LM3875 speaker pop when powered on

[JDawg]

I built an LM3875 kit from audiosector and its sounds really good so far. When I turn it on I hear a pop sound from my speakers. Is this normal? Is there a way to remove this popping noise during power up?

[AndrewT]

I don't think the 3875 has a mute facility built into the chip.

You can add an output relay to isolate the speaker from the amp at switch on switch off, but this would completely defeat the Sound Quality philosophy followed by the designer/Manufacturer.

[Peter Daniel]

There must be something wrong with your setup, wiring or grounding as normally that circuit does not produce any pop at turn on or off.

How is it with no source connected?

[JDawg]

It only makes a pop when I turn it on. I'm not sure if it does it on both channels or just one of them so that's something I'll have to check later tonight.

It looks like everything is connected properly otherwise I don't think I would be getting good sound out of it. I'm using the star grounding system with the ground from the mains and two amp boards connected to a single point on the case. I did add the preamp buffer circuit found on the Decibel Dugeon website and I have a seperate power supply board to power the two buffer circuits. Would I have to connect the ground from the two buffer circuits and its power supply board to the same ground point as everything else?

[mikje]

Just a guess... could it be the on/off switch? Dirty maybe?
Just my 2 cents.
Mike

[JDawg]

I just checked for the popping noise again and it comes out of both speakers. I don't think its the switch because if I turn it on, then off, and on again after a few seconds there's no pop from the speakers.

[nycavsr2000]

Originally Posted by JDawg

I just checked for the popping noise again and it comes out of both speakers. I don't think its the switch because if I turn it on, then off, and on again after a few seconds there's no pop from the speakers.

I have the same issue as you, but it doesn't happen on every switch on or switch off. I'm using some pretty expensive Nikkai switches which are way overrated for the application. Doesn't seem to hurt the speakers, but one thing I haven't tried is placing a small ceramic cap across the switch.

Anand.

[audiophilia]

The LM3875 has 10mV of input offset. Multiply this times your configured gain and you may have quite a bit of output offset. This will cause an audible transient at power on as the output stage suddenly jumps to the offset voltage. This is a common problem in higher power amplifiers with no soft start circuitry. You can measure the output offset if you are so inclined by grounding the audio input, turning on the amplifier, and looking at the DC level of the output. My guess is it's a pretty large offset, which is definielty audible during turn on.

Another common issue is that the external caps are not charged at power on. If the amplifiers output stage is enabled while the caps are still charging, the output will swing around and create a pop sound (or some other sound...). JDawgs comment seems to point to this being a problem. You can test by turning it on for a while, then off and right back on. Then try turning it on for a while, then off for a while, then back on. If it clicks when you wait a while to turn it back on but not when you do it quickly, this would point to the cap charging issue.

Hope this helps... I couldn't advise you on any solutions unless I saw a schematic.

[JDawg]

I actually tried turning it on and then off and then doing the same thing again but waiting a while before turning it on again. If I did it right away then the pop isn't there. If I wait a while to turn it on then I get the noise. So I think you're probably right about the capacitors. Its not a loud pop or anything but is there a way to fix this?

[tauro0221]

Hi,
Try to use an inrush current limiter like the GE CL30 between the rectifiers and the filter capacitors. Add it to both negative/positive voltages. This will slow down the inrush current in the power up.