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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 25th Jul 2020, 3:07 pm   #21
merlinmaxwell
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Default Re: Speaker cable used in commercial cinemas

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Domestically, the main requirement is end to end connectivity and concealing it.
That sums it up perfectly.
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Old 25th Jul 2020, 4:31 pm   #22
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Default Re: Speaker cable used in commercial cinemas

The attached table of cinema speaker cable practice is from the JBL Cinema Sound System Manual (2003). The sole criterion is cable voltage drop: power loss in the cable should be no more than 0.25 dB.

Depending on cable length and speaker impedance, wire sizes from 0.7mm to 6mm are specified.

I must confess that I've found it surprising that it's still customary to install the power amplifiers in the projection room and connect to the speakers behind the screen using a low impedance connection. This applies even when, as is customary today, active crossovers are used. I guess it makes sense from the maintenance aspect to have all the electronics racks centrally accessible, but the connections are expensive in copper in a big cinema!

It seems likely that cinema installations didn't always use a low impedance speaker connection. I do have in my amplifier collection a pre-war cinema amplifier (on the 'round tuit' pile) where the output connector is wired straight from the output valve anodes and HT: it seems likely that the output transformer was located with the speakers behind the screen. The HT connection may also have been used to energise the speaker field magnet, but these are just my guesses.

Whether conventional 70V or 100V line operation was ever used in cinemas I haven't been able to find out. Those old 2000 seat auditoria must otherwise have used some heavy cables to connect up 3 ohm speakers. Does any forum member happen to know about historic practices?

Martin
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Old 25th Jul 2020, 5:06 pm   #23
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Default Re: Speaker cable used in commercial cinemas

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I must confess that I've found it surprising that it's still customary to install the power amplifiers in the projection room and connect to the speakers behind the screen using a low impedance connection.
If the distribution were performed on the input to the power amplifiers, wouldn't that mean that anything picked up by the distribution cables (such as mains hum) would also be amplified?
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Old 25th Jul 2020, 5:27 pm   #24
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Default Re: Speaker cable used in commercial cinemas

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I must confess that I've found it surprising that it's still customary to install the power amplifiers in the projection room and connect to the speakers behind the screen using a low impedance connection.
If the distribution were performed on the input to the power amplifiers, wouldn't that mean that anything picked up by the distribution cables (such as mains hum) would also be amplified?
Well, yes, true, but avoiding picking up mains hum on audio cables is as ancient an art as the earliest broadcasting and recording studios. Decent balanced screened cables do the job fine. Think of BBC Broadcasting House.

Martin
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Old 25th Jul 2020, 5:31 pm   #25
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Default Re: Speaker cable used in commercial cinemas

How would the cable costs compare?
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Old 25th Jul 2020, 5:49 pm   #26
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Default Re: Speaker cable used in commercial cinemas

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I must confess that I've found it surprising that it's still customary to install the power amplifiers in the projection room and connect to the speakers behind the screen using a low impedance connection. This applies even when, as is customary today, active crossovers are used.
I suppose it's a tradition locked in from days of yore

A) so the projectionist could smartly change any valves that expired mid-showing

B) so hot stuff that could catch fire was kept away from screens and curtains in the auditorium.

David
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Old 25th Jul 2020, 7:40 pm   #27
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Default Re: Speaker cable used in commercial cinemas

Old Western Electric Cinema Speakers were high sensitivity Horns. I think they used SET Amplifiers (please correct me if I am wrong) that could drive the speakers with only a couple of watts of power, and fill a large cinema with sound. That I think is very impressive.

The speaker cables Western Electric used were 16 gauge tinned copper, with a cloth sleeve. Highly sort after today, and silly expensive.

Western Electric in the UK was Westrex. Does anyone know much about Westrex ?
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Old 26th Jul 2020, 12:43 am   #28
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Default Re: Speaker cable used in commercial cinemas

Re #22, the 1936 Bulgin catalogue lists a 4 pin plug and socket (with flat, angled, pins), said to be suitable for use with extension speakers having energised field magnets.
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Old 26th Jul 2020, 1:29 am   #29
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Default Re: Speaker cable used in commercial cinemas

I acquired a pair of 25W amplifiers that are branded Paramount that have the fixing holes for the projector unit on the top. They are switchable 7.5/15 ohms. I bet that is really 8/16 excluding cables.
They had two and switched amplifier/projector each time they switched reels.
A breakdown meant a gap in the show for each reel change.
Who would want to root around behind a screen over a dead amplifier at show time.
Lamps were always a looming menace.
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Old 26th Jul 2020, 6:23 am   #30
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Default Re: Speaker cable used in commercial cinemas

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I must confess that I've found it surprising that it's still customary to install the power amplifiers in the projection room and connect to the speakers behind the screen using a low impedance connection...
With large live amplified concerts I think it's been customary for some years for the power amplifiers to be located on or near the stage, close to the Front of House speakers, and of course these days even many portable PA speakers are self powered, which has other design advantages beyond saving running expensive low Z cables from amps to speakers. EQ and crossover can be not only active but DSP based.
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Old 26th Jul 2020, 10:51 am   #31
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Default Re: Speaker cable used in commercial cinemas

If more than 2 cores are needed, I would agree with Lucien that YY cable would be a good choice, if agree is the right word since I would think he is much more experienced than me in these matters. It is also useful for organ installations due to its compactness and neatness. At the loudspeaker end, a wall box with a suitable number of Speakon connectors allows for distribution to each loudspeaker cabinet.

For single pairs, automotive flat twin can be a good choice and is available in a number of CSA sizes. This is the range from one supplier selected at random https://www.electricalcarservices.co...-0-168-238-489. They also sell specific loudspeaker cables but I wouldn't expect to hear any difference in qulaity, or at least not for long in the case of some of their customers.

PMM
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Old 26th Jul 2020, 12:48 pm   #32
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Default Re: Speaker cable used in commercial cinemas

Thank you for all the replies. The connection between an amplifier and speakers is electrical, so shouldn't it be treated like any other electrical connection used around the home ? Surely 2 core 1.5 mm electrical cable should therefore be more than adequate. It comes to mind then, what is so special about dedicated speaker cable ?

Looking at the electrical specifications of 2 core electrical wire, it far exceeds the electrical specifications of standard speaker cable. So what is so special about speaker cable ?
Is inductance and capacitance, and quality of copper better on speaker cable ?

How much of a role does inductance and capacitance play in the overall performance of your hifi ?

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Old 26th Jul 2020, 12:54 pm   #33
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Default Re: Speaker cable used in commercial cinemas

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what is so special about dedicated speaker wire ?
Nothing electrically at all, as said, my reason for getting "'speaker cable" was only for flexibility and at a bargain price. I do like the 300OHM ribbon for under carpet use, ribbon cable (with cores paralleled) is also good for that.
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Old 27th Jul 2020, 11:46 am   #34
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Default Re: Speaker cable used in commercial cinemas

Re post #27. The early amplifiers (pre 1935) were Westen Electic which then became Westrex. The early ones are particularly sought after. Some years ago a night club in Caernarfon had a fire. It used to be a cinema, and the projection room was still there and relatively undamaged after the fire. The owner let my colleague in to take pictures of the Westen Electric sound equipment which I could have taken, but then the demolition men moved in, the safety fencing went up and the equipment vanished, probably into a skip. Still annoys me that we didn't act sooner!
In the studio where I worked in the Eighties the sound transfer bay used a Westrex transfer unit. The size of a fridge-freezer and many times heavier, the output from a Nagra 1/4" was fed into it and it was transferred onto 16mm mag "film" for editing on the Steenbecks. Must have been quite old then but i never had any trouble with it.
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Old 27th Jul 2020, 2:45 pm   #35
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Default Re: Speaker cable used in commercial cinemas

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... How much of a role does inductance and capacitance play in the overall performance of your hifi ?
As far as cable inductance and capacitance go, with properly designed amplifiers the answer is 'not very much'. Conventional hi-fi uses negative feedback. As the signal frequency climbs the fed-back phase will tend to move away from the ideal 180 degrees relative to the input. If it moves too far away while the amplifier still has more than unity gain then the kit may become a high-power oscillator instead, and destroy itself or the speaker it's connected to or both. The difference between the actual fed-back phase at a given frequency and the phase at which oscillation starts is called the phase margin. This will be a feature of the amp's design. Adding extra capacitance in the form of long runs of speaker cable can shift the output signal phase and eat into the phase margin. But competently designed equipment shouldn't have a problem driving many metres of 'ordinary' twin-core cable of almost any kind.

Cheers,

GJ
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Old 27th Jul 2020, 3:35 pm   #36
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Default Re: Speaker cable used in commercial cinemas

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Thank you for all the replies. The connection between an amplifier and speakers is electrical, so shouldn't it be treated like any other electrical connection used around the home ? Surely 2 core 1.5 mm electrical cable should therefore be more than adequate. It comes to mind then, what is so special about dedicated speaker cable ?

Looking at the electrical specifications of 2 core electrical wire, it far exceeds the electrical specifications of standard speaker cable. So what is so special about speaker cable ?
Is inductance and capacitance, and quality of copper better on speaker cable ?

How much of a role does inductance and capacitance play in the overall performance of your hifi ?
At audio frequencies, the impedances of the inherent inductance and capacitance of typical cables are negligible compared with the plain old resistance. 1.5 sq mm cable will be fine (the above JBL table confirms it's good to 7.5 m length). When we had our living room wired many years ago, I arranged for the electrician to run four speaker 'ring mains' of standard 2.5 sq mm cables from the amplifier location to a number of optional speaker positions, terminated in blanked off junction boxes. These permanent connections have proved invaluable and are still in use 35 years later. Of course, hiding cables in the walls and above the ceiling does extend the length required, hence my choice of 2.5 sq mm cable.

Whilst some power loss in the cable may not seem important, and cable inductance and capacitance have neglible effect, the 'elephant in the room' here is the impedance variation of the speaker with frequency. This is typically about as flat as the Lake District. As an example, the attached graph (solid line) shows the measured impedance plot for the very respectable Bowers & Wilkins 802: it varies between 3 ohms and over 20 ohms. With such drastic impedance variations with frequency, it's obvious that significant cable series resistance will affect frequency response because the cable voltage drop will be influenced by the inverse of the speaker impedance. This fact probably has much to do with differences in the sound of different cables.

I guess that the ideal cable is the one that the designer used when evaluating and measuring the speaker.

Martin
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Old 27th Jul 2020, 4:24 pm   #37
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10 Miles = 16093 Metres.

Regards,

Dave.

Did they place it all on cable lifters to stop floor capacitance effects
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Old 27th Jul 2020, 4:55 pm   #38
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Default Re: Speaker cable used in commercial cinemas

Therein lies the delight of having amplifiers right at the speakers. Have one poweramp per driver and a low level crossover. You lose all the problems of high level crossover components, you can even have the low level cables from the preamp as true matched transmission lines, if that takes your fancy.

The best speaker cable is no speaker cable. The idea of having all the power amps near the listening position is like the idea of putting the engine at the front of a motor car "Because that's where the horse used to go!"

Just look at that B&W impedance plot and the Q factor of those peaks. Wow. Must have been fun stopping those showing through into the acoustic output.

And as Martin says, the influence of the erratic Z depends on the cable Z (chiefly resistance) and the amplifier output Z. If you don't equal what values for these were designed into the crossover equalisation of the speaker, you are letting an unwanted factor in.

A fair bit of the 'sound' difference attributed to cables may be from amplifier stability suffering with some cable/amp combinations.... nothing that sane engineering can't fix/ But sanity is not a common factor in a lot of audio.

David
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