I've always wanted to make digitally programmable tube preamp like the Mesa Triaxis. As I gear up to do the high voltage parts of that project I decided to start at the back end and make a 5 band graphic eq that was programmable. The Triaxis cuts some corners and prewires the EQ to a popular curve, then the "dynamic voice" control sets the amount that is applied - I want a real 5 band.
Since the EQ is at line level I'm using AD5293 digital potentiometers. It's mostly surface mount and small parts so a it's little scary if you haven't done it before. Fortunately I've been slowly moving all my projects to more SMD and, frankly, I like it a bit better in some regards. The board works (mostly*) but I haven't set up a good control scheme so the pot shown below is either adjusting one band or the whole "voice" depending on what parts of the code I activate. Most importantly it's pretty transparent and when set "flat" it sounds the same as my normal guitar tone. This proof of concept opens the door to digitally controllable tone stacks and maybe even front-end drive circuits if the gain doesn't cause digital noise to leak in.
* "mostly" because I implemented the zero cross circuit in the data sheet for the AD5293; one of the components in the sub circuit was damaged by heat I think and was clamping the 5v rail to .9v. So I had to rip out the sub circuit and bypass it. Perhaps you can see the 30awg jumper wire. Regardless, the popping from updates to the pots is only noticeable in some circumstances but I'm going to replace the bits and see if I can get it working.
Since the EQ is at line level I'm using AD5293 digital potentiometers. It's mostly surface mount and small parts so a it's little scary if you haven't done it before. Fortunately I've been slowly moving all my projects to more SMD and, frankly, I like it a bit better in some regards. The board works (mostly*) but I haven't set up a good control scheme so the pot shown below is either adjusting one band or the whole "voice" depending on what parts of the code I activate. Most importantly it's pretty transparent and when set "flat" it sounds the same as my normal guitar tone. This proof of concept opens the door to digitally controllable tone stacks and maybe even front-end drive circuits if the gain doesn't cause digital noise to leak in.
* "mostly" because I implemented the zero cross circuit in the data sheet for the AD5293; one of the components in the sub circuit was damaged by heat I think and was clamping the 5v rail to .9v. So I had to rip out the sub circuit and bypass it. Perhaps you can see the 30awg jumper wire. Regardless, the popping from updates to the pots is only noticeable in some circumstances but I'm going to replace the bits and see if I can get it working.
No real updates beyond what’s in the first post. I haven’t yet deployed it into a preamp project as it was originally intended. It sounded pretty transparent and the zero cross circuit remains bypassed. When I get to use it as intended I’ll get a sense for if zero cross was ever necessary.