12 speakers .....Impedance Matching?

[stizzswizz]

Hello,

I have a member of my family who purchased a large sail boat and asked me if help fix their stereo on it. Currently they have a 4-channel car stereo connected to a Radio Shack speaker selector to control 12 speakers over several rooms.

They want to get rid of the speaker selector and just have seperate volume controls for each set of speakers for each room. So I understand that a car head unit can not power 12, 4Ohm speakers due to a couple reasons, the main one is probably power output from the head unit. So, if I use say a 250W(RMS) x 2channel at 4Ohm amplifier, can I arrange the speakers in a parallel/series configuration and still have quality sound? I also would like the ability to decrease the volume for one room, without affecting the volume of another room.

Is there any way to "trick" an amplifier to seeing a constant 4Ohm load?

I showed them a multi-channel home theater amplifier that would work perfectly but they didnt want to dish out $700.

I guess Im looking for a semi-simple passive circuit idea...

Thanks,
Steve

 

[dknguyen]

Circuit:
Speaker->Volume Control->Impedence Matching Transformer -> Amp

Each circuit runs in parallel with every other circuit leading away from the amp.

Is that what you mean?

Transformers step up/down voltage and down/up current. And since V=IR, you can sort of see how a transformer might be able to "trick" something on one side of the transformer to effectively see a different resistance.

 

[Nigel Goodwin]

The techniques is called '100V line' in Europe, I think the USA might use a slightly different standard?. Essentially it's like electricity distribution - use a high voltage and a low current to prevent loss over long wires.

So at your 4 ohm amplifier you have a 100V line transformer, that converts the 4 ohm impedance to 100V line (the impedance doesn't matter, that's the whole point of it). Then at each remote speaker you have a 100V line transformer (except a smaller one) to convert it back down again, the transformer will normally have multiple taps, so you can adjust the power output to each speaker individually - the taps are labelled in watts. As already suggested, instead of using the taps, you can use a large wattage wirewound potentiometer between the transformer and speaker to give fully variable levels.

Calculations are simple - again, that's the whole point!. Just add the wattages of the transformers together, so if your main amp is 100W, you need a 100W to 100V line transformer to feed the line. You can then have a maximum total of 100W of speaker transformers - so perhaps ten 10W ones. If you wanted to connect twelve 10W transformers you would need a 120W amplifer and transformer.

 

[audioguru]

Stereo shops sell little stereo dual transformers with a dual volume control built in.
Because they are small they messup the bass frequencies.
If the amplifier is rated for 200W into 8 ohms then its output has a max voltage of 40V RMS. Then it won't need a stepup transformer.

In Canada and the US, 70.7V is used to distribute most PA systems and 25V (safer) is used in schools.

 

[stizzswizz]

Thanks for the info, I wasn't aware that could use a line transformer on a regular car amplifier.

Thanks for your input,
Steve