Stereo Amplifier

[David G4EBT]

Forum member Joe, (’Pillenwerfer’) tipped me off some time ago about a neat little stereo amp that he'd built using a surplus module bought from Bardwell's on Abbeydale Rd Sheffield. The modules are based on a pair of Thompson (French outfit) TCA940 ICs at just £3.99 each. (Worth more than that just as a donor for the pots).

http://www.bardwells.co.uk/product/s...s-per-channel/

These NOS stereo amplifier modules are based on a pair of Thompson (French) TCA940 ICs. They supposedly need 24V to power them, but they'll work down to about 6 Volts, and at 9V they'll blow your socks off. Quite handy as a bench amp, signal tracer, MP3 player amp, guitar practice amp, etc. (For mono, just common the inputs so that each IC is in use). Ironic that some restorers say "I only play with valves - I don't touch 'modern stuff'". The TCA940 had its 40th Birthday last year!

http://www.datasheetcatalog.com/data...9/TCA940.shtml

The TCA940 datasheet shows that it's quite a capable IC. It features supply overvoltage protection, output short-circuit protection, and protection from overheating. The frequency response is from 40Hz - 20 kHz, and distortion is stated as 0.3%. The supply voltage can range from 6V - 24V DC. The volume control on the module has a double pole mains rated switch (which has no connections to it), which could be used to switch a mains PSU, to provide a DC supply from 9V - 20V - well within the stated voltage range, built into an amplifier cabinet.

As supplied, it's intended for use with 8 Ohm speakers, but by changing the two caps shown on the attached pic, which also shows the connections, from 500uF 15V to 1,000 uF 15V, the amp is suitable for 4 Ohm speakers, but given that the IC is fairly well bomb-proof, I doubt it would come to any harm if the caps aren't changed.

The datasheet on the TCA940 states that it is capable of 'up to' 10Watts, the data and graphs on the characteristics indicate and output of 5 Watts per channel. (More than enough to fill an average sized room).

(The TCA940 IC is pin-compatible with the more familiar TBA810 of the same era, which often featured in magazine projects as the output stage of receivers, or in such things as intercoms, baby alarms, guitar practice amps etc).

I've made aluminium temporary front and back panels and mounted mine on a little baseboard (about 7" x 4"), and have fitted the stereo input sockets (RCA and 3.5mm), a power socket and spring-clip speaker sockets on the rear panel. It's functional, if not pretty! When I get time I'll make a nice wooden case for it and a front panel with lettering etc. I've fitted some small splined knobs from Spiratronics which just push on.

http://spiratronics.com/three-colour...nob-green.html

It's a bit awkward to mount the amp as the PCB at the front protrudes further out than the threaded bushes of the controls, so I bent the front mounting panel to enable the controls to be mounted and to clear the PCB. I could possibly have got away with using two nuts on the control bushes to bring a flat from panel far enough forward to clear the PCB. (Further down the line, I may make a flat front mounting panel and see if it will give clearance with two nuts per shaft.). At the rear of the PCB I drilled three holes through it and made three 6mm diam brass pillars, tapped 4BA with bolts through them to secure the rear of the PCB.

When I get round to making a cabinet and front panel I'll post some more pics.

Hope that's of interest.

[reelguy]

I bought a couple of these some time ago but haven't tried them out yet.
I like the rear panel you have made and will have to check my oddments box for the various speaker and other connections and do something similar.
I think they are handy for all sorts of stuff.
Reelguy.

[Nickthedentist]

Very interesting, David.

Any idea when these date from? Surely 1970s by the look of the components.

And does anyone recognise what they were made for? It looks like a British-designed product with some Far Eastern components.

Nick.

[Steve_Bell]

Nice metal bending. I particularly like the front panel.

I've been inspired to buy a couple of these modules to play around with.

[Andrewausfa]

Thanks David (and Joe), just the thing I was looking for.

Andrew

[dave walsh]

I agree with Nick. Maplin and others do more recent modules I think! It's back to Clive Sinclair in a way when my stereo comprised of a Tape recorder valve amp left Ch, ZX12 right Ch.

Although I like the valve sound, I did once postulate that you could put a couple of IC modules in a vintage amp case [I've got one or two] or a Radiogram and see if anybody noticed or said "thats' a great valve sound" . Arguably the main component in any amp is psychology! I have a contact in Sheffield. Will there be any left at Bardwell's? Have I missed this somehow or what's the watts at 9v/24v? [Yes I have 5 watts in the photo is that at 9v?]. Thanks for posting.
Dave W

Just noticed Peter has some in Bexhill [p2]. How did he sneak in there without me noticing?

[Ti Pwun]

Great little test amp, if nothing else. I ought to grab a couple of these. Thanks for the info.

[David G4EBT]

Originally Posted by Nickthedentist

Any idea when these date from? Surely 1970s by the look of the components.
And does anyone recognise what they were made for? It looks like a British-designed product with some Far Eastern components.

Late 1970s I think, given that the IC went on the market in 1974 and there will have been a short lapse before equipment that used it went into production. There are no markings on the PCB to identify the maker and the only name on any components is on a couple of the electrolytics - 'Bellcon'. Although I said the amp uses ICs as indeed it does, there are also two BC239 transistors. The maker's logo on the pots is a stylised 'dp'.

Bardwells is a long-established shop, started by the late Norman Bardwell after WW2 to deal in ex WD and other surplus equipment and is still run by family members. They've do idea where the PCBs came from but have had them in stock for decades and seemed to have plenty of them when I last popped in some months ago when over in Sheffield. I imagine that the PCBs were perhaps for a budget 'mini hi-fi' system, given that the PCB measures only 16cms x 8cms, and the controls are only 4.5cms apart from each other.

Certainly a bargain!

[David G4EBT]

Originally Posted by Steve_Bell

Nice metal bending. I particularly like the front panel.
I've been inspired to buy a couple of these modules to play around with.

Thanks!

I bent the front panel as a means of supporting the PCB and controls, because as explained in my first post, the PCB at the front protrudes forwards of the mounting bushes on the controls, so you couldn't for example simply use the nuts on the control shafts to mount the PCB directly onto a front panel. I also set the PCB with the controls positions far enough back so that the knobs would slide home on the splined shafts without the need to either leave a gap behind the knobs, or to have to saw the shafts down.

I intend to make a little wooden cabinet (or 'case' - maybe 'cabinet' is too grand a word!), and want the front panel controls to be at centre height, which is why I lifted the PCB up on pillars and a wooden base. I've drawn a rough idea of a front panel, which will be 19cms long x 9cms high, with the controls at 4.5cms, and the box will be about 12cms deep, probably in comb-jointed mahogany.

I'll post some pics when I've finished it, but meanwhile, I've attached a couple of pics that show the protrusion of the PCB beyond the control mounting bushes, and a mock up of a front panel showing the dimensions. Apart from the mounting bushes on the controls, there is no other means of supporting the PCB so I drilled three holes along the rear edge of the PCB in places where there are no tracks. (I did buy two PCBs in case I messed on up, but as I didn't, I've given that to another forum member).

Maximum enjoyment for minimum expenditure is the aim - a rather abstract concept in today's world!

[G4XWDJim]

David,

Many thanks, you've inspired me. When I saw the amplifier in your first post I thought it looked familiar. I bought one from Bardwells probably 20 years ago when I was buying a lot of stuff from them. It was £2.75 according to the price on their single sheet documentation that came with it.

I tested it once many years ago and was quite impressed with the results. A little stand alone amplifier like yours would be very handy. I'll follow your progress with interest.

Jim G4XWD

[David G4EBT]

Yes, that's the one Jim!

I got the hand-drawn sheet of paper with mine, but annotated the photo I took of the PCB to make the connections a bit clearer.

Great little amp for all sorts of applications really.

[Steve_Bell]

I bought two of these and the one I tested sounded OK. At 24v supply voltage , it plays very loud. I'll probably build one into a box as a test amp.

I notice the price at Bardwells is now increased to £4.99 each!

regards,
Steve.

[David G4EBT]

I think they have a price differential online as compared to personal visits to the shop to try to keep their P&P price down. Don't know that for a fact, but even at a fiver, they're a gift, and virtually bomb proof. I tried mine at voltages down to 6V and audibly, there was little discernible difference, (though of course there would be on a 'scope or Wattmeter). With a stated frequency response of up to 20kHz at 0.3% THD, and treble & bass controls, I think it's performance will very respectable.

[Steve_Bell]

Inspired by David's construction skills, I've built one of my amplifier boards into a standard metal box. I mounted the PCB on pillars, using two already available mounting holes and creating a third one.

I'm using a 20v SMPSU to power the amp and that provides more than adequate output.

[David G4EBT]

Excellent Steve - I must get on and finish boxing up mine!

[Biggles]

Well.....I couldn't resist getting one of these after reading this thread. I powered mine up on a 24V supply using a transformer I had and an LM317 regulator. I intend building the mains supply into the case and I am going to add a Velleman kit built RIAA pre-amp so it can be used with a magnetic cartridge. I will add a selector switch for tuner, cd etc. I have a couple of stand alone decks which I can use so it will be interesting to hear what it sounds like. I will make a one-off case using my metal bender. Bardwells are probably wondering why there is suddenly so much interest in their amps. One thing I have noticed is that the volume control appears to be after the tone pre-amp stage so it is possible to overload the amp by feeding too high a signal into it.
Alan.

[Nickthedentist]

I've kindly been given one of these modules by Alan SP15.

Does anyone know whether they're designed to accept the input directly from a ceramic cartridge?

Nick.

[David G4EBT]

No Idea Nick - I can only suggest you give it a try.

If you want to hear what they sound like, have a listen to this:

https://www.youtube.com/watch?v=5gZXleVOTIs

Not a bad four quid's worth eh - it would certainly de-motivate me from building a 3/10.

(Mine's still waiting to be boxed up!)

[paulsherwin]

If they weren't actually designed for a ceramic cart then the input impedence is probably too low. You could always add a 470k or 1M resistor in series with the input if there's sufficient gain.

[bikerhifinut]

Originally Posted by paulsherwin

You could always add a 470k or 1M resistor in series with the input if there's sufficient gain.

Could you make a simple buffer with an op amp (5532/TL072 etc) Paul?
was thinking it could be quite simple, set the input resistance at 1 meg and adjust feedback to give unity gain, and if you need gain the same trick sets whatever gain you want? It shouldn't be too complicated.
Andy.