The Lunchbox of Doom

While my transformer based amp awaits some more soldering work, I nabbed a tin lunchbox and built an enclosure for a previous build, build around the Acupulco Gold/Tufnel/Sunny T dual LM386 design.

Mine includes a Big-Muff style tone control in between the two power amps, which has some mixed results. All the way to the low side, it produces some nice tones, but to make the high side listenable, I may have to solder a low-pass filter on the output. Small, full-range speakers do get shrill compared to guitar amp speakers.

Sound sample here. build pictures below.

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.

Transformer Progress

I've been posting somewhat haphazardly with regards to my amplifier goings on, and am trying to wrap my head around the concept of audio transformers. I promise this will settle down once I stop barfing my random technical thoughts online.

Anyway, I think I made a breakthrough today by running some white noise through the 1:1 transformer I pried out of an old digital TV box.

I know this is not the way to do stuff, but I don't have an oscilloscope and Waveosaur is free.

What you see there is the frequency response for white noise, and a telltale peak at 1242 Hz. The transformer was being used in an RLC high-pass configuration, with 240Ohms and a 100nF cap, which would indicate my transformer is roughly 0.16 Henries. Different cap and resistor values seemed to be consistent with this pattern, so at least I have that one part of the amp somewhat figured out.

Now to drive it from the 386 without blowing another power cap...

Rat's Nest

Threw together some proof of concept. The noise it makes is... noisy.

Transformer Based Saturation

Just a little something I'm working on. I got a transformer hooked up to a small LM386 guitar amp I built, and so far the sounds its producing aren't unpleasant. More to follow (including some Python code, because I got tired of manually calculating the 386's gain and bass boost frequencies).

Disclaimer: not built and tested. Yet.