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Author Topic: 70 volt system - what am I doing wrong?  (Read 1095 times)

Mike Caldwell

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Re: 70 volt system - what am I doing wrong?
« Reply #10 on: June 28, 2021, 11:37:00 PM »

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

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Re: 70 volt system - what am I doing wrong?
« Reply #11 on: June 29, 2021, 10:26:31 AM »


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.
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Othmane Alaoui

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Re: 70 volt system - what am I doing wrong?
« Reply #12 on: July 26, 2021, 01:30:59 AM »

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...
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Re: 70 volt system - what am I doing wrong?
« Reply #12 on: July 26, 2021, 01:30:59 AM »


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