I have taken the 1980's Maplin 225WRMS disco amplifier and brought it up to date with modern transistors and a better circuit.
Constant current source on front end. Better front end decoupling.
Decreased power output to 100 watts RMS.
Output transistors are MJ15003/4.
Constant current source on front end. Better front end decoupling.
Decreased power output to 100 watts RMS.
Output transistors are MJ15003/4.
An externally hosted image should be here but it was not working when we last tested it.
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The amp is an easy build.
The challenging part was drilling the heat sink using the pcb as a template.
The screw holes are easy but the transistor lead holes were harder to drill spot on.
I learned I can do screw holes with drill press but transistor lead legs need doing using drill by hand technique. Put drill in pcb hole then start drill slowly to keep accurate.
Its a bit of a juggling act to get the plastic bushes into the heat sink while juggling screws, isolating tab and transistor all at the same time. Then the nuts have to be put on.
The challenging part was drilling the heat sink using the pcb as a template.
The screw holes are easy but the transistor lead holes were harder to drill spot on.
I learned I can do screw holes with drill press but transistor lead legs need doing using drill by hand technique. Put drill in pcb hole then start drill slowly to keep accurate.
Its a bit of a juggling act to get the plastic bushes into the heat sink while juggling screws, isolating tab and transistor all at the same time. Then the nuts have to be put on.
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I wanted to be true to the original amplifier this was based on which used TO3 packages.
I made another heat sink today and this time it was much closer to being spot on. I did the 6mm holes on the drill press and the 2mm holes were done with a hand held drill. Once the 2mm holes are done I can unbolt the pcb and make the holes 6mm for clearance between transistor legs and heat sink.
I also built the exact same one and replaced the 3055/2955 with a pair of 15003/15004 it worked well however I could not the bias current to adjust . Any chances of posting your circuit Nigel ?
I used mj15003/4 too.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Yes I found I couldn't set the bias.
It seems to not have crossover distortion at zero bias.
Hi Nigel
Is the PCB fully debugged ? all works OK ? also any reason why you only used one pair of output transistors instead of 4 like the original ... I would be interested in buying a few PCB from you .. any chance of some PCB pictures ?
best regards
Is the PCB fully debugged ? all works OK ? also any reason why you only used one pair of output transistors instead of 4 like the original ... I would be interested in buying a few PCB from you .. any chance of some PCB pictures ?
best regards
Not such a challenge. TO3 has yet to be bettered for heat transfer
The circuit works and sounds good.
I thought 2 To3's would be easier to drill than 4.
100WRMS is good for most peoples use.
I have some pcb's unpopulated if you want any.
£5 for each pcb and £2-20 first class signed for UK p+p by paypal.
I will include parts list, wiring diagram and circuit diagram.
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ok great ... also can we have a 4 output transistor version ? how is the best way to purchase please PM me or put on EBAY ?
I already have a four transistor design and pcb laid out.
I don't have any unpopulated pcb's at the moment.
I had a look through the original Maplin disco amp write up and the bias is set by measuring power supply current and it should be 70mA.
But that's with four output transistors.
Here is a page that I made up about the amp, the Maplin article is in pdf form right at the bottom of the page.
I am finding my amp sounds ok and doesn't have crossover distortion even at zero bias voltage.
Maplin 150 / 225 watts RMS amplifier from the 1980's.
But that's with four output transistors.
Here is a page that I made up about the amp, the Maplin article is in pdf form right at the bottom of the page.
I am finding my amp sounds ok and doesn't have crossover distortion even at zero bias voltage.
Maplin 150 / 225 watts RMS amplifier from the 1980's.
I built up another pcb today.
This time I used 2N3055 and MJ2955 as output transistors as per original design by Maplin.
I was worried they wouldn't have enough gain but my semiconductor analyser says 2N3055 has gain of 65 and MJ2955 has gain of 273 !
Sounds ok. So got my self a retro amp.
This time I used 2N3055 and MJ2955 as output transistors as per original design by Maplin.
I was worried they wouldn't have enough gain but my semiconductor analyser says 2N3055 has gain of 65 and MJ2955 has gain of 273 !
Sounds ok. So got my self a retro amp.
I see a lot of hatred for the 2955/3055? What’s with that? I reckon they’re possibly the most important transistors ever - used in absolutely everything.
Snobbery, pure and simple.
I used TO3 2N3055 from 1969 to about 2004/2006 to build some 10.000 PA, Guitar and Bass amplifiers, so abused by definition.
2 to 6 transistors per amplifier, so do the Math.
Had to stop *only* because fakes popping out all over the place, including supposedly "serious" suppliers, made them unusable in production.
A beast of a transistor.
I used TO3 2N3055 from 1969 to about 2004/2006 to build some 10.000 PA, Guitar and Bass amplifiers, so abused by definition.
2 to 6 transistors per amplifier, so do the Math.
Had to stop *only* because fakes popping out all over the place, including supposedly "serious" suppliers, made them unusable in production.
A beast of a transistor.