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Telephone Microphone Circuit Bent POTENTIOMETER HUMMING

bzblues

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Hello everyone! First of all, I'm very new in electronics, I'm experimenting and learning.

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 thing is the I mounted a volume potentiometer or a lowpass filter, and the issue appears. When I turn the pot up or down, it hums, like a mosquito going by your ear. When its on 10 or 0, the hum dissapears. I've tryed different wirings but nothing seems to work out. Also try to put a resistor, wich turned the hum down, but just a bit, it is still there. I'm using a B100K pot.

Cuold anybody know how can I solvethis problem? I'll attach the wiring that I'm using.

Thank you in advance!
 

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Audioguru

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A microphone signal level is VERY low so its amplifier needs a lot of voltage gain to be sensitive enough. But the sensitivity picks up the electricity in the air and is all around you. If ordinary wires are used they act like antennas to the electricity hum so shielded audio cable must be used. The cable shield blocks the interference.
 
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bzblues

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Great and clear answer! I'll try to use some shielded cable to see if that hum goes away. I've also been told to use a 47pF capacitor on the pot, but maybe that will act as a lowpass filter instead of eliminating the hum, right?

Thank you!
 

Audioguru

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47pF is almost nothing and will reduce pickup of radio interference. A lowpass filter passes low frequencies like hum.
All consumer audio products use shielded audio cables (usually with RCA plugs) to prevent hum.
 

bzblues

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Ok, I understand. The thing is that I want to add the lowpass filter since the sound frequenzies from the earpiece are very high range, very "telephonic" so to say haha, so it tends to feedback. So I put the potentiometer with a capacitor on the center leg soldered to the back of the pot to roll off a bit of the high frequiencies. It works fine, but the problem remains: going up or down, midway the hum appears. In 0 or 10 the hum dissapear. So, it can be solved by using shielded cable? I now remember that I use shielded cable on one handset, but I think I did something wrong. The cable had 2 thin cables and one bare wire. I cut the bare wire and solder the the two remained cables to each terminal of the earpiece. Maybe I should solder the bare wire to the negative terminal and the hum would dissapear for real? Should I use a Faraday Cage aswell?

Thank you!
 

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Since you are using the earphone from a telephone as a mic then of course it sounds "tinny" like a telephone.
A real mic has a wide smooth frequency response.
A cheap little speaker also sounds tinny, especially if it is not in a suitable enclosure.

The bare wire in a shielded cable is the shield wire that you connect to the circuit ground.
If the pot is not beside the mic then use another shielded cable between them.
It is good to see you also connected the metal housing of the pot to the cable shield and the circuit ground.
Then the entire system is a Faraday Cage.
 

bzblues

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I was going to use the real microphone from the phone, but its a carbon mic and needs extra power, sadly I can't fit a 9V battery or I'm not experienced enough to do other kind of power supply. I see that I've made a mistake in not soldering the bare wire! I'll try that and let you know if it works. Thank you so much for the information, its been very usefull!
 

bzblues

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hahah I know! but it sounds really cool if you want to recreate a 1920s or 30s sound, its a cool toy ro play with.
 

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The most common phases spoken on old telephones was, "What? What did you say?" over and over because the carbon mic produced no low voice frequencies, no high consonant frequencies and had lots of distortion.
 

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midway the hum appears. In 0 or 10 the hum dissapear.
Try a lower ohm value in your potentiometer. My guess is your pot is a very high resistance. The middle region leaves your outgoing wire (wiper) almost floating, and susceptible to electronic noise.

The hum disappears when the wiper gets close to the extremes of travel, because it sees the mic through low impedance, or ground through low impedance.

Very often I've seen 5k used in volume controls.

Or as an alternate, ground your outgoing wire through a 5k or 10k resistor. If you do this then the mic is audible only when you dial the pot very high.
 

bzblues

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Thank you! I'll try a smaller pot to see what happens. Also I'll try use shielded wire, that I think it will solve the problem.
 

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Of course using shielded audio cable will eliminate the hum. From the mic to the pot and from the pot to the amplifier input.
The rows of contacts and wires all over the place in a solderless breadboard pickup hum and all kinds of interference.
 

bzblues

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Well, I tryed the shielded wire and it didn't work out at all. The thing is that I've used shilded wire before on another telephone mic project, cutting the bare wire and it didin't work out, noise was still there. I tryed now soldering the bare wire to the negative terminals and, not only did not work, but the pot didn't do its job, I turned to 0 and the signal from the mic was still there + the interference noise!

I know the earpiece has a very low reading, like 20.5 OHMS wich is a very low signal of course, but when its directly soldered to the jack and into the amp, it souns very low, without interference!

So, I'll post the wiring again for you guys to see if I've made a mistake with the bare wire. Also tryed a lower value pot, but still nothing.
 

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bzblues

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Of course using shielded audio cable will eliminate the hum. From the mic to the pot and from the pot to the amplifier input.
The rows of contacts and wires all over the place in a solderless breadboard pickup hum and all kinds of interference.

OK! Ditched the idea of the pot. Too much trouble. I tought that maybe I can use a switch with a capacitor to apply a lowpass filter to the signal, that was the whole idea in the first place. But I have to put a resistor aswell, right? Here is what I was thinking
 

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andre_teprom

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I tryed the shielded wire and it didn't work out at all
Just out of curiosity, did you try grounding the shield mesh to the earth ? A long time ago I had noise problems with a device for reading analog signals in the range of a few millivolts that picked up noise from a nearby switched power supply, solved only after doing that.
 
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You show a mic, a switch and a jack that all have long unshielded wires that are guaranteed to pickup hum.
Why do you switch a capacitor?
Why does the circuit have a jack? The amplifier should have a jack and the mic should have a plug.

1) Does the amplifier produce hum when this circuit is disconnected?
2) Connect a plug to a piece of shielded audio cable without the mic on its other end. Plug it into the mic input on the amplifier. Is there hum?
3) Connect the mic to the shielded audio cable above. Is there hum?
4) Add the switch and capacitor. Is there hum?

I show shielded audio cable with a braided shield and another with a metalized plastic film shield. I could not find a clear photo if a shield that has many strands of wires wrapped around the inner conductor. Which type of shielded audio cable do you have?
 

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bzblues

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Hello everyone. Ok, the drawings are not the most accurate. It is a plug. I'll attach a picture of the shielded wire I'm using.

Here is step by step what I did:

The result when I use a potentiometer as a volume knob, or adding a capacitor to make a lowpass filter, even with shielded wire is the same all the time. When the pot is on 10, it does NOT hum, the signal is loud and clear, no problem. When I star to turn the pot down, it starts HUMMING.

With the shielded wire 1:
I connected the red to possitive, white to negative and the bare wire to the negative terminals in all the components The result: STILL HUMMING when starting to turn down the pot.

With the shielded wire 2:
I connected the red to possitive, white to negative and I CUTTED the bare wire and added a bit of solder to the remaining tip of the bare wire as I saw in some videos. The result: THE SAME, STILL HUMMING WHEN I TURN THE POT DOWN.

So, shielded wire or not, still the same results.

I've tested the mic on:

- Portable guitar amp, on batteries (more quiet hum of course) and plugged to the wall (louder hum)
- Audio interface: humming

So, my first intention was to add a low pass filter to the mic, as I said before, to roll off the high end frequencyes that could cause feedback with loud volume. Bassically the pot with a capacitor soldered in the middle lug of the pot.

Since I had the same humming problem with different models of telephones and different pot values and different wires, I tought that I could eliminate the potentiometer to eliminate the humming problem and add the lowpass filter as a switch so you can select between straight signal and filtered signal. I took an ON/ON switch and solder a resistor and a capacitor, but it didn't work. Then I solder a 0.1uF capacitor and a 1M resistor directly to the plug to see how it affected the signal, in theory the cuttoff freq. would be 15Hz with is SILENCE! aaaand NOTHING HAPPENED. At first, with the potentiometer, the filter worked fine (just roll a bit off the high end freq) but THE HUMMING!!! HUMMINNNNNGGG!!! (sorry, I'm getting really frustrated with this)

This is the website that I'm using to calculate the low pass filter: http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

I'm gueessing that I should put the components of the same value that worked in the first place, but change the 100K potentiometer for a 100K resistor. Right?


Thank you!
 

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andre_teprom

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Then I solder a 0.1uF capacitor and a 1M resistor directly to the plug to see how it affected the signal, in theory the cuttoff freq. would be 15Hz with is SILENCE! aaaand NOTHING HAPPENED
If even with the addition of the well-sized filter the noise has not disappeared, apparently the interference reaching the amplifier is of the common mode type. This reinforces the belief raised above that (not grounding, but) earthing the shield mesh could somehow improve the noise atenuation. In addition, you could chek if it is possible to add the Capacitor not only close to the Mic, but also another one or even itself as close as possible of (or within) the connector at the end of the cable, although clearly difficult.
 

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