CONNERLABS

Category: Electronics

  • 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.

  • To EQ or not to EQ, that is the Q

    If you’re a follower of hi-fi trends, you might have noticed that tone controls don’t seem to be cool any more. High-end amplifiers have become very minimal, with only a volume control and power switch.

    Well, there’s high end and high end. Richard Burwen’s system would certainly qualify as high-end by almost any definition, if only because of its 20,000 watts of amplification, 169 speakers, and 34 channels of active crossover and EQ, which Burwen claims to have spent a year adjusting by ear for a flat overall frequency response. In other words, the very opposite of a minimal system.

    Moving on to my own hi-fi system, which has 60 watts of amplification, 2 speakers, and no tone controls or EQ whatsoever, and has been plagued by annoying room modes. I’ve tried several different speakers, but the bass on all of them just sounded terrible in there.

    Recently, I realised that the fundamental problem is that my living room has the two large opposing walls made of solid brick. These two reflective surfaces allow a standing wave to develop between them, and the result is a highly resonant bass boost of about 9dB at 60Hz. I fed the measurements of the room into an online room mode calculator, and it agrees that I should get terrible bass.

    So, what to do about it? The minimal solution would be to damp the resonance by installing bass traps. This would be a major carpentry project, though, and good bass traps have to be physically large, so I would lose space in the room. And, I hate working with rockwool, it makes me itch all over.

    So, like Richard Burwen, I decided to install an EQ, and “tune by ear for flat frequency response”. My first attempt was with an Alesis PEQ-450, which is a 10-band digital parametric EQ. It only took about 5 minutes to find the offending frequency and notch it out, and I was amazed by the improvement. I found myself getting out those old hip-hop and drum’n’bass CDs that had just sounded offensive on the system before.

    However, I wasn’t too happy with the sound quality of the PEQ-450. Maybe I was imagining it, but I felt that it grunged up the treble somewhat. It was also too big and I had nowhere to install it. I ended up buying two (they’re mono) Presonus EQ3Bs on Ebay, and again, tuned them by ear for best bass. I could have gone digital with a Behringer DEQ2496, but in the end, I decided that the simpler solution would be better.

    So now, I find myself in the situation of committing audio sacrilege (tone controls are bad! So EQ must be worse!) in order to get a major improvement in the performance of my audio system. Mmm, sacred cowburger.

    EQ

    The EQ3Bs in place

    The system

    The system. The graffiti paintings are by my brother, and help to break up standing waves too. 🙂

  • My first EMC test pass!

    It had to happen sooner or later… Something I built had to get EMC tested! In this case it was the Doble PDS100, for which I did the firmware, digital and power electronics, and PCB layout. The happy news came back from York EMC today. Hopefully this will be the first of many 🙂
    I am the one on the right

  • Transformerectomy!

    No, it’s not some sort of futuristic android sex change.

    I’ve lived happily with the Crown SXA for about a year now, and by and large, it’s been a pleasure. It sounds great, has more than enough power for me, and now winter is coming, I can warm my feet on it!

    My main worry, though, was always the power transformers. Not only are they something like 50 years old, but they were never designed to handle 50Hz power, which is what we have in the UK. So, even with RG Keen’s Vintage Voltage adaptor installed inside the chassis, they hummed really loudly, got really hot, and gave off worrying smells.

    My first port of call was Sowter Transformers, a custom winding company in Ipswich. I sent them the dimensions of the original trannies, and got the reply: “We don’t do metric sized transformers”. They were made in Indiana in 1960. How can they possibly be metric? But none of Sowter’s standard core sizes were even close.

    So I thought, maybe “metric” just means “Not British”. A quick peek in the Hammond catalogue (sorry, catalog!) and sure enough, there were several transformers of the right size. I settled on the 290KX, which is Hammond’s replacement for the Marshall JCM900 guitar amp, and Bluebell Audio in Dundee supplied me with a pair.

    After purchasing some end bells from TAD and some M4 threaded rod from “boltmeup”‘s Ebay store, it was time to pull the poor Crown apart!

    dismantled!

    This took most of the day. The new trannies had lugs sticking out of their bobbins, and needed some trimming to get them through the chassis holes. “It’s just like trimming a toenail”, as Philip from Bluebell told me, but much more expensive if you get it wrong!

    New vs. old

    After much fiddling, rewiring the rectifier to a bridge, wiring one of the transformers up backwards (thank goodness for the old light bulb limiter trick!) and pulling out the redundant Vintage Voltage unit, she was good to go!

    done!

    The bad news is, the new trannies still hum a bit. They’re quieter than the old ones, but not silent. They also still run fairly hot. The good news is, they don’t smell!

    I finally painted the OPTs black to match