Maplin "50W HiFi" amplifier module instructions.

It seems stable enough. I've driven it pretty hard at times, including

80W RMS per channel continuous sinewave into 8R dummy loads for half an hour or so when I first built it.

There's just a faint click at switch-on, and the power supply rejection is fine. With the input shorted, I have to stick my head right into a speaker to hear anything, then it's just some hiss as I recall - no hum. Mind you, It's been a long while since I stuck my head in a speaker with the input shorted.

Mine has even less protection since I've shorted out the output fuse in the name of lowering the output impedance and I've up-rated the power supplies. I like to sail close to the wind :o)

Cheers,

Colin.

Reply to
Colin Stamp
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You jest, but I sat in a development lab at Marconi once watching someone do almost exactly this with a 2kW RF amp. It was working nicely, but then someone in marketing sold the navy a version of it that was significantly quieter (in an acoustic sense) than before. Problem was it had two stonking high speed fans to keep itself cool, and these made a bit of a racket. Hence some poor engineer was given the task to keep it working while making it quiet enough. He spent a couple of weeks tweaking the fans to get as much airflow as possible, within the available noise budget. Then tweaking heatsink design, then titting about with copper inserts in the ali heatsink etc. He managed it in the end, but got through a significant pile of output transistors in the process (at £50 a set IIRC).

Reply to
John Rumm

I think they sold a separate kit to provide some form of switch-on delay for the speakers (and various other goodies, too).

I remember thinking about building one of those amps, but then I ended up going for a pair of their 150W ones instead; I had an old 19" instrument chassis that was just crying out to have a large amp built into it. It was a reasonable beastie, but I think the only thing I have left from it is the transformer...

Reply to
Jules Richardson

The last ever 150W design I did had everything. Down to the point where even in the hot African sun, it would deliver into any load we threw at it, for as long as the loudspeaker cones lasted, was fully short, half short, open circuit protected, and was on the ragged edge of heat before the thermal cutout went.

It had no switch on effects at all, because there was no current delivered to the outputs beyond a mA or so in order to stabilise the feedback before any serious power was applied to the speakers.

It was about my tenth or eleventh design for high power professional use. After that I ended up designing software instead.

I cant remember the specs, but short of MOSFETS it was as good as any bipolar design on the market.

Removing the fuse will not affect the output impedance whatsoever.

That is a function of the feedback in the amp, and is likely to be well below the resistance of the RF stabilising choke and the loudspeaker cables themselves.

Reply to
The Natural Philosopher

The fuse is outside the feedback loop on this design but of course shorting it out was indeed fairly futile. My defence is that I was young. It isn't the silliest thing about the amp anyway. I had to use a blow-torch to solder the power supply and speaker cables.

Cheers,

Colin.

Reply to
Colin Stamp

Many years ago I built a couple of amps. The first couple were Wireless World designs (Bailey, I think), the last, which I'm still using, is supposedly a Quad

405 copy, the current-dumper design. Someone at the place I was working had made some circuit boards for it. My days of caring much about hi-fi are long gone, but I'd be curious to hear your comments on this amp, which once was highly regarded. I got rid of the pre-amps (a very old WW design) a few years ago, and stuck in op-amps with only a volume control. I've never missed the tone controls or filters.
Reply to
Gib Bogle

The difference between the circuits in magazines, and the circuits you put in production are simple.

A magazine circuit worked well for one man, once.

A production circuit has to work well for all spreads of semiconductors under all customer conditions.

The very first circuit I ever built from a magazine failed when I left in in the sun. It was fine when I put it back in the shade. I was IIRC

  1. That's when I realised that I had better start understanding what I was doing. Since the guys in the magazines patently didn't.

I probably built half of all the HiFi amps of the period. Pretty sure I did a Linsley hood was it a Bailey. Did a class A as well. Worked for Uncle Clive, then on into HiFi proper for a couple of years, then the music business for disco and PA and guitar usage..waste of my time really. Only one of the firms I worked for still exists. (Actually only one of the firms I worked for in my entire career doing electronics still exists in this country, and that's Marconi Elliot Avionics, which lingers on as SELEX Sensors and Airborne Systems. The other one still exists in S Africa)

It was post Uncle Clive when I realised finally that technicians could put circuits together and fiddle with them till they worked without really understanding why they did, or the implications of their design choices, that I finally answered the question of how alleged experts who wrote for magazines could produce stuff that didn't work properly. Because they were of the 'fiddle till it works' brigade.

Now we all did that,. but some of us with more insight started from better places that required less fiddling to get to better results.

The better results in power amps came really from better output devices. Essentially the better those were the more feed back you could slap round the whole thing, without going unstable, and the lower distortion you could get, and the higher frequency the nasties in the crossover region could be pushed.

And 99% of the circuits were in fact mild adaptations of the application notes supplied by the semiconductor magazines. That's where the better fiddlers started their fiddling from.

Ultimately I realised that all a PA is, is an arrangement of power devices that is broadly symmetrical to drive a complex load. and simply work back from there. Those devices themselves have to be driven, safely and securely so that all devices are inside the SOAR, and then they need phase splitting if its class B, and then some kind of reasonably low noise input stage.

Then try and keep the whole arrangement stable into just about any possible load, with enough feed back to get distortion down..early stuff we could do 0.1%, pretty soon I was hitting 0.025%, and by the time I finished it was pretty much off the scale in the noise. Even low level crossover was getting to be almost unmeasurable, compared with the 1-2% it had been on e.g. a Quad 303. Frequency responses likewise went up as better devices came along.

Hitachi complementary FETS were the best devices I worked with..back about 1980 or so.

Really, there wasn't anything left to do then. You could produce an amplifier that was better than any valve amplifier, ruler flat and totally distortion free, in a back shed. You couldn't sell it though, without employing the sort of spin merchant that made Alistair Campbell look like Honest John, Or putting valves in it.

I realised from my time building high distortion guitar amplification that people actually like distortion. It's something they get used to and it sounds 'right' to them. Likewise natural resonances and colouration in Loudspeakers.

Put them in front of a flat amp and really good loudspeakers and they feel something is missing: It is. All the distortion and colouration they are used to. The sound becomes naked and sterile, because all they now hear is what is actually on the CD..

Well, the CD was in by then, and the cassette tape had taken the big money out of the music business, prejudice and religion ruled the HiFi world, and I was still broke, and no one wanted a circuit designer outside of Silicon Valley or Japan so I started playing with microcomputers and turned my back on it all.

I was in fact pretty disillusioned. I'd given them 'the closest approach to the original sound' made em rock solid, and idiot proof, and realised that that wasn't actually what they wanted. They wanted style, bragging rights, fashion statements, a brand..not what I could provide really. I majored in proper engineering.

At least in the music business they were more honest. 'Just so it sounds like a Marshall turned up to 11, I don't care what that means, that's what I want'.

I got a Technics amp once. Looked at the circuit diagram and thought 'hey, That's the way *I* broke the patent on that clever AAB design I stole..and they integrated the whole lot on a chip to get temperature stability' It was a ripoff of my ripoff!!..

And I wouldn't mind betting that the output of any Peavey guitar Amp to this day is exactly the way I designed it (but not for Peavey!). So some of the work lives on I suppose.

.
Reply to
The Natural Philosopher

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Colin Stamp saying something like:

Glad to see you got your priorities right. A wife can be replaced.

Reply to
Grimly Curmudgeon

That's a very interesting post and echoes a lot of my experience over the years. Thanks for that.

I recently came across this (it's been around for ten years, but passed me by):

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it all really. HiFi as a concept is pretty dead for me. Almost everything audio sounds 'good enough' these days (if used in an appropriate context, natch).

Reply to
Dave Osborne

well those chips are not that much good for top end applications really.

They are good enough for guitars though, and I used a few in my time..hung some weird feedback round em to do high output impedance and soft clipping a la vox AC 30..

But they are good enough to be called 'hifi' for sure.

And ten times better than most loudspeakers..:-)

Reply to
The Natural Philosopher

New to me. Thanks.

The 'HiFi' world has become one gigantic scam, or comedy, depending on your level of cynicism. Gold-plated speaker cables costing more than my amplifier cost to make, and having, of course, no audible effect. Take a glance at one of the up-market hi-fi mags, the level of ridiculousness is beyond belief, surreal.

I remember many years ago reading an interview with a British designer-builder of high quality cheap speakers. He wasn't doing very well selling his stuff, which was very good value for money, until he raised his prices significantly. This caused his sales to increase markedly, but he was so disgusted with this evidence of what the market cared about that he quit.

Reply to
Gib Bogle

I have to say, I was more or less in that camp.

I could have made money with the contacts in the music biz by making a crappy overstyled valve guitar amp and selling them for a grand apiece.

Think Mesa boogie.

Oh they are actually £1500 new these days.

there must be all of 80 quids worth of parts in em.

But as Keith Richards said 'its less about who you sleep with, than sleeping with yourself, for the rest of your life, so to speak'

Reply to
The Natural Philosopher

Speakers still matter. The rest is just past caring, and has been for

30-ish years,

Except MP3s, which do still sound a bit limited if you're from Bristol and have a decent system to compare on.

Reply to
Andy Dingley

Tragicomedy, surely? Hear that rumble? It's the sound of Gilbert Briggs, Harry Leak and Peter Walker tuning in their graves...

Reply to
Andy Wade

I agree completely. I'm using old Tannoy Monitor Golds that my dad bought in the 70s (I think). Unfashionably big boxes, but I have a big room.

Reply to
Gib Bogle

Pretty good, even today.

Reply to
The Natural Philosopher

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Gib Bogle saying something like:

Ditto, but with Leak 3080s here.

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need resuspending now, but were utterly great in their day. I will probably spring for a suspension kit, as they're worth it.

Reply to
Grimly Curmudgeon

Should you want of pass them on to a good home for a crate of beer tokens drop me a mail:))...

Reply to
tony sayer

I'm considering offloading my old Active Linn Isobariks along with the tri-amp stack of Naim 250 replicas I home-built. ( schematics and layouts supplied! )

But I'd only be prepared to offload them to a loving home.

Do you reckon anyone would be interested in such audio relics?

Reply to
Ron Lowe

Yes just need to advertise in the right place!..

Ebay would be a good start..

Reply to
tony sayer

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