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'[EE] Audio distribution'
2009\03\17@083536 by Phil Keller

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All,

I am looking for some assistance from the collective:
* Background: I work with a local charity that puts on a Ghost House
that every year raises money for various non-profit organizations. The
ghost house consists of 10 rooms in two 66ft trailers. We are utilizing
PC style amplified speakers and plan on running the leads (most likely
16 gauge “ZIP” cord) back to a centralized location which will be
utilizing an MP3 style player.
* Problem: As I understand it, and I am not an audio kind of guy, the
MP3 player will not be able to drive the 6-12 PC speaker loads, thus the
need for an Audio Splitter/driver distribution system. The problem is
that the Audio drivers that I have found run several hundred dollars and
are really designed for multiple speakers, not earphones. The advantage
we have is that the input impedance to most PC speakers is in the 10K
range so that the output drive requirements of the splitter is reduced.
The sounds do NOT need to be of great quality, this is a ghost house
after all, but the reproduction should be “GOOD”.

Since cost is an issue (every $ spent means a $ less to give), does
anyone have any designs that they would like to share, ideas of how you
would solve this problem, pointers or better solutions.

Your ideas & comments are appreciated.

-Phil

2009\03\17@084941 by Dario Greggio

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Phil Keller ha scritto:

> * Background: I work with a local charity that puts on a Ghost House
> that every year raises money for various non-profit organizations. The
> ghost house consists of 10 rooms in two 66ft trailers. We are utilizing
> PC style amplified speakers and plan on running the leads (most likely
> 16 gauge “ZIP” cord) back to a centralized location which will be
> utilizing an MP3 style player.
> * Problem: As I understand it, and I am not an audio kind of guy, the
> MP3 player will not be able to drive the 6-12 PC speaker loads, thus the
> need for an Audio Splitter/driver distribution system. The problem is
> that the Audio drivers that I have found run several hundred dollars and
> are really designed for multiple speakers, not earphones. The advantage
> we have is that the input impedance to most PC speakers is in the 10K
> range so that the output drive requirements of the splitter is reduced.
> The sounds do NOT need to be of great quality, this is a ghost house
> after all, but the reproduction should be “GOOD”.

hmmm, I'd use cheap PC speakers, make the out-going impedance "low" (to
avoid picking hum and noise), and simply done.
I've been doing the same in my home for some time, to drive wall
speakers from my PC.


2009\03\17@090651 by Alan B. Pearce

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>* Problem: As I understand it, and I am not an audio kind of guy,
>the MP3 player will not be able to drive the 6-12 PC speaker loads,
>thus the need for an Audio Splitter/driver distribution system. The
>problem is that the Audio drivers that I have found run several
>hundred dollars and are really designed for multiple speakers, not
>earphones. The advantage we have is that the input impedance to most
>PC speakers is in the 10K range so that the output drive requirements
>of the splitter is reduced. The sounds do NOT need to be of great
>quality, this is a ghost house after all, but the reproduction should
>be “GOOD”.

Are the speaker units you want to use self powered? If so I would take the
PC output into a standard Hifi amp, and use the speaker output of that to
drive the other units. This may be enough to drive the speakers even if they
are not self powered, if the amplifier has sufficient current grunt. I would
have thought something in the region of 30-50 watts would suffice.

2009\03\17@090744 by Jake Anderson

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Phil Keller wrote:
{Quote hidden}

If the speakers are the kind that plug into the wall you might well get
away with it.
Worst case a small amplifier (1W range) might be needed mainly to get
the run your after.

Try to get speakers *without* the earth plug, that way there
manufacturers cant do crazy things with earth and ground.
Run a shielded cable, twisted pair inside a shielded cable would be even
better.


2009\03\17@104017 by Harold Hallikainen

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The fact that the speakers are amplified and have a 10k input impedance
makes me think you can bridge a bunch of them across a single line and not
worry about loading it much. In broadcast, we used to have 600 ohm sources
and loads (brought over from telephone lines). Inputs were either
terminating (600 ohms) or "bridging" (10k or higher). You could hang a
bunch of bridging loads across the terminated line with no problem. One
other issue is ground loops since both inputs and outputs are unbalanced.
But I don't think even that is an issue if the amplified speakers are
powered by ungrounded wall warts, so the inputs to the amplifiers are
floating.

So, overall, I don't think you need to add any hardware. How about testing
it and seeing?

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\03\17@110215 by alan smith

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You could always ask your local TI rep to see if they can sample you several demo boards.  So you have a single MP3 player needing to distribute to several speakers...they make distribution amps for exactly this thing, I used one last year for a client.  I don't have the part number handy, and they provided it but it did exactly that. See of one of the local audio shops might consider donating one to the cause?


--- On Tue, 3/17/09, Phil Keller <spam_OUTPhilTakeThisOuTspampkeller.net> wrote:

{Quote hidden}

> -

2009\03\17@110424 by Dwayne Reid

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At 06:35 AM 3/17/2009, Phil Keller wrote:

>All,
>
>I am looking for some assistance from the collective:
>We are utilizing
>PC style amplified speakers and plan on running the leads (most likely
>16 gauge "ZIP" cord) back to a centralized location which will be
>utilizing an MP3 style player.
>* Problem: As I understand it, and I am not an audio kind of guy, the
>MP3 player will not be able to drive the 6-12 PC speaker loads, thus the
>need for an Audio Splitter/driver distribution system.

I may be able to offer some help here.

From the above, it appears as if you want to drive multiple
amplified speaker systems (with built-in amplifiers) from a single
audio source.

That should be no problem but there are a couple of things to look out for.

1) make sure that all of the amplified speaker systems do NOT have a
ground pin on the power plug.  You are running unbalanced audio over
a distance and would most likely have a problem with ground loops
that would cause hum and other undesired noises from the speakers.

2) use shielded audio cable for your connection between the MP3
player and the amplifiers.  I assume that you are sending a stereo
signal - that means that you should use a shielded twisted pair
cable.  A typical cable number is Belden 8450 (solid conductors) or
8451 (stranded).  You don't have to use those exact numbers - lots of
other cable suppliers make similar cable a LOT less expensive than
Belden.  But you can use the Belden numbers to cross to other cable
manufacturer's part numbers.

That's really all there is to it!  Oh - if I was doing this, I'd most
likely be balancing the audio using some of my transformer cards or
active balancing cards but that should not be necessary if your
distances are short.  You will know that its a problem if you have
undesired noise coming out of the speakers that you can't get rid
of.  Write back if you have that problem.

The reason the above should work just fine for you is that modern
audio gear is set up for what's called "bridging mode" instead of
"terminating mode".  Bridging mode connections rely on the audio
source having a low output impedance and the audio inputs having a
high input impedance.  The output impedance of most consumer audio
gear is 47R to 100R.  The input impedance is usually 10K or so.

So: just run shielded audio cable from each amplifier-speaker input
to where the MP3 player is located.  Put 3.5mm stereo plugs on the
ends of the cable so that they can plug into the amplified speaker
inputs.  At the MP3 player end, just use pigtail splices to join all
of the shield, Left, Right conductors together to the cable coming
out of the MP3 player.

Hope this helps!

dwayne


--
Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2009\03\17@110748 by William \Chops\ Westfield

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On Mar 17, 2009, at 5:35 AM, Phil Keller wrote:

> * Problem: As I understand it, and I am not an audio kind of guy, the
> MP3 player will not be able to drive the 6-12 PC speaker loads, thus  
> the
> need for an Audio Splitter/driver distribution system.

Perhaps you should try it out.

In the worst case, I suspect you can use one set of "computer  
speakers" as a driver for all the others, given nothing more than a  
couple resistors at each speaker to move the levels back down.

BillW

2009\03\17@114712 by Bob Blick

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Hi Phil,

In supermarkets they use either "70 volt line" or "25 volt line"
systems. At each speaker is a small transformer with many taps, and you
select the speaker's impedance and how much relative power you want it
to have, and connect to the big long speaker wire that runs through the
store.

At the driving end of the wire you connect an amplifier. If you are
using a 70 volt system you have an amplifier connected to a large master
transformer(some amplifiers meant for supermarkets have the transformer
already built in). With 25 volt systems you just connect the speaker
wire directly to any amplifier. So nowadays people usually do 25 volt
lines.

You can get speakers with transformers already mounted to them, with or
without enclosures, for under $10. If you just want 25 volt
transformers, they are about $2 or $3.

MCM Electronics carries that kind of stuff. Officially they are "70.7
volt" and "25.2 volt". It's been a while since I did any of this but
it's been popular forever, and fidelity is reasonable.

BTW, the only reason they have that voltage rating is so the power
levels printed on the transformer relate to something. The audio you
pump into the line varies in voltage, unless you listen to test tones :)

Cheerful regards,

Bob

On Tue, 17 Mar 2009 05:35:35 -0700, "Phil Keller" <EraseMEPhilspam_OUTspamTakeThisOuTpkeller.net>
said:
{Quote hidden}

> --

2009\03\17@114728 by olin piclist

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Harold Hallikainen wrote:
> The fact that the speakers are amplified and have a 10k input
> impedance makes me think you can bridge a bunch of them across a
> single line and not worry about loading it much. In broadcast, we
> used to have 600 ohm sources and loads (brought over from telephone
> lines). Inputs were either terminating (600 ohms) or "bridging" (10k
> or higher). You could hang a bunch of bridging loads across the
> terminated line with no problem. One other issue is ground loops
> since both inputs and outputs are unbalanced. But I don't think even
> that is an issue if the amplified speakers are powered by ungrounded
> wall warts, so the inputs to the amplifiers are floating.
>
> So, overall, I don't think you need to add any hardware. How about
> testing it and seeing?

I agree.  I don't see why this is supposed to be a problem.  As Harold says,
the real issue is picking up common mode noise, like power line hum, that
gets interpreted as differential signal at the powered speakers.  Shielded
cable will probably help some, although it may actually make things worse if
the speakers aren't isolated.  If you do have hum, it is probably cheaper
and likely more effective to put small 1:1 audio transformers right at the
inputs to the powered speakers.  Then you should be able to run cheap
twisted pair out to each set of speakers.  I haven't used audio transformers
in many years, so I'm just assuming these things are still available
somewhere.

OK, I go curious so I checked.  Jameco has a "audio transformer" at $28 for
10 (630459), and a "modem coupling transformer" at $17 for 10 (125357).
Both are 1:1 at 600 ohms both sides.  The modem transformer probably was
designed for a narrower frequency range with more slop allowed, but most
likely will work fine for your needs.  A PC sound card output or anything
designed to drive headphones should be able to easily drive these.  The
extra cost of the transformer will be more than offset by being able to use
cheaper cable if you do have a hum problem.

However as Harold said, try it first.  You may not need to do anything
special.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\03\17@130842 by Bob Blick

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Here's an example of a 25/70 volt audio transformer:

http://www.proacousticsusa.com/productdetail.php?pId=3179

Prices are higher than when I used them, now $3.95

Cheers,

Bob

--
http://www.fastmail.fm - A fast, anti-spam email service.

2009\03\17@131913 by olin piclist

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Bob Blick wrote:
> In supermarkets they use either "70 volt line" or "25 volt line"
> systems. At each speaker is a small transformer with many taps, and
> you
> select the speaker's impedance and how much relative power you want it
> to have, and connect to the big long speaker wire that runs through
> the store.

I can picture it now.  Howls, clicks, rasping noises, growls, and other
spooky sounds all around, then suddenly "Price check, register 5".


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\03\18@084809 by Phil Keller

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 Thanks to all for the great suggestions. I am leaning towards the idea
of just placing a standard speaker amplifier (small maybe 1-2 watts) in
line and having it drive the powered PC speakers.  I will keep the idea
of an isolation transformer as a back-up in case I find that noise
enters the system.  I "think" that the PC speaker are ground-less but I
will have to wait and check as they are in storage right now.

 Thanks again for the suggestions.

-Phil-

Phil Keller wrote:
{Quote hidden}

--
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Long ago when men cursed and beat the
ground with sticks,  it was called witchcraft..

Today, it's called golf.

2009\03\18@110231 by Dwayne Reid

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At 06:48 AM 3/18/2009, Phil Keller wrote:
>   Thanks to all for the great suggestions. I am leaning towards the idea
>of just placing a standard speaker amplifier (small maybe 1-2 watts) in
>line and having it drive the powered PC speakers.  I will keep the idea
>of an isolation transformer as a back-up in case I find that noise
>enters the system.  I "think" that the PC speaker are ground-less but I
>will have to wait and check as they are in storage right now.

My suggestion: DON'T do that!  All you are going to do is add
significant noise to the signal.

If you really feel the need to buffer the output of the MP3 player,
do so with a decent op-amp.  NE5532 comes to mind - its good for at
least 30 mA output and is relatively quiet.  The old audio standbys
of TL072 are both noisier and have much less drive than the 5532.

But: I really don't think that you need a buffer.  As I mentioned in
my previous post, the output impedance of your MP3 player is most
likely in the region of 47 through 100 Ohms.  That's plenty low
enough to drive several hundred feet of shielded cable.

Do make sure that the PC speaker power supplies are ungrounded, though.

dwayne

--
Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2009\03\18@115327 by Bob Blick

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On Wed, 18 Mar 2009 05:48:09 -0700, "Phil Keller" <@spam@PhilKILLspamspampkeller.net>
said:
>   Thanks to all for the great suggestions. I am leaning towards the idea
> of just placing a standard speaker amplifier (small maybe 1-2 watts) in
> line and having it drive the powered PC speakers.  I will keep the idea
> of an isolation transformer as a back-up in case I find that noise
> enters the system.  I "think" that the PC speaker are ground-less but I
> will have to wait and check as they are in storage right now.

Noise will always enter the system, it's whether it reaches your
tolerance level :)

You have four things working against you, because you are driving an
unbalanced line, non-shielded cable, at low levels, into an unknown
impedance. Fix the dominant problem and you are still left with three
compromises.

But if you run at high signal levels, terminate and attenuate at each
station, you can probably get enough quality to be satisfied.

Cheerful regards,

Bob


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                         love email again

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