Maplin 160W (225W) bipolar amplifier design

Hi bluevanman63

Think this was the one with two pairs 2N3055/MJ2955 in the output?

If so the one I'm thinking of had BFY50's and suchlike. BUt these were never 80V transistors, had to use Vcer spec. which seemed dubious.

You could probably use 2N3019 for all NPN's and 2N4033's as PNP's for input & driver in TO-39 cans or with a bit of persuasion BD139/BD140's for all small signal devices.

Maplin 160W (225W)

At last, someone who has the same AMP. I build mine back in 1995 i have used it for my mobile disco since. it's been a fantastic Amp until now. The bias circuit has gone a bit off track and as the amp warm up the bias reduces and turns off the transistors. cut a long story short while in a hurrt trying to fix it pick up the speaker leads and pluged it in. Puzzled by the no output turned it up only to find out that i had connected both channles together and burnt out two resistors on the board. Do you have any info on the amp such as a schematic and parts list. This has been such a good amp i really want to get it fixed. PLease help
bluevanman63 said:
Does any one have a copy of the parts list for this design (as published in old copies of the maplin catalogue and used in their diy disco consule many years ago). I have a schematic and a number of broken modules. Any help welcomed

I am pretty sure it was 225WRMS.
I got told to turn it down on a few occasions at discos !
Are you talking about the famous Maplin MOSFET design, based on Hitachi apps data? If so, you can find versions of it here:

These amps can be uprated to offer very low-distortion performance. David White of White Noise Audio used to sell a CCS kit for them. The CCS prevents the VAS being influenced by loading from the OPS. If you fit the CCS, but still go on the basis that the PSRR probably isn't the best in the world, i.e. use plenty of smoothing in the PSU, you can attain good results. Contrary to much opinion, the amps are pretty stable as well - as long as you respect the laws of laying out MOSFET amp PCBs (MOSFETs are capable of operating at very high frequencies, and will oscillate if the layout permits them to do so).

Rod Elliot also has a MOSFET design that's loosely based on the Hitachi apps data - many here have built it and attained good results.

Note that you can directly sub Exicon MOSFETs for the Hitachi (which now sell for silly money).

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There were three Maplin Amps as I remember.
The MOSFET amp which used Lateral FET's and was based on an Hitachi application note. That was the best of the bunch. Then there was a 50 ? watter using bjt. I can remember the wording now, " we threw away are technical specification and designed an amp that was musically perfect " or something along those lines. Then there was the muscle job, erm right then ;) using 2N3055/2955 I think.
The laterals and Hexfets are different animals. The Exicons are laterals, no problem there, D Whites design used HEXFET's ( well the one I have seen does ) and has a thermal Vgs multiplier on board.
ooh, that takes me back to being a spotty teenager in the 80's...

I was dismayed to learn that our useless Ferguson "music centre" produced a staggering 6w / ch and sounded like a fart in a phone box. Then I discovered the Maplin's catalogue - this when you still had to get everything by mail order (no shops), and most times your order arrived with a note to explain half the stuff was out of stock..
However, this is what got me into Hi Fi and further into electronics. I built a very nice amp around the 50W modules with a chunky power supply, which is still part of my main system - they really are very good. That was the "We threw away all our technical books" model. The assembly instructions were just a photocopied page from the sales catalogue, but were just enough to go on and the boards were well made. 50W seemed amazing enough, but the 150W amp and 225W models were out of this world to me.

I hope you find the info needed to fix the amps. I may have it but it is in the UK and I'm away at the moment.

i'd be interested to see this.. as a kid i used to see the amp modules for sale in the catalogue, even though there was no way on earth i could afford them or had the skills to build them!!

The best they have now, if you can find them, is the lousy Velleman amps - the circuit in them is ultra simple ultra cheap garbage... or an ST chip of some kind.
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The MOSFET one is well documented as you probably know, with a lot of copies and variations as well.
The 50 watter, not seen a circuit for that for many years.
The big one. That was a really basic circuit I seem to remember, a classic textbook simple amp. Something like an input pair, VAS and drivers, nothing else, no CCS or anything. You just hung outputs on the end of it.
I think I have the 50w circuit around somewhere, as I have a couple of the PCBs built up.

It's a fairly normal amplifier with a differential input (though with a resistor rather than a current-source in the tail of the LTP), together with a Linsley-Hood style gain stage (*) and a CFP complementary output stage.

(*) the compensation is applied as in most JLH amplifier designs, rather than as a simple C-B capacitor in the VAS.

Very much a 1980s text-book design, but it's a pretty good amplifier for general use, and the FR4 pcb is well laid out.