# How to share one Stereo Input with several Amplifiers which would feed their own mono speakers.

#### deepmagenta

Joined Jul 26, 2019
65
Hi all,
We have this Stereo Sound system which plays music in an office environment. With 8 rooms. In each room there is one 4 Ohm speaker. It is not working properly (long story).
What I need is as follows:
1- One Stereo Audio source (cell phone / SD card etc) shared with a bunch of (similar) amplifiers inside a box ( I see all sorts of Chines products available which I have purchased some already).
2- Each speaker in each room should get a mono signal.
3- Each speaker has to have its own volume control on amplifier-box (sitting in Server Room ), so that the level of sound in each room can be controlled independently from the server room.

What is the best practice to implement this? What are the potential issues:
- in sharing the same input for all those amps?
- in sharing the same power supply for all those amps
- in converting stereo signal, coming out of each amp, into mono signal.

I would appreciate any input on this. Naturally if there is a totally different approach to this, I would love to hear that too.
Cheers

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#### Irving

Joined Jan 30, 2016
794
Put the amplifier & speaker for each room in its own box with local volume control and power supply.

Converting stereo to mono, just join the channels together at the output of the source with a resistor in each channel; resistor value will depend on type of source and next stage.

Depending on distance you might b able to get away with a small preamplifier next to source driving a cable to each room amplifier, all in parallel. If not you'll need a proper balanced audio line driver and a receiver in each room.

Need to know about distances.

Any idea about power output from amp needed?

#### deepmagenta

Joined Jul 26, 2019
65
Thanks Irving for your prompt reply. here is the answers:
1- Put the amplifier & speaker for each room in its own box with local volume control and power supply.
- I am trying to avoid this. I will end up having 8 amp boxes etc. I did edit my original post to mention that I need to control all speakers from one spot in the server room (the way it is now, and avoiding new wiring). I envisioned a box with 8 amp inside, and 8 volume control sticking out to replace the existing amp. OR put each pair of speakers in series and utilize 4 amps and 4 volume controls, so that each volume control takes care of 2 rooms.

2-
Need to know about distances......Any idea about power output from amp needed?
- Rooms are distanced from 2 meters to about 6 meters.
- The level of music we play is very low( a bit louder than a whisper ) .
- The existing system worked for 13 years (Philips MCM108D/98 ) with total 4(four) RMS output, still we never needed to turn the volume more than 30 percent, but not perfect, in that, rooms were not playing music at the same level, some louder than others (annoying).

- I don't know how the wiring to 8 speakers thru the ceiling and walls is done. For all those 8 speakers, only 2 pair of wires appear at the back of Philips unit (the R and L channels of amp). So, it seems that those speakers are connected in parallel/Series configuration by the installer( don't count of his competence at all). I measured the resistance of each pair of wires. It is about 4 Ohms.

So, my ideal solution is to avoid having one amp in each room. The single speaker in each room is in the dropped ceiling, hence independent amp in each room would require wiring thru the wall to get to the cieling to feed the speaker with the room amp. I don't wish to touch the walls. I need to utilize the existing wiring in the ceiling to get to the speakers (if I have to run 8 pairs of wires to those speakers, so be it), with one amp in server room.

#### schmitt trigger

Joined Jul 12, 2010
277
What you require was invented many years ago, probably over 60 years.

It is called a 70 volt audio distribution line. Basically with a transformer you convert the audio amp's output to a 70 volt signal, distribute it to wherever you want, and then on each room each speaker has another transformer that steps down the voltage.

Amazon and others sell them:
https://www.amazon.com/70-volt-audio-transformer/s?k=70+volt+audio+transformer

#### deepmagenta

Joined Jul 26, 2019
65
What you require was invented many years ago, probably over 60 years.

It is called a 70 volt audio distribution line. Basically with a transformer you convert the audio amp's output to a 70 volt signal, distribute it to wherever you want, and then on each room each speaker has another transformer that steps down the voltage.

Amazon and others sell them:
https://www.amazon.com/70-volt-audio-transformer/s?k=70+volt+audio+transformer
It seems wonderful schmitt trigger. I will look into it and learn how it works. Meanwhile I purchased a Pioneer AVR system( VSX-933) to have some music at work. I guess I have to bring it home now. This technology you alloded to seems to address the issue. I have to admit thought, although I am not an electronic guy, I was itching to get my hands dirty and built some thing with these little Chinese amplifiers, and learn something in the process. Thanks for your reply

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#### Externet

Joined Nov 29, 2005
1,518
I see no need to go to a 70V distribution line for distances of less than 6 metres.
A rheostat 50-100 ohm in series with each speaker line (total 8) will control the loudness from the server room.

#### deepmagenta

Joined Jul 26, 2019
65
Thanks Externet. I guess we would have not have problems with heat (very low volumes used) and frequency response of the speakers (classical music being played all the time, of relaxing type, with limited frequency range). Thanks for Reply, much appreciated.

#### Irving

Joined Jan 30, 2016
794
As @schmitt trigger suggested, that's the 'proper' option, but doesn't generally allow individual volume per room (though you can fit one at about $20 each speaker plus the room wiring you didn't want to do) and in your case you'd have to go into the drop ceiling to wire up each transformer to its speaker - or put them all side by side in the server room which rather defeats the point of a 70v distribution system. Plus the$20+ for each transformer, plus one extra for the amp output to drive the line (assuming you have an amp and are not reusing the Philips box) is a fair expense for something that'll work but may not meet your needs. The line distribution system is best suited to environments where you want the same sound level everywhere, e.g. PA and supermarket muzak systems.

As @Externet suggests, you could have 1 amplifier and a 'volume' control for each speaker, but you still have the issue of matching 8 speakers + volume controls to an amplifier, and it's likely the series/parallel combination will have interesting side effects between speakers. You still need to source an amplifier, though as you said, there are plenty on eBay.

OK, don't know what your level of 'engineering skill' is, but here's my suggestion, FWIW:

8 off these 10W mono amps
at $7 each approx 1 Power supply, 24v/5A at$10 approx

Stick it all in a old PC case, wire all the inputs together board to board with some screened cable, and then out to a 3.5mm jack to your sound source.

Wire each amp back to power supply +/- directly.

Will give you 5W, individually adjustable per channel (power supply current limited, but you could go for a bigger supply)

#### Irving

Joined Jan 30, 2016
794

#### Externet

Joined Nov 29, 2005
1,518
...you still have the issue of matching 8 speakers + volume controls to an amplifier...
Hi. Thanks. What is there to match ?

#### deepmagenta

Joined Jul 26, 2019
65
As @schmitt trigger suggested, that's the 'proper' option, but doesn't generally allow individual volume per room (though you can fit one at about $20 each speaker plus the room wiring you didn't want to do) and in your case you'd have to go into the drop ceiling to wire up each transformer to its speaker - or put them all side by side in the server room which rather defeats the point of a 70v distribution system. Plus the$20+ for each transformer, plus one extra for the amp output to drive the line (assuming you have an amp and are not reusing the Philips box) is a fair expense for something that'll work but may not meet your needs. The line distribution system is best suited to environments where you want the same sound level everywhere, e.g. PA and supermarket muzak systems.

As @Externet suggests, you could have 1 amplifier and a 'volume' control for each speaker, but you still have the issue of matching 8 speakers + volume controls to an amplifier, and it's likely the series/parallel combination will have interesting side effects between speakers. You still need to source an amplifier, though as you said, there are plenty on eBay.

OK, don't know what your level of 'engineering skill' is, but here's my suggestion, FWIW:

8 off these 10W mono amps
at $7 each approx 1 Power supply, 24v/5A at$10 approx

Stick it all in a old PC case, wire all the inputs together board to board with some screened cable, and then out to a 3.5mm jack to your sound source.

Wire each amp back to power supply +/- directly.

Will give you 5W, individually adjustable per channel (power supply current limited, but you could go for a bigger supply)
My "Engineering Level", as for this project, is adequate. I did manage to fix up that Philips Unit (CD had died+some Caps on Main board), also bypassed the Tuner on it (never used) and directed it to an Aux input. My technical background is Mechanical and SW.
- As for your suggestions, I have no issues with getting into the ceiling and running pairs of wires for each speaker. Each speaker is 4 Ohms, with wires in varying lenghts to each room, still will be within 4 ~ 8 ohms, I suppose. It seems that on the input side of the amps, you see no need to put some resistors in series with each line before feeding them into each of those amps.
- As for 70V system, based on what I just read, the speaker transformers provide the ability to set individual power to each speaker. In general, we don't play with the volume. We set it at level we want, and let it be there. Also, I guess the 70V amp has individual Volume Controlls for each line as well, so I guess we have control for volume in each room.

#### BobTPH

Joined Jun 5, 2013
2,429
It if is really whisper level, the little 5V 3W amplifiers you can get for $1 a piece would be all you need, one for each speaker. Here, you go: The$11 solution to your problem

Bob

#### Externet

Joined Nov 29, 2005
1,518
...I did manage to fix up that Philips Unit...
That is the way to go ! Well done.

#### Irving

Joined Jan 30, 2016
794
It if is really whisper level, the little 5V 3W amplifiers you can get for $1 a piece would be all you need, one for each speaker. Here, you go: The$11 solution to your problem

Bob
Yes, if you like listening to classical music with 10%THD. Plus those boards are really only good for 0.5W max before the chip overheats on a 4ohm load...

#### Irving

Joined Jan 30, 2016
794
- As for 70V system, based on what I just read, the speaker transformers provide the ability to set individual power to each speaker. In general, we don't play with the volume. We set it at level we want, and let it be there. Also, I guess the 70V amp has individual Volume Controlls for each line as well, so I guess we have control for volume in each room.
Sort of... The 70v system is distribution only between high voltage parallel wired transformers making a multidrop system. There is a facility with transformer taps to adjust maximum sound pressure at each 'drop', but individual volume controls are fitted between transformer and speaker. The amp provides a master volume control, and connects to the HV distribution by its own matching transformer, unless the amp is designed to drive the HV line directly.

#### Irving

Joined Jan 30, 2016
794
My "Engineering Level", as for this project, is adequate. I did manage to fix up that Philips Unit (CD had died+some Caps on Main board), also bypassed the Tuner on it (never used) and directed it to an Aux input. My technical background is Mechanical and SW.
- As for your suggestions, I have no issues with getting into the ceiling and running pairs of wires for each speaker. Each speaker is 4 Ohms, with wires in varying lenghts to each room, still will be within 4 ~ 8 ohms, I suppose. It seems that on the input side of the amps, you see no need to put some resistors in series with each line before feeding them into each of those amps.
Great, then this should be a piece of cake for you!

The input side of the amp is the volume control, so is a moderately high impedance >50k. It should be OK just to wire all in parallel as your sound source will be a low-impedance headphone output. As long as it can provide about 100mW into 1kOhm say it will work fine. Will need a couple of 100ohm resistors to connect L & R together.

#### BobTPH

Joined Jun 5, 2013
2,429
Yes, if you like listening to classical music with 10%THD. Plus those boards are really only good for 0.5W max before the chip overheats on a 4ohm load...
The datasheet shows 2.5W at 4 Ohms with 1% distortion and 0.5W with 0.15% distortion. I am using one of these for my computer speakers with 8 Ohm speakers powered by USB. It produces way above whisper level with no noticeable distortion. These are speakers that I built and the sound is WAY better than the $100 Bose bluetooth speaker I also use. Bob #### Taff The Soundman Joined Jul 16, 2020 3 Yes, if you like listening to classical music with 10%THD. Plus those boards are really only good for 0.5W max before the chip overheats on a 4ohm load... These little amps have a suprisingly good sound and can be powered by any 5v wall PSU. I have used them in many projects, and unless you want to drive an 18" speaker, they work well. Thread Starter #### deepmagenta Joined Jul 26, 2019 65 It if is really whisper level, the little 5V 3W amplifiers you can get for$1 a piece would be all you need, one for each speaker.

Here, you go:

The \$11 solution to your problem

Bob
@BobTPH
- it seems like a good plan, except it would require providing 5v power supplies to each speaker in the ceiling (I don't want to do anything to the walls). Plus, if the amps are supposed to be attached to each speaker, then the input signal has to be traveling from the server room down to each room (diff. length wires, interferences or any other mismatch issues that I have no idea about). Perhaps there are products (perhaps headphone slitter boxes), which might be helpful in distributing the input signal in a balanced fashion to each room.

#### deepmagenta

Joined Jul 26, 2019
65
Sort of... The 70v system is distribution only between high voltage parallel wired transformers making a multidrop system. There is a facility with transformer taps to adjust maximum sound pressure at each 'drop', but individual volume controls are fitted between transformer and speaker. The amp provides a master volume control, and connects to the HV distribution by its own matching transformer, unless the amp is designed to drive the HV line directly.

View attachment 212305
Great, I did not know that, amoung other things.

70 V Dist. System seems very elegant and enticing solution to this, yet I have to admit, it takes the joy of tinkering with other ideas which we all have discussed here away.
To all of you, wonderful helpful souls:
- I am inclined to run one pair of wire to each room (as mentioned before)
- Put 8 mono amps in one box, as @Irving suggested, feed them all by the same Power Supply, and see how it goes. This way I can take the Pioneer AVR that I just purchase and installed there back home and watch some movies with.

PS. This is actually my wife's Physiotherapy Clinic. I want to dazzle her with a home made 8-amp system that she can show off to her friends, as a sign of the ineffable love her husband has for he .