Noise when using input selector switch

[Nuuk]

I'm getting a loud noise when I use the rotary selector switch on a pre-amp that I ahve recently built.

I have checked that there is no DC on the input. The switch is a new one and is non-switching. It is a 4 pole 3 way and I am switching both signal and ground.

The signal goes from the switch via stepped attenuator to the input of the pre-amp circuit with no cap between them.

Can anybody suggest how to stop the noise please?

 

[AndrewT]

Hi,

The switch is a new one and is non-switching.

what did you mean to say?

Is the switch make before break or break before make?

Break before make may allow a DC bias to be set up just during the instant of switch over.

Can you monitor the input signal immediately after the switch using a scope to see if a glitch is present?

Can the amp see/feel the presence of an open circuit on it's input during the switch over?

 

[jan.didden]

What is "loud noise"? Really noise (wideband), which could be oscillations, or hum, or what? Or is it a switching impulse noise?

Jan Didden

 

[Nuuk]

Oops, I did mean non-shorting rather than non-switching

I don't have a working scope here at present (just three old ones that need making into one working one when I can find the time).

The noise is what you get if you plug or unplug something in while the system is powered up. It's momentary and goes once the switch is in the next postion.

Would some 100K resistors across the input phonos help?

 

[AndrewT]

Hi,
I just posted another thread suggesting they fit 1M0 to 2M2 across the input/output RCA sockets.

 

[Nuuk]

Thanks Andrew, I'll give it a try.

 

[damianl]

try another its new but that dosnt mean to say its not broken

 

[jan.didden]

Oops, I did mean non-shorting rather than non-switching

I don't have a working scope here at present (just three old ones that need making into one working one when I can find the time).

The noise is what you get if you plug or unplug something in while the system is powered up. It's momentary and goes once the switch is in the next postion.

Would some 100K resistors across the input phonos help?

OK, what may be the case is that while switching you do momentarily interrupt the DC path from the input pin through ground, which normally is implicite via the switches and/or level control. I guess that is also what AndrewT is thinking?
Try the 100k (or better 10k) but directly from the input pin to ground. See if that makes a difference.

Jan Didden

 

[Nuuk]

Thanks Jan,

I tried 1M on all input and output pins but it made no difference. I'll try 10K and see what happens.

 

[jan.didden]

I mean input pin directly on the amp board, AFTER the switch, not the RAC input jack on the box. Just to be sure. Do you have a coupling cap?

Jan Didden

 

[Nuuk]

I mean input pin directly on the amp board, AFTER the switch, not the RAC input jack on the box. Just to be sure. Do you have a coupling cap?

Aha! OK, I already have 47K directly on the input after the attenuator. No, there is no coupling cap.

 

[moamps]

Does it happen when you do not switch the sources' grounds?

 

[Nuuk]

Does it happen when you do not switch the sources' grounds?

Somebody had suggested not switching the source grounds Moamps and that would have been my next job.

But I tried something else first! I was using the 4 pole 3 way switch for two inputs and using the third position for mute. I had 47K resistors soldered across the pins of the switch. I changed the arrangement to make the switch a 2 way and did away with the resistors. And voila, no more noise when switching.