Impedance matching speaker selector without resistors? - diyAudio
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Old 27th January 2012, 03:28 AM   #1
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Default Impedance matching speaker selector without resistors?

I'd like to build a speaker selector and control it from an arduino. The arduino will be controlled through rs-232 and/or TCP. I bought a cheap speaker selector from monoprice and opened it up. It was nothing more than a 5Ω resistor in series with 4 parallel DPDT switches to turn the speakers on and off. I don't like this design because of the power wasted across the resistor. It seems that the higher end units use transformers. Here's my question:
Using this page as a guide, Impedance Matching Transformers,
I get the following. My amp can deliver 100w into an 8Ω load. This is 28.3 volts. If I now use a 1:2 transformer and wire all 4 speakers in parallel (32Ω), my V will = 56.6. 32Ω at 56.6 V will be the full power of 100W that the amp can deliver. I can do something similar for 2 and 3 speakers but I will use a different ratio transformer. My question in this scenario is, can a speaker be damaged by a higher voltage, or is it only the net power that it cares about? I'm looking at this transformer, model 117K32: Hammond Mfg. - Sound Distribution and Speaker Matching Transformers - (117 - 119 Series) I think it will give me the flexibility to use one transformer for the circuit of 2,3, or 4 speakers. (none needed for one since they are 8Ω speakers)

I am very new to all of this.
Thanks
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Old 27th January 2012, 03:35 PM   #2
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Higher voltage and higher power are intrinsically locked together. If you increase the voltage at the speaker terminals the power will increase. In fact the only way that you can control speaker power is through speaker voltage.

You might think about using a 70 Volt system.
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Old 27th January 2012, 03:57 PM   #3
sreten is online now sreten  United Kingdom
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Hi,

You have to wire the speakers in parallel, and 4 8 ohms speakers are 2 ohms.
You need a 2:1 transformer to convert a 2ohm load to a 8 ohm amp load.

As each speaker will only get a quarter of the amplifiers output, its simply
easier to use say 4 x 25W amplifiers, than a 100W with the transformer.
(You can build one power supply for all the amplifiers.)

You can control digital volume controls for various levels for each speaker
and don't need high current switching relays to turn them on and off,
just mute the inputs. If you don't need volume control, simple FET
muting circuits on the inputs could be used for on/off only.

rgds, sreten.
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Old 27th January 2012, 05:28 PM   #4
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So, it would work though? I already have the amp, so I'm not sure that getting 4 new 25w amps would be cheaper. The one I have is an Onkyo M-282, Onkyo M-282 - Premium Two-Channel Amplifier | Model Information | Onkyo USA Home Theater Products
The speakers I'm using are the Niles OS6.5's, Niles Audio Corporation

They are nice speakers so preserving the sound quality is important. What kind of 25w amps did you have in mind? Do these little guys sound like junk? 2x25W @ 4 Ohm TPA3123 Class-D Audio Amplifier Board 320-304

Thanks for your help. I'm a pretty serious DIY'er so I can handle this with no problem, but audio is still somewhat foreign to me.

Thanks for your help, I really appreciate it.
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Old 27th January 2012, 05:33 PM   #5
macboy is offline macboy  Canada
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Quote:
Originally Posted by n8huntsman View Post
I get the following. My amp can deliver 100w into an 8Ω load. This is 28.3 volts. If I now use a 1:2 transformer and wire all 4 speakers in parallel (32Ω), my V will = 56.6. 32Ω at 56.6 V will be the full power of 100W that the amp can deliver. I can do something similar for 2 and 3 speakers but I will use a different ratio transformer. My question in this scenario is, can a speaker be damaged by a higher voltage, or is it only the net power that it cares about?
A not so small mistake in your reasoning is that 4 parallel 8 Ohm loads will result in a 2 Ohm total load, not 32 Ohm. Putting 4 8 Ohm loads in series gives 32 Ohm. When in series (assuming all are identical and perfect resistors), each load will 'see' 1/4 of the voltage. When in parallel, it is the current that is 'shared', with each load seeing the same voltage.

You can still do what you want with a 1:2 transformer, but wired up the other way to deliver 1/2 the voltage (not 2x the voltage). Then it is safe to connect up to 4 loads. Keep in mind that with only 1 load connected, the amp will see a 32 Ohm load; with all 4 connected, it will see a 8 Ohm load. Due to the limited voltage at the transoformer output, the amp can only deliver (roughly) 25 W into a single speaker, even if it is the only one turned on at the time.
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Old 27th January 2012, 05:59 PM   #6
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Doh! I totally know that, just a brain fart. The picture is a little more grim with that correction. Assuming the amp puts out 100w @ 8Ω, that would be 28.3V. Now run that the other way, 2:1 stepdown, I will end up with 2Ω, 100w (25 per speaker) but my voltage will only be 14.1V. I'm okay with the 25W per speaker. They are good down to 5W. What I'm not sure about is the 14.1V. The furthest speaker is about 200 feet away from the amp. Would this present a problem?
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Old 28th January 2012, 01:47 AM   #7
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I think I should explain my system a bit. My main AVR has multi-zones. I'm running the zone 2, pre-amp out, to a 2-ch, 100w @ 8Ω amp, which will power 4 pairs of 8Ω speakers in my backyard. The speakers say, "Recommended Amplifier Power: 5W To 125W." The volume is controlled on the main AVR pre-amp, not on the amp itself. My end result needs to be the ability to turn on and off whichever pairs I choose so that I only have music in the backyard where I want it. If I can get by with the transformer method, I plan to use an auto-transformer with multi taps so that I can program the arduino to change the tap based on how many speakers are on, in order to keep the amp load at 8Ω. If there is a better way, I'm all for it.
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Old 28th January 2012, 03:33 AM   #8
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Google '70V sound distribution system'.

If you read some of this stuff it will give you an understanding of distributed sound systems.
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Old 28th January 2012, 04:09 AM   #9
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Thanks. I have read about them, but was trying to avoid having to use transformers at both ends. Since my runs are not as long as what most 70v systems are, I'm wondering if I can get by with the lower voltage. I did come across this device; MultiMatchit 300S
It seems to do what I'm looking to do and I don't see any magic in it. I suspect that it is what they use in this product, Audioplex Speaker Selectors - PT Series
I think this may be one of those try it and see if it works instances. I'm not in love with my amp so if it blows it blows. I'll post a BOM here before I buy my stuff so others can make suggestions on the components. Thanks for your help.
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