Short Circuits 3 - KC5519 Kit (auadio amplifer with pre-amplifier

Is anyone familiar with this?

I am trying to get this working with 2 x electret microphone attached to PCB connectors 2 & 3.

I have added 2 wires to the positive supply pin, connected the end of these to 2 x 27k resistors and then connected the other end of these to the positive of the positive of the PCB connectors.

This gives me about 6V on the positive of the electrets when they are clamped into the connector.

I am getting output on my speaker (which came from a television set) but the output is very scratchy and talking into the electret microphones does not produce recognizable voice output on the speaker.

The circuit is meant to take 2 x microphones (L & R), since it has two input connectors on the pre-amplifier stage.

Sorry but I can't find a schematic of this online - I will have to scan the printed schematic that came with the kit.

But I am hoping that some one is played with this kit and is familiar with it.

Can you bypass the preamp? If so, you can test the power amplifier section by connecting the headphone-output from your computer, or the line-output from your TV, etc. (You might need a volume control/pot at the input.)

You can test the whole thing in a similar way if you add an approximate 1:100 [u]voltage divider[/u] at the preamp input. (Again you might need to add a volume control.)

If that works you know the amplifier is working and you’ve got a problem with your microphones. They might be defective or wired wrong (maybe simply wired backwards).

[u]Here is an example electret preamp schematic[/u].

I have added 2 wires to the positive supply pin, connected the end of these to 2 x 27k resistors and then connected the other end of these to the positive of the positive of the PCB connectors.

One resistor on each mic?

Two equal value resistors in series will give you a 1:2 voltage divider, but I’d expect that to change when you connect the microphone. And, you DO NOT want that bias going into the preamp… Only the mic should be biased, but that bias should be blocked from the preamp by a capacitor. (There may already be a series capacitor built-into the preamp input but if you’re not sure, add one.)

This gives me about 6V on the positive of the electrets when they are clamped into the connector.

The bias resistor should be connected in series with the mic. 27K seems high, but it could be right since we don’t have any specs for the mic.

I may have given you some bad advice before… Maybe 6V it too high. These things are typically powered from 5V and biased at 2.5V, so maybe 2.5V should be the target. (That would required a higher-value resistor, which would be even stranger.)

I am getting output on my speaker (which came from a television set)…

The circuit is meant to take 2 x microphones (L & R)…

In that case you should have left & right speakers too. And if you have only one speaker, make sure you are testing the microphone that’s connected to the same channel as the speaker. :wink: