Bent Audio / Slagel Auto-former Passive Preamp

After hearing consistently positive comments about the Slagle Auto-formers I decided to take the plunge. At RMAF in 2010 I was wowed by the terrific auto-former implementation that John Chapman of Bent Audio came up with. Using a set of 4 PCBs from John I had all of the parts to put together a world class attenuator with a nice remote and lots of flexibility. The components 2 - Slagle-former attenuators, 1 - Front panel PCB and one RCA input select PCB. This provided a remote controlled, 2 channel attenuator with 6 inputs.

An enjoyable part of this project was designing and building the chassis. I had recently obtained a Bridgeport CNC milling machine and the new Preamp casework was one of my first milling projects.

I installed a 4 way line level crossover in the same chassis as may be seen in the final picture. Placing the crossover in the same chassis eliminated a set of Interconnects and RCA jacks. Whenever feasible I try to simplify and remove components from the signal path. The Preamp may be used with or without the crossover as there are RCA jacks that carry the output of the attenuator, bypassing the crossover.

This new passive Preamp was a surprisingly large upgrade over my previous high quality DACT attenuator / 26 tube line stage. I suspect that some of the benefit is due to removing the tube line stage and some to the superior auto-former attenuator. Regardless it was a notable improvement. Oh yes, I underestimated how much I would love having a remote.

Below are pictures of the Bent Audio components.