Excessive electrical switching noise in a Voice Modulator design

Thread Starter

Aussie Kev

Joined May 4, 2023
Looking to lower the switching noise and have a quieter Voice Modulator.
Signal switching design to turn the voice On and Off very quickly. Well a lot faster than by just pressing a button.
2 separate parts to the design with a 1.25M inter-connecting cable.
1st half has 2 tone boards with Op-amps. 1st tone board is used as a microphone input pre-amp. 2nd tone board is used as a microphone output pre-amp. Then a separate audio amplifier.

2nd half is the oscillator box. Push to operate switch to turn On the DC power. The momentary push button is to turn the power On and Off.
7806 voltage regulator.
555 duty cycle module kit in astable mode. Has LFO speed and duty cycle adjustments.
555 timer is used as a low voltage pulse generator. These pulses then trigger the BC546 transistor. The transistor then switch's the 5 volt 1 amp mini relay.
Common terminal is for the microphone voice input.
N.O terminal is the output for the modulated voice.
N.C terminal is the bypass. So the microphone voice will travel from the input to output with the DC power turned Off for the oscillator and relay circuit.

Many months this design has been up and running.
So many changes have been made to lower the switching noise.
The high pitch treble sound of the switching noise has been lowered in volume level and changed to a lower pitch bass sound to the switching noise. Now up to version 4 of the relay board. Sort of run out of idea's.

Need - Some other people to look over the design and see what Kevin can't see to lower more of the switching noise.
Works okay. Just need some help to get it to work better.
Here's some photo's to get a closer look.



Joined Mar 14, 2008
Pictures of what you have aren't really helpful, as it all looks like a hot mess.
Schematics are the language of electronics.
Can't really help without a complete schematic of your latest circuit.


Joined Apr 5, 2008
Hello Kenin,

A schematic overview would give us a better insight on what you want to try to do.
What are the signal levels you want to switch?
Is a switch-over needed?
A bilateral switch , like the CD4066 (on/off) or CD4053 (switch-over) could likely be used.


Audioguru again

Joined Oct 21, 2019
Years ago I worked with a high quality intercom system that had hundreds of stations. It had up to 12 communication paths that used high frequency switching so that many conversations were on each path using time division modulation.

To avoid switching noise, when it was sequence time for one of the conversations, the previous sequenced conversation sound levels were quickly ramped down and disconnected then the new sequenced conversation sound levels were connected and quickly ramped ramped up from zero level to full level.

Abrupt switching would have produces clicks and pops, ramping levels up and down allowed the switching to occur during no sounds.

Thread Starter

Aussie Kev

Joined May 4, 2023
Hello bertus.
Here is a schematic. Well the best from my limited ability. May not be up to the standard that other people may expect.
Still a learner.

Audioguru again

Joined Oct 21, 2019
Of course you have switching noise. You are switching a voice on and off. When there is a voice and it is switched off then you hear a BANG as it suddenly goes from a signal to silence then it makes another BANG as it suddenly goes from silence to the voice signal.

As I said, if you ramp down the signal volume to zero then switch, or ramp up the signal volume then switch, there will be no switching noise.
Hey, if you ramp the signal up and down then the switching is not needed.

schmitt trigger

Joined Jul 12, 2010
As Bertus mentioned, a solid state switch will be far better.
A mechanical relay will exhibit contact bounce. That on itself will generate noise.
AG’s suggestion is also good, but more complicated.

Thread Starter

Aussie Kev

Joined May 4, 2023
How fast do you want to switch?
A mechanical relay will not be fast.
The chips I mentioned are much faster.
Somewhere between 10Hz to 100Hz for the switching speed would be ideal.
The mechanical relay can be driven a lot faster than what I need. Have driven it too fast in the speed and it lowers the volume level of the voice and creates a continual buzzing noise.

As Bertus mentioned, a solid state switch will be far better.
There is also the reed relay and a solid state relay.
Just don't have the experience to know if the reed relay and the solid state relay can be driven slow enough to switch the voice. Something that I have not tried yet.

A tremolo "pedal" for an electric geetar will do it.
I have a tremolo guitar effects pedal and a vibrato guitar effects pedal.
The tremelo is an ultra slow speed. Vibrato is a very slow speed and a little quicker in oscillator speed.
Really looking for something a little quicker like in the ring modulator speed.

Thank's guys for your help so far.

Thread Starter

Aussie Kev

Joined May 4, 2023
No luck with the Tremelo PCB. It contains a lot of tiny SMD components. It will not be an easy R/C change for my current ability.
It would be safer to just leave that PCB alone while it's still working.

The above part of this topic was about model 1 and from version 1 to version 4 that used a SPDT 5 volt relay. It will not fit a DIL socket, so it has to be permanently soldered in position. It is only 5mm wide and not 7.62mm to fit the DIL socket.
So I have been making more changes to get model 2 and model 3 and model 4 by using a DPDT 5 volt relay. This will fit a 16 pin socket and make it a lot easier to replace.
Model 2 was a DPDT relay with a transistor. Well that was so noisy and I went backwards.
Model 3 was just with the relay. After doing the maths from milliamp to milliwatt, I increased the 555 power supply from 6v to 12v. Then pin 3 output was increased from about 3 volt to 5.5 to 6 volt. Then I took the risk to get the 555 IC output to directly drive the relay coil. So it has survived many hours of part time usage. Now less noisy.
Model 4 was with the DPDT relay and the 4066 switch. So very noisy.

DC switching a DC signal is easy.
DC switching an audio signal is so much more difficult. The audio amplifier picks up all the DC noise and sends it through to the speaker.

Temporary solution has been found. By sending the audio through the normally closed contacts of the relay instead of the normally open contact will remove a lot of the electrical noise. This is a compromise at the moment. We have less audio volume level through the circuit with a lot more distortion noise added to the audio.
The audio signal is out of phase with the ocscillator speed.
555 will at best be a 50/50 ratio detween the ON cycle and the OFF cycle.
555 can be at 60/40 ratio with 60% ON and 40% OFF.
Problem that I may face is that the relay in the NC position, is the voice can be ON for 40% of the time and OFF for 60% of the time.
The initial power switching noise is at a low level.
The electrical switching noise is at a low level.
The modulator bleed through is at a low level.
Still a compromise until I can come up with a better solution.