Switches at the input of an amplifier

[Arielle]

Hey guys,
could someone explain to me, why it is not sensible to use switches at the input of an amplifier. I was told to be careful with it but not why. Do they generate noise or distort the signal or sth. like that?
Thanks,
Arielle

 

[Audioguru]

I have always used switches that have gold plated contacts at the input of an amplifier. The gold costs the same as high current silver plated contacts but the gold does not corrode like silver does.

 

[betwixt]

The main reason is that all the gain is after the switch. Differences in voltage at the switched inputs could make large changes in the output as the switch is moved. In the case of an audio amplifier, consider what would happen if say you had two small signal sources but one had a 1V DC offset from the other. The amplifier would likely hit its maximum output as it saw the sudden change in input. As Audioguru pointed out, a good quality switch is essential, corrosion in the contacts can (and does) cause non-linearity and hemnce distortion in the signal.

Brian.

 

[Audioguru]

When silver contacts on a switch turn black then it crackles the audio, gold contacts don't.

 

[dick_freebird]

Especially at very low input amplitudes, contacts' rectifying
behavior (corrosion may look like a II-VI semiconductor) can
ugly up the signal. Less of a problem when you have higher
amplitudes. Using a semiconductor switch is another way to
get rid of that phenomenon.

 

[Arielle]

Hey guys, thank you for your replies. I should have mentioned that I wanted to use a capacitor array in front of the input of the amplifier and the different capacitors are supposed to be shortable using switches (simple PMOS Transistors, which are then in the signal path). Is it still the right explanation, that one wants to prevent large voltage differences between the inputs and therefore shouldn't use switches?
Regards,
Arielle

 

[Audioguru]

I should have mentioned that I wanted to use a capacitor array in front of the input of the amplifier and the different capacitors are supposed to be shortable using switches (simple PMOS Transistors, which are then in the signal path). Is it still the right explanation, that one wants to prevent large voltage differences between the inputs and therefore shouldn't use switches?

What are the capacitors for? To cut low frequencies? Use a bass tone control instead. To reduce hum pickup? Use a shielded input cable instead.
The Pmos transistors will probably cause distortion.

Where will the "large voltage differences between the inputs" come from?

 

[Arielle]

It's actually not an audio amplifier but a variable gain amplifier (for biosignals) which employed an input cap array, a feedback cap and a feedback resistor. Unfortunately, I'm past the deadline to change anything, just need to explain why I discarded the input cap array and used a variable feedback cap instead.
regards, arielle

 

[Audioguru]

Variable feedback caps parallel to a resistor makes a simple variable frequency lowpass filter.
Variable input caps that feed a resistor make a simple variable frequency highpass filter.

I do not know why you need variable cutoff frequencies.
If the frequencies of the filters overlap then the gain is reduced at the peak frequency.

 

[dick_freebird]

Sounds more like a switched capacitor amplifier with gain
varied chunk-wise by front end cap switching. In this case
matching goes to gain error / gain repeatability / gain
linearity w/ code. Sorta like a CDAC.

A voltage variable feedback cap, of course, may have its
own linearity issues (all the ones I'm aware of, do) if the
voltage swing is significant.

Sounds like what is wanted, is a post facto justification
for a choice of convenience?

 

[Jibby Benjamen]

Wrong assumption