Tuesday, July 24, 2012
Jet Phaser Clone
My main thing is guitar pedals. This is one of the most ambitious ones I have done and I still have a lot of work to do on it. It's a clone of the AP-7 Jet Phaser from Roland. It's from the 70s and if you can find one on eBay it would cost you around $800. This pedal is really old school and sounds awesome!
I'm close to getting that sound, but there are a few problems I have to figure out first. The preamp stage which you can see is off the pcb is part of the original schematic, and it wasn't on the main pcb either. I added that to beef up the input since only the distorted setting sounded loud enough. Adding the input stage didn't make much difference. I think the problem is because the schematic asks for a high gain RF PNP transistor, and I used a 2N3906. Yeah... It didn't help the volume, but the tone got a little worse.
I also tried modifying the gain of the opamp in the mix stage, but that made the distorted setting too loud, and the phase effect couldn't be heard in the normal setting. I tried changing the resistor values in the output, and nothing. I have a feeling that if I use a different transistor in the input gain stage it would solve my problems. I bought 30 new transistors (2N5087) I'll replace for the 3906 soon.
For the JFETs, I used 2N5457s and they work well and sound fine (very clean without the 3906 input stage). The problem I had was that the circuit needs 8 matched JFETs and I only bought 30. I chose the ones that matched best, but the average VGS was around -0.4V. I think the circuit was designed for -4.0V VGS so the biasing was off. I just modified that without a problem by replacing one of the fixed resistors tied to the bias trim pot.
The distortion, on the other hand, sounds SO good. It has a really vintage sound and I love it. I think I'm gonna take it out and make a distortion pedal out of it. Maybe.
Anyways, it phases, sounds good, but not good enough for me. I designed the circuit board and got the PCBs made through DorkbotPDX (OSHpark), so I have three boards and I'm going to populate the next board soon. I bought 100 new JFETS (BF245) with around -4V VGS. I hope they make it better.
I also put a ZVEX fuzz factory distortion on the circuit board because it is so small and I always wanted to adapt that pedal for bass. I haven't tested the fuzz factory since I have no Ge transistors. I'm not an intense audiophile, but I'm pretty sure this pedal needs germanium to operate properly. If anybody knows how to get it to work with another transistor, please tell me.
Anyways, I put the fuzz factory on because the circuit is super simple and could easily fit onto my board. It would make my pedal look pretty funny with 10 knobs! I was thinking I can use that circuit for the gain stage for V2? Here are some pictures. I'll put up a video or sound file when I get it working the way I like.