small stone "phase-filter" mod

[bassmeister]

I saw a description a while back on how to turn the last two sections on a Small Stone into lowpass filters for a more subtle phaser effect.  This was done with one SPDT switch, and the real swithing (two caps to ground and at the same time remove the dry signal) was done with a 4016, 4053 or 4066 (I think).

I can't find this description now, but I guess someone knows about it. Could "someone" please post that thread?

[Mark Hammer]

Can't give you the link, but I suppose I'm as good a source as any to explain how it's done.

There is a 6800pf (.0068uf) cap that goes from the output of the OTA back to a 27k resistor that feeds the input of the OTA.  If you have a stock 3094-based Small Stone, this cap goes from pin 1 of the 3094 to the 27k resistor tied to pin 2.  If you have built the Tonepad/Pena clone of the SS based on CA3080's, the cap goes from pin 6 of the 3080 back to the 27k resistor feeding pin 2 of the OTA.

When that cap is tied to the 27k resistor, the circuit forms an allpass stage.  When the cap is tied to ground instead of the 27k resistor, it forms a a single-pole (6db/oct) lowpass filter section.  So, if you ground that cap on two stages, you have a 2-stage phaser and 2-stage lowpass filter.  You can hear some samples at Charlie Barth's site (www.moosapotamus.com; though I don't think he squeezed the best sounds possible out of the unit - it does more tricks than you hear), and you can read a review of a similar commercial unit in DEVICE, posted at my site (http://hammer.ampage.org).

The phasefilter sound does not require that you break the connection for dry signal.  It can still be heard with a 50/50 combination of wet and dry.  However, if you have a phase/vibrato switch installed in your phaser (i.e., break/restore dry signal), you will find the phasefilter setting provides some really interesting sounds.  At medium speeds it adds a pleasing swampiness to vibrato that provides a really interesting blend of subtle wah, pitch shifting, and tremolo I haven't heard anywhere else.  In that regard it is an even better effect than the DEVICE article would make it out to be.

Since you're simply directing two component leads to go either here or there, a DPDT toggle switch will do fine.  Because DPDT functioning is relatively easy to emulate with CMOS switch chips, you can also try doing it with them if you need to be able to switch remotely, for example.  If you have an already built stompbox, however, it should be possible to position a low-profile slide switch over where the caps are, without having that switch and the stompswitch interfere with each other.  Provided the leads don't run all over the place, the switch should not result in any noise or interference issues.

Hope this tells you what you want to know.

[moosapotamus]

Hey Mark!

You can hear some samples at Charlie Barth's site (www.moosapotamus.com; though I don't think he squeezed the best sounds possible out of the unit - it does more tricks than you hear)...

Actually, I got to agree with that. Those clips were done right after I finished doing the mods, some time ago, and it does have more tricks up it's sleeve.

Also, consider using a center-off DPDT toggle to switch those caps. The center-off position breaks the connection, disabling any filtering in the last two stages, giving you a two-stage phaser sound. If you also do the vibrato mod, this will give you a nice alternate vibrato sound, too.

~ Charlie

[puretube]

http://www.pat2pdf.org/patents/pat3470495.pdf

this was probably before Oberheim/Curtis/ARP...