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Impedance Question: Speaker cable plugged into TRS w
Old 8th September 2012
  #1
Here for the gear
 
🎧 5 years
Impedance Question: Speaker cable plugged into TRS w

Can someone please explain the problems created by the following situation, and how to correct them?
Overdubbing guitars I would like to have the amplifier in the control room and the speaker cab in the live room.
The studio has 1/4 TRS inputs and outputs in the wall connecting both locations.
What would be the result of plugging unbalanced speaker cable out of the guitar amp into this balanced and shielded input, then connecting an unbalance speaker line on the other side of the wall and into the speaker cabinet?
I'm certain this would effect the load and perhaps be harmful to the amp.
Is there fix for this besides a drywall saw and some more appropriate wiring?
Thanks in advance

Last edited by themikedean; 8th September 2012 at 06:43 PM.. Reason: user error.
Old 8th September 2012
  #2
Lives for gear
 
🎧 10 years
Don't do this.

Line level is 1.23volt and requires ground/sheilding.
The 3 conductor TRS lines are very small guage light weight for low voltage.

Speaker wires are heavy guage unsheilded 2 conductor made for much higher
voltage.
Old 8th September 2012
  #3
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johnnyv's Avatar
 
13 Reviews written
🎧 10 years
The best solution is to somehow run a heavy gauge speaker wire into the other room, or to put the complete amp in the other room and use the TRS jacks to patch the guitar to the amp. This could cause a change in your tone as long guitar runs are not a good plan either.
Old 10th September 2012
  #4
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🎧 5 years
<Line level is 1.23volt and requires ground/sheilding.
<The 3 conductor TRS lines are very small guage light weight for low voltage.>

<Speaker wires are heavy guage unsheilded 2 conductor made for much higher voltage.>

I'm aware of all of that...but
What would the minimum gauge of wire be for speaker cable?
Is 16 gauge good enough?
And if it is couldn't you convert the 2 wire speaker cable to 3 wire TRS and back by connecting the ground to of the three wire balanced loom in the wall to the negative side of the 2 wire speaker wire?
Old 10th September 2012
  #5
Here for the gear
 
🎧 5 years
( Crickets)
I guess I'll turn off the computer and dig around in the trunk of my car for the yamaha PA bible.
Thanks.
Old 10th September 2012
  #6
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Lotus 7's Avatar
The OP asked "Can someone please explain the problems created by the following situation..."

Do you know the actual wire gauge of the TRS connections in the studio?

Although it's obviously not good engineering practice to use a TRS interconnect to send speaker level current it's potentially not as dangerous to the equipment as might first appear.

From an engineering point of view, the significant issues are:

A. Safety

1. Is the potential voltage high enough to damage any insulation?

Answer: If the amplifier is pumping 128 watts into an 8 ohm speaker the maximum RMS voltage is only 32 volts. Even the poorest quality bell wire will handle 32 volts and certainly any balanced audio cable will also.
The fact that normal balanced line signal voltages are only a volt or two is irrelevant.
No safety issues here.

2. Current capacity

This factor is directly related to the wire gauge. If the TRS wiring is via 22 Ga. wire the maximum recommended current limit is 7 amps continuously. If it's 24 Ga the the max recommended current is 3.5 amps. A speaker amplifier putting out a continuous 128 watts into a 8 ohm speaker would produce 4 amps, but no realistic guitar signal would be that steady or continuous, so again a power limit of 128 watts is possible.
No safety issues here.

3. Amplifier loading issues


The small gauge wire has much more resistance than the normal 12 or 14 Ga speaker wire normally used in this application. The added wire will raise the load impedance the amplifier sees and reduce the damping factor the speaker sees. At the wire lengths involved any inductive and capacitive reactance added by the cable is not significant at speaker impedance levels.

If the through-the-wall line length is 40 ft. the total series wire length is 80 ft. If the wire is 22 Ga (resistance 16.14 ohms/1000 ft.) then the total series resistance ie (16.14/1000*80) =1.29 ohms. Not great from a damping factor point of view, but it won't raise the load impedance enough to cause any possible output transformer over-voltage issues on a tube guitar amp. At worst, the slightly reduced damping factor may make the speaker a little bit more resonant at very low frequencies. You might even like it!
No safety issues here.

B. Other audio issues

1. Crosstalk

This will be highly dependent on the type of wire and how it's installed.

If the TRS lines are all independent 3-conductor cables (with separate electrostatic shields) that are loosely bundled together, the high speaker currents may induce a small crosstalk signal magnetically in a nearby line. However if the line is fully balanced, the induced signal probably won't be a problem.

If the TRS lines are part of a multiconductor "snake" cable, the likelihood of induced crosstalk signal is higher and may be a problem. especially if that snake also contains any mic lines.

The only way to really tell if this will be a problem is to test it.
Possible crosstalk issue

If the wire gauge is smaller (24 or 26 GA) a way of lowering the line resistance might be to run the Tip and Ring lines in parallel and to use the (probably much lower resistance) shield as the speaker return. That connection method will lower the series resistance but will also increase the magnetic field around the wire, so will increase any potential crosstalk.

If the wire gauge is very small (thinner than 26 Ga) it's really not practical to try to push the required currents through it.

Bottom line: As long as all the connections are "solid", you won't risk any amplifier, wiring or speaker damage by trying it. The worst that can happen is that you might get some crosstalk in other lines running between the control room and the live space.

Here is a link to a wire gauge resistance and current capacity chart.

Hope this helps - let us know how it works.
Old 10th September 2012
  #7
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AlexK's Avatar
 
🎧 10 years
I've done this before when there really is no other option, and it hasn't presented any major issues. I could see it causing issues with valve amps due to the impedance change, but that's up to you to use your multimeter to measure the load being presented across the TS cable before you connect it to the amp.

There may be a sonic difference, I've heard it with some amps, less with others...
Old 10th September 2012
  #8
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🎧 5 years
Thanks to Lotus7 and everyone for their input.
Is it crazy to build a "super-adapter" using the carrying capacity of TWO 3 wire headphone lines to make ONE very capable mono path?
Old 10th September 2012 | Show parent
  #9
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Lotus 7's Avatar
Quote:
Originally Posted by themikedean ➑️
Thanks to Lotus7 and everyone for their input.
Is it crazy to build a "super-adapter" using the carrying capacity of TWO 3 wire headphone lines to make ONE very capable mono path?
That's actually a good idea. If there is any crosstalk coupling to the other lines it may increase that a little. It's certainly a way to increase the current carrying capacity of the lines.

You're not going to hurt anything so just do it. For the first try, you may want to bring up the mic lines slowly with the speaker being driven at high level, just to be safe. If you're micing the speaker with a high output capacitor mic you should have plenty of signal from the mic and the mic cable(s) will be driven from a low impedance.

If you're using a low output ribbon on the speaker cab, (I know-very unlikely) and have the desk gain cranked way up you may have a more severe crosstalk problem.

One thing to remember:

Since you will be running the guitar amp output through TRS plugs at either end, be very careful to NOT unplug any connections while the guitar amp is being driven. If it's a typical vintage style tube amp, totally unloading the output transformer is not a good idea and can result in a damaged transformer and/or the output tubes. You are probably more than aware of the potential danger, but it's very easy to forget what's happening and to pull a TRS plug without thinking.
Old 11th September 2012
  #10
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🎧 5 years
I'll report back when we wrap up this little session.
And thanks for the warning about abruptly "unloading" the amplifier.
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