Telephone Microphone Problem with low pass filter

[bzblues]

Hello everyone!

First I must tell you guys that I'm very new at this, so sorry in advance!

I'm starting to put together this Telephone Microphones. They are very easy to do, just grab the two wires from the earpiece and solder them to a 1/4 inch jack, and viola!

The idea es to use it as a lowfi tool, but this earpieces of the phones tends to lead to some feedback so mounted a 100k potentiometer and a capacitor to make a RC lowpass filter to roll off a bit of the high frequiencies...aaaaand the issue appears. When I turn the pot up or down, it hums. When its on 10, the hum dissapears. I've tryed different wirings but nothing seems to work out. The impedance of the earpiece is very low, 20.5ohm, but I've try other earpieces with much more higher readings, and the same happens. Also, I've tried shielded wiring, but...THE SAME HAPPENS.

Now, since I couldn't solve this problem, I thougth that I could make the lowpass filter using a ON/ON switch, attaching the capacitor and a 100k resistor to one part of the switch, so you can select between the "normal" signal or the "filtered" signal.

I calculate that using a 100k resistor with the 2nF capacitor I can get a cuttoff frequency of around 800Hz, wich is what I was looking for.

So I tried to solder the components directly to the input jack to see if it worked out. The resistor in series and the capacitor in pararlel. Aaand it didn't affect the signal at all. I use a 0.1uF cap wich it should cuttoff the signal by 15Hz, basically killing the signal...aaand NOTHING! the signal was still there, unaffected.

So, I really don't understand why it hasn't work. It maybe be the impedance of the mic, but I don't know anything about impedance or how to make it work, I'm barelly know what a capacitor and a resistor do! I'm telling you this because some of you are more experienced and write content that gets my head spinning (I'm looking for an basic electronic course so I can learn propperly) so, sorry in advance.

Hope you guys can help me solving this out!

Thank you!

 

[PRR]

Why are you using a 100,000 Ohm resistance with a 20 Ohm source?

Usually the loading should be similar or a few times larger than the source impedance.

Find a 100 Ohm pot/resistor.

Or put a 10uFd cap directly across the 20 Ohm transducer. (This might be unwise with high powered sources but a mini transducer can't burn itself out.)

 

[bzblues]

Hi! thank you for replying. I saw some tutorials on youtube and they where using a 100K pot, so I did the same.

So I should use a similar o the exact value as the source and that could solve the problem?

If I wanted to make a lowpass filter I should use a 100ohm resistor or potentiometer and a 10uF capacitor? The lowpass filter calculator shows me that with this components it will not cut any high freq.

To make a cuttoff frequency of around 700Hz I would need a 2uF resistor. Is that correct?

 

[Pano]

I'm not sure I understand what you are trying to do. Are you using the element in a telephone handset as a microphone? And you plug this with a 1/4" plug into - what? A mic input on a mixer or recorder? A guitar amp? Or are you using it as some other pickup?

What's causing the feedback? Is it the speaker in the telephone handset or something else?

 

[duncan2]

I cant figure out why you are using a telephone receiver insert as a microphone .


Having worked for BT (UK) for a long time the mechanical construction and FR are different .


Does the BELL Telephone Company in the USA use the same type both as a microphone and a receiver ?

 

[PRR]

> Does the BELL Telephone Company in the USA

There is no functional Bell Telephone (/System) in the US. It is only a trademark now. We have not made telephone gear here in many years.

I see no clue that bzblues is in the USA.

I have no idea why bzblues has not linked to the YouTube, FaceBook, or Blog where this idea was found.

 

[PRR]

Found a TapeOp article which says the MOUTHpiece has screws and is a microphone. Normally that mike needs power, not mentioned in the essay.

How to Make a Telephone Microphone! | LearnAudioEngineering.com - YouTube
This one correctly takes the EARpiece. This is nearly a dynamic speaker (not moving coil but near-enough) which will function as a un-optimum dynamic microphone.

I have no idea where feedback is unless it is simply monitoring the mike back to the room.

 

[Pano]

Dude!! That was my XLR cable. Now what do I do?

Back in the 80s we used to take the earpieces out and use them as magnetic pickups. Worked like a charm and would pick up vibrations in metal things. Much like a single coil guitar pickup.