8mm Forum: Projector Speaker Extension Leads

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Posted by Simon McConway (Member # 219) on October 03, 2006, 03:55 PM:

I have been asked to give a show in a church. The speaker lead on my projector isn't long enough, so thought I'd make a simple speaker extension lead, using proper speaker cable. But, if I make it fairly long (say 15 metres) could this damage the amplifier in the projector. I will be taking the sound straight out of the projector amp, no external amp.

Posted by Steve Klare (Member # 12) on October 03, 2006, 04:31 PM:

I guess if the series resistance was high enough, the amp would have to pour out significantly more power to get the same output at the speaker (because of the line losses), but the effect could be minimized by getting the fattest gauge speaker wire you can get your hands on: something meant for much higher power than you need.

It wouldn't be an issue at all if you sent the signal out at a low level and then amplified it near the speaker.

Two weeks ago I did a cartoon show at my church for about 40 little kids. What a hoot! I've already signed up to do another one next winter.

Posted by Gordon Hunter (Member # 679) on October 05, 2006, 06:07 PM:

Provided the extension speaker you use is of the correct impedance (the same as the projector speaker's impedance) a cable length of 15 metres should not be a problem.
Ideally, you should use screened cable with the screen connected to earth. I gave a show the other evening in a small hall with a cable length of some 10 metres and used a two core cable. This arrangement worked satisfactorily. Try however, to use the correct screened cable.

Posted by Rick Skowronek (Member # 385) on October 06, 2006, 09:59 AM:

Hi guys,

The Electronics Engineer part of me forces me to reply to this question. Speaker cable is pretty much not at all touchy. The speaker outputs of any audio amp are low impedance, 4-32 ohms. As such they are not usually impacted by hum and noise from sources near the speaker wire. Simple two conductor wire such as used for lamps, doorbells, etc., is satisfactory.

As to the possibility of damaging the amp of the projector, won't happen. Actually the longer the speaker wire the more impedance you add to the output and the less power from the amp is used (also why speaker volume may also go down somewhat). Typically, the only thing that may damage a transistorized amplifier is a pure short circuit across the speaker terminals.

As to your length requirement of 15 meters, that is nothing in speaker parlance and you should notice almost no degradation in sound output and quality. Just try and find/use 2 conductor stranded 14 guage wire for your add-on length. If you're not projecting in stereo then polarity won't matter at all.

Hope that helps you.

Rick

Posted by Steve Klare (Member # 12) on October 06, 2006, 11:45 AM:

The Electrical Engineer in me agrees with a couple of "buts".

1) In order to get the same power out with extra impedance in series, the operator is going to crank up the volume higher.

2) Being that the speaker is quite a distance from the operator, he, she, it or they are going to crank it up until their perception of the volume is satisfactory, even if the people seated near the speaker will think it's too loud.

Now! Since the amp should be designed to drive the speaker close up with zero line drops at full blast volume, it shouldn't kill the amp anyway to do the same thing through series resistance and therefore slightly lower power, but it will stress it more (given the same output level from the speaker)and conceivably reduce its service life.

If this was a fighter jet or a pacemaker, we'd be extra concerned here. For the sake of a once in a while operating condition it probably won't hurt (...much).

A debate among engineers is never a good thing. People who give a fig about the 8th digit of Pi can get wrapped up in details nobody else cares about!

(It's "6", by the way!)

Posted by Rick Skowronek (Member # 385) on October 06, 2006, 05:25 PM:

Hi Steve,

Enjoyed your comments and we are in agreement, not that I wasn't with your original post. Just that 45 feet is extremely minor in a speaker feed and that the amps will hardly know the difference.

Let's take a quick look at the 14 guage wire I suggested. Standard 14 guage has a resistance of 0.025250 ohms per 10 feet. Let's take that out to 15 meters (roughly 45 feet) and we now have an additional .11 ohms. Out 8 ohms, which is pretty much an average speaker impedance, that figures out to less than a 1.5% increase in impedance. Believe me, no amp in the world or human ear will ever know or hear the difference for that small change. The volume control will stay about the same for equal sound levels for the same type of speaker. What may tax the amp more is filling a larger space with enough sound for all to hear clearly. This, of course, will force a higher volume level. In that case, you may wish a more "potent" amp to drive larger speakers.

Thanks for keeping me honest.

Rick

Posted by Steve Klare (Member # 12) on October 07, 2006, 05:34 AM:

Rick, agreed (-no "buts"!)

The number of 15 meters is kind of interesting. That's 50 feet(ish). Kind of a big dimension where projecting Super-8 is concerned.

Is this to allow the line to safely go around the perimeter of the hall, rather than snake up the middle where it can be stepped on and tripped over?

Wires are the curse of any portable presentation. When I did my show at church, I found that this big room (maybe 30 by 40 feet) had one (1!!) outlet, and powering up the projectors meant snaking an extension cord. Throw in the fact that the audience had about 15-20 little boys (including mine!) running around the place whenever the room lights were up, and it meant either there had to be an adult standing guard over the wire or it needed to be disconnected and moved.

Oh! For a projection booth!

Posted by Tony Milman (Member # 7) on October 07, 2006, 07:21 AM:

Guys

I haven't the foggiest idea what you are talking about but enjoyed it nevertheless.

Just nipping out to buy a stack of wire and try out the theories....... [Wink]

Posted by Craig Hamilton (Member # 258) on October 07, 2006, 07:41 AM:

Rick & Steve

The electro pneumatic engineer in me is well & truly confused now. [Big Grin]

To think that for the last twenty years I have added & shortened speaker leads to suit my requirements with no problems at all. As they say, ignorance is bliss. [Wink]

Craig

Posted by Steve Klare (Member # 12) on October 07, 2006, 08:27 AM:

Hmmm...

..in pneumatic terms:

Remote Device=Speaker
Pipe=Extension Wire
Pump=Amplifier
Flow=Current
Pressure=Voltage
Back Pressure=Resistance
Power=Power (that's the beauty of it!)

You have a device with a certain back pressure and a certain flow required through it. You mount it remotely so you have to add pipe.

I'm saying that because of the extra back pressure associated with the pipe, you need additional total pump pressure to maintain the same flow. This means more power required from the pump.

Rick is saying that the additional pressure isn't enough to cause a real problem, since the pump doesn't really need to work that much harder.

We're both right, just groovin' on the theory!

Posted by Tony Milman (Member # 7) on October 07, 2006, 12:11 PM:

Now why didn't you say that the first time [Big Grin]

Posted by Jan Bister (Member # 332) on October 08, 2006, 10:28 AM:

quote:
A debate among engineers is never a good thing. People who give a fig about the 8th digit of Pi can get wrapped up in details nobody else cares about!

(It's "6", by the way!)

Only if you count from the "3" before the period [Big Grin]

Posted by Rick Skowronek (Member # 385) on October 09, 2006, 08:36 AM:

Sorry guys about confusion city we may have created here. I hope that the gist of the whole conversation did make it through all the mumbo jumbo. That is, it isn't critical, won't over tax your projector and is easy to do.

Aside from the true comments of Steve about watching errant feet tripping over the cords and short of trying to power up a concert hall with sound from your projector, most anything with good wire and connecting practices will work just fine.

Loved your Hydraulic/Pneumatic explanation Steve. Priceless.

Rick

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