Son of a Ben - Cloning the Benson Preamp

Benson Amps is an amazing boutique amplifier and pedal builder based in Portland, OR (very near to where Steggo is based!). One of their guitar pedal offerings is a pre-amp based on the Chimera 30 amplifier, and it is an awesome pre-amp. I have a friend who has an original unit, but was looking for something a little smaller to put onto his pedal board for gigging, so he asked if I could build him a version in a smaller form factor. Fortunately PedalPCB offers a board that fits in a 125B enclosure, so the project was a go!

For such a punchy pre-amp, the circuit itself is fairly straightforward. The PedalPCB Son of Ben board is well laid out and compact (a hallmark of PedalPCB boards). There are a couple of modifications to the circuit for ease of build, however. The original pedal uses a single B2M potentiometer for the bass control (which can be hard to source), so the PCB includes an option to use a B1M dual potentiometer. If you have a B2M (which I did) you just use a solder bridge to hook up that option. The board also includes trimmer resistors to set the bias on the three J201 transistors (and there are several great threads on the PedalPCB forum that will walk you through the process). The original appears to use transistors hand selected for bias and fixed resistors. 

Most of the rest of the parts are just simple 1% metal film resistors and good quality film and electrolytic capacitors. I did go ahead and spring for silver mica capacitors for the <1nF values as one of them was an oddball value anyway. I'm not sure what the real thing uses, but since this was going to be taking the place of the real thing on someone's gig board, I didn't want to cut any corners. The enclosure assembly was mostly straightforward - the jacks are all insulated with heat shrink tubing per my normal standard. Unfortunately I'd reversed the treble and bass knobs on the enclosure, but my friend was okay with my hand wiring those two pots.

My friend wanted a metallic purple enclosure. Unfortunately Tayda - my go to source for UV printed enclosures - doesn't offer anything in metallic purple. I was, however, able to find something that would work at Stomp Box Parts. Unfortunately they don't UV print, so I pulled out the laser etcher and went that route for the enclosure. I think it came out pretty cool! 

I'm going to work with my friend to get a tone demo, but when I handed it off to him, we were able to directly compare it to the real thing and within the tolerance of the potentiometers it seems to be dead on target. Now I just need to finish up one for me - though I'm not going the purple route. I have another limited edition enclosure I'm going to use... stay tuned!