Optical Gain/Lead Footswitch Mod

Year: 2021

Problem

A customer came to me with a Soldano Lucky 13 guitar amp, which had been extensively modified. One of the mods had added additional gain stages, and a footswitch to turn the gain stage on and off, as is typical for clean/lead. The implementation of the footswitch however, made the amp almost unplayable. 

The preamp signal wire had been run directly to the footswitch and back, about 15 feet one way. That 30' of unshielded cable generated a huge amount of electrical noise. 

Strategy

Now, the original designers of this amp had already solved this problem- all the channel switching within the amp was handled by a low-voltage optical relay system. This is what the second button on the footswitch already controlled. But sourcing more of those same optical relays was impossible, they had gone out of production decades ago, and then worshipped by some sect of the HIFI community, making them expensive in addition to rare. 

Why not a normal relay? Besides trusting the engineering judgement of the original designers, I was worried about switching noise on this high gain section, and also that I did not know the limitations of the voltage rail supplying the original relays. Better to draw as little current as possible, and avoid switching noise. 

After consulting Mouser electronics, I selected a series of affordable optical relays which had quick switching times, comparable voltage requirements, and, most importantly, almost negligible resistance when on, and many megaohms of resistance when off, to best approximate a true relay. 

As these relays were essentially SPST switches, I would need two to approximate a true DPST switch. This also meant I needed to design an inverting logic gate so one relay would always be in the opposite state, given a single input signal. Fun!

Prototyping

Initialed circuit was breadboarded, and connected to the amplifier. After the circuit was verified, I transferred it permanently to a breadboard, and installed it into the amp. I also maintained compatibility with the LED light on the footswitch. 

Results

A very happy customer, and I got to do transistor bias calculations for the first time, a rare event for a Mechanical Engineer.