CrazyOldGuy

I've been piecing together a Public Address system using off the shelf kits and modules, I've got it working ok with input from a MP3 player, but when I hook up a microphone pre amp I get a low frequency noise out of the speaker. The mic is one from a CB with an in-line switch, the noise is there when the switch is open, i.e when the microphone is off. When the mic is on it works ok.

I was wanting help on how to get rid of this noise, would a filtering cap be needed somewhere?

I'll also be hooking this up in a car, so would a filter cap on the power input be a good idea? and what rating would be best for that?

The pre amp kit link
The Amp module Link

The speaker is a 8 ohm 30 Watt unit, Vcc is currently 12V

Thanks for your help.

Attached Images

PA amp.png (51.6 KB, 21 views)

Nigel Goodwin

Your volume control is in the wrong place, it needs to be between the preamp and power amp (replace R9 with it).

Also the preamp is designed for an electret mike, which is why it has a resistor feeding power to it - your CB mike might not be suitable?.

The switch on a CB mike is also for PTT, NOT for muting the microphone.

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CrazyOldGuy

I probably should have been more clear, the CB mike is just an attachment to a CB radio, from the look of the PCB inside of it, the switch just interrupts the track leading to the mic (which looks to be an electret)

Thanks for the tip on the volume control, it isn't actually in place yet, I was just guessing as to where to place it, though now I think about it, I did put it in the wrong place

audioguru

5cm of unshielded cable between the mic and the preamp will pickup mains hum. The entire length of the mic cable must be shielded cable.

The first transistor in the preamp is a BC548 which is NPN. Your schematic shows a PNP transistor there.

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