#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.

My solution to this problem, in the end, was to remove the coupling capacitors going to the two inputs of the internal rectification circuits (to pins 2 and 15, respectively), which provide the control signal for the gain cell. At least the signal's nice and consistent now.

That being said, i kinda wouldn't mind if it could go a bit louder though. Towards that goal, i ended up deactivating the "channel balance" trimpots from the circuit (by removing the 4.7k ressitors in series with them), and reducing the 4.7k in series with the expander outputs down to (by "piggybacking" a couple of 4.02k resistors on them). That didn't turn out to do much, but there's also a group of resistors, capacitors and a couple clamping diodes between the tuner chip and the expander, and i've got a hunch there's some attenuation going on there, as well. I oughtta see what the scope says about the signal levels in that area... If push comes to shove, i'm tempted to tack on some dead-simple P-channel MOSFET switch to power the headphone amp chip straight from the lithium cell (ie. before the series diode i had tacked on its output, to bring the voltage more in line with the series pair of AA's it would normally use).

Upon further investigation, there's a good chance the power-mod won't be necessary. Did a quick little test with a wire soldered on to the expander chip's supply line, and the other end manually temporarily connectable to one of the rectification circuits' "storage" capacitor (in this case, the one connected to pin 1). Result - right side went REALLY damn loud. Perhaps a little bit too much so, actually...

Normally there seem to be about 0.53V on both those pins / capacitors, regardless of the signal level applied to the transmitter. Since the 3.3V or so that were on the Vcc line proved to make things a bit TOO loud, i'm thinking i'll tack on some resistive dividers there, to provide maybe 1V or so on those pins. I'll have to first mess around with a trimpot and find something close to what could be called a "sweet spot".

Some 'scoping later, and after tacking on a 10k trimpot to adjust the input voltage to pin 1 of the compander chip (which handles the right output channel), it turned out that, for a seemingly maximum-before-the-transmitter-overcompresses level, about 0.95V are needed on pin 1 to just-about drive the output into clipping, at full volume. Given a 3.2V or so Vcc supply for the chip, that would mean the voltage needed would be aboout 30% of the supply voltage. I have some 4.02k resistors, and some 10k, and those would seem to be juuuust the right ratio (29.7% of Vcc calculated required voltage; 28.6% with the 10k/4.02k divider). I'm also thinking i should be able to get away with only using one divider for both halves of the chip, since they're supposed to do the same job (no expanding, just straight-up gain).

<~10mins later>
Hunch confirmed, mod-wiring done, results satisfactory - we now have the maximum volume in the "pretty damn loud, gonna have my ears ringing tomorrow" territory. The first volume step (out of a total of 16) above "off" is perhaps a bit less than what you'd need as a "normal" level for headphone listening, in a quiet environment. Damn i love DIY... <insert demonic laugh here>

Right, now on to applying the mod to the other two...