Line out for MicroBeast (v2)

After the success of Line out for NanoBeast the MicroBeast's line output was starting to sound distinctly ropey. This is because when I bodged it on many months ago, I didn't actually think about it or design it properly: I just tacked wires onto whichever component in the audio pathway was easiest to solder to.

The MicroBeast's audio output circuit looks like this:

('scuse my annotations).

The "easy to solder to" component in question was C37, which as you can see is part of the low-pass filter, and it's on the input side of the NPN transistor. This turns out to be a spectacularly bad choice, as we'll peak at 5v, and anything we load the circuit with will almost certainly pull too much current from that AUDIO line (which is actually the OUT2 line of the 16c550 UART) possibly resulting in damage to the output driver - more on this later.

Another drawback of the original mod is the common ground, and you can definitely hear all sorts of noise, particularly when the LED displays are active.

I can't use the NanoBeast circuit as the MicroBeast's signal is already low-pass filtered, but we can use a simpler variant of it that omits the filter:

I've moved the tap on my NanoBeast to be the collector of Q31. I also need a ground, which is the emitter of Q31, so that's a nice neat mod, secured with a blob of hot snot.

Mono vs TRRS

I've only just realised that my el cheapo Chinese 2 channel mixer is in fact a stereo two channel mixer, and I've just been sending LEFT all this time with no corresponding RIGHT signal. This is extra confusing because I'm using a 4-pole socket (TRRS) on my interfaces as that's all I have to hand.

My solution is to wire Tip and Ring1 together (so both LEFT and RIGHT come from the same mono AUDIO signal), and to wire Ring2 and Sleeve to ground, as shown in the above circuit. I modified the NanoBeast line out to be the same.

A/B listening test

A colleague was casting nasturtiums on my efforts, saying it was impossible to tell in A Feersum orchestra that there were actually two 'Beasts playing in unison. Therefore I present definitive proof! This time with fresh MicroBeast and NanoBeast ports of the povver engine, courtesy of utz from the Irrlicht Project.

I used the mixer's faders to isolate each 'Beast so you can hear it solo. The MicroBeast sounds a little distorted and gutless. I think maybe I need to change the 2.2k resistor for a 3.3k to bring the level down a bit, but I suspect that it might actually be output driver damage in the 16c550 from my earlier ham-fisted efforts.