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Telephone Microphone Circuit Bent POTENTIOMETER HUMMING

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If the mic and pot are connected with ordinary wires then they are antennas that pickup hum.
Shielded audio cable MUST be used.

The lowpass filter is designed to cut 973Hz a little but gradually reduce higher frequencies and still produce 9700Hz.
The lowpass filter is not a filter if it feeds a normal low impedance mic input on an amplifier.
 

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    mic and pot wiring.png
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IF the amplifier has a very high input impedance of 2M or more (but a mic input might be only a few thousand ohms) then you can use a 100k series resistor then a 2nF capacitor ground to cut all mid and high frequencies above 800Hz.
But a normal low impedance mic input will attenuate the signal level to almost nothing.

You said the mic is 25 ohms. Then to cut all frequencies above 800Hz, connect a 8uF capacitor parallel with the mic.
Why do you want to cut all mid and high frequencies? Then speech will be only vowels, AY, EE, EYE, O, and OO.
 
Audioguru said:
If the mic and pot are connected with ordinary wires then they are antennas that pickup hum.
Shielded audio cable MUST be used.

The lowpass filter is designed to cut 973Hz a little but gradually reduce higher frequencies and still produce 9700Hz.
The lowpass filter is not a filter if it feeds a normal low impedance mic input on an amplifier.
Click to expand...

Yes ! I've been using shielded wire, forgot to mention that on the picture. So, if I understood correctly, if I put the resistor in series and the capacitor paralel just like you told me to, the filter wont work?

"
The lowpass filter is designed to cut 973Hz a little but gradually reduce higher frequencies and still produce 9700Hz.
The lowpass filter is not a filter if it feeds a normal low impedance mic input on an amplifier. "

I didn't quite understand what you said above. The only way for a lowpass filter to work is with a potentiometer?
--- Updated ---

Audioguru said:
IF the amplifier has a very high input impedance of 2M or more (but a mic input might be only a few thousand ohms) then you can use a 100k series resistor then a 2nF capacitor ground to cut all mid and high frequencies above 800Hz.
But a normal low impedance mic input will attenuate the signal level to almost nothing.

You said the mic is 25 ohms. Then to cut all frequencies above 800Hz, connect a 8uF capacitor parallel with the mic.
Why do you want to cut all mid and high frequencies? Then speech will be only vowels, AY, EE, EYE, O, and OO.
Click to expand...

Great! I'll try the 8uF capacitor. How can I calculate what capacitor valur should I use for the specific reading of a microphone?

The thing is this mics are not only for singing. You can use them to record whatever you want. The goal is to obtain a low-fi sound. Maybe the filter is not that much suitable for singing, altough I've recorded some tests and EQ them around 800Hz and it still sounds nice, just a bit darker. Its a fun mic to play and experiment with, just to have fun!
--- Updated ---

Audioguru said:
IF the amplifier has a very high input impedance of 2M or more (but a mic input might be only a few thousand ohms) then you can use a 100k series resistor then a 2nF capacitor ground to cut all mid and high frequencies above 800Hz.
But a normal low impedance mic input will attenuate the signal level to almost nothing.

You said the mic is 25 ohms. Then to cut all frequencies above 800Hz, connect a 8uF capacitor parallel with the mic.
Why do you want to cut all mid and high frequencies? Then speech will be only vowels, AY, EE, EYE, O, and OO.
Click to expand...


What its still not clear to me is that I'm using this calculator to see what resistor and capacitor I need to made the cuttoff freq:

Low Pass Filter Calculator

This is a Low Pass Filter. It calculates the range of low frequency signals that a filter passes through.
www.learningaboutelectronics.com

You only have 3 values, so thats why I'm asking how can I calculate the values of the components suitable for the reading of the mic?
 
Last edited:

A resistor in series then a capacitor to ground is a lowpass filter that can feed an amplifier that has a very high input impedance. If the resistor is 100k ohms and the amplifier input impedance is 5k ohms (a normal mic input) then the output level at low passing frequencies is reduced 1/21 times.
The 10k input resistance of the amplifier loads down the signal level to near nothing.

The mic already has a fairly low resistance of 25 ohms and can feed a 5k amplifier input resistance with almost no signal level loss. Then filter the mic with a capacitor in parallel with the mic like I said in your other thread about it.
 

Audioguru said:
A resistor in series then a capacitor to ground is a lowpass filter that can feed an amplifier that has a very high input impedance. If the resistor is 100k ohms and the amplifier input impedance is 5k ohms (a normal mic input) then the output level at low passing frequencies is reduced 1/21 times.
The 10k input resistance of the amplifier loads down the signal level to near nothing.

The mic already has a fairly low resistance of 25 ohms and can feed a 5k amplifier input resistance with almost no signal level loss. Then filter the mic with a capacitor in parallel with the mic like I said in your other thread about it.
Click to expand...


Ok, So when you talk about the amplifier input, you can be talking about a guitar amplifier, a mixer console, an audio interface? any of those? I'm guessing that each one of those has a specific impedance?
 

If an amplifier has a microphone input it probably has an input resistance of 5k ohms for a dynamic (coil and magnet) mic. It has a very high gain.
A an electric guitar input input has a resistance of a few million ohms and not much gain.
Aren't you connecting your mic to a mic input?
 

Audioguru said:
If an amplifier has a microphone input it probably has an input resistance of 5k ohms for a dynamic (coil and magnet) mic. It has a very high gain.
A an electric guitar input input has a resistance of a few million ohms and not much gain.
Aren't you connecting your mic to a mic input?
Click to expand...

I'm pluggin the telephone-mic with a normal guitar plug cable, not an XLR cable.
 

You can use any shielded cable you want. I am talking about which input on an amplifier, not which cable.
Usually a mic is plugged into the mic input on an amplifier.
The output level of a mic is about 0.01V feeding about 5000 ohms. The electric guitar input on an amplifier is about 1V at 3 million ohms. A huge difference of signal level and resistance.
 

Audioguru said:
You can use any shielded cable you want. I am talking about which input on an amplifier, not which cable.
Usually a mic is plugged into the mic input on an amplifier.
The output level of a mic is about 0.01V feeding about 5000 ohms. The electric guitar input on an amplifier is about 1V at 3 million ohms. A huge difference of signal level and resistance.
Click to expand...
Oh, sorry.

I have a Roland Street Cube portable amp tha has a mic and instrument input, I've plug it into both and sounds ok as long as I'm using the battery and not plugged into the wall, otherwise, it will hum when I turn the pot down.

I plug it into mi audio interface to record in my computer, and it hums, just like the recording that I sent you a few posts ago.

What happens if I use a 100 ohm resistor or potentiometer with a 2uF capacitor? according to the lowpass filter calculator, that is the capacitor I would need for the filter to work with a 100 ohm resistor. I've been told that the resistor needs to be close to the value of the mic.
 

With a 100 ohm resistor or pot as a rheostat in series with the 25 ohm and a 2uF capacitor to ground then mid and high frequencies at and above 640Hz will be cut. With the pot turned down all the way then the 2uF capacitor parallel with the mic will cut high frequencies at and above 3200Hz.
 

Hi,

I'm not sure if this has already been suggested as a solution to the problem described in post #1 regarding the humming when turning the trimpot from 0 to 10 or viceversa, I'm a bit lost in this thread by now, interesting as it is. Out of curiosity, have you tried a 55HZ or a 100Hz band-reject filter across that potentiometer and turned the potentiometer from 0 to 10 and back again to see if it does anything to get rid of the noise/hum. Probably won't work, and it would attenuate any signal of interest at the same time, but just to see if the noise/hum goes away at all.

I did a simulation for 55Hz. You could use 1k and 1.47uF for around 100Hz.

I used this calculator called Band Stop Filter Calculator for the original values, the simulation begged to differ a little about RC values needed, normal. While not a great representation of your actual set-up, at least it's helpful to see what the band-stop should do.

BAND-STOP 55HZ SCHEMATIC AND FREQUENCY SWEEP.JPG
 
Hi,

The circuit for the pots is obviously wrong.
What is your idea? How should they work? What should they do?

Klaus
 

Hi Klaus,

KlausST said:
Hi,

The circuit for the pots is obviously wrong.
What is your idea? How should they work? What should they do?

Klaus
Click to expand...


You are addressing me in your last post, aren't you? What's obvious to one person may not be to another.

Obviously wrong - okay, but why is the idea wrong? I can learn something, and probably a lot from the reason(s) why.

My thought was that - going back to post #1 that describes the problem as being hum/noise appearing only during the turning of the potentiometer but not present with the wiper at either extreme/end, to place a band-stop filter parallel to the potentiometer in order to see if the hum/noise went away or was at least attenuated at all whilst turning it from one end to the other. The idea isn't for the band-stop filter to be placed there permanently, just for it to be connected from the top of the potentiometer/on the signal path and to ground for the duration of the test to see if the hum could be attenuated and to locate its actual frequency.

Pardon me for speaking...
 

Hello everyone! GOOD NEWS! it worked! Adding a 100 ohm resistor in series and a 2uF capacitor (Its polarized, I don't know if it makes any difference with one thats not) I can make the filter. I took an ON/ON and put it there so you can select between the signal and filtered signal. Thank you so much for your patience and help, really appreciate it!

From now on I have to read the earpieces to know what resistor should I add and calculate what capacitor value suits it.

I'll keep you guys post with the advances!
--- Updated ---

d123 said:
Hi,

I'm not sure if this has already been suggested as a solution to the problem described in post #1 regarding the humming when turning the trimpot from 0 to 10 or viceversa, I'm a bit lost in this thread by now, interesting as it is. Out of curiosity, have you tried a 55HZ or a 100Hz band-reject filter across that potentiometer and turned the potentiometer from 0 to 10 and back again to see if it does anything to get rid of the noise/hum. Probably won't work, and it would attenuate any signal of interest at the same time, but just to see if the noise/hum goes away at all.

I did a simulation for 55Hz. You could use 1k and 1.47uF for around 100Hz.

I used this calculator called Band Stop Filter Calculator for the original values, the simulation begged to differ a little about RC values needed, normal. While not a great representation of your actual set-up, at least it's helpful to see what the band-stop should do.

View attachment 165212
Click to expand...
Hi Daniel! I haven't try it, but I should give it a try! I'll let you know what the results are. Thank you!
 

Hi,

obvious...
* they are not related to the filter
* both pots are in parallel
* they just act as a variable ohmic load to the input signal.

and if the wiper is at one side, it will short circuit any input signal. (at least for the simulation, but not for a real pot)
For sure there will be no hum anymore.

And you have the simulation tool. So use it.
--> Does the pot position make any difference from 1% to 100%?. I doubt it.

d123 said:
but why is the idea wrong?
Click to expand...
I don´t know what your idea is. That´s why I asked for it. I still don´t know - or not understand.

Klaus
 
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KlausST said:
Hi,

obvious...
* they are not related to the filter
* both pots are in parallel
* they just act as a variable ohmic load to the input signal.

and if the wiper is at one side, it will short circuit any input signal. (at least for the simulation, but not for a real pot)
For sure there will be no hum anymore.

And you have the simulation tool. So use it.
--> Does the pot position make any difference from 1% to 100%?. I doubt it.


I don´t know what your idea is. That´s why I asked for it. I still don´t know - or not understand.

Klaus
Click to expand...

Hello Klaus! the idea first was to put a 100k potentiometer with a capacitor to act as a lowpass filter for the microphone. The main problem was that when I turned the pot down to "0" it started to hum. I couldn't figure out how to solve this problem, tough now I think that the problem was the value of the pot it was to much for the 20.5 ohms of the mic. So, I figured (in my short experience) that I can get rid of the huming problem by removing the potentiometer and use a switch as and ON/OFF selector between the "normal" signal and the "filtered" signal. So following "Audioguru" advice, I put a 100 ohm resistor in series (I couldn't find a potentiometer with that low value) and a 2nF capacitor to ground and it worked out!

If I find a 50 ohm or 100 ohm potentiometer maybe I can solve the huming problem.
 

Hi,

bzblues said:
the idea first was to put a 100k potentiometer with a capacitor to act as a lowpass filter
Click to expand...
You know there is no capacitor involved in the pot circuit that can act as low pass filter ?

bzblues said:
The main problem was that when I turned the pot down to "0" it started to hum.
Click to expand...
With the pot lower_end connected to the star point and nowhere else? I still don´t see this in your circuits.

bzblues said:
I put a 100 ohm resistor in series (I couldn't find a potentiometer with that low value) and a 2nF capacitor to ground and it worked out!
Click to expand...
I don´t see this. What schematic are you talking about?

Klaus
 

d123 said:
Hi,

I'm not sure if this has already been suggested as a solution to the problem described in post #1 regarding the humming when turning the trimpot from 0 to 10 or viceversa, I'm a bit lost in this thread by now, interesting as it is. Out of curiosity, have you tried a 55HZ or a 100Hz band-reject filter across that potentiometer and turned the potentiometer from 0 to 10 and back again to see if it does anything to get rid of the noise/hum. Probably won't work, and it would attenuate any signal of interest at the same time, but just to see if the noise/hum goes away at all.

I did a simulation for 55Hz. You could use 1k and 1.47uF for around 100Hz.

I used this calculator called Band Stop Filter Calculator for the original values, the simulation begged to differ a little about RC values needed, normal. While not a great representation of your actual set-up, at least it's helpful to see what the band-stop should do.

View attachment 165212
Click to expand...
Your filter reduces 53Hz only -3dB which is barely noticeable. Besides, a video of the hum makes it sound like a 100Hz triangle waveform that has many harmonics.
--- Updated ---

KlausST said:
Hi,


You know there is no capacitor involved in the pot circuit that can act as low pass filter ?


With the pot lower_end connected to the star point and nowhere else? I still don´t see this in your circuits.


I don´t see this. What schematic are you talking about?

Klaus
Click to expand...
KlausST said:
Hi,


You know there is no capacitor involved in the pot circuit that can act as low pass filter ?


With the pot lower_end connected to the star point and nowhere else? I still don´t see this in your circuits.


I don´t see this. What schematic are you talking about?

Klaus
Click to expand...
He says the hum sound is high frequencies so he is making a lowpass filter with a 100 ohm resistor in series with the 25 ohm mic and a 2uF capacitor to ground for a 640Hz cutoff of all higher frequencies.

He said this, "The idea is to have the option of the filter, to roll off the higher end freq. and also have a bit different sound, just that. This hum is not low freq, it is very audible, it sounds like a wave of mosquitoes!".
 
Last edited:
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Hi,

My post#52 refers to the circuit of post#51
My post#55 refers to post#53
Thus I´m a bit confused about post#56.

Maybe it´s just time for me to be quiet.

Klaus
 

Hi Klaus,

KlausST said:
Hi,

obvious...
* they are not related to the filter
* both pots are in parallel
* they just act as a variable ohmic load to the input signal.

and if the wiper is at one side, it will short circuit any input signal. (at least for the simulation, but not for a real pot)
For sure there will be no hum anymore.

And you have the simulation tool. So use it.
--> Does the pot position make any difference from 1% to 100%?. I doubt it.


I don´t know what your idea is. That´s why I asked for it. I still don´t know - or not understand.

Klaus
Click to expand...

Thanks. The two trimpots/identical circuits are to show that the RC values are interchangeable.

I simulated at 90%, 50% and 10% wiper position with the frequency sweep of 1 Hz to 40 kHz and the results were the same as the graph in the attached schematic.

From your reaction and observations, I see adding a 55 Hz or 100 Hz band-pass parallel to the bass potentiometer is pointless even as an experiment to see if it attenuates the hum. Okay.
 

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