Maplin "50W HiFi" amplifier module instructions.

Hi all,

A long long time ago, I made an amplifier using two of these modules. Since I made it in my yoof, I'm quite attached to it. Recently though, my wife bashed her head on the underside of the shelf it was on. There followed loads of crackling from the speakers accompanied by the disturbing hum of power rails coupled directly to speakers. I sprang up and flew across the room to switch the amp off (which wasn't easy since I had to be careful to avoid my wife who was now rolling around on the floor, clutching her head) but I was too late and the left channel was toast.

I'm a bit mystified as to how the head-bashing could have caused any damage but, aside from that, repairs are progressing and new bits are on order; I only lost two of the output transistors, some resistors and a couple of polystyrene caps. The only problem I'm expecting is how to set up the quiescent current, since I can't find the original instructions for the modules.

I know it's a bit of a long shot, but does anyone here happen to have a set of instructions hanging around? I think it was quite a popular module for quite a long time. The order code was HQ68Y.

Oh, and the wife's making a full recovery, by the way.

Cheers,

Colin.

Reply to
Colin Stamp
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how the **** did you lose polystyrenes?

Don't have instructions, but there should be a trim pot pot somewhere. Turn it so the current drawn (remove line fuse on the low voltage side and meter across) is as low as you can get, the turn the pot up to give

5-10mA more than that.

Reply to
The Natural Philosopher

They copped a bit too much heat from nearby resistors that were busy converting themselves to smoke. Now they're a very funny shape.

Thanks for that. Sounds like a good plan if I can't find the specific magic numbers from Maplin.

Cheers,

Colin.

Reply to
Colin Stamp

Still got one of those knocking about in a bits box somewhere... never did get round to turning it into a fully fledged amp alas. Note sure about the instructions - I will have a look.

How is the shelf? ;-)

Reply to
John Rumm

If one channels still ok and one toast, not hard to copy what the other one's doing current-wise.

NT

Reply to
Tabby

I hope she's going to stump up for a new amp!

-- Halmyre

Reply to
Halmyre

It was normal to swap the fuse for a very low power sacrificial resistor when setting the quiescent current, so that you can't draw too much current to damage output stage, and if you try, the resistor alone goes up in smoke.

Reply to
Andrew Gabriel

Hi Colin I'm pretty sure I have a scan of the original construction article if you still need it - assuming we're referring to the 50W MOSFET module that was very much based on the Hitachi Mosfet data sheet. it may take me a day or so to track it down...

Regards J^n

Reply to
jkn

Good plan.

Reply to
The Natural Philosopher

If its hitachi mosfets, they like a bit more current- 30-50mA increase over minimum.

Reply to
The Natural Philosopher

I've found a copy of the mag. Seems they reprinted the same project a number of times it was so popular.

I found it in ...

Electronics - The Maplin Magazine

50W HiFi Power Amplifier July 1993 issue 67

Scanning it right now ...

It's not a MOSFET amp.

Reply to
Adrian C

Reply to
Adrian C

Good work Adrian. I have that scan as well, but you beat me to it ...

J^n

Reply to
jkn

That's perfect. Many thanks for sorting it out. It came just at the right time too; I changed the output transistors this evening and tried the set-up procedure. It lives once more! It's only on the bench at the moment and I didn't have a speaker handy, but it works fine on a scope. I'm well pleased. I might even forgive the wife :o)

Thanks again,

Colin.

Reply to
Colin Stamp

well I am not surprised it blew up. Pretty amateurish design.

Reply to
The Natural Philosopher

FWIW I've found an old Maplin catalogue from 1985 with the original project. (The days when the catalogue had projects and data sheets printed inside that you could actually dream & build from!)

First paragraph of that reads ...

"A superb quality 50W power amplifier. We threw away all our technical specification handbooks and designed an amp that just sounded musically perfect. When we'd finished we found that we'd got a pretty impressive technical spec as well."

Reply to
Adrian C

except temperature stability ;-)

The design is utterly conventional expect the temperature coefficient of the output stages is highly positive, which is rather dangerous.

I can write bullshit like the above too.

I am a bit worried about it's hum rejection and switch on thump characteristics as well.

Distortion and frequency response will be relatively good, for the day, but when you do this for a living, that's just the start of the game. There is no overload protection whatsoever apart from the fuse, no thermal cutout, no SOAR protection, and I am not convinced that they have got the hum as low as they should..nor arranged for the feedback to start working before any serious power is delivered to the speakers, to eliminate 'thump' on switch on.

Reply to
The Natural Philosopher

That's why Q6 is thermally coupled to the output transistors.

It uses a split power supply, and so long the input 0Volts, thw output will very quickly go to 0V. I would expect more a click than a loud "plop".

It was the circuit in its day, as you say there's no output current protection, but with two output transistors in parallel, each with a 12A continuous rating it could cope with some mistreatment. Whether the fuse would go first would be questionable!

Most complementary circuits would stick to npn's in the top half and pnp's in the bottom, this circuit has an inversion in both.

However most designs had non-complementary npn output transistors, typically with an inverter in the "top" half.

I recall building one (non-complementary style) and it was exceptionally good with regards to hum. I could get very cheap 2N3055s!! I feel the long-tailed pair in the input stage was inherently insensitive to power supply variation, which is the section with all the voltage gain.

Reply to
Fredxx

It's better than anything I could have done and, to be fair, my pair did last over 20 years before one of them blew up. Even then it took a good whack to do it.

I'm wondering if it was a collector-heatsink short on one of the output transistors that did it, due to a long-forgotten bodge when I built it.

The holes in the heatsink bracket were too small for the insulating bushes and, rather than drill them out, I cut down the bushes to match the transistor tab thickness. Looking back, it was a pretty stupid thing to do as it didn't leave any margin for error. I reckon one of the transistors shifted a bit upon cranium-to-shelf contact.

Cheers,

Colin.

Reply to
Colin Stamp

Yep. I remember that quote from the catalogue well. It gave me a mental picture of some bloke beavering away in a Maplin back-room:-

Randomly assemble some components. Flick switch - puff of smoke. Bugger! Randomly assemble some components. Flick switch - puff of smoke. Bugger! Randomly assemble some components. Flick switch - puff of smoke. Bugger! Randomly assemble some components. Flick switch - horrible noise, but no smoke. I'm on to something! Make random changes. Flick switch - puff of smoke. Bugger! . . .

Mind-you, I still bought two kits.

Cheers,

Colin.

Reply to
Colin Stamp

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