[RuudNL]

I have recently built two LA502 opto compressors.
As such they work great, I even managed to get the stereo tracking within 0,5 dB over a range of 20 dB compression.
So far, so good.
Recently I did some measurements. The audio chain measures completely 'flat', so no problem here.
But...when I feed a 1 KHz. signal into the input, dial in 10 dB gain reduction and change the frequency to 10 KHz (same level of course!), I notice that the gain reduction is much lower than the 10 dB I had at 1 KHz. (10:1, 10 mS.)
At 10 KHz. the gain reduction is 5,5 dB now, at 20 KHz only 2,5 dB.
So, the higher the frequency, the less signal is produced by the side chain, it seems.
It is questionable if this is a 'problem' in reality, because most of the energy is usually in the lower frequencies, but it surprised me.
I didn't calculate it, but I don't think the combination C14 (470 pF) and R20 (470 ohm) is responsible for this.
Is it the behaviour of the rectifier? Both compressors do exactly the same thing.
Just curious!

[JPK]

You are correct, this effect is caused by C14.
It mimics the limited bandwidth of the tube sidechain of the LA2A.
It is possible to eliminate/reduce this effect by replacing C14 by a 100pF or 220pF instead of 470pF.

[RuudNL]

Thanks, I will try that.
(Although I don't think it will seriously affect the audible result.)

[Erik L]

Thanks for this information!
I changed to 100pf and prefer how the compressor now grabs cymbals for instance.