Small Stone feedback control and other mods?

[shawsofhell]

I've previously built a small stone phaser using the tonepad layout but I have never been overly impressed with it. Do you guys think I might have luck with the phase 90?

I was also thinking maybe I could remove the colour switch as it annoys me that there is no adjustment and there is a volume drop. I could replace it as follows to allow adjustable feedback?

http://pub21.bravenet.com/photocenter/album.php?usernum=1772399634&cpv=2

the only thing is what do i do with the red part of the cicuit?

also what can I change to get atleast unity gain when the effect is turned on?

[shawsofhell]

please can anyone help?

[puretube]

try some >search< in this forum (+archives...)
should be a lot in there

[Mark Hammer]

First off, don't count your Piedrita out just yet.  There is a *lot* of potential in there, and if you don't believe me just check out Charlie Barth's "Frankenstone" (http://www.moosapotamus.com).

The "color" switch on the SS is somewhat analogous to the 4-way switch on the Phase 100 in that it changes both resonance/feedback and sweep width simultaneously.  While that obviously WILL change the feel and sound of the unit dramatically, it also seems to rob the user of choice.

The sweep width is, frankly, something that has always mystified me in this unit, so I'd suggest leaving one half of the switch's function intact for now.  But  the feedback adjustment CAN be tinkered with.

Let's stroll through what that half of the switch does.  Using the Tonepad schem as reference point, take a look at the signal path. coming out of the darlington pair of the last phase-shift stage (IC4).  The phase-shifted signal goes to the passive mixing/output stage via a 27k resistor.  It also goes back to the input of the pedal via a resistive-divider pair made up of a 3k3 and 4k7 resistor to bring the level down a bit to avoid oscillating feedback.  From there it goes through a 27k resistor and a 0.1uf cap to keep DC out.

Okay, now to the switch.  In one position ("ON"), the wet signal bypasses the 270k resistor and goes directly back to the input.  Well, not *directly*, since it does go through the network just described, but as directly as most feedback paths tend to be, and if you redrew it without the switching it would look quite familiar.

In the other switch position, two things happen.  One is that the 270k resistor now becomes a path to ground from the pedal's input.  The other is that the feedback signal itself also gets grounded through the 0.1uf cap.

Okay, so let's assume we want to maintain the feedback path and never ground it out.  First, the 270k resistor is replaced with a straight wire shunt.  Varying the amount of feedback can take place in two ways.  One is to vary the proportional mixing of input and feedback signal by changing the *series* resistance of the feedback path.  So, consider replacing the 27k resistor with maybe an 18k resistor and a 100k pot wired up as a variable resistor (wiper and one lug, with the other lug shorted out to the wiper).   One of the interesting features of this is that the cap and resistance interact to change the low end rolloff, which may or may not be something you want.  In this instance, using the parts indicated, low-end rolloff would vary from 88hz at max regen to 13hz at min regen, which is actually not going to be that audible a change.  Drop the cap to .047 or .039uf and you'll get an interesting function in which max regen has more impact in the midrange and up than in the low end (with .039, lowend starts to roll off around 226hz at max regen).  I haven't tried this optin so I can't speak to how much variation in feedback it will provide, but it WILL vary feedback amount.

The other option is to vary how much signal there is to mix in the first place.  I mentioned the 3k3/4k7 resistive divider.  This drops the signal level to keep the feedback from oscillating.  If you leave the 27k and 0.1 parts as is, but stick a 5k pot in conjunction with the 3k3 and 4k7 resistors, you can get some useful variable attenuation of feedback.  Here, ends of the two mentioned resistors should be lifted from where they normally meet, and the wiper of the pot would go there.  One outside lug of the pot goes to each of the two freed-up resistors.  What you have now is a variable resistive divider.  At one extreme, the attenuation is given by 3k3/(5k+4k7), and at the other it is given by (3k3+5k)/4k7.  That may or may not be enough range.  You might try a 10k pot as well.

Finally, I *think* I once stuck a 25k pot in series with the 4k7 resistor and used that to some productive end, but don't quote me.

These are all ways you can vary the amount of wet signal fed back to the input.

[shawsofhell]

can anyone comment on my mod? i just added the feedback control from the ross phaser with out making any changes?

can i replace the switch in the lfo section with a pot so i can control its function as well?