Replacing Electret Microphone With Audio Input?

I found a circuit that does use a regular audio input, but I can't find a couple of capacitor values at Dick Smith. The caps have to be ceramic. I can find a 3.3 pF for the 3 pF, but I can't find a 10 nF ceramic unit. Would it be OK to just parallel 10 1 nF caps, until I can find a 10 nF unit?

Of course. You wouldn't have stereo with a simple one or two transistor transmitter - so you may also want to combine both channels going in.

Electret's have built in preamplifiers or impedance matching amps and they usually impress a DC voltage on the audio connection to the electret mic.. You just need to disconnect that resistor and not use it.

There also may be a need for a capacitor to couple audio in without upsetting the DC bias of the transmitter.

Forgot the link:

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I understand that, but my radio receiver is a mono type anyway, so I'm not looking for a stereo version.

Yeah the 3.3 is good enough. Isn't 10 nano farad just another way of saying .01 microfarad? .01 microfarad is very common.

sorry I somehow got the idea that the Ipod would be the music source.

But if you don't combine the two channels at the receiver, you'll only be getting some of the content.

IN order for there to be stereo, there has to be difference between the channels. How it's arranged can be quite complicated and depends on what was recorded and how the recording engineer felt things should sound.

If you only have one channel of a stereo signal, you will lose content. So that trumpet over in the right channel that is only on the right channel to give a sense of space, will be missing if all you feed into the transmitter is the left channel. Other things may still be there, but lacking in volume because the intent was to place it somewhere between the two channels.

Michael

1nF= .001

Thus you need the extremely common .01uF capacitor. And I suspect that's not even critical, such a large value is bound to be a bypass capacitor and hence something around that value will do fine.

Michael

iPod is the music source, it will be broadcasting to the mono radio. I will use that circuit I linked to before, seems to have the audio channels combined by 10K resistors so I don't have to worry about that. Even though I've already got a device that combines left and right with 1.8K resistors.

I somehow managed to read the chart wrong @

You have a better idea of what you are doing than I do. The Ipod output has to (should) be mixed to drive the transmitter with the contents of both channels. That can be as simple as two resistors from each channel to a common input.

Not aware of the circuit you linked to before. You aren't referring to the one where you ground the two signals to control volume are you?

No, the one @

The article linked to previously mentioned using a 5-20 pF trimmer cap instead of a fixed capacitor. What capacitor do I need to replace to install a 4.2pF-20pF trimmer? This would make it easier to tune.

The 12 pf cap is the tank circuit tuning cap; spreading or compressing the coil will also tune it.

The 3 pf is a feedback cap and gets it oscillating. The .01 is a bypass cap and not critical to anything. The 1 nf on the base is a bypass but will affect audio frequency's as well.

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So if I got this right, I replace the 12 pF?

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Yes, put a variable in place of it. The plate of the variable that the adjustment screwdriver touches should be the ground side so the adjustment tool doesn't cause it to change freq..

Just look at the cap to determine which side should be ground, It is normally obvious.

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Thanks, I don't really like the idea of fiddling with the coil, I like to leave coils alone. The trimmer I'm going to use is a PCB mount one, from Dick Smith of course Cat # R2945

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The website also mentions replacing a 10K resistor, with a pot. Would I need this if I drive the transmitter from an ipod headphone output, and if so, what resistor should be removed? There are 5 10k units there.

You see the vertical resistor just on the input where the LR channels come together? Put a 10K pot in place of it, with the junction of the LR 10K's to the full clockwise position and ground on the CCW position. Change the position of the 100 nf to the wiper of the pot so it connects there and nowhere else.