My Blameless is working!

I’m so happy! Ever since I was a humble EE student, I’ve wanted to design my own hi-fi power amp based on Douglas Self’s “Blameless” philosophy. I now proudly present the prototype.

doesn’t look goodbut seems to work well so far

I’ve built power amps from other people’s designs before, but this is the first one I’ve designed, albeit with a lot of help from Self’s “Power Amplifier Design Handbook.” It’s a modular system, with a driver board that can be hooked up to any kind of output stage, to make different kinds of experimental amps.

It’s still not finished: the protection circuitry and THD need testing. But it’s passed the first hurdle, in that it can run with a good DC offset (I measured 16mV), stand +/-60V rails and drive a load without blowing up.

The last three amps that I’ve built were powered by valves (tubes?), and the two before that had MOSFET output stages, so working purely with BJTs was a bit of a culture shock. Self always argued that they were the best amplifying devices, and they certainly seem pretty good. The OnSemi MJ15024/MJ15025 pair of transistors I used in the prototype cost a few dollars each, less than half the price of equivalent MOSFETs, and they make as much Umph as a pair of KT88s. They didn’t want to oscillate or explode, and the whole thing generally just worked first time. Apart from that evening I slipped with the scope probe and took out half a dozen trannies in the driver board.

This is something of a multicultural project. The output trannies were made by Motorola in Mexico, all of the other ones came from Continental Device in India (that’s what you get when you buy Farnell’s “Multicomp” value brand transistors) and the whole mess was assembled by a Scotsman wired on Fairtrade coffee beans.

And yes, Self convinced me to buy a distortion analyzer. So far all it’s told me is that I need to get a better signal generator.

3 Replies to “My Blameless is working!”

  1. That’s a beautiful thing! I love the heatsink. I’m guessing it came from a mid-80’s Ducati Desmo. Sweet. What kind of THD are you looking for?

    Everything I try to design ends up looking like a 5F6A, but I know nothing. I wonder if its more difficult when you have a good brain and good gear, when you have concrete goals in mind and will be working with fairly narrow tolerances? I admire your skills.

    Not knowing any better I’ve always thought that a tube pre amp and a solid state power amp would be the cat’s meow. I’d love to hang an SS power stage on one of my contraptions.

    Hey, will you be plugging your bass in?

    Can’t wait to see the schematic and hear the clips. You’ll know its good when it can do both ‘Billie Jean’ and Mahler’s Tenth.

    But a sawtooth is fine for now. Baby steps.

  2. Hi Ken, long time no see! The BAGA is still my benchmark for metal tone. 🙂

    I tried playing guitar through the Blameless, and it sounded… clean! A hi-fi listening test will have to wait until I’ve built a stereo pair (where did I put that other heatsink?!) but I’m sure it can cope fine with Billie Jean, Mahler, or heck, even both at once! It’s all just a time-varying voltage, after all.

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