MOSFET's interaction with audio signal

[LukeeeeBennett]

If I pass an audio signal through a MOSFET will it be affected? Coloured in any way?

 

[ronsimpson]

yes..............

 

[LukeeeeBennett]

yes..............

In what way?

 

[ronsimpson]

Even a wire can colour a signal.
A MOSFET is not linear. Its output is not the same as its input.
Did someone tell you MOSFETs are better than Transistors. I think that is some what true. That does not make them perfect.

 

[LukeeeeBennett]

I was thinking more when you're listening back to the audio, is it blatantly obvious that the signal has been changed slightly.

 

[ronsimpson]

If I pass an audio signal through a MOSFET will it be affected? Coloured in any way?

Audio passed through a MOSFET leaves me not knowing what you are doing with the audio.
There are MOSFET audio amplifiers. There are "MOSFET switches" that turn on/off the audio.
If you make a one transistor-MOSFET audio amplifier it will have distortion. There are some very good transistor-MOSFET audio amplifiers (with many MOSFETS)that sound good. Even the best amplifier has some distortion.

 

[LukeeeeBennett]

Audio passed through a MOSFET leaves me not knowing what you are doing with the audio.
There are MOSFET audio amplifiers. There are "MOSFET switches" that turn on/off the audio.
If you make a one transistor-MOSFET audio amplifier it will have distortion. There are some very good transistor-MOSFET audio amplifiers (with many MOSFETS)that sound good. Even the best amplifier has some distortion.

Ahh, yeah sorry, I should've stated it was being used as a switch.

 

[ronsimpson]

Anything adds distortion. In this case it is small it you do it right. Then passing audio the MOSFET is on. It can not be a perfect switch. It might have 50 ohms on resistance. That 50 ohms and the 10k input resistance of the next amplifier makes a voltage divider where a very small amount of the audio is lost. This is OK. The 50 ohms is not constant under all conditions. Example: the 50 ohms (at 0 volts) might be 40 ohms with the positive half of the signal and 60 ohms with negative half. Hopefully the ratio of 50 to 10,000 is so small that 40:10000 and 50:10000 and 60:10000 all sound the same.

If I have time I will try to find real numbers tonight.

 

[ronsimpson]

Below is from the data sheet on a CD4051 switch.
The right half shows resistance at three different temperatures.
The left half shows three different supplies voltages and what that does to resistance.

With a 15 volt supply at 25C and a signal of 0 volts the resistance is 100 ohms. If the signal is at -5 volts the resistance might be 125 ohms. This is non linear. If the next amplifier has in input of 100 ohms this causes large distortion. If the next amplifier has an input of 100,000 ohms this is almost no distortion.

Hope this helps!

 

[LukeeeeBennett]

That's a massive help! Thanks a lot man.

 

[audioguru]

Few people can hear distortion in music or speech that is 0.1% or less.

 

[ronsimpson]

AudioGuru,
I agree with your 0.1% comment.
Many people spend money on specs. If twice the money will get 0.05% then why not 0.01%.
In the audio world there is a contest. My amp is bigger than yours. I spent more money then you. etc.

 

[simonbramble]

Didn't Doug Self replace his directional speaker cable with mains cable from his lawnmower and measure an improvement in audio quality? I knew a guy who actually bought a gold plated mains plug and swore there was a difference in audio quality.

If you are switching line level signals, a low ON resistance analogue switch with good matching and flatness is the best way to go.

 

[crutschow]

...........
Did someone tell you MOSFETs are better than Transistors. I think that is some what true. That does not make them perfect.

I know this is a nit but I'm rather a picker of nits. A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a type of transistor as is a BJT.