Quite a lot of amps have a 70V tapping, apart from 'international compatibility' it can be useful if you want to run lots of speakers at low power, as 70V amp into 100V speakers matches at half the power.
Owain
On Oct 4, 1:34=A0pm, The Natural Philosopher wrote:
SNIP
Well I have moved forward somewhat. Following the bundle of cables through a partition I found they all terminated in a box with jack sockets labelled mic1, mic2 etc etc. It goes to show with old stuff - Assume Nothing. - I cant really see the point of the short leads from amplifier to connection box when the amp position is said to have been at the stage side of the partition? However moving on - Hidden behind a curtain cord behind the partition I found the dangling end of a two core flex. With a torch and steps I followed this across the top of the stage curtain rails and found it headed off in the direction of the first speaker. A single AA cell scraped across the ends of the flex resulted in all the speakers in the main room making a noise so I borrowed an amp with an 8 0hm output and an MP3 player. Lo and behold the speakers produced music. I had to cut the investigation short as the carpet bowls club turned up but things are looking up. I need to measure the resistance presented by the speakers to the ends of the flex and decide whether I need to dig deeper or if its in "expected" range. I believe a 100volt line system will present significantly high resistance but a low impedance system should be in single figures of ohms although resistance is not likely to be the same as impedance in this case. If I do find about 8 ohms at the flex ends it would seem ohms law and parallel impedance trickery indicate 6 speakers of about 50 ohms each - were speakers made with that impedance? I've heard of 4 ohm 8 ohm and 16 ohm but its not my normal line of work.
I have been offered the loan of an adastra pa amp with both 8 ohm and
100volt line outputs but until I've looked further into it I'll keep this away from the ends of the flex cores
Parallel would be likely in an installation of that type and age. Also, depending on age, you'll find that the speakers are more than likely 15 ohms each, so if it's not 100 Volt, you'll get two sets (One each side of the hall) of 15 ohm speakers in parallel, giving you just under 3 ohms in total, which would match a lot of valve amplifiers, as most valve PA stuff had a 3 ohm tap on the output transformer. Or, if you split the cables, 5 ohms per side, which you can drive quite well with an amplifier rated for 4 ohms.
Connect a 100 ohm 10 watt resistor in series with the 100 volt output of the amp, and if you get something you can hear clearly at a low volume setting, they're 100 volt line speakers. If you get a whisper, and the resistor gets hot, then you've got a low inmpedance system, in which case, a 4.7 ohm resistor in series with the 8 ohm output will give you enough volume for testing.
In article , Bill scribeth thus
Yes remember them now!. Don't think there're around anymore 'tho....
Or it may be a series-parallel mixture.
3 x 8R in series =3D 24R. Two sets in parallel =3D 12R.4 speakers in series =3D 32R. 2 speakers in series =3D 16R. Both sets in parallel =3D 10R.
There's a handy calculator at
It's unlikely to be 100V line with a total of 8R unless there was about a kW of audio power!
Owain
They might be wired in a series/parallel combination to keep the load reasonably close to what a domestic amps might expect. Not ideal, but then quality probably wasn't that important. And of course if you have several
100 volt line transformers in parallel, you'll get a much lower reading than with one.If the resistance looks roughly like one ordinary speaker, you won't do any damage to the amp by connecting them to the low impedance output.
The name is familiar Dave. Was he the stores manager? Trouble is it was a long time ago now. Little things are coming back though, like taking the hand made amp chassis down to the plating works in Merton High Street to be cadmium plated and passivated :-)
"Long" being in the order of several miles for large factory systems and railway long-line public address.
Need to start loading the lines for inductance after about half a mile though.
Owain
I dunno - although he gave the impression he sort of made things there. But it's too late to ask him - he sadly passed away a couple of years ago.
Still some useful industry around there even today. I particularly like the Tenable Screw Company. ;-)
I have a Vortexion 'portable' PA amp/mixer. Early transistor design, mains or 12v. Weighs a ton - due to the massive transformers.
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Thanks to everyone with pointers and suggestions. I bit the bullet and dismounted one of the wall speakers. Inside there was an 8 ohm speaker and a 100volt line transformer - yippee!
I have now bought a very cheap second hand amplifier InterM A-120 with common, 4 ohm, 70 volt and 100 volt line outputs. The outputs are within a linear socket arrangement with sunken pins. For testing purposes I have made up a couple of single pole connectors by butchering an under-cupboard flourescent plug connector for its metal links, and the amplifier works ok. To make things more appropriate for the users I would like to get hold of a proper connector plug but have had little success in searching for the correct item in various catalogues. So for a last question does anyone know this amp and can tell me if the connector is available or if it is a manufacturers special?
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Full data here
Best to post a link to a pic of same.
Or if there's room, fit Speakon connectors. They fit the same sort of space as an XLR and are pretty well bulletproof.
SNIP
The picture is a bit confusing. The actual outlet is a chain of holes with pins inside a green plastic strip. I could strip it out and fit a screw terminal block but it seemed simpler to just get hold of the correct plug if it is relatively easy. - Possibly the change from the more conventional connector shown on the date sheet drawing was an "upgrade somewhere along the way?
post a picture of what you have. Someone might recognise it.
I've uploaded it to here
In article , cynic scribeth thus
Dunno what its called but seen those on the RS components website!....
Philip
Oh bosoms. That is rubbish isn't it?
Surely the amp came with a mating other half?
Looks like a generic sort of Molex thing to me..
See if a typical Molex 8981 Series (the sort of thing that plugs into qa disk drive for power) is anything like it)
I'd contact the manufacturers and ask what fits it - or take it apart and fit something you HAVE got a pair for.
The other possible is to use bullet sockets if you can match the pin dia.
I suspect I would take the bloody thing apart, throw that crap away and make a plate with 4 banana terminals on it.
Philip
options. RS sell them - and likely all the other places - CPC, Rapid etc.
If it's been discontinued I'm fairly sure I have some here. They're a convenient medium power connector for say between mains transformer and PCB.
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