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[Brad Worrell]

I'm new to 70 volt systems but I know how do to math but this one has me stumped. I've installed a 4 speaker system of JBL Control 25-1's tapped to ask for 105 watts, powered by a 120 watt JBL CSA 1120Z; 16 gauge install cable with the longest run being around 70'.

The amp runs for maybe 45 seconds to a minute then shuts off. It's hard to tell but sounds like the fidelity may not be the highest during that time either.

I've tried a backup of the exact same amp, removed the volume control, tried it in 8 ohms via series/parrallel wiring and tried with just two speakers hooked up (70 volt), all to no avail. I disconnected the speakers and checked for continuity between the two wires to see if there was a short, but no. A digital multimeter shows me 4.9 ohms of resistance at the beginning of the line.

[Mike Caldwell]

Check the wiring connections at the volume control, actually bypass it and test the system.

[Brian Jojade]

The first thing I'd do is test the speakers one at a time connected directly to the amp.  If you have one shorted speaker, you'll quickly be able to find out which one is causing the problem.  If they all work individually, then wire up all 4, again right next to the amp and see if the test passes.  If it does, then that tells you there's an issue in the wiring.  Start putting them back in place one at a time until it fails.

[Milt Hathaway]

First thing: Resistance measurements won't tell you anything useful on a 70v system. If you're going to be doing 70v work often enough, I highly recommend purchasing an impedance meter such as the Goldline ZM1. It's pricey, but worth every penny when troubleshooting 70v system issues.

Second: You may have a feedback issue. Not the usual acoustic feedback, but feedback induced from the 70v cables into any mic level cables going into the system. Typically this feedback is at frequencies too high to be heard, but still low enough to find it's way through the mic preamps. It will heat an amp up to shutdown levels pretty quickly. See how long the amp stays on with nothing hooked to the inputs. If it stays on, you've found your problem and will need to re-route some cabling.

Of course there could be a bad transformer on either the speakers or the level control. Isolating sections of the system is the only way to find that.

[John Roberts {JR}]

+1 to suggestions already offered...  divide and conquer.... disconnect everything then, reconnect one at a time.

JR 

[Mike Caldwell]

What outputs are you connected to on the amp?
Do you have it switched to 70 volt mode, high?
What is the input source?
How loud are you trying to run it at?
Are you trying to make the system thump by boosting a lot of low end?

[Brad Worrell]

The connections were all done properly and I was running some pop music thru at moderate volume, both via a directly attached iPhone and also from RCA "record" outputs on a mixer. While it's a long run for 8ohm, I also tried that, wiring speakers parallel/series to get 8ohms, bypassing transformers, etc. Same result.

The "Divide and Conquer" is obviously going to be the way to go. I've performed most of that but the one thing I haven't done is connected all speaker to amp separate of the installed wiring grid.

Thanks for all the prompt and helpful replies. I've got my work cut out for me... 

[Mike Caldwell]

With the system properly connected and the speaker tap settings set for 70 volt operation
when it's turned up is the ring around the amp level control flashing red?

 

[Brad Worrell]

With the system properly connected and the speaker tap settings set for 70 volt operation
when it's turned up do you the ring around the amp levels control flashing red?

Nope. At no point was I clipping the input.

[Brad Worrell]

You may have a feedback issue. Not the usual acoustic feedback, but feedback induced from the 70v cables into any mic level cables going into the system. Typically this feedback is at frequencies too high to be heard, but still low enough to find it's way through the mic preamps. It will heat an amp up to shutdown levels pretty quickly. See how long the amp stays on with nothing hooked to the inputs. If it stays on, you've found your problem and will need to re-route some cabling.

I think you called it. The amp wasn't showing any changes in signal lighting when shutting off. It just went silent, so I couldn't try your suggestion, BUT, after trouble shooting everything else, with the feedback thing in mind, I freed up a balanced output from the mixer and used that (to the balanced inputs of the amp) and VOILA! everything works fine.

[Mike Caldwell]

I think you called it. The amp wasn't showing any changes in signal lighting when shutting off. It just went silent, so I couldn't try your suggestion, BUT, after trouble shooting everything else, with the feedback thing in mind, I freed up a balanced output from the mixer and used that (to the balanced inputs of the amp) and VOILA! everything works fine.

Did the lighted ring around the level control show the presence of signal when you were not playing any audio prior to going to the balanced connection?

How long was the cable run using the unbalanced line connection?

[Brad Worrell]

Did the lighted ring around the level control show the presence of signal when you were not playing any audio prior to going to the balanced connection?

How long was the cable run using the unbalanced line connection?

No sign of signal at the level control and the unbalanced RCAs were only 3' although there are a lot of other audio and AC lines within a few inches of them. Whether high-frequency feedback or something else, unbalanced outputs on that mixer (I tried 2 and the headphone out) are NOT to be used with this amp.

[Othmane Alaoui]

First thing: Resistance measurements won't tell you anything useful on a 70v system. If you're going to be doing 70v work often enough, I highly recommend purchasing an impedance meter such as the Goldline ZM1. It's pricey, but worth every penny when troubleshooting 70v system issues.

Second: You may have a feedback issue. Not the usual acoustic feedback, but feedback induced from the 70v cables into any mic level cables going into the system. Typically this feedback is at frequencies too high to be heard, but still low enough to find it's way through the mic preamps. It will heat an amp up to shutdown levels pretty quickly. See how long the amp stays on with nothing hooked to the inputs. If it stays on, you've found your problem and will need to re-route some cabling.

Of course there could be a bad transformer on either the speakers or the level control. Isolating sections of the system is the only way to find that.

This is right, there is resonance in the network...

[ProSoundWeb Community]