Simplest, easiest, cheapest way to sum to mono?

Lisa Lane-Collins

Lisa Lane-Collins

Sophomore
Dec 9, 2012
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Adelaide, Australia
Running some cables for a very stingy client, we're trying to facilitate remote input points for an ipod, a computer and people's laptops being used for powerpoint presentations with sound.

The client is happy to use the one input line for ipods and laptops, and we have a second line run for the computer (microphone cable from the source to the mixing desk behind the bar). I know I can sum the stereo image to mono in software on the pc, what I'd like to know is how to sum to mono the output from the laptop/ipod so I don't have run a 3rd line, and don't lose half the stereo data?

This came up in another post that it's not a good idea to just physically link the two outputs together...something about needing resistors..but I can't find any off the shelf devices that will sum to mono for me nor information about how to make my own devices (must be using the wrong buzz words). What do I need to do?
 
Re: Simplest, easiest, cheapest way to sum to mono?

Lisa Lane-Collins said:
Running some cables for a very stingy client, we're trying to facilitate remote input points for an ipod, a computer and people's laptops being used for powerpoint presentations with sound.

The client is happy to use the one input line for ipods and laptops, and we have a second line run for the computer (microphone cable from the source to the mixing desk behind the bar). I know I can sum the stereo image to mono in software on the pc, what I'd like to know is how to sum to mono the output from the laptop/ipod so I don't have run a 3rd line, and don't lose half the stereo data?

This came up in another post that it's not a good idea to just physically link the two outputs together...something about needing resistors..but I can't find any off the shelf devices that will sum to mono for me nor information about how to make my own devices (must be using the wrong buzz words). What do I need to do?
Click to expand...
Simple easy and cheap are different things.

The cheapest way is to get 2 1K (give or take the value at bit) resistors (should cost about $0.1 US ea) and put one in series with each "hot" and tie the other end together into the plug of your choice. Takes a couple of minutes to put together. You an assemble it inside an XLR shell.

What I do is get a premade 1/8" cable to "anything" and chop it off. Build it all inside a MXLR with the new "hot" going to pin 2 and the shield going to pin 3.

But that is not easy or simple.

You can get various summing devices (Rapco makes one just for that purpose and has a balanced output), but it is a lot more expensive than a couple of resistors. It has a 1/8" plug on one end and XLR on the other. Real easy.

So I guess it depends on what you call "cheap" and what you call "easy".
 
Re: Simplest, easiest, cheapest way to sum to mono?

Lisa Lane-Collins said:
Running some cables for a very stingy client, we're trying to facilitate remote input points for an ipod, a computer and people's laptops being used for powerpoint presentations with sound.

The client is happy to use the one input line for ipods and laptops, and we have a second line run for the computer (microphone cable from the source to the mixing desk behind the bar). I know I can sum the stereo image to mono in software on the pc, what I'd like to know is how to sum to mono the output from the laptop/ipod so I don't have run a 3rd line, and don't lose half the stereo data?

This came up in another post that it's not a good idea to just physically link the two outputs together...something about needing resistors..but I can't find any off the shelf devices that will sum to mono for me nor information about how to make my own devices (must be using the wrong buzz words). What do I need to do?
Click to expand...

Whirelwind PODdi sells in the US for about $73 or a cheaper unit is the Rolls AV Presenter DB14b just under $50. The rolls DB14 isn’t flat on the bottom end, it rolls off a little bit around 150hz but for most things it works ok. The one I have has an 3.5mm jack on it along with the RCA jacks but there seems to be a version without the 3.5mm jack so if you order one be careful of which one it is.

http://www.amazon.com/dp/B0006NAMFE...vptwo=&hvqmt=b&hvdev=c&ref=pd_sl_1zeg1bi3i8_b

http://www.amazon.com/Whirlwind-Pod...08137&sr=1-1-catcorr&keywords=whirlwind+poddi
 
Re: Simplest, easiest, cheapest way to sum to mono?

Am I right in saying that the mono summing device needs to be cheaper than just running one extra cable in order to achieve an overall reduction in cost? How much money are you saving by running one less cable, ie. what's the budget for the mono sum to be viable?

Chris
 
Re: Simplest, easiest, cheapest way to sum to mono?

Chris Nixon said:
Am I right in saying that the mono summing device needs to be cheaper than just running one extra cable in order to achieve an overall reduction in cost? How much money are you saving by running one less cable, ie. what's the budget for the mono sum to be viable?

Chris
Click to expand...

Perhaps it's an input limitation? Now that I think about, is the cable run too far that an unbalanced connection is going to cause significant issues?
 
Re: Simplest, easiest, cheapest way to sum to mono?

Max Warasila said:
Ivan, I'm assuming you're using the full 1K because you don't have the resistor to the shield, correct?
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The resistors are in series with each hot lead. Nothing to ground.

The idea is that with the series resistor the load of one channel will not affect the output circuit of the other side.

If you just tie them together the output impedance of the L channel will load down the drive circuit of the R channel an vice versa.

This can cause distortion and possibly damage the output circuit-because it is driving an impedance much lower than it was designed for-just like loading a "power" amp down to around .01 ohms. They don't like it very much
 
Re: Simplest, easiest, cheapest way to sum to mono?

Ivan Beaver said:
The idea is that with the series resistor the load of one channel will not affect the output circuit of the other side.
Click to expand...
Well, "not affect" is not entirely true. Each side will see a load of 2k + the output impedance of the output circuit. Better than tying the outputs together without any resistance, but it's still a load, just to clarify.
 
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Re: Simplest, easiest, cheapest way to sum to mono?

Ivan's advice is solid and cheap...

The ASSumption is that outputs are low impedance and inputs are high impedance. !k Ohm should be high enough impedance to not load down the output significantly, and the two resistors summed will only add 1k to the source impedance, not crazy for driving a high impedance line input.

Try maybe making one from an old bad cable and a few cents worth of resistors (they can be low power 1/4W or so).

Do not over promise to your client in case this doesn't work as planned, and tell him he could pay $50+ for a commercial solution that probably does the same thing. If it works you are hero (boffin?). If not you tried to save him $50...

It will probably work, it's what I would try.

JR
 
Re: Simplest, easiest, cheapest way to sum to mono?

A quick bit of Googling found a third party measurement of less than 2 ohm output impedance for an iPod Five. 1K should be a bridging load.

http://www.kenrockwell.com/apple/ipod-touch-5g/audio-quality.htm

Ivan Beaver said:
This can cause distortion and possibly damage the output circuit-because it is driving an impedance much lower than it was designed for-just like loading a "power" amp down to around .01 ohms. They don't like it very much
Click to expand...
 
Re: Simplest, easiest, cheapest way to sum to mono?

John Roberts said:
The ASSumption is that outputs are low impedance and inputs are high impedance. !k Ohm should be high enough impedance to not load down the output significantly, and the two resistors summed will only add 1k to the source impedance, not crazy for driving a high impedance line input.

Try maybe making one from an old bad cable and a few cents worth of resistors (they can be low power 1/4W or so).

JR
Click to expand...
I was "assuming" that the 1/8" outputs on things like phones-computers-MP3 players etc are headphone jacks and not line jacks.

I use 1/4 watt resistors (to keep the size down) and they have always been fine. Especially since the output power of those devices is very low and the impedance of the summing resistors very high (in relation to the load that will be on them-which is usually 10K or so).

It is a bit tight inside an XLR-but can be done. Heatshrink is your friend.
 
Re: Simplest, easiest, cheapest way to sum to mono?

How far is this running and into what kind of input? After it's summed to mono you may need to go through a transformer to make it balanced line.
The number of "professional" installations I have seen where they have taken a 1/8 inch stereo jack on a plate and wired it directly to a balanced input as in tip to +, ring to - and sleeve to ground is unbelievable.
I try to get away from 1/8 inch connectors as soon as possible in an installation, any place there is a consumer equipment input connection I at least go with RCA connectors, between the two I think the RCA is a more durable choice.
If your going to mount the connector ( 1/8, 1/4, RCA) directly in a wall plate and the wall plate is metal, using a metal wall box, fastened to metal wall studs and or metal conduit you just about need to insulate the connector from the plate to stay away from ground loop problems. Neutrik makes RCA and 1/4 inch jacks in a D series body that are insulated from the body.

Rapco makes this handy piece that does everything you need, mono sum, ground lift switch, handy level control and standard XLR output so you can use any mic jack in the room as aux source input.
http://www.rapcohorizon.com/p-379-ltiglblox.aspx
 
Re: Simplest, easiest, cheapest way to sum to mono?

I keep the Radial Stagebug-2, Stagebug-5 and the Rapco LTIGLBLOX in stock for this purpose. As mentioned in previous posts they all provide summing, transformer isolation and ground lift switches. The Radials have a switchable -15 dB pad and the Rapco has an adjustable pot.

For your purpose I think the ground lift switch and isolation are essential.
 
Re: Simplest, easiest, cheapest way to sum to mono?

Ron Kimball said:
I use one of these:
http://www.pimfg.com/Product-Detail/PVS-582-PT
But it looks like it's been recently discontinued :(

This looks like the same item:
http://www.ebay.com/itm/Audio-convertor-lead-3-5mm-Jack-to-XLR-Male-Plug-30cm-/190948278382
Click to expand...

There is already in the driver chip protection from shorts and cross wiring in the output of most devices. I've been slamming the hots together for years and never blew out any. Even those cheesy MP3 players with no name.

I never heard any ill effects from doing it even wide open. And you will have more than enough level.

Ivans suggestion is best if you are worried. 1k in line with each hot is more than good enough.
 
Re: Simplest, easiest, cheapest way to sum to mono?

You do have to be careful of ground loops with any device that is AC line powered (like a laptop that is plugged in). But for those I usually go stereo and just use an ART DTI . One of these days I should open up one (or all) of mine (I have three) and add a ground switch (they are always ground lifted stock) and a stereo to mono switch w/ resistor passive combiner. I've had noise problems with my camera when fed from a board with one when the camera was on its wall wart - the noise went away when I kludged a ground connection between the input and output of the DTI.
 
Re: Simplest, easiest, cheapest way to sum to mono?

Glenn Adams said:
There is already in the driver chip protection from shorts and cross wiring in the output of most devices. I've been slamming the hots together for years and never blew out any. Even those cheesy MP3 players with no name.
Click to expand...
You are welcome... Because of customers like you large (experienced) manufacturers add build out resistors in series with the outputs, Often larger values than are optimal for clean transfer of signal (causes HF loss from long cable capacitance). But the customer is always right even if they do things like shorting active driver outputs together.
I never heard any ill effects from doing it even wide open. And you will have more than enough level.
Click to expand...
Funny I've heard of gear failing, but maybe I've been doing this longer than you.

Most modern gear is designed to be "customer-proof", high on the list at Peavey. So I would not expect Peavey gear to blow up, but I still do not advocate shorting L and R (or any) outputs together on any brand gear.
Ivans suggestion is best if you are worried. 1k in line with each hot is more than good enough.
Click to expand...

I worry about giving advice to large audiences via the WWW that could damage their equipment.

JR
 
Re: Simplest, easiest, cheapest way to sum to mono?

John Roberts said:
You are welcome... Because of customers like you large (experienced) manufacturers add build out resistors in series with the outputs, Often larger values than are optimal for clean transfer of signal (causes HF loss from long cable capacitance). But the customer is always right even if they do things like shorting active driver outputs together.

Funny I've heard of gear failing, but maybe I've been doing this longer than you.

Most modern gear is designed to be "customer-proof", high on the list at Peavey. So I would not expect Peavey gear to blow up, but I still do not advocate shorting L and R (or any) outputs together on any brand gear.


I worry about giving advice to large audiences via the WWW that could damage their equipment.

JR
Click to expand...
True, that is why the first 5 pages if any manual are warnings.
Note to my advice- don't try this at home.
 
Re: Simplest, easiest, cheapest way to sum to mono?

John Roberts said:
Most modern gear is designed to be "customer-proof", high on the list at Peavey. So I would not expect Peavey gear to blow up, but I still do not advocate shorting L and R (or any) outputs together on any brand gear.
Click to expand...
I was once using some gear that could power external equipment and they told me that its psu was well protected against short circuits and such and you did not have to worry if wires was crossed or something malfunctioned with the external stuff.

It took me just a couple of minutes to release the magic smoke. They had overlooked the scenario where their -12v was crossed with +12v. 8O~8-O~:shock:
 
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