A Working Prototype

There have been a few random posts with pictures of wires, some random schematics and some screenshots, so it might be time to give things some narrative.

For a few weeks, I've been trying to build a tiny 5 watt amp out of ICs with one particular feature: after hitting a small power-amp (an LM386), the signal gets routed through a transformer. This serves a couple of purposes, the primary being the emulation of the power amp transformer saturation that happens with an overdriven tube amp. After the transformer, the signal gets routed to two LM380 IC's, which bump the signal's power back up to 5 Watts or so.

Now, with the kind help of the folks at www.diystompboxes.com, I've managed to put together a working prototype on breadboard with some sensible component values. And it works.

Quick sketch. One day, I will learn Eagle.

Now, it produces a usable tone, but recording a demo has been hampered by having no heat sinks for the LM380's. I can play for a few seconds, and then the chips get too hot to be healthy, and I have to power down. This will be remedied in the future, and then I will record some tone demos.

But I can give some details on what's happening with the most critical component, the transformer. The unit I have is the Midcom 671-8001. Now, this was pulled out of an old Digital TV box someone was throwing out, so it's not really a high quality component. Its primary purpose was to figure out how such a component would fit into the signal chain.

The configuration I've settled on is a combination of two things. The transformer, the DC coupling capacitor for the 386's output and a small resistor form an RLC high pass filter. The coupling cap is necessary to remove DC from the amp's output, and the resistor is needed to ensure a stable filter (I think. Don't quote me on that.). Secondly, there's a cap on the output forming a low-pass filter to take some of the really nasty fizz from the LM380's input stage. After trying out several cap values, the transformer seems to behave like a 240Ohm resistor, which is close enough to the sum of its primary and secondary DC resistance values (108 + 120 = 138 Ohms). I'm probably missing some math that would explain it with more certainty, but it does seem like a significant coincidence.

Running a white noise signal from my phone to measure the frequency response while testing caps.
Expedient. Not scientific.

There are other things which remain unknown. The primary one is that I have no idea if the transformer is saturating. The LM386 puts out a lot of distortion, and the filtering effect of the transformer and its neighboring components is further altering the signal. For all I know, all I have is a needlessly complex filtering setup for my signal.

Nonetheless: I have sound. Baby steps.