I have bought a Maplin 100WRMS mosfet amplifier off ebay that hadn't been built, it was still in kit form. As rare as rocking horse droppings !
So I built it up tonight and it worked first time after all these years.
Sounds very good.
So I built it up tonight and it worked first time after all these years.
Sounds very good.
An externally hosted image should be here but it was not working when we last tested it.
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Positive karma - I hope.
This is exactly the board and components (even to the the "staining" on the o/p Hitachi TRs) that you very kindly helped me be confident enough to power up last November.
Mine continues to do excellent duty driving a pair of same vintage Castle Conways thus making my Television sound much better than it did.
This is exactly the board and components (even to the the "staining" on the o/p Hitachi TRs) that you very kindly helped me be confident enough to power up last November.
Mine continues to do excellent duty driving a pair of same vintage Castle Conways thus making my Television sound much better than it did.
Probably only steel could be as thin as the cover and still be welded consistently to the baseplate. It certainly is steel, even being a little rusty, hard and magnetic so you can lift them even with a weak fridge-magnet....
I have a 2SK135 and a similar few vintage Hitachi TO3 transistors that have just as thick a base (3mm) that is light and soft enough to be scraped like aluminium. I'd guess they are an aluminium alloy suited to the type of weld used.
Perhaps the "insert" on the base is just an indentation mark of the bottom die of the welder. That can be any size or none at all depending on the die and whether it's for attaching a heat spreader, or the cover.
I have a 2SK135 and a similar few vintage Hitachi TO3 transistors that have just as thick a base (3mm) that is light and soft enough to be scraped like aluminium. I'd guess they are an aluminium alloy suited to the type of weld used.Perhaps the "insert" on the base is just an indentation mark of the bottom die of the welder. That can be any size or none at all depending on the die and whether it's for attaching a heat spreader, or the cover.
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Trip down memory lane.
I built more than 20 of these kits for me and friends when I was a teenager. Each one worked flawlessly. 6 modules were employed as Pro amps and used to power the speakers in the school hall where school dance parties were held. Never did one fail, that is upto the time I left highschool. Thinking of which, they probably still there.
I built more than 20 of these kits for me and friends when I was a teenager. Each one worked flawlessly. 6 modules were employed as Pro amps and used to power the speakers in the school hall where school dance parties were held. Never did one fail, that is upto the time I left highschool. Thinking of which, they probably still there.
I had to fix a amp for a church about 10yrs ago.
Turns out it was a DIY amp, based on the Maplin MOSFET amp.
The two modules were attached to the same heatsinks, with the one module mounted upside down.
One channel still worked, turns out it was the upside down module that was faulty.
I measured the output fet to be a short.
When I took out the fet, I saw that one pin was soldered to the case.
I cleaned of the solder and put it back in, working still today.
Turns out that the module got so hot that the solder on the pin ran down the leg, shorting it to the case, very hardy amps.
This was also the first discrete power amp kit I built while at school.
Turns out it was a DIY amp, based on the Maplin MOSFET amp.
The two modules were attached to the same heatsinks, with the one module mounted upside down.
One channel still worked, turns out it was the upside down module that was faulty.
I measured the output fet to be a short.
When I took out the fet, I saw that one pin was soldered to the case.
I cleaned of the solder and put it back in, working still today.
Turns out that the module got so hot that the solder on the pin ran down the leg, shorting it to the case, very hardy amps.
This was also the first discrete power amp kit I built while at school.
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The top metal has oxidised just slightly since the 1980's so I would guess it is aluminium. The base looks aluminium too.
The transistors are just standard TO3.
The heatsink had quite big holes drilled in it for the isolation bushes.
Hi,
You need to uprate the series resistors to the Mosfets or
it will inevitably die at some point, I recall 100R to 270R.
Fine at low volume, but it dies being being hammered.
rgds, sreten.
YMMV, but I've had bad experiences driving said amplifier
hard. The flange in the kit I had I recall being aluminium.
The devices being steel T03's.
You need to uprate the series resistors to the Mosfets or
it will inevitably die at some point, I recall 100R to 270R.
Fine at low volume, but it dies being being hammered.
rgds, sreten.
YMMV, but I've had bad experiences driving said amplifier
hard. The flange in the kit I had I recall being aluminium.
The devices being steel T03's.
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Hi,
A Vbe multiplier is chasing your tail as there is no optimum
bias point that needs to be maintained for a Mosfet pair.
The bias point is so arbitrary it has no sensible purpose.
Similarly source resistors serve no useful purpose for bias.
Mosfets are not BJTs and cannot be treated the same.
In the Maplin circuit the output will only swing to 7V or
more (depending on the load) below each rail at best.
The high voltage drive needed for each device renders
any sort of BJT bias schema completely irrelevant.
rgds, sreten.
A Vbe multiplier is chasing your tail as there is no optimum
bias point that needs to be maintained for a Mosfet pair.
The bias point is so arbitrary it has no sensible purpose.
Similarly source resistors serve no useful purpose for bias.
Mosfets are not BJTs and cannot be treated the same.
In the Maplin circuit the output will only swing to 7V or
more (depending on the load) below each rail at best.
The high voltage drive needed for each device renders
any sort of BJT bias schema completely irrelevant.
rgds, sreten.
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I need to set the current through the Vbe multiplier to around 7ma to keep the transistors working within their spec.
I am using 600mW transistors in the VAS and LTP stages.
I have designed a few hexfet amplifiers and they do need a bias voltage to stop crossover distortion. Having said that I found I could get away with just 10mA to stop crossover distortion.
I am using 600mW transistors in the VAS and LTP stages.
I have designed a few hexfet amplifiers and they do need a bias voltage to stop crossover distortion. Having said that I found I could get away with just 10mA to stop crossover distortion.
An externally hosted image should be here but it was not working when we last tested it.
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Hi,
There is some point where biasing resolves into utter
nonsense trying to control the consequences of poor
heatsinking, good luck with that, I'm not interested.
Decent heatsinking and simple is my opinion.
BJTs need optimum bias for distortion, not thermal
issues, and Mosfet bias doesn't, only thermal stuff.
rgds, sreten.
rgds, sreten.
There is some point where biasing resolves into utter
nonsense trying to control the consequences of poor
heatsinking, good luck with that, I'm not interested.
Decent heatsinking and simple is my opinion.
BJTs need optimum bias for distortion, not thermal
issues, and Mosfet bias doesn't, only thermal stuff.
rgds, sreten.
rgds, sreten.
Ahh, remember the Maplin MOSFET modules too, used them in my first DIY build as a spotty 20-something engineer Rather ambitious project because I built a seperate PSU along lines of Cyrus PSX and used identical case for the amp. Front-panel mounted heatsinks that looked (and ran) cool even if there was nothing else apart from amp modules. Don't think the THD figure was particularly great but boy did they sound good Will definitely be interested in your progress and results.Maplin MOSFET amp revival long overdue
Rather ambitious project because I built a seperate PSU along lines of Cyrus PSX and used identical case for the amp. Front-panel mounted heatsinks that looked (and ran) cool even if there was nothing else apart from amp modules. Don't think the THD figure was particularly great but boy did they sound good Will definitely be interested in your progress and results.Maplin MOSFET amp revival long overdue- Status
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