#24 SM Pro Audio TB202 dual tube preamp / channel strip modding & upgrade, part 4

On the note of front panel switches, you might've noticed two extra ones, in the previous post (the part 3). One other idea had crossed my mind, already long before this second stage of modding. It took me a little while, but i managed to get my head around how to implement this. The idea was to be able to swap the order of the two processors in the signal path.

From the factory, as described in the "part one" post, the processing order is input - gain - tube - compressor - EQ - output. But in some cases, it can be desirable to have the EQ come before the compression stage. That way, one can attenuate certain frequency ranges that one would not want the compressor to react to (or conversely, boost ranges for the compressor to react to).

#22 SM Pro Audio TB202 dual tube preamp / channel strip modding & upgrade, part 2

Once the "adventures" from the previous post were completed, i proceeded to hook up one channel to my audio interface and feed some test-signals into it. This revealed a few things which didn't sit with me all that well.

First of all, the compressor didn't seem to be doing much of anything. No level reduction with the knob turned up to full (even with full input gain and clipping the snot out of the tube), the associated LED stayed green (instead of going red), nothing. A bit of oscilloscope-probing in the signal rectifier area quickly pointed out the issue - there was no real rectification going on. The signal coming out of that stage was still largely sinusoidal, albeit with a small kink at the zero-crossings. Well, that's no good...

Removing the diode in the signal-rectifier for testing didn't reveal anything, it measured as a normal 1N4148 both in and out of circuit. My aforementioned buddy pointed out that the arrangement they had used in this design, a "precision diode" circuit, was somewhat flawed as a concept, as is described in this link over here. I then proceeded to upgrade both channels to the "precision rectifier"circuit (see link above).

#21 SM Pro Audio TB202 dual tube preamp / channel strip modding & upgrade

<Further developments can be found in parts two, three and four>

I'll admit, this was a bit of a lucky catch, in that it was mistakenly listed in the wrong eBay category, but one i "stalk" for good deals on certain faulty bits of audio gear. It was up for auction, there didn't seem to be much demand for it, and i ended up winning it for a mere 21 euros. About as much as the shipping for it ended up costing, oddly enough.

But still, even for some 40-odd bucks, not a bad little unit. Two channels, a (bare-bones-ish) compressor on each (1.5-10:1 ratio, 1ms/5ms attack, 500ms/1500ms release, at least according to the specs in the manual), as well as 3-band EQ (80Hz shelving / 1.8kHz bell / 8kHz shelving), and there's a tube / valve involved in the circuitry as well. If nothing else, i figured it would do nicely as a bass DI (or even an overdriven preamp, once some internal re-wiring is figured out and done).