TG-Music Analog Chorus
Analog chorus has been on my to-do list for a long time. These days MN3007 is way too expensive or counterfeit, so I decided to use MN3207 and Boss CE-3 as a starting point.
First things first: BBD type doesn't have as big impact on sound as people on the internet are saying. And those who are adamant on this are often the ones selling either products with MN3007 or the chip it self. What a shame, and now even MN3207 (or V3207) has had increase in price as well.
One thing that definitely has impact on sound is input buffer. Boss CE-2 has bipolar transistor buffer with approximately 400kΩ input impedance. This is very low impedance in guitar world. However Boss CE-3 has JFET buffer with pretty much standard 1MΩ impedance. Higher impedance will retain high frequencies much better, and I think this is why Boss CE-2 is often said to be more “mellow” or softer. There's a high cut on input!
So I made my pedal to have bipolar transistor input buffer. Apart from CE-3 stereo option, this is the biggest difference between the two Boss pedals. As you can see in the schematic, I did few small mods here and there, but nothing too big. Most notable change is C28. This capacitor determines LFO frequency range. So, making this capacitor smaller will give higher maximum rate speed.
Other changes are more subtle. C29 and R51 are in CE-3 to round up LFO wave shape. Difference isn't really that big. C25 could be called “intensity capacitor”, making this value bigger will increase the perceived intensity of the effect. Just don't go crazy with it, as you might encounter problems with clock signal bleeding through the low pass filter. I would say 100pF is maximum value without filter redesign.
C21 is output capacitor from output filter. CE-3 has 68nF capacitor here because it's stereo effect. For non stereo pedal CE-2 value of 33nF is ok. Making this capacitor bigger will slightly improve the bass response.
Apart from adjusting the BBD bias correctly with the trimmer that's it. This is one of the very few cases where signal generator and oscilloscope is used in effects pedal world. Input 200Hz +2dBm sine wave to pedal input and connect oscilloscope probe to emitter of filter output transistor. Adjust bias trimmer for clean sine wave.
Analog chorus is not too difficult project. It's definitely advanced project, but not unreachable by any means.
