Power On/Off Thumps and Plops

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buckethead

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Hi guys,

after searching the internet for infos about power on/off thumps/plops without success, I wanted to ask what you guys think about it.

I'm working on a compact monitor controller (2 stereo in/2 stereo out) with a Muses 72323 volume chip. For buffering/debalancing/balancing/mono-summing I'm using opamps - everything works great so far. The problem is powering the circuit on and off. With speakers attached and powered on, powering on the controller results in a huuuuuuuge thump/plop, seriously enormous. Powering off the controller is not as bad, but still quite noisy. It's not that I'm surprised about it, I'm more in search of why this actually happens or rather if there's any way to minimize power up/down thump noises in amplifier circuits (besides something like delayed relay muting stuff etc.).

Any input would be highly appreciated :)

Thanks and best regards,
Mathias
 

john12ax7

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Check the outputs for DC, and also with a scope to see what transient voltage there is.

Some pop and thump is often inevitable unless you get really creative. Best practice is generally turn the monitors on last and off first.
 

Brian Roth

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Add a muting relay for the outputs with a time delay circuit driving the coil? I've seen equipment with a "slow on/fast off" muting relay circuit.

Bri
 

buckethead

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Check the outputs for DC, and also with a scope to see what transient voltage there is.

Some pop and thump is often inevitable unless you get really creative. Best practice is generally turn the monitors on last and off first.
There's just a tiny amount of DC offset on the outputs, nothing that makes me worry. The transient after turn on is huge, I'll check it again on the scope tomorrow for an exact number.

What kind of creative ways are coming to mind? I'm thinking about some "soft-start" circuitry for the power rails, but it feels like overengineering and probably won't change the thump too much.

Proper power-on sequence is mandatory, but I have these "ooooops-days" that force me into thinking about foolproofness. :)

Add a muting relay for the outputs with a time delay circuit driving the coil? I've seen equipment with a "slow on/fast off" muting relay circuit.

Bri
I'm working on a new PCB version at this very moment, fighting with limited PCB real estate and thinking "Do I seriously need two additional relays or is there any other way?". But I can't think of a better solution...
 

JohnRoberts

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Over the decades I thought about this a lot.

Customers don't like loud turn-on/off clicks, thumps, farts..... They are generally considered a sign of inferior design while there is no actual correlation other than that premium products design in benign turn on/off characteristics as a feature because customers expect it from premium designs. Actually customers expect it from all designs.
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For today's TMI I recall years ago when dbx designed their first active PA system processor. While not new to product design it was their first foray into sound reinforcement and when their line level box is connected to thousands of watts and PA speakers, turn on clicks and pops matter. I don't recall them actually harming any speakers with the spurious turn on/off noises, their customers were disappointed and they corrected it by their next generation design.
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There are multiple ways to deal with this... the premium cost is no object approach is to use a bypass relay that connects input to output when power supply is removed, and connects to the active output after a suitable settling time delay. The control circuity wants to turn the relay off immediately when power is removed and on slowly after power is applied.

The more cost effective approach (and I used lots of these for Peavey) is to use JFET shunts*** on the output with similar on slow/off fast characteristic. For the JFET to offer effective attenuation you want some series output resistance. In extreme click/thump mitigation you can cascade multiple series resistor and JFET shunts for more attenuation. The downside to this is that you get increased output impedance. Generally not an issue for line level SKUs. The shunt JFETs can be damaged by static and often diode clamps on output lines protect against some of that. Customers care far more about clicks and thumps than output impedance.

JR

**** in the wayback machine cheap tape recorders and the like used bipolar transistors for output mutes. These mute transistors were special design devices for low saturation voltage, and high reverse base-emitter zener voltage. I used bipolar transistor mutes in my old 4TR tape recorder for Peavey(AMR). I inherited that design that was pretty much cut and paste from a Dolby NR chip set app note.
 

buckethead

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Wow, very interesting read! I've been successfully using JFETs for momentary muting while channel switching in high gain guitar amps - although I remember having problems with measureable noise/distortion (probably bad mixed signal GND layout) + those JFETs .

For now I'm going the cost-no-object-route with relays
 

JohnRoberts

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For my 4x250 home brew power amp I built back in the 70s P1010072.JPG
I built my amp into an old Western Electric chassis (from the 30s). The WE PS chassis had a 3 position on-on-off power switch that allowed old school designs to turn on the tube heaters to warm up, before applying full power. I used the first on position to charge the amp power supply through a resistor to limit inrush current. I didn't energize the 120vac loudspeaker relays until the 2nd on position. The speakers disconnected immediately upon power off. Click and pop free...

JR
 
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