self powered speakers

I hope you won't take this amiss, but I thought it would be best to first clarify what you wrote.

Are you talking about the power connection, the audio signal or both?

Are the amplifier-speakers designed to be driven from a microphone or from the line level output of a preamp/mixer? 1K is quite low for the line-level input of a power amplifier. It's more in the range of an input for a low impedance mike. Or did you measure the power supply input?

Sorry if you know your stuff about these things and what you said are what they appear to be. But I thought it would be best to make sure.

Is 25W the power consumed from the power supply or is it the output power rating?

Are the amplifiers specced to be powered from an external 48Vdc source?

More details about how the speakers are to be located with respect to each other will also help.

When I was a kid I built a "Sweet 16" out of 6"x9" ovals... infinite baffle.

Really sounded super.

If I had somewhere to put it I'd do it again, with individual amplifiers for each speaker... shades of the "audio wars" on East Campus at MIT... which I usually won with my exponential horn that fit the dorm window... raised the Cambridge cops all the way from Central Square ;-) ...Jim Thompson

both

Well the 1 K is for the transformer which connects to the speaker inputs. You can change the transformer settings though to vary the output audio power.

Consumed by speaker and I would guess the amplifier unit though the data sheet does give sufficient detail on that.

Yes the 50A source mentioned above

Thanks.

H.Johnston

Are you sure that you are talking about active speakers and not about remote speakers intended for 70 V or 100 V line operation ?

Your descriptions fits quite well to remote loudspeakers as found in schools and other large buildings for PA applications.

These contain an ordinary 4-16 ohm speaker, bit if it is driven by long lines directly, most power would be lost in the cabling due to the high current. To avoid this, each speaker contains a transformer, which is intended to fed from the nominally 100 V audio line. The speaker unit usually contains a step attenuator implemented by selecting the appropriate winding tap from the transformer.

A 1000 ohm impedance on the 100 V line would draw 0,1 A, thus the speaker could deliver up to 10 W. Thus for 50 units, a single power amplifier would be required with 500 W output power. If the current really is 0,1 A for each speaker, the total current would be 5 A, possibly distributed along several branches, so wires less than 1 mm thick should be sufficient.

Maybe I wasn't clear enough. No I'm familair with the units you're talking about and we've set up a lot of those at 70V /100V and as a matter of fact for PA systems they are the most used and the literature is full of advice on setting those up Self powered speakers or active units are not as common in PA , at least for me, which is why I was asking . The attenuating unit on the output of the inbuilt amplifier is a transformer or inductor is what the manufacturer is using to reduce the voltage instead of a resistor. The data sheet shows a combination of 6 dip switch settings that are set on this unit to vary the output power on the loudspeaker from values ranging at 0.76W to 25W. The datasheet gives the speaker impedance as 16Ohms, I measured it during audio reproduction and it varies from 1 Ohm to 25Ohms depending on frequency.

The loudspeaker+amp unit have practically speaking 5 wires entering into them 2 for the 48v source which powers the internal amplifier unit

The input impedance to the amplifier , which I think is more important then the figure I gave earlier as the audio signal passes through here, is 11Kohms before getting to the speakers.

Thanks again.

Johnston

WHEW! For a (very) brief time i thought you had an example of a perpetual motion machine equivalentactually generating power! "Self-powered" - NO! Solar-, battery-, line- powered, YES!

I had a powered amplifier-speaker set (stereo) that was meant to be driven from the line out of a (generic) PC, and its input impedance was

Well looking at just the power your looking at fifty 50W loads (assuming the amps are 50% efficient which is typical for class AB) That's 2500W which is ~52A from your 48V power supply at full out so you don't have much margin.

You will more design for voltage drop than anything else. Assume a 10% voltage drop due to the resistance of both the 48V and the return leg.

Assume each set of to amps gets its own power cable from the supply. At 11 amps (500W/48V) the resistance would be 4.8V(10% drop)/11A = 0.43 ohms. This is the total loop resistance of the wires from the power supply to the ten amps and back (70 there and 70 back so 140 meters total length).

Looks like you'll need 10 AWG wire to meet that specification.

You'll need 5 runs of this to accomodate your 50 speakers.

.14Km * 3.27 ohms/Km = .45 ohms

If you can allow the 48V to sag 20% say down to 40V then you can get away with 12 AWG wire.

All of this assumes you need max power from the speakers, if you know you'll need less you can get away with smaller wires.

As for input signals that is more tricky because noise pickup and crosstalk are a bigger concerns than line resistance.

For the input signals, a shielded microphone cable to each speaker will do.

The power supply cable is more involved as we have to work out the best way to group, separate and run the wires from the common power supply to the speakers. We need to know the distances involved as this is crucial to calculating the size of wires needed. It's vital to know how far the speakers are spaced from each other and from the common power supply. We have to consider voltage drops over the wires, not just the current carrying capacity.

And that is assuming all 50 speakers are bunched close together, powered from a common supply 70 meters away. I'm not sure that's what the OP is trying to do. The speakers or groups of speakers may be scattered over a sizeable area.

Thanks there for the insight on calculating the power cable size though one thing I didn't mention was that the amplifiers were class D which should give a higher efficiency figure . Well still the exact power is still unknown because before the signal goes to the amplifier it passes through a mixer and attenuator unit (there is one in an audio rack cabinet ... whatever the power consumption is here is unknown to me ... I'll see if I can find the documentation somewhere. The attenuator units (who knows why its called so ) from the documents I got are supposed to amplify the signal to sufficient levels as it comes from a microphone so as to get to the amplifier + loudspeaker units sufficiently intact. But actually what goes on in these units is still unknown to me at the moment.

I was thinking of for the sake of simplicity using a 4 wire cable (2 twisted pairs, each sized 10AWG if the 500W power consumption assumption holds) with a shield for both power and audio.

H. Johnston