I always wanted to have a decent analogue phaser with true stereo options to use it with Rhodes piano or string synthesizer sounds. Two years ago I purchased a BOSS RPH-10 Micro Rack Phaser to find out if a pair of these (two of them can be linked) could fill that gap. Unfortunately none of the three modes convinced or satisfied me, although they all sounded pretty good. It just wasn't my sound. Some of you know that problem with personal preferences when it comes to overdrive, distortion or whatever effect types.
I was able to hunt down three modules of the KORG KPH-401 Phaser. I was curious if there could be a way to modify two of them to work in stereo.
In June 2022 I asked our dear member Laurie if he could come up with something. I sent him the schematics and he saw a way how to make one of the modules work as a master unit to control the speed of the other one. And to my big joy he also said that he saw no problem to implement an inverted signal, thus making the one phaser unit go up while the other one goes down to achieve spacious stereo effects.
A strange problem occured when I finally tried doing this modification a few months ago. The normal master signal produced stuttering noise with the slave unit, while the inverted signal didn't work at all. We weren't able to make it produce the right voltages, no matter what Laurie tried to figure out.
That's why I sent the modules to Laurie in Canada. With his oscilloscope he immediately saw what the problem was: unlike any other phaser unit that Laurie knows, the KPH-401 produces pulse signals at the certain point that we wanted to use for the control signal. Only a few days later he has finished the modification which then worked exactly as intended.
The modules arrived at my place a few days ago and I'm extremely happy with the result! Many, many thanks, Laurie! It is a dream come true!
This is how the modules look like. I keep a third unmodified KPH-401 for my collection. But the modified units don't look bad at all. The small switch of the slave unit bypasses the external control signal and puts it back to normal working mode with independent speed.
The modules can be linked with a wire with mini plugs. For true stereo I have to use two PME-40X boards. They can also work in unison - or with inverted signal - on a single board for mono sounds. Especially with the inverted signal and two different phaser settings they can produce some really cool morphing sounds that I haven't heard before. A nice side-effect that I haven't had on my radar when initially thinking about linking these modules. It is really nice that Depth, Manual and Feedback can be set independently. I hope that I can provide some sound clips, soon.
In the end the modification was quite easy to do. A small board provides the inverted signal.
A tedious thing was to file down the potentiometer boards until the mini jacks and the small switch finally fitted inside.
