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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 30th Mar 2011, 11:19 pm   #21
Phil G4SPZ
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Default Re: Sinclair Z12

A friend and I used the later, higher-powered Z30s to build a disco amplifier. After many, many burn-ups we got a reliable working amp that sounded really good. In its final version, we mounted beefier output transistors on big heatsinks, and used two Z30 PCBs in a bridge circuit. This worked really well, and as far as I know, this beast is still in one piece. My friend took it to the States with him when he emigrated in 1980.
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Old 31st Mar 2011, 2:23 pm   #22
terrykc
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Default Re: Sinclair Z12

The secret with the Z12, as with so many Sinclair products (and others) was in interpreting the specification correctly!

If you look at the Z12 advert in the link that Baz posted, it says that it can be run on 6 - 20 volts and feed 3 - 15Ω speakers but then goes on to say that two 3Ω speakers can be connected in parallel!

So, the minimum load is really 1.5Ω - something that most people overlooked. Of course, nowhere does it say that the output power is both supply voltage and load impedance dependent which probably fooled a lot of people reasonably confident with valve technology, not to mention many more who had no technical knowledge whatsoever - remember that Sinclair didn't confine his advertising to technical publications!

To get 12W output with a 1.5Ω load requires about 4.25V rms at just under 3A.

4.25V rms is 12V peak to peak, so the supply needs to be about 13V. However, as the power supplies were simple, unstabilised designs, the offload voltage would probably have not been all that far short of the maximum 20 volts!

With a single 3Ω speaker, the requirement is 6V rms @ 2A but, as this needs an 18V supply, it's unlikely that it could be met with a nominal 20V (max) supply!

Right, that's the background dealt with - now back to the plot! Trevor wanted Z12 stories - so here's mine.

I was asked to go to Southborough in Kent to look after the PA for a Folk Concert in the hall there. Fortunately I had the forethought to slip a simple passive 3-way mixer into my overnight bag * ...

Top of the bill was a group I new very well who surprised me by turning up with their own PA gear - and its builder who had ideas of grandeur well above his station!

This gear was set up and they started to rehearse. Confident smiles vaporised as a horrible distorted racket emanated from both speakers ...

(Anybody ahead of me on this yet?)

I was summoned up to the stage to investigate and looked inside this marvellous box of tricks where, as you've probably guessed, sat not one but two Z12s - each driving its own speaker. 24W rms would have been ample but a quick meter check showed that the speakers were both 15Ω!

About the best you'll get out of a Z12 with a 15Ω load is about 1.5 - 2W - 3 watts if you've got a good stabilised 20V supply - which this thing most definitely didn't)

The three microphones they had were a bit much for the simple house PA - and their 'expert' had built his mixer into the marvellous box of tricks, so my simple passive job came into play and the whole thing went off very well indeed. I don't know what happened to the Z12s or their proud owner - they disappeared long before curtain up and were never seen again!
___________________________________

* Oh yes, the overnight bag. Did I say I wasn't getting paid for this? I'd travelled down by train and bus from my home in Essex, expecting to spend the night on someone's floor ...

The top of the bill also came from Essex - though nowhere near to me. However, for my efforts, I got a lift back - right to my own front door ...
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Old 31st Mar 2011, 7:29 pm   #23
Trevor
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Default Re: Sinclair Z12

TerryKc
What year was this ?
I love the story and have done similar work myself!
I am aware re the load impedance etc and the Z12 however as a 15 years old I had no idea!
I really don't mean to knock Clive what he sold me was a life long hobby and a dream of a better future
His work allowed a lot of us to learn at a minimal expense it could not have bben all bad as some units still work 40 years later
best regards Trev
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Old 31st Mar 2011, 9:25 pm   #24
roffe
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Default Re: Sinclair Z12

The specs
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Old 31st Mar 2011, 9:58 pm   #25
terrykc
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Default Re: Sinclair Z12

Since writing my previous post, something has occurred to me.

At the time, I pointed out to the error of his ways to the over-confident builder, using much the same figures as I used here - except that I used mental arithmetic with a bit of rounding then, rather than the spreadsheet I used today!

< cheat!>

However, in those days, I'd never encountered the concept of using two amplifiers in a bridge configuration to increase power with a limited supply voltage. In fact, I think a generation must have passed before I did!

Using my fantastic powers of hindsight, I think I might possibly have been able to dig him out of the mire ...

Tobacco tins for storage were mentioned here recently but I had another supplier who smoked slim cigars - cue: Air on a G string ...

I found that, by folding and glueing a sheet of stiff paper to produce stiff ribs, into which slits were cut using side cutters, I could get 12 slits into one 10 cigar tin - each exactly the right depth to hold two 0.5W RS resistors! A bit like this:
_______
| ---------|---[_______]----|-------- |
| ---------|---[_______]----|-------- |
|__________|________________|_________|

Thus one tin held a decade and a handy little plastic case I found was three tins deep and two tins wide - thus I could conveniently carry two of every value from 10Ω to 22MΩ (plus a few extras) in a very small space, which fitted in neatly with my multimeter, soldering iron, etc.

So, I could (had I known/realised at the time) have turned the one transistor preamp on one Z12 into a phase splitter, with equal collector and emitter resistors, driving the two Z12s in anti-phase, in turn driving the two 15Ω speakers in parallel.

Had the PSU been up to it which, of course is debatable, a 38 volt swing (from a 20V supply) @ 1.8A would produce 24W!

With a PSU sagging to around 15 - 16 volts, a reasonable 15W would have been available.

Adequate for vocal accompaniment to acoustic instruments, I think. (How many pop groups in the 60s used a Vox AC30 for PA, competing against another 3 AC30s plus drums?)

On the other hand, my recollection of the appalling distortion that I heard that day doesn't convince me that the PSU was capable of anything much at all so, perhaps, it's just as well I didn't try ...

Does anyone know when the bridge configuration was first used? And, more to the point, when t became widely known?
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Old 31st Mar 2011, 10:00 pm   #26
terrykc
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Default Re: Sinclair Z12

Quote:
Originally Posted by Trevor View Post
What year was this ?
As near as I can recall, 1968.

This would place it at the end of the Z12 run.

Terry
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Old 31st Mar 2011, 10:29 pm   #27
terrykc
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Default Re: Sinclair Z12

Thank you Roffe.

I allowed for a one volt drop in my spreadsheet, which agrees with the 5V figure (5.04W) but Clive obviously upped the stakes to two volts above that!

My figures for 15V and 19V are 8.17W and 13.5W respectively, which agree with the 16V and 20V figures in the original spec.

The current demand shown in that spec is not for rms output power though!

The real figures should be like this (the PSU and available output swing figures have been modified slightly to agree with the published spec):

PSU Available_Output_Swing
12V 11V = 3.89V rms @ 1.3A = 5.04W
16V 14V = 4.95V rms @ 1.65A = 8.17W
20V 18V = 6.36V rms @ 2.12A = 13.5W

Quite a difference!

Terry
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Old 30th Apr 2012, 9:11 pm   #28
RPEDUNIX
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Default Re: Sinclair Z12

Note PZ3 was 15v @ 1.5a
PZ4 was 18v @ 1.5a
Both were actually fully stabilized power supplies.
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