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Summing iPad output to Mono via a TRS to TS lead

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Summing iPad output to Mono via a TRS to TS lead

Postby Murray B » Sun Apr 25, 2021 1:16 pm

Hi All,

I wondering if anyone might be able to help with some advice about this one:

I work for a music education charity and one of the projects we do is an iPad orchestra involving lots of young people making music on iPads. We have a large mixing desk for the purposes of amplifying the final performance by the group, but given the number of participants the system relies on summing the iPads to a mono output to keep the channel count down.

I've had a request to get some new leads for the project and in my search I discovered a small note from an amazon seller and then an article about the risk of burning out the output section of the iPads when summing the left and right outputs of the iPad via a TRS to TS cable because of impedance issues when you connect the left and right outputs together.

According to the article this can be avoided by adding resistors within the physical connections to maintain the required loading on each side of the output.

I've had a search and can't find an off the shelf cable that has the required resistors in place. So I am wondering...

Is the burn out risk a real thing? (it kind of makes sense that it might be, but I am no electronics engineer!)

Is there anywhere I can buy the 'safe' cables without resorting to having them custom made?

Many thanks
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Hugh Robjohns » Sun Apr 25, 2021 1:35 pm

Yes, it's a real thing., and a very real risk.

No, I don't know of an commercial manufacturer of mono mixing cables either.

The best solution, as you are in the UK, would be to contact John at Orchid Electronics and ask him to make you some custom cables with mixing resistors. He knows what he's doing and does a good job. Lead times can be a little long, though (no pun intended), depending on what else he has on.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Murray B » Sun Apr 25, 2021 1:43 pm

Thanks Hugh,

I need to have a good look at the cable we are currently using for the project and see how they are configured.

Shame nobody makes a stock version, I have some Orchid kit already and it's brilliant so I will indeed contact them.

Could be a small order of a couple of replacements or about 60 cables for the whole kit - as the Musicians who run the project have reported a number of problems with connections to the mixer I'm wondering if any of our existing cables have the required protection :think:

Could get expensive this one...
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Sam Spoons » Sun Apr 25, 2021 1:50 pm

John made me a summing lead for a very decent price a couple of years ago, in my case the lead time was very reasonable.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby James Perrett » Sun Apr 25, 2021 2:27 pm

I see that Fentronix also sell a small stereo to mono combiner box

https://www.ebay.co.uk/itm/184363451093

although it doesn't specifically mention whether it incorporates any resistors.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Mike Stranks » Sun Apr 25, 2021 2:41 pm

I can recommend Fentronix... I have a few of their useful little gizmos...

I've just put one of these into an install I've just completed:

https://www.studiospares.com/studiospares-red503-passive-line-combiner_458220.htm Completely transparent and fuss-free.

I also have one of these: https://www.img-stageline.com/products/audio-tools/line-combiner-line-splitter/lc-31/

and one of these: https://www.img-stageline.com/products/audio-tools/line-combiner-line-splitter/mc-31/... although can't for the life of me remember why I bought it now! :lol:

Rule of thumb for splitting and combining... 1 to 2 is OK (usually) ; 2 to 1, you need gizmos or resistors...
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Wonks » Sun Apr 25, 2021 3:02 pm

Would one of the output channels on the iPad being grounded cause an issue? You aren't then feeding an electronic signal back into it.

If it's not an issue, can the iPad music application output be set to mono and you then use a 1/8" TS to 1/4" TS mono lead?
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Murray B » Sun Apr 25, 2021 3:47 pm

James Perrett wrote:I see that Fentronix also sell a small stereo to mono combiner box

https://www.ebay.co.uk/itm/184363451093

although it doesn't specifically mention whether it incorporates any resistors.

I think the attenuation effect might imply that resistors are inside but I could be wrong.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Murray B » Sun Apr 25, 2021 4:03 pm

Wonks wrote:Would one of the output channels on the iPad being grounded cause an issue? You aren't then feeding an electronic signal back into it.

If it's not an issue, can the iPad music application output be set to mono and you then use a 1/8" TS to 1/4" TS mono lead?

I like your thinking, but I'm not sure if I can make GarageBand or the iPad just do a mono output from the headphone socket. If anyone knows how to achieve this it would be the most cost effective solution.

Thanks Mike for the links and suggestions...

The solution needs to be simple and not require further adapters, hence having a custom built cable that straight from the iPad into the mixer rack. I haven't seen the full rig in operation yet so I can't answer why we are using TS connections rather than XLR's for example as the system was set up before I joined the charity. Buying cables + combiner boxes might push the non existent budget much too far unfortunately as I might have to buy 60 sets of cables and boxes etc if / when I discover that our existing cable set has the potential to break all the iPads :headbang:

Thanks to all for sharing your thoughts and ideas for this though - also if you know any manufacturers / suppliers who have a tendency for a little bit of philanthropy than let me know - we are a charity after all :D

Mind you given the state of the Live Music scene and the impact this must have had on businesses - this may be an ask too far.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby tomas » Sun Apr 25, 2021 4:05 pm

Wonks wrote:Would one of the output channels on the iPad being grounded cause an issue?

It probably won’t, but why risk it? The cheapest and safest option is to get cables with TRS in both ends, and snip the connection to ring in one of the jack plugs. Of course, this requires plugs that can be opened..

Here’s one that perhaps will be fine without any modification.
https://www.thomann.de/intl/cae_15562_audiokabel_30.htm
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Murray B » Sun Apr 25, 2021 4:18 pm

tomas wrote:
Wonks wrote:Would one of the output channels on the iPad being grounded cause an issue?

It probably won’t, but why risk it? The cheapest and safest option is to get cables with TRS in both ends, and snip the connection to ring in one of the jack plugs. Of course, this requires plugs that can be opened..

Here’s one that perhaps will be fine without any modification.
https://www.thomann.de/intl/cae_15562_audiokabel_30.htm

The cable in the link is the type that can cause the problem unfortunately.

If I snip the ring connection will the iPads channel amp like having a very low impedance (kind of zero) connection made to it? I wish I knew more about this stuff but it kind of seems like it might create the same problem but just on the channel output on the ring?
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Sam Spoons » Sun Apr 25, 2021 4:19 pm

I've emailed them but I suspect it will be wired with the tip and ring shorted which is not a safe method for summing a stereo output to mono.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby tomas » Sun Apr 25, 2021 5:54 pm

Murray B wrote:The cable in the link is the type that can cause the problem unfortunately.

I’m not so sure about that. It does say ‘unbalanced’, which kind of indicates that the manufacturer may know that the TRS end can be used for a balanced connection (albeit unusual for a 3.5mm), and if you short out the hot and cold leads in a balanced connection the result is no signal at all. It also says ‘made in Germany’, and that could mean that the manufacturer knows a thing or two about what they’re doing. Lastly, it’s a brand of cable that is used by many musicians and you may find some relevant use cases in the reviews of the article. :P

Murray B wrote: If I snip the ring connection will the iPads channel amp like having a very low impedance (kind of zero) connection made to it?

Nope, quite the opposite. From the iPad, the ring is the right channel output. It will see an open circuit = infinite impedance. The problem with shorting the tip (left channel) and ring is that one channel can present a kind of “load” to the other channel. Think for example if the user is playing something where the channels are out of phase with each other.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby Sam Spoons » Sun Apr 25, 2021 6:06 pm

Snipping the ring connection will leave the R side OC and the left only going to the TS jack. I would wager a very small amount of money that the tip and ring are shorted and connected to the tip of the TS ¼" jack with the potential to cause the problem you refer. The only correct way to sum the L and R outputs is using a special lead or summing box with resistors to prevent one output 'driving/loading' the other.
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Re: Summing iPad output to Mono via a TRS to TS lead

Postby tomas » Sun Apr 25, 2021 6:11 pm

tomas wrote:you may find some relevant use cases in the reviews of the article. :P

....having now done so myself, it is clear that tip and ring are shorted out... however, also many users have no issue with hooking it up to phones and tablets. If you’re worried, it’s easy to open the 3.5mm jack and cut the shorting wire. You probably should buy 20-30% more cables than you need, to have some spares when the others disappear or break.
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