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Old 8th March 2005, 12:47 PM   #1
rho is offline rho  Belgium
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Default Contact bounce

I have a question about using relais in signal-lines at line-level.
At the output of the phono-stage I'm currently designing I would like to put a relais to switch off the output during start-up.
The contact will be open untill the power-supply is stable. Then it closes. At shut down the contact opens before the
supplyvoltage drops too low. This is to prevent plops at start-up and shut-down.
Now I'm worried that opening and closing of these contacts in the relais will introduce noises.
These are mechanical contacts and they will bounce.
Is this a problem or will this be hardly noticeable?
An option is to use mercury wetted contacts but these are much more expensive and harder to find.
Are there any other options to do this, without influencing the signal (reistive and/or capacitive load) too much?
Relais-contacts have very low ON-resistance and capacitance.
I was thinking about analog switches first (like the SSM2404, ADG333A)
but these ask for a lot more circuitry to ensure correct supply-sequencing or need an extra power-supply.
So they fall off from the option list.

A while back, I've read something about LDR's that could be used for switching audio.
Anyone know what company makes them and/or where to get these (Europe)?

Any help is welcome. Thanks.
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Old 8th March 2005, 01:27 PM   #2
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Default Re: Contact bounce

Quote:
Originally posted by rho
I have a question about using relais in signal-lines at line-level.
At the output of the phono-stage I'm currently designing I would like to put a relais to switch off the output during start-up.
The contact will be open untill the power-supply is stable. Then it closes. At shut down the contact opens before the
supplyvoltage drops too low. This is to prevent plops at start-up and shut-down.
You mix two problems in a single question.
The first problem is about to prevent pops at start-up and shut-down.
The first option to prevent pops is to proper sequence the power supplies. It has the great advantage not to insert any contact in the signal path.


Quote:

Now I'm worried that opening and closing of these contacts in the relais will introduce noises.
These are mechanical contacts and they will bounce.
Is this a problem or will this be hardly noticeable?

If the first option is not available (e.g. the circuit has a single power supply nor it cannot be turned on very slowly) you have to use some form of switch on the output.

The best option is a relais for low level signal (not a mercury one). You have to chose it very carefully but if you get the proper one and use the "right" circuit topology the bounce will not be a problem.

Regards
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Old 8th March 2005, 04:49 PM   #3
rho is offline rho  Belgium
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What do you think of the reed relais of Hamlin Electronics?

I would pull the output to gnd at startup. (after the series resistor at the output of the last opamp)
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Old 9th March 2005, 01:44 AM   #4
anatech is offline anatech  Canada
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There are many very high end preamps that use gas-filled relays for all switching. They are very reliable and do not affect the sound badly. If you are paranoid, short the output to ground as rho suggested.

These relays are better than most switches, I wouldn't worry about it as you have to have much worse problems than the relay. I'd worry more about your power supply, layout and circuit.

-Chris
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Old 9th March 2005, 05:18 PM   #5
rho is offline rho  Belgium
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gas-filled relays? Can you give me a manufacturer-name or/and type of such relais? Could be very interesting.
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Old 10th March 2005, 12:32 AM   #6
anatech is offline anatech  Canada
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Hi rho,
Most telecom relays (read reliable) are gas filled with nitrogen or something similar. NEC made some, Siemens T85 series, Omron G6A series. These may not be gas filled but they are sealed. NEC does have gas filled (or used to). These are also used in Studio recorders.
-Chris
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Old 10th March 2005, 08:31 PM   #7
rho is offline rho  Belgium
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Thanks for the most useful info.
I'll have a look at their web-sites.
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