#23 SM Pro Audio TB202 dual tube preamp / channel strip modding & upgrade, part 3

Even before having received the thing, i had already started thinking of daisy-chaining the two channels. In the context of something like a bass preamplifier, for example, one would want a "clean" channel, perhaps with some heavy compression, in order to have a consistent signal (especially in the lower registers), as well as a "dirty" channel, with a high-passed signal having some distortion applied to it.

#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).

#20 SM Pro Audio TB202 power supply repair

<Some sort of continuation from another post>

Buuuuut that brings us to what might as well amount up to the proverbial "elephant in the room" - the power supply. No markings or label anywhere on it - if i didn't know any better, i could almost say it never had any (no adhesive residues or anything). Fortunately, it was held together with four Phillips-head self-tapping screws, so "disrobing" it wasn't unecessarily traumatic. That being said, what greeted my eyes, sort of WAS.

#19 Takstar / Gear4Music WPM-200 Wireless Monitor System part 3: transmitter mods

And at long last(?), we've reached the point where we take a look at the transmitter end of this wireless monitoring system. Not a whole lot to see on the outside, really: power switch, power LED, channel select rotary switch (1 to 6) on the front, and the DC input barrel socket and the 1/4" stereo input jack socket on the back.

#18 Takstar / Gear4Music WPM-200 Wireless Monitor System part 2: receiver mods

Ok, turns out i lied in the previous post, as this part 2 won't be about the transmitter - so sue me...

Either way, into the meat of it: upon some listening tests with music, it turned out the expanders were misbehaving just awfully. They made the volume jump up and down virtually all the time, in a very disconcerting manner - basically briefly increasing the volume (noticeably) upon every louder bass note (and only for the duration), and dropping back down in between.

I took apart one of the receivers, and lifted the schematic of the expander chip and the associated passives around it. Pretty much exactly what's in the datasheet of the TA31101, with the exception of having omitted the coupling capacitor going to the "Comp In" (pin 11), thereby turning the compressor side of the chip into another 1:2-ratio expander, just like the other half.

#17 Takstar / Gear4Music WPM-200 Wireless Monitor System part 1: receiver teardown / li-po upgrade

This "part one" will be dealing only with the WPM-200 receivers and the lithium-upgrade pertaining to them. The transmitters and subsequent mods will be dealt with at a later date.

Fine, i'll admit, i miiiiiiiight be ever so slightly guilty of the whole "if all you have is a hammer, everything around you looks like a nail" thing, what with these nifty little one-cell charger & protection boards, but hey - at least it kinda validates(?) my "stockpiling" of laptop and phone batteries along the years. Now, i finally have the excuse to put some of them to good use and all that...

During my UK eBay browsing sessions a month or two (or seven?) back, i happened to come across a few listings of faulty wireless monitor sets (transmitter & bodypack-receiver) from this British music gear retailer. A bit of googling later, revealed to be straight rebadges of Takstar WPM-200 sets - nope, they hadn't even bothered changing the model number. The one thing they might've customized, though, are the radio frequency ranges / channels (since there are different regulations on different continents, and so on).

#16 Cheap chinese BM700 / BM800 microphone modding / upgrade, part 2 (CK12 & K47 capsules)

<Continuation from part 1>

After quite a while, i "finally" got around to modding the other three BM800 mics i had. Just for the hell of it, i decided to make a matched triplet (to use as, say, three overheads on a big drum set).

I went with more of my modded-Schoeps boards, and matched components between them, as closely as i could (within reason, at least). Resistors were no big deal, and neither were the capacitors; one of those cheapo chinese ATMega328-based "component testers" helped with matching the JFETs and PNP transistors. No, i didn't necessarily care about the absolute values, just that they're the same (or as close as reasonably possible).