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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

relay as source selector - pop issues
relay as source selector - pop issues
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Old 14th July 2019, 12:05 AM   #21
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by Fast Eddie D View Post
Ground leak resistors should be as large as possible. Example if input impedance is 47K you could use 470K leak down resistor.

If you make it too big, it will take a long time to pull down the capacitor. 470K is fine for say a 2.2 uF coupling cap.

For active filter circuits, you can go even bigger. It depends on the values of the capacitors and resistors in the circuit. Example if the largest resistor in the circuit is 100K then by the "rule of tens" you might choose a 1 meg pull down resistor. But even this value might significantly alter the transfer function of the circuit. However the capacitor values are typically smaller in such a circuit than coupling caps. Say you're switching between a 0.1 uF and a 0.068 uF capacitor. With these values a 2 to 5 meg pull down resistor will be fine, and will not change the circuit operation in any significant way.

Also it isn't a good idea to switch direct coupled audio circuits unless you have the offset very low and stable. A pull down resistor won't help you here.
I have limited options if I want to have a direct input. This is retrofit, not a new design, so I am limited by the existing case design and existing features.

If you refer to the amp DC offset, that is extremely low to nil (@ +/-2mV)

I will go to the breadboard and do more troubleshooting.

Thank you!
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Old 14th July 2019, 02:53 AM   #22
sangram is offline sangram  India
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Quote:
Originally Posted by rottalpha View Post
tried with both sources shorted to ground.
added pull-down resistors for both inputs.

do you mean an analog MBB? have a part number?
do you have a circuit you I could reference?


Many thanks!
MBB = Make before break

BBM = Break before make

Even with close to zero DC offset you may get a small click or a loud pop when the switch is BBM. Another term for these is shorting (MBB) vs non-shorting (BBM).

If you have pop even with a shorted source, check the offset at the input pins.
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Old 14th July 2019, 03:54 AM   #23
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by sangram View Post
MBB = Make before break

BBM = Break before make

Even with close to zero DC offset you may get a small click or a loud pop when the switch is BBM. Another term for these is shorting (MBB) vs non-shorting (BBM).

If you have pop even with a shorted source, check the offset at the input pins.
yes, understood, but do you have a part number or a circuit for reference?

Many thanks!
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Old 14th July 2019, 04:34 AM   #24
sangram is offline sangram  India
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56S36-01-2-04S Grayhill | Mouser India

You can use parametric search to narrow down the options for your use case.
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Old 14th July 2019, 05:33 AM   #25
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by sangram View Post
56S36-01-2-04S Grayhill | Mouser India

You can use parametric search to narrow down the options for your use case.
thank you!

I am bound to using a push switch given the current front panel design of the amplifier. The mechanical / push switch contacts close the supply circuit to the relay coil. The relay does the actual signal switching.

I am trying to tame the pops associated with the relay signal switching.

At the moment, I removed the entire PCB circuit and have PTP wired the relay directly to the input RCA coax and no source attached on either input. I only have a flyback diode and powering up the relay from a 9v battery to rule out any PSU interference.

Yes, pops still there so I am suspecting my input coax wires playing a number on me. RCA from real panel directly coupled on both inputs now
....or the relay being really bad for this duty...
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Old 14th July 2019, 06:41 AM   #26
sangram is offline sangram  India
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When using a single relay with a pushbutton, there is no way to ensure a shorting/MBB operation.

All relays needs about 50-100ms to switch between contacts, thus ensuring that even the lightest offset will be grounded through the pull-down resistors after a momentary excursion to a higher value, as the relay actuates. This change in voltage at the inputs is what causes the pop due to base/gate current changing.

To avoid this we use one relay for each signal and the actual switching operation is not broken before the next source is selected.
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Old 14th July 2019, 08:35 AM   #27
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by Chris Daly View Post
Agreed In a normal audio system there should not be DC present.

If the contact is some how making noise due to opening and closing, a option is Optocoupling using a NSL32SR3 per audio channel, which neatly removes the contact in switches, due to its Resistance Off figure of 25M ohms.
https://lunainc.com/wp-content/uploa.../NSL-32SR3.pdf on the downside is 60 ohms when ON, but is usually not problematic.
I thought or octocouplers but wont they be adding any noise to the signal?
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Old 14th July 2019, 08:45 AM   #28
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by sangram View Post
When using a single relay with a pushbutton, there is no way to ensure a shorting/MBB operation.
I believe your assessment is very accurate, no mater what I tried so far did not get rid of the pops

Quote:
Originally Posted by sangram View Post
To avoid this we use one relay for each signal and the actual switching operation is not broken before the next source is selected.
won't that create any cross-talk?
do you have a reference circuit with the separate relays?


I am also thinking that a timed sequence where the speaker relays are turned momentarily OFF, until the source select ends, is probably the ultimate solution....along the lines of @RickTH suggestion.

In the mean time I do have the option to turn the speaker relay off manually.

Last edited by rottalpha; 14th July 2019 at 08:51 AM.
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Old 14th July 2019, 10:25 AM   #29
sangram is offline sangram  India
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Originally Posted by rottalpha View Post
won't that create any cross-talk?
do you have a reference circuit with the separate relays?
The reference circuit is a 1P3T (or 4T as occasion demands) switch with each throw connected to one relay and the pole to the relay supply. It's that simple, there's really no electronics involved. The NO contact is hooked to the signal in and the NC contact is left open.

We do however add pull-down resistors on both ends and a reverse diode across the speaker relay for housekeeping. There is a provision for caps on the output side if needed.

Haven't noticed any crosstalk, and if there was any it would still be better than a single relay as the signals are physically further apart.
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Old 14th July 2019, 01:30 PM   #30
DF96 is offline DF96  England
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Quote:
Originally Posted by rottalpha
I have tried ground leak resistors on both inputs.
You need ground leaks on the outputs too. And coupling caps both sides.
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