LM393 comparator misbehaving and causing issues

Question

I've been stuck on (what seems to be) a very unusual issue with a little audio circuit I’m designing. I wonder if anyone here can quickly highlight the flaws of my design and/or offer an insight into what’s going on.

So, the issue is as follows. The circuit pictured below is a simple pair of comparators (LM393) indicating a 'healthy' signal level going into the two respective stereo inputs of an audio mixer. The LEDs illuminate and indicate signal as expected. The issue arises with the second channel, using comparator U5.2. When there is a strong enough signal to illuminate the LED it couples what I would call a 'breakthrough' noise onto my mix bus, which is very unpleasant at high gain levels (the noise is post-channel fader so cannot be faded out). For example, if the music had a strong drum beat each transient would couple a pop/click onto the mix bus.

I cannot work out why this is happening. In an attempt to fault find I wired the IC to swap the two comparators inside the package across the two channels and the fault moves (keeping all external components the same per channel), i.e. the top channel bleeds through distortion and the bottom channel is now clean. This left me very confused as it almost seems like any intrinsic flaws in the circuit are only bad enough to cause the second half of the comparator to misbehave. I also swapped in different manufacturers ICs and this made no difference.

I have not been able to fully understand the mechanism that couples these breakthroughs to the mix bus with my simple test equipment, so I am asking here in a hope that someone more intelligent than I can help.

Many thanks.

EDIT: I'm adding my PCB layouts below. In both cases the side of the comparator which has the longest trace to the LEDs current limiting resistor is the one at fault (R42, R57 / R15, R21). I will try to reduce the trace length and see if that makes a difference.

Answer 1

The output of the comparator will swing many volts with a distorted version of the audio input signal. It will have very fast rise times that can easily couple though stray capacitance to other parts of the circuit.

I would suggest putting a capacitor of a few microfarads from the output of the comparator to ground. A small resistor (eg 47 ohm) in series with the comparator output will limit the current when discharging the capacitor.

That will do two things;

1. When triggered the comparator will discharge the capacitor via its open-collector output and stretch the signal so that it will light the led brighter than the very small duty cycle that you currently have.

2. The output signal will now change slowly and not couple into the audio output signal.

I use an almost identical circuit to turn on my power amplifier in a house audio distribution system for the last ten years.