Jonny Tubetronics: amps and synth stuff now in one thread

Maplin 150W MOSFET amp:

Back in the 90s when I was a teenager, I used to do house parties and a bit of DJing. It kind of led to my stereo growing into a pretty big soundsystem and I used to do house and hardcore events in pubs and halls. It was all home made kit except for my 12" flight case speakers. Something I built a number of, were these 150W MOSFET amplifiers from Maplin. They were pretty bombproof and would survive no matter who was at the mixer.

The one here is the sole survivor, which I built when I was about 15. I can't remember exactly how many of these I built, but it must have been 6-7. I know I blew up the first one I built, because I remember connecting the power supply backwards during testing and all the caps exploded. There's the stereo pair I'm going to talk about here. There was a pair that I bridged to make a 400W sub amp, using an audio-grade op-amp to invert the signal. Those ones eventually blew. Then, there's a very crusty 47K dual potentiometer in my electronics box which is evidence of another stereo pair which had a volume control. I don't remember that one or what happened to it.

I built it in two cases to keep the noise down - one for the amplifiers and one for the power supply:







There was no volume control because they were fed directly from my mixer which controlled the individual levels. There were two speaker outputs because these could drive 4 Ohms and I could hook up two sets of 8 Ohm speakers in parallel. +55V/OV/-55V DC was taken from the PSU to the amps by the kettle plug; not ideal, but I couldn't think of a better way to do it at the time and still be able to separate the boxes for easy transport.

Everything was sourced from the Forest Hill Maplin in SE London, which was on the bus route on the way home from school. It was an EXTREMELY rare thing to be near a physical, actual Maplin shop back then and this one was complete with all their books on the shelves, wooden hatch at the back where you asked for components and all the staff were electronics experts (well, to me, anyway). No silly cheap plastic radio controlled cars, no rubbish plastic 100000W PEAK MAX PMPO speakers, just the nerdy electronics stuff.

So bear in mind that I made these around 1993 and as far as I can remember I haven't opened them since I built them. This is what I found inside:



A little bit messier on the wiring than I expected, but I was only 15. Looks like cutting holes wasn't my strong point:



I say that, but I didn't have a small battery drill or sharp drills like I do now. I'd have done this with dad's Black and Decker power drill, chain-drilling with whatever drills he had knocking around and a hacksaw. I don't remember having a file. That was my own pop rivet gun.

The amps I noticed are not the same length or the same height. I assumed this was more of my poor drilling, but both the amps are slightly different. This might have been because I blew the first one up and had to get a replacement, so they might be from different batches. Have a look at the size of the heatsink plates under the MOSFETS and they're slightly different too:



I know I etched a number of circuit boards for the PSUs using photoreactive PCB sheets in a new UV light box my school had bought. I guess I would have printed the circuits onto OHP acetate, slung it into the light box for a couple of minutes, into developer fluid, then in to the etch tank. God knows what chemicals we used with no gloves or safety specs. I was hoping to find one of these in the PSU box, but it's unfortunately just tag-wired together. I am slightly disappointed because I know I made at least two of these and it would have been great to see them again.

The plan. Rebuild. Slightly safer, slightly different layout and with a volume control. Re-use the cases but tidy them up a bit first. One amp module is loose (probably from being dropped and bashed about) so I'm going to come up with a slightly better mounting system. I was thinking of swapping the PSU connections to DIN rail power distribution blocks, but I think it might not be worth it. I've got some old-school tag boards which might be more fitting for the build:



On top of that, they need some new feet which I might 3D print. Some fuses and better earthing might be nice. After that I'll fire them back up and re-set the bias and quiescent current (which I never actually did properly back in the day). Then they're going back into use in the living room with a nice retro knob on the volume control.

References.

Pretty sure this is the 90s Maplin Magazine article which accompanied the kits:

bhiab.se/data/files/03667481__skanna0060.pdf

This is the earlier iteration with the lower output and resistor inside the output inductor:

www.muzines.co.uk/articles/mosfet-amplifier/2651

I've got one of those in a box in the loft somewhere, brand new and never used!!

I've got a Leak TL12 monoblock valve amp that has bulging caps that needs rebuilding. Could do with a second one to make a stereo pair but they are getting a bit epensive now.

I built an EPE TENs unit, after my then GF was complaining about some ache or other. It worked quite well, though of course, she was afraid of using it, so I did most of the initial testing! Nowadays, you can buy them on E-bay for a tenner or so.

I’m still using a JLH designed 15W class A amp that I built in the mid 90s based on a Williams Hart kit. I made the frame, case and heat sinks. Heat sinks made of stacked copper sheets and bus bars soldered together. That was a mission. I was particularly pleased with the blue led power light….. in my defence they were new and exotic at the time!

Fed from an ebay-bargain Audiolab 8000 preamp, which is mostly fed by an iPad these days, though I still have turntable (Rega Planar 3) cassette and CD sources available.

Nick

Got a bit of time to strip this down. First thing I did was de-solder the wires from their pins and cut them free from the other ends:



There was an unmistakable smell of old solder flux. It doesn't smell like that any more. Undid the screws and popped the amp modules out:



Amazingly, the heat transfer compound was still greasy and sticky. So although one of the amp modules had shaken loose, it was still in there trying to do its job. On the underside, I wasn't exactly sure what I was going to find. It wasn't fantastic, but nothing a bit of TLC couldn't fix:



First thing I noticed was that I hadn't cleaned the flux off after soldering. I'd heard that it can be quite corrosive and it's turned the PCB resin brown where it's been sitting on it. I had a skim read through the instructions the other lunchtime and blah blah blah it said something about soldering a thick solder fillet along the track between the bolts that hold the MOSFETS on. One of them has been done but not the other, so maybe I got lucky here. One side has definitely got hot and looks a bit scorched. I'm guessing this was the side which was flapping about on its bolts. So I cleaned both boards with some IMS to try and get the old flux off. It was a bit stubborn but I got most of it. Then I clipped the unclipped MOSFET legs on one of the boards and filleted the tracks to the output:



Then I drilled out all the pop rivets and you can see the true state of my hole cutting!



The plan is to cut most of that side of the case out. I'm going to join the two halves to make a single case and retain that side of the PSU half for shielding reasons. In the PSU side I'll add a power switch and kettle cable mains power input. The PSU could also use a fuse and could really do with the bleed-down resistors moving into that box. In the amp side I'll add a volume control and relocate the inputs/outputs. The heatsinks will then be on the far right hand side. If I've got a bit big enough, it might also get a gloss carbon fibre panel on the front. I'll bond that on with what is currently one of the strongest structural adhesives in aerospace and is used to hold modern aircraft structures and F1 cars together.

Progress yesterday and today. Rebuilt the power supply, mounted the amps and wired it up. Here we are testing on the bench:



I should have taken a pic of the PSU before I started unsoldering it. It was basically hard wired, with the caps wrapped in gaffer tape and stuck down on double-sided adhesive pads:



I rebuilt that on a strip board with the caps held on with ties and mounted properly on nylon PCB stand-offs:



Cables all go into screw-down terminals for easier connections and testing. Fired it up and nothing exploded which was good.

IMHO safety concerns should come before performance, so the case and any other isolated, exposed metalwork must be connected to mains earth. To do anything else is asking for trouble...which could be of the fatal variety!

I would say that 0Vdc and mains earth shouldn't be connected together, especially on an audio project.

The drain (shield) of the signal cables should be connected to 0Vdc at the PCB-end (or a slight improvement might be made connecting to the 0Vdc star-point on the PSU), and insulated at the other.

Depending on what your signal source is, you might need to add isolating transformers on the signal side before the pots.

Yeah I can't remember what the reason is but I'm pretty sure this is the case. I seem to remember it was from a safety perspective rather than an earth loop / noise thing but I might well be wrong there.


That's the only thing I didn't try. Annoyingly, once I'd worked out it was the earth on the shielding making the Zobel get hot, I cut the tags off and cut the shielding connecting the runs together (to shorten the runs) to see if it was that picking the noise up like an aerial. I'll have to re-wire the inputs to try it.