Removing an audible “pop” from a CMOS switch – Ramping Power?

Hi again,

I have a simple audio switching circuit that is giving me a problem. I am using a CD40106 Schmitt Trigger in conjunction with a CD4066 CMOS switch to route up to two audio signals between "wet" & "dry" circuitry.

The circuit is DC and will run on 9 - 15 volts. It will always be under 100 mA so we're not talking a lot of power.

The circuit works well, except that with an amplifier turned way up I can hear a very small "pop" in the audio when the circuit gets engaged. I know, most of you think "Well turn the amp down dummy!" but I am trying to totally isolate all the sound so there is a seamless switch from one state to another.

I know that the issue could be taken care of with pull down resistors or capacitors. However these seem to remove some volume and degrade the sound quality a bit, the goal is to have the volume level that enters the unit match the output volume so I am trying to avoid that route.

I have read that this is due to a split second of Capacitance Coupling into the signal line. I believe that Ramping the control signals voltage up and down with Triangle waves to the CMOS can eliminate the issue, this but I cannot find an example of how to really implement this concept.

So what do think? Would an Astable Multivibrator make the difference? And where would it go in the circuit?

Thanks in advance for ALL of you out there helping.

Danny

Hey Danny,

I have worked on a problem very similar to this in the past. Using a dual-rail VCA (SSM2164) to control the mix of multiple audio sources into a single output, I had originally used a simple switching circuit to drive the control input. Unfortunately, I had the same problem you are having now.

The way I was able to remedy it was to use an integrator circuit and op-amp follower to drive the ramp into the control input of the VCA. I was using a +/-15VDC supply, and the VCA requires an inverted control. i.e. 0V = full output signal. I've attached the schematic here and hopefully it will help you. The input is a logic 5V control and the output ramps from 15V to 0V when activated.

This definitely fixed the issue in the system I was working on.

First off, thanks.

Is it possible to work this without using +/- 15VDC? My power source is a +12VDC or +15VDC and I cannot get that much power to a +/- VDC setup.

I would really like to use it with just voltage and ground, do you think this would work in my situation?

Use a DC-DC Converter

Here's a quick and simple answer that I found. It worked right off the bat!

When you have equal DC potentials on both sides of the 4066 switch, the "pop" will be lower.

The resistors can be in the range from 10 to 100K.
There will be no DC jumps on the capacitors on both sides.