Audio Signal Switch

[Shahriar]

Hi

I am working on a project and I want to select a microphone through other 19 Mics to connect it to PreAmp. I want to select
Mics By a 89c51, what kind of relay or switch can i use...

ThankX in Advance

P.S. if I use ordinary switch to select Mics Manually, Does Ordinary swiches make the Audio signal Noisy?

[GPG]

Google 74HC4051. 3 will get you 24 channels

[audioguru]

Hi GPG,
I think that a 74HC4051 or regular 4051 will be noisy and make switching "pops" when used with mic levels.

Shahrier,
What is an 89c51?
Since microphones have a very low signal level, relays or switches should have gold-plated contacts. It doesn't cost much more since the layer is very thin.
Use good-quality shielded cable for all microphone wiring.

[Shahriar]

Thank you so much for your good replies Audioguru... ;)

[MP]

shahriar, I use 4016 for switching audio with microprocessor such as you have described. Also, 4066 is a good switch, depending upon impedance needed. Study the data sheets for these two.

Hope it is helpful.

Many music effects use the 4016 for switching between different effects in a rack mount mobile type of unit. You can get around the pop with an RC time constant so that the open and close actions are not at exactly the same time.

MP

[audioguru]

MP,
Yes, 4016, 4066 and 4051 chips switch line-level audio, music and effects quite well. But since a microphone has a level that is 100 to 200 times smaller, then noise and pops are very noticeable.
On TI's datasheet for their CD4016, page 3 and figure 20 on page 5 show that the Crosstalk from control to output is typically 50mV, which is a lot more than a 5mV or less mic signal. This crosstalk "pop" can be reduced with capacitors across the outputs, but the high frequency audio will also be reduced. The 4066 and its sister, the 4051, have the same crosstalk spec.
Noise level is not specified since it is not designed for low noise.
The datasheet is here:
http://www.cedmagic.com/tech-info/data/cd4016.pdf

[MP]

audioguru, I have never used the chip for line level.
I was not talking about capacitors on the output or across the output. In fact, I was not talking about adding capacitors on the audio side at all. I was also not talking about theory of how to do this, but was speaking from many years of experience. Believe me, guitar effects are very prone to noise and pops as well as microphones. The 4016 works well with both. I have never had problems with noise or cross-talk in any of the devices I have built using this chip. The trick is how you condition the control line.
I might add, that there are touring bands using some of my custom effects ;D.
Good shielding practice and good board layout practice are also essential.

MP

[audioguru]

MP,
I dunno about guitars, but my experience has proven that 4016, 4066 and 4051 chips make pops in microphone switching circuits. The "charge injection" is internal and cannot be slowed external to the control pin.
In 1996, MAXIM introduced their low-cost improved MAX4066, with much lower "charge injection" that causes pops, and lower distortion, than the 1975-designed ordinary 4066 chips.
Maybe this circuit will switch microphones well, but I haven't tried it. Their datasheet is here:
http://pdfserv.maxim-ic.com/en/ds/MAX4066-MAX4066A.pdf

[GPG]

Charge injection can be thought of as a very small capacitor connected between the control and the switch channel.
As a result the pops are related to the rise time of the control signal
which can be reduced.

[audioguru]

GPG,
You are talking about charge injection between external pins of the IC. I agree that the control voltage can be slowed down to reduce that, as well as proper PCB layout.
I was talking about charge injection occuring internal, from the output of the IC's controlling high-gain inverting and non-inverting amplifiers (the high gain speeds-up the transition time, and the voltage swing is the entire supply voltage) to the MOS transistor switches.
It doesn't take much capacitive-coupling for 10V control pulses to couple into a 5mV microphone signal. That's 2K to one, and maybe only 2M to one is audible.
Maybe MAXIM re-designed the IC's internal layout so that the opposing capacitive pulses cancel each other.

[GPG]

GPG,
You are talking about charge injection between external pins of the IC
No

[MP]

but my experience has proven that 4016, 4066 and 4051 chips make pops in microphone switching circuits.

It is because you have not made the circuit correctly. You would have the same problem with a mechanical switch. I would not bother with 4066 or 4051 for high impedance audio signals, but the 4016 works fine.

MP

[audioguru]

MP,
A mechanical switch doesn't have a control voltage swing to couple into the audio path, but a relay does and therefore needs careful PCB layout like these electronic switches.

A dynamic microphone is low impedance and the switch's load must also be low impedance for low noise when the mic is turned off.
A 4016 isn't suitable for low impedances like the 4066 or 4051 are designed to cope with.

Maybe you are talking about switching during a loud transient sound, which makes a "DC" pop. Some high-tech electronic switches use "a delay to zero-crossing" circuit to avoid that.

[MP]

A dynamic microphone is low impedance and the switch's load must also be low impedance for low noise when the mic is turned off.
A 4016 isn't suitable for low impedances like the 4066 or 4051 are designed to cope with.

audioguru,
Many microphones have a switch for low or high Z or can be set inside the device. A Dynamic microphone is not specifically a low Z device. I have several that are switchable with high Z or low. Nowhere in this thread did the original post specify a mic with a low Z. ::)
BTW...89C51 is a microproccessor since you asked.

Shahriar:
Like I said, it works for me; and very well. I won't continue with audioguru's theory argument since there is no point. A working circuit speaks for itself. Check the data sheet of these devices and I am sure you will come up with something better than a relay.

MP

[GPG]

Came across this:
pdfserv.maxim-ic.com/en/ds/MAX4617-MAX4619.pdf
Has a spec for charge injection p6, if you are switching low lvel it looks like a bias of ~.6v minimises. 4617 is x8 addressable

[audioguru]

Hi GPG,
Good find! That new MAX4617 is even better than the improved MAX4066 or MAX4051. You're right, its charge-injection is minimum with a bias of 0.6V. The resulting 1.2Vp-p is plenty of headroom for low impedance microphones.