Gearslutz (
-   Geekslutz Forum (
-   -   500 series hacks and the Radial Omniport (

David Rick 29th December 2019 02:59 AM

500 series hacks and the Radial Omniport
2 Attachment(s)
If you're like me, you've probably added an extra switch or connector to a piece of rack gear at one time or another. It might be a side-chain insert, a switch for a lower-speed fan mode, or a switch to dim that horrible blue power LED that's been blinding you. Whatever it the reason, you can usually find space on the back panel (or perhaps a side panel) to add what's needed.

What are we to do in the case of a 500-series module? The only accessible surface is the front panel, and it's usually already chock-full of other stuff. But I recently bought a Radial Engineering chassis in which each slot includes a TRS jack called on Omniport. This connects to a couple of (typically) unused positions on the card edge connector as follows:

Tip -- Pin 9
Ring -- Pin 7
Sleeve -- some kind of ground, probably Pin 5

Some Radial modules use these connections to good purpose: usually as an unbalanced insert loop or a direct out. But most other makers don't connect anything to these pins, which means you can use them for whatever you want.

What I wanted to do was to add provision for a mute foot switch to the Buzz Audio Elixir preamp I use in my live performance rack. (Sorry Tim! Your baby just became a test subject.) The Elixir's DI input sounds really marvelous for the piezo pickup I use on my cello. It sounds full, fast, and it doesn't go quack. But I only use the piezo for pizzicato work and I need to mute it when playing arco. The Elixir has a front panel mute switch, but I need a foot switch because I've got my hands full.

Many preamp designs are internally unbalanced, so it's pretty easy to interrupt the signal path somewhere to make an unbalanced insert point. Not so with the Elixir: it's fully-balanced all the way through. After a bit of circuit tracing, I realized that the front-panel mute switch actually controls the coil of a DPDT mute relay. It's easy to understand that if you leave the front panel switch open, you can strap a foot switch in parallel with it and achieve the desired result.

Not so fast, though! The relay coil circuit runs from the +/- 16V supply rails, and if you route it out and back though a TRS jack, it would be really easy to short a supply rail to ground when plugging or unplugging the footswitch cable. Also, running the coil current down a long cable risks inducing clicks in your audio. So I decided to use a local switching transistor and run its control current through the Omniport. A hand-drawn sketch of my circuit is attached. Note how I've split the base control resistor in two, so neither supply rail can short. I did need to build a special TRS-to-TS cable to connect the Omniport jack to my unbalanced foot switch.

I've also attached a photo showing my hack. Note the TO-92 transistor added near the upper-most red toggle switch and the two new resistors added next to the card edge connector.

I don't know whether any manufacturers besides Radial have included Omniport-type connectors on their racks. But it would be easy to add yourself in a rack like those from BAE that have a hand-wired bus.

David L. Rick
Seventh String Recording