So almost 2 years ago I was scouting around for something to build - something really different other than a restoration. I began looking into a bunch of old electronics magazines (found here: World Radio History) and eventually came upon this: K6 Amplifier. What intrigued me about this was the SMPS power supply. To digress for a moment, most of my career was spent in avionics where SMPSs are heavily used due to the weight savings. So I had some experience. This looked like an opportunity to build a big amp, but not give me a hernia every time I wanted to move it. Plus the amp itself was interesting - power MOSFETs. Now this is an old design - early to mid 90's. I contacted Reinhart Metz - the designer and owner - he still had some parts for the SMPS, not the pulse transformers, but had a suggestion for a commercially available suitable sub. So I ordered a shitload of parts from Mouser, Digikey, Newark, EBay, and others.
I started on the SMPS. I turned it on - no smoke - voltmeter readings looked ok. And then I looked at the plus & minus output with a scope. There was this 2 V p-p ripple at roughly 8 Hz. Long story shorter, I futzed with this for a year. I tried this, I tried that, maybe this will fix it. I tried breadboarding it. I bought every pulse transformer I could (they are not expensive). No joy. Damn near pitched it out the window. Finally I gave up. Put it in a box and set it on my shelf.
So I switched to building the amp and built a conventional power supply with a toroidal transformer. Got both amplifier boards together and turned them on. They made nice 1.5MHz oscillators (roughly 1.5MHz, each had its own special freq. We use to say if you want an oscillator, build an amplifier.). Ugh. Finally tamed these beasts - turned down the loop gain and reduce the overall bandwidth (originally it was about 750KHz - not much musical content there). Mounted the boards, heat sink, conventional supply in a chassis - weighs about 40lb - gives me a hernia every time I move it. But it works - nicely. I've been using it (listening to it now) regularly for the last six months. (There's more to this story, but I'm cutting it short - let's just say "hum".)
Some time ago on this forum someone posted a link about capacitors. I learned that at some frequencies some capacitors look more like inductors than capacitors and it got me thinking that my SMPS filter capacitors are trying to filter 77kHz. One of the improvements I made to the SMPS was to use 200vdc electrolytic capacitors instead of the spec'd 100vdc caps. Investigating this further I discovered that SMPS in particular are sensitive to the filter caps and you should use low ESR caps. New order to Mouser and new caps installed and the SMPS works. At least to the loads I'm able to test - 150 ohm load @ 80vdc = 43 watts x 2 (plus and minus 80vdc) = 86 watts. It's supposed to be a 1kw power supply.
I started on the SMPS. I turned it on - no smoke - voltmeter readings looked ok. And then I looked at the plus & minus output with a scope. There was this 2 V p-p ripple at roughly 8 Hz. Long story shorter, I futzed with this for a year. I tried this, I tried that, maybe this will fix it. I tried breadboarding it. I bought every pulse transformer I could (they are not expensive). No joy. Damn near pitched it out the window. Finally I gave up. Put it in a box and set it on my shelf.
So I switched to building the amp and built a conventional power supply with a toroidal transformer. Got both amplifier boards together and turned them on. They made nice 1.5MHz oscillators (roughly 1.5MHz, each had its own special freq. We use to say if you want an oscillator, build an amplifier.). Ugh. Finally tamed these beasts - turned down the loop gain and reduce the overall bandwidth (originally it was about 750KHz - not much musical content there). Mounted the boards, heat sink, conventional supply in a chassis - weighs about 40lb - gives me a hernia every time I move it. But it works - nicely. I've been using it (listening to it now) regularly for the last six months. (There's more to this story, but I'm cutting it short - let's just say "hum".)
Some time ago on this forum someone posted a link about capacitors. I learned that at some frequencies some capacitors look more like inductors than capacitors and it got me thinking that my SMPS filter capacitors are trying to filter 77kHz. One of the improvements I made to the SMPS was to use 200vdc electrolytic capacitors instead of the spec'd 100vdc caps. Investigating this further I discovered that SMPS in particular are sensitive to the filter caps and you should use low ESR caps. New order to Mouser and new caps installed and the SMPS works. At least to the loads I'm able to test - 150 ohm load @ 80vdc = 43 watts x 2 (plus and minus 80vdc) = 86 watts. It's supposed to be a 1kw power supply.
