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I have some loud speakers connected to an amplifier. The problem is when I turn the preamp off, it sends a loud POP noise through the speakers. The turn off of the amp is delayed by 1 or 2 seconds, but that's not enough to kill the amp. So here's what I'm looking to achieve:

I'd like to slowly fade the speakers out as soon as as a remote wire goes LOW (it is 12V when preamp is on, but immediately becomes 0V when preamp turns off). And by slow I mean the whole thing happening is roughly half a second.

The speakers are pretty high powered. They draw 30 - 75 watts. So this circuit should withstand that.

If that's too difficult, I can try to implement this fade out function on the preamp outputs. It would be similar to a relay, except instead of a sudden off, I'd like to bring the voltage down to 0 gradually, so it gives the impression that audio is fading out.

Is this too difficult to achieve? Would a digital potentiometer connected in series to preamp signal, controlled by a Arduino, and then fed to amplifier be enough to achieve this? What are the repercussions? Can this be nicer implemented with capacitors? Is there a good way to switch the audio with a relay or something?

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  • \$\begingroup\$ Just to make clear, I'm not interested in a relay sudden switch off, but was asking for a gradual fade out. \$\endgroup\$ – darksky Mar 25 '17 at 5:49
  • \$\begingroup\$ OK something is missing here... If you turn off the pre-amp there should be nothing to fade... at the amp. Further, the pop is caused by the transient of the pre-amp shutting down, and is short, so by the time you fade out the amp it's too late. \$\endgroup\$ – Trevor_G Mar 25 '17 at 5:49
  • \$\begingroup\$ IS the preamp separate from the amplifier? Different power supply /box? \$\endgroup\$ – Trevor_G Mar 25 '17 at 5:50
  • \$\begingroup\$ Hah. I didn't think of that. Of course the amplifier and preamp are two separate boxes with their own power supplies. But it seems the preamp signal seems to last a little while after the audio source is shut down, then it stays silent and finally sends a lout POP noise after 2 or 3 seconds. \$\endgroup\$ – darksky Mar 25 '17 at 5:55
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    \$\begingroup\$ @Trevor, I think you're right. By the time remote wire is low, there is probably no preamp signal to fade, so I was just going down the wrong path here. Probably should just put simple relays before the amplifier. To get the fade effect, I'd have look more into my preamp. \$\endgroup\$ – darksky Mar 25 '17 at 6:05
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OK something is missing here... If you turn off the pre-amp there should be nothing to fade... at the amp. Further, the pop is caused by the transient of the pre-amp shutting down, and is short, so by the time you fade out the amp it's too late.

Since they are powered separately in different boxes I think you need to make the AMP mute the output when the pre-amp shows it is off. There may be a way in the pre-amp to fade out before it sets the shutdown line though.

I'd be interested to see if that line from the pre-amp really "immediately becomes 0V when preamp turns off" or if it actually fades out with the preamp power supply. It it's the latter you could use that to turn off the input more gracefully than a relay.

Perhaps with an LM13700 Dual Operational Transconductance Amplifier wired as a unity gain voltage controlled stereo volume control between the pre-amp and the amp. RC shown here would need to be adjusted for your output signal from the pre-amp

enter image description here

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  • \$\begingroup\$ Thanks. It's hard to imagine the duration of pulldown is more than a few micro seconds, so probably the fade wouldn't be audible. Anyway, I don't have a good way to measure it. \$\endgroup\$ – darksky Mar 25 '17 at 6:37
  • \$\begingroup\$ Yup you would need a scope, A scope would also help you identify EXACTLY when the pop happens in that sequence. \$\endgroup\$ – Trevor_G Mar 25 '17 at 6:40
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Why not use a digital pot, right before the PowerAmp (so you need 2), and immediately move the "wiper" to 100% attenuation?

We MUST keep the MCU logic0 and logic1 noise [digital LOW and HIGH levels] out of the DigiPot. This noise comes from internal clocking, memory fetches/writes, IO activity, etc which cause 0.2 to 0.5 volt spikes on MCU GND and on MCU VDD.

If MCU outputs directly connect to the DigiPot, that digital noise MUST have a return path. Even if there is LogicGND in the DigiPot, digital trash will cross into the resistive-network because the analog switches need to digitally controlled.

You will end up with MCU_clock_rate buzzing and hissing in your audio, and you PowerAmplifier will attempt to amplify that trash.

How to avoid this? SLOW DOWN the digital edges, with a dedicated RC LowPass Filter, with the Capacitor tied to the MCU gnd or gnd plane. Then run a high value resistor (this is a CMOS DigiPot, right? with no steady state currents other than 1nanoAmp leakages, so even 1MegOhm would suffice except the edge into 1pF would be 1uSec) over to the DigiPot region, and include a 2nd Capacitor to absorb most of what trash comes from the first Filter; this trash may be nothing more than a noise difference between the MCU gnd and the Audio gnd, but we don't care about the source; we just don't want any trash entering the DigiPot logic pins or its VDD.

The DigiPot has its own private capacitance from Logic Inputs to its GND, because ESD structures/diodes/etc have 3pf to 10pF capacitance. Hence I use 10,000 pF in the MCU trash filters; little trash actually gets into the DigiPot.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I was worried when attenuation is at 0%, the digital pot might be a noisy closed circuit. I'm just imagining it might distort the signal when its supposed to be just a wire. Also I was a little worried about it overheating. \$\endgroup\$ – darksky Mar 25 '17 at 3:26
  • \$\begingroup\$ The digipot can be "noisy" if the MCU's clocking noise enters the Audio line. \$\endgroup\$ – analogsystemsrf Mar 25 '17 at 4:40
  • \$\begingroup\$ Interesting. So you send the Gain Set from MCU through an RC low pass filter, and then to another low pass filter again through the audio ground? I'm just trying to understand what this is doing. \$\endgroup\$ – darksky Mar 25 '17 at 5:07
  • \$\begingroup\$ By setting the DigiPot to 100% output, you can view it as a resistive-load to GND, not actually performing any attenuation. This switch, of course, need not be a DigiPot; you can use 2 analog multiplexors, one ON during AudioListening and then OFF when PowerOFF be selected; the other mux connects from AudioIn to gnd, with this switch/mux being OFF during AudioListening and then ON when PowerOFF be selected. Whether you use a DigiPot or dual-analog-mux, you will have some broadband thermal noise when listening to music. 60_Ohms and 1MHz bandwidth will yield 1nV*sqrt(1Mhz) = 1microVolt RMS. \$\endgroup\$ – analogsystemsrf Mar 26 '17 at 3:24
  • \$\begingroup\$ Thanks for the explanation. So as I understand, softening out the digital edges going into the digipot's control pin is aimed at eliminating high frequency noise from MCU showing up in Vout. I wonder if the control pin, or what you call "Gain Set", is something MCU just sets and forgets, or it's something that has be constantly updated -- because in the duration of audio being on, I was looking to keep the wiper at a constant value. \$\endgroup\$ – darksky Mar 26 '17 at 4:39

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