BinaryMike
Pelagic EE
CatSkin grew out of a fascination with MCM’s very low cost 555-7125 “30W” speaker line matching transformer, which was used successfully as an audio output transformer in a couple of earlier push-pull triode power amp projects (Perveant & Bonseki). Because of its excellent high-frequency characteristics, I began to wonder if this transformer could be shoehorned into an even less likely role; as the OPT in a five-watt single ended triode amplifier. MCM’s low-frequency spec for this item is 50Hz, which, by the power-proportional-to-frequency-squared rule, should extend down to 20Hz at 5W. MCM doesn’t disclose how the 50Hz cutoff was determined, but a little negative feedback can do a world of good in this department as long as the core doesn’t saturate. It’s not challenging to find power triodes with anode resistance in the 400~500Ω range, to work with the MCM transformer’s 1,000Ω nominal primary impedance, although this impedance reduces considerably due to insufficient inductance at the low end. The biggest problem is that single ended amps task the transformer with a formidable magnetic bias, which normally compels the use of gapped cores, and that’s akin to sharkbite on important aspects of transformer performance. One would think that cheap transformers are tragically unfit for this job.
Well, as my dad was fond of saying, there’s more than one way to skin a cat. I chose the outlandish solution documented here because I enjoy hacking my way through brambles and learning a thing or two along the way. In the end, I produced an amplifier with excellent sonic qualities and a bit more power than initially planned. A brief investigation of low-frequency distortion effects of DC bias in the 555-7125 transformer was published over at diyAudio in December of 2015. A follow-up description of CatSkin amplifier breadboard results was presented in June of 2016. My decision to publish the end product here was influenced mostly by the convivial atmosphere.
CatSkin isn’t a project for beginners. It does use inexpensive components, but it’s more complex than most SET amps in this power class and it lacks refinements that could improve reliability as well as temperature stability. At this juncture, CatSkin is more like an advanced experiment or early prototype than a production-ready design. Better output transformers could improve low-frequency performance, but would likely require major changes to the flux cancellation circuit and may not improve sonic results because the high-efficiency speakers normally used with SET amplifiers don’t often perform well below 40Hz. Advanced DIYers who are curious about the sonic impact of magnetic bias in OPTs could find a lot to interest them here.