Audio Equipment and relay switching inputs

[Norm Dresner]

Will I need to provide any special protection to high-quality audio
equipment (currently in the design stage) if I know that the remote inputs
will be switched with (reed) relays? I am thinking, of course, of the
effects of contact bouncing producing spikes of voltage at supersonic
frequencies. Would I need much more than a resistor and capacitor from the
input to ground to shunt rf energy away from the circuitry?

TIA
Norm

 

[Joerg]

Hi Norm,

Reeds are pretty quiet. I'd be more worried about the open lines running
from a remote location into the equipment. Someone turning on a large
bank of fluorescent lights would at least produce a nasty sequence of
crackles in the speakers. A nearby lightning hit could actually fry
things. But that's what diode pairs such as the BAV99 plus a series
resistor are for.

Regards, Joerg

 

[Stefan Heinzmann]

Will I need to provide any special protection to high-quality audio
equipment (currently in the design stage) if I know that the remote inputs
will be switched with (reed) relays? I am thinking, of course, of the
effects of contact bouncing producing spikes of voltage at supersonic
frequencies. Would I need much more than a resistor and capacitor from the
input to ground to shunt rf energy away from the circuitry?

Do I understand correctly that you want to protect the inputs on your
high-end device from any effects that might be caused by the upstream
device's output? For example you are designing a power amp and the
upstream preamp has reed relays on its output?

Your input will presumably have a defined input impedance. In case of a
single-ended input this will probably be the standard 47kOhm,
implemented as a resistor to ground. This will keep the input at zero
level whenever nothing is connected to it. You will also have a
capacitor in series with the input to block any DC voltages.

If the upstream device is switched on (or switches its relays on) it
needs to make sure that it doesn't suddenly apply a DC signal, as this
would produce a thump. You probably can't protect yourself against this
without side-effects.

Regarding supersonic effects from bouncing: Either the effect is outside
of the bandwidth of your system and will thus be attenuated, or it is
inside the bandwidth and then it can't be distinguished from a wanted
signal. Bouncing is so short, however, that I wouldn't expect trouble.
If the relay contacts are mercury-wetted they won't bounce.

Regarding RF interference: It wouldn't hurt to provide some filtering at
the input. A T-filter can be bought as a single device (i.e. Murata) for
little money. Other than that, take care with grounding and shielding.

What exactly are you building, and are you worried about damage or about
unwanted noise?

 

[Norm Dresner]

Norm Dresner wrote:

What exactly are you building, and are you worried about damage or about
unwanted noise?

I'm building a fairly complex preamp with individual plug-in cards for each
external device. The output of the cards is switched to a common bus by a
microprocessor which addresses cards one at a time. Of course, downstream
from this is a power amp and speakers too.

As far as the noise in long cables is concerned, I've had cables taking
separately buffered output from am old preamp throughout the house -- cable
runs are up to 50' but are all shielded twisted pair into a moderately low
termination resistance. This has worked exceptionally well since (roughly)
1983 with several satellite equipment upgrades. IIRC (and I probably
don't), each output is buffered by a separate LM310 unity gain buffer. I
haven't seen any RFI on these cables in the last 20 years.

 

[Rich Grise]

I'm building a fairly complex preamp with individual plug-in cards for
each
external device. The output of the cards is switched to a common bus by a
microprocessor which addresses cards one at a time. Of course, downstream
from this is a power amp and speakers too.

As far as the noise in long cables is concerned, I've had cables taking
separately buffered output from am old preamp throughout the house --
cable runs are up to 50' but are all shielded twisted pair into a
moderately low
termination resistance. This has worked exceptionally well since
(roughly)
1983 with several satellite equipment upgrades. IIRC (and I probably
don't), each output is buffered by a separate LM310 unity gain buffer. I
haven't seen any RFI on these cables in the last 20 years.

Oh, yes, then, definitely, reed relays are OK. It's no different,
electrically, from the rotary switch they used in the toob age.

Are these things hot-pluggable? If so, out of curiosity, how do you
do it?

 

[Norm Dresner]

Oh, yes, then, definitely, reed relays are OK. It's no different,
electrically, from the rotary switch they used in the toob age.

Are these things hot-pluggable? If so, out of curiosity, how do you
do it?
--

I hadn't thought about making them hot-pluggable but that may be a
really neat feature to add. I'll have to think about it for a day or two
before rejecting the idea as too complex for this project. OTOH, since all
outputs from each module are relay switched in the module, and the inputs
are hard-wired from the back panel to the individual connector, it would
just be a matter of making the power supply connections safe for insertion
and removal. Just a few more relays would do the trick here, though I'm
certain that there are much better ways than that. My main concern here is
that the +-15V supply to each board is very carefully regulated and filtered
and I'm not sure what a relay switching a load in and out might do to the
it, though all of the op amps in the project should have sufficient PSRR to
handle a glitch or two and there are "bypass" caps on each board as well.
AFAIK, the only way I could switch the boards electronically with no noise
or degredation of the regulated voltage to the board is to give up central
PS regulation and have regulators on each card. That's certainly doable but
a definite change in scope for the modules. I'll have to think about how
much it buys us before making a decision. Thanks for the suggestion.

Norm

 

[Rich Grise]

I hadn't thought about making them hot-pluggable but that may be a
really neat feature to add. I'll have to think about it for a day or two
before rejecting the idea as too complex for this project. OTOH, since
all outputs from each module are relay switched in the module, and the
inputs are hard-wired from the back panel to the individual connector, it
would just be a matter of making the power supply connections safe for
insertion
and removal. Just a few more relays would do the trick here, though I'm
certain that there are much better ways than that. My main concern here
is that the +-15V supply to each board is very carefully regulated and
filtered and I'm not sure what a relay switching a load in and out might
do to the it, though all of the op amps in the project should have
sufficient PSRR to handle a glitch or two and there are "bypass" caps on
each board as well. AFAIK, the only way I could switch the boards
electronically with no noise or degredation of the regulated voltage to
the board is to give up central
PS regulation and have regulators on each card. That's certainly doable
but
a definite change in scope for the modules. I'll have to think about how
much it buys us before making a decision. Thanks for the suggestion.

Would it be worth looking into making the boards less dependent on such
a stiff supply? I know there's a term for it - ah! supply rejection.
It kinda sounds like you have some control here.

The only thing I'd worry about is if an input can be damaged (or an
output, for that matter) can be damaged by being connected to its normal
thing without the circuit being powered, i.e., if the signals make contact
first. But I'm sure somebody else would be much better at coming up with
a good scheme than I am.

 

[Norm Dresner]

Would it be worth looking into making the boards less dependent on such
a stiff supply? I know there's a term for it - ah! supply rejection.
It kinda sounds like you have some control here.

The only thing I'd worry about is if an input can be damaged (or an
output, for that matter) can be damaged by being connected to its normal
thing without the circuit being powered, i.e., if the signals make contact
first. But I'm sure somebody else would be much better at coming up with
a good scheme than I am.

I'll have to ask around about hot-pluggable considerations.

Norm