Isolators seem to be all the rage at the moment, thought I would make one up.
I decided to split the circuitry across 2 PCBs, one for the filtering and volume control – another for other tasks like input and output circuitry, buffering before the filters as well as the summing amplifiers for the three bands. I wasn’t worrying too much about including a power supply as I tend to get the best results from using an external power supply. This adds considerable cost to the build, as power cable, connectors and another enclosure for the PSU are big ticket items, but it is worth the trouble.
Don Lancaster’s Active Filter Cookbook is always a great read for these kinds of things, and Rod Elliot’s Project 153 was an influence on the design too.
The filter PCB contains all the state variable filters, trimpots for setting the cutoff and the volume control.
I like Alps RK27 pots so used those as I do for almost all stereo audio control. I’d love it if I could get some with a centre-dent but I haven’t come across them before. As an aside, I was in the famed Akihabara neighbourhood of Tokyo and managed to buy a decent stash of the 2xA10K models for less than $10 per unit. RS components will charge you $44 for the privilege and the units off ebay at half the price are of dubious origin.
The filter PCB looked like this:
Next the other PCB. As far as the Input and Output goes, I wanted to offer the option of balanced or unbalanced connections so added this using the THAT Corp 1246 and 1646 balancing/unbalancing chips. I haven’t tested this yet as I don’t have any of the unbalancing amplifiers on hand. These are a generic part with SSM (at one point) at Texas instruments making a pinout compatible version. This PCB looked like this (right):
And when sandwiched together the the PCBs look like this:
As you can tell, it is a very tight fit! But it all works nicely which is good. I still haven’t sorted out an enclosure for this yet, I’m looking for something that won’t add much girth to it. I’m after something truly pocket sized (except the power supply). I’ll be adding a DC power output connector from the DJ mixer I posted earlier to supply power and also building a small power supply for occasions when I’m not using them together.