Oddity with A&H PA20-CE

[dbuckley]

Last week I mixed a show on a PA20-CP, and noted something odd.

 

When the mute buttons on FB1 or FB2 were operated, there was a nasty click in the foldbacks. Experimentally holding the button part way produced horrible humming. The channel and master mutes were fine.

 

Is this a feature or a fault?

[Mixermend]

Does it click when PFL'ing on the FB outputs?

 

All the muting push button does is to disconnect the Foldback output and connects it to earth (towards the output socket) It is unlikely that you have defective IC's (with DC offset) on both FB outputs - as they are on different PCB's.

 

Is it possible that you have a DC coupled power amplifier connected for foldback use? If this has only a few millivolts of DC present on the input - the output coupling capacitor in the mixer will charge up slightly and will charge and discharge when the MUTE button is pressed - causing a click.

 

I am presuming that you are using a separate power amplifier - if you have the mixer with a built in power amp, switch the foldback outputs to this and see if it clicks perhaps?

 

To prove whether it is the external amplifier - try a different amplifier - or put a transformer isolator in between the mixer output and the power amplifier - which will preclude any DC interaction.

 

Please report back - we would all like to know the answer!

[dbuckley]

No clicks on AFLing.

 

The foldback outs went to a active monitors, balanced all the way. Both made by Alto, but different models.

 

Unfortunately I'm unlikely to see this desk again for a while, so won't be able to report back. But if the mute button works directly on the oputputs, rather than on an internal mix bus, then yes, all sorts of possibilities occur. So that'll probably be it; it's designed to click.