The Sunstone OTA Phaser is an adaptation of the classic Electro-Harmonix Small Stone, perhaps the second-most famous phaser of all time next to the MXR Phase 90.
Originally released in 1975 as a more compact alternative to the earlier Bad Stone, the Small Stone used operational transconductance amplifiers (OTAs) instead of the matched JFETs used in the Phase 90 and original Bad Stone.
In 1977, EHX made some improvements to the Small Stone that resulted in the most well-regarded version of the circuit, called “Issue J” after the text on the PCB.
The Sunstone is based on Issue J with a few changes. It replaces the CA3094 OTA with the LM13700, which is essentially two CA3094s in one package and is still in production. The output buffer from the CA3094 has been added in discrete format (Q6/Q7 in the Sunstone) since the built-in Darlington buffer in the LM13700 is not compatible with the way the LFO is configured. With these conversions, it will perform exactly the same as an original 1977 Issue J unit, but using modern parts.
The Sunstone also adds an output volume control to help compensate for the change in volume between the two Color switch settings for which the original unit was notorious. This is preceded by an added gain stage based on the EHX LPB-1 booster so that there is plenty of extra signal to turn down.
The Small Stone Issue J is also the base circuit for the Blackout Whetstone (available from Aion FX as the Redshift Deluxe Phaser), so if you like the sound of this one and want more features, the Whetstone has just about every modification imaginable.