]

That IS odd.  I certainly never noticed it.  I can't imagine why it would make a difference, but here goes: which end of the resistor did you lift?

[Fp-www.Tonepad.com]

The one going to pin1

[Mark Hammer]

Looking at the schem now, I can't see any reason why lifting either or both ends of the 27k resistor would cause the behaviour described.  Does it do this when the LED is NOT part of the switching action?  When I've implemented a vibrato option, it has always been with a toggle, rather than stompswitch, so I've never really felt the need to use any sort of indicator LED.  Maybe that's why I never got the effect you note, although I still don't know why any effects of increased current drain would linger post-reswitch (is that even a word?).

Here's something to consider.  The usual means of producing vibrato is to entirely disable the dry signal, but really all one wants is to put it way in the background.  I wonder if it might not be better to replace the 27k resistor with a 27k and 470k in series, and use the switch to shunt the 470k (normal wet+dry mix) or enable it (way more wet than dry, given the 27k path of the wet and nearly 500k path of the dry).

Really and truly, though, I am mystified.

[puretube]

I`d like to look at that, if there were a direct link to the schem in quest...

[Fp-www.Tonepad.com]

Mark, I had thought of that, but it still sounds odd, I will try it soon. The pedal is not in my hands right now. Could it be a bias thing? I appreciate your help! (I knew you'd be the first to jump in!)

Pure Tube, here's the link:

http://www.tonepad.com/project.asp?id=25

Fp

[puretube]

ehemmmm.... sounds like a "latch up" problem... (be it the ICs or an electrolytic cap "remembering" a voltage, and hanging on to it...);
that whole thing is DC coupled;
IC1b might see different DC at its inverting input when "vibrato" or "phaser";
or is it just the fact, that "+V" varies, when the vibrato-LED is "on" (decoupling of audio and signaling path?);

first I`d advice to check what happens without the extra LED....

[Fp-www.Tonepad.com]

I'll do the led thing first, I'm using a regulated power supply BTW.

I'll post back when I try without the LED.

Fp

[james]

Hi,

I put a DPDT switch in my old phase 90, and I'm having a similar prob.
The effect volume is considerably lower than when its off. Also, it sounds
much muddier to me now than before. Any suggestions? Thanks!!!

James

[puretube]

Hi James: of course you gotta be aware of what Fp stated above:

there's a volume drop (which is kind of normal

his Q. is not about that fact, that in vib-mode, you get only "half" of the signal...

the reason for your mentioned "muddiness" may just be the fact, that in phase-mode you have enhanced harmonics swirling around in the spectrum, causing a pleasing tone;
once you switch over to vib, the magic is gone and you in fact hear the original "clean" tone, which compared, sounds "duller"...

[puretube]

Fp: I`m missing any big el.-cap across the power rails (and maybe a little resistor in front of that...

(I bet you can also raise the vol back up, when switching off the supply for a short moment?)

...does the LFO show start-up problems, sometimes?

[Fp-www.Tonepad.com]

puretube,

I did add a 47uF cap across the power rails, but I didn't add the resistor. When I made the layout for the Ross phaser, I decided I would add pads for a little filtering to the LFO part of the circuit, that's why there's a jumper that jumps nothing and a couple of adjacent pads where an electrolytic cap would fit.

I'll try unplugging the power and plugging it back in and see if that brings the vol back up.

The LFO doesn't have startup problems, it works all the time as far as I've noticed.

Fp

[Jay Doyle]

FP,

Man this one is weird!

If I were to guess I would say that it has something to do with the way you switch the signal out of the mixing stage.

I would try moving the pole of the switch from the output, pin 1 of IC1a to the end of the 27k resistor that you switch in and out. Then try switching between pin 1 and Vb, instead of leaving the end of the resistor floating.

That is my guess at this issue.