Sunday, August 12, 2012

versatile EH small stone

smallstone1 smallstone2 This Electro-Harmonix Small Stone phaser has components from eras of both the second and third Russian-made versions. The PCB layout seems to be the third version, though the schematic and part values are similar for either.

I've always thought it had a great sound.. but the major drawback is its extreme lack of flexibility. A stock Small Stone has only two controls- a potentiometer to vary the rate and a 2-position "color" switch.

First, I added a CV input- a switched 1/4" jack which interrupts the connection between the LFO's output and the inputs to the four OTA phase shift stages. Clamping diodes and a current-limiting resistor protect the OTAs. This modification allows the phase to be controlled by a synthesizer's LFO, an expression pedal, random S/H, analog sequencer, etc.

In the original design, engaging the DPDT "color" switch adds a lot of feedback, increases the depth of the LFO, and also slows its rate slightly. I disconnected the feedback portion of this switch, and installed a pot to vary the amount of feedback- from none to near-oscillation.

The other half of the original "color" switch (which affected LFO depth and rate) is now on the top left of the pedal and is labeled "deep/shallow". The switch on the upper right, which is labeled "phaser/vibrato" simply removes the dry input signal from the mix, creating a nice mild vibrato effect.

The four original OTA caps were all .0068uf. I replaced C4 and C5 with .022uf and .22uf. A DPDT toggle switches the third and fourth caps (C8 and C9) between the stock value and .00047uf / .0047uf.

My unit did have a very noticeable volume drop when engaged. On web forums, many people suggest replacing R42 (4.7k) with a 10k (or even 12k) resistor, however many also complain about reduced headroom and bad distortion once they've made this modification. R42 affects transistor biasing and will cause distortion if its resistance is greater than 8 or 8.5k. Based on my experience, an 8.2k resistor would be the most ideal replacement. I used a 10k trimpot (set to approximately 8.3k).

Also, that annoying 1/8" DC power jack has been replaced with a standard 2.1mm barrel jack.

Here are a few sound samples:
- smallstone1.mp3
- smallstone2.mp3
- smallstone3.mp3

All component names refer to these schematics (via
- ggg_smallstj_sc.pdf

And here is the documentation for all of my modifications:
- my_smallstone_mods.pdf

E-mail me if you'd like to have your Small Stone modified similarly!