Silent switching of audio signals (revisited)

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Some time ago I started a discussion about problems I was facing with audio switching. I never ended up finding a satisfactory solution and now I have tried a slightly different approach which works better but I still feel there is something I'm missing out of.

 

Attached is a schematic of the switching portion of the system.

 

  • Audio in is an audio signal. Usually it comes from another device. AC coupled.
  • Audio out goes to another device or an amplifier, sees high Z (500k and above)
  • Ctrl is either high or low (5V-0V), together with the cap. it enables driving a single coil bipolar latching relay using a single pin.
  • U4 is an opto relay (TLP222G)
  • The Ctrl signal is fed with an RC delay line in parallel into a XNOR gate which will create a pulse.
  • The pulse length is variable using the RC values.
  • The pulse drives an LED which in turn opens the mosfets and shorts the signal to ground faster than relay is switching (that's the idea anyway), the relay action should start after the signal is muted and finish before it is un-muted.

 

I have tested the opto relay on it's own and the switching action is silent. On the scope it's smooth with no sudden transient. I've tested the relay switching on it's own and it does cause a 'thump' noise when switching. I did try to bypass the relay only leaving it attached to the circuit and that did not cause a noise which switching. Together (relay + mosefts) does not mute the signal while switching and I can hear some noise. I am not sure of the source but it is not the opto relay and it is not the ctrl signal itself.

 

 

 

Last Edited: Fri. Aug 25, 2017 - 04:31 PM
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I'm not sure what U3 is, or if "Return" is a signal or GND... If it is an output, are you sure you want the junction of the three signals entering the relay?

 

Edit: I see you have now modified your original image...

David (aka frog_jr)

Last Edited: Fri. Aug 25, 2017 - 04:36 PM
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frog_jr wrote:

I'm not sure what U3 is, or if "Return" is a signal or GND... If it is an output, are you sure you want the junction of the three signals entering the relay?

 

Fixed schematic. Send is the input of the circuit, return is the output of the circuit.

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What effect does adding something like a 10k pull-down resistor to 'audio in' have?

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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When the relay is in bypass (Audio_In directly connected to Audio_Out), the state of the "Send" pin of U3 will be floating. What does that do to the "Return" of U3?

David (aka frog_jr)

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Brian Fairchild wrote:

What effect does adding something like a 10k pull-down resistor to 'audio in' have?

 

It does help, it also lowers the volume because the source sees a lower resistance.

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frog_jr wrote:

When the relay is in bypass (Audio_In directly connected to Audio_Out), the state of the "Send" pin of U3 will be floating. What does that do to the "Return" of U3?

 

I've tried the following variant for switching, it sounds about the same.

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slow_rider wrote:

It does help, it also lowers the volume because the source sees a lower resistance.

 

So what on earth is the source resistance? For audio you normally aim to have the lowest possible source resistance.

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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Brian Fairchild wrote:

So what on earth is the source resistance? For audio you normally aim to have the lowest possible source resistance.

 

I think it's about 10k (normally) but might be lower for some devices

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Clicks often come from voltage level shifts when the switch happens. You can improve it a lot if you insure same level on both sides of the output switch. One of the other things I have done is to use a "digital potentiometer" after the switch. Ramp down the "volume" just before the switch event, then ramp it up after. Even this, however, is fraught with caveats. You don't want much DC voltage across the pot, otherwise you will get a "thump" during each ramp.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
use a "digital potentiometer" after the switch
Beware "zipper noise" when using digital pots.

 

Edit: A solution is to only switch the signal when at a zero crossing...

David (aka frog_jr)

Last Edited: Fri. Aug 25, 2017 - 06:56 PM
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Zipper effect is most apparent when there is a DC voltage on the input of the digital pot. 

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net