Sound Skulptor forums

Build help and troubleshooting => LA502T Tube Optical Compressor => Topic started by: Perihelion on September 30, 2026, 10:34:40 PM

Title: Release time, and a couple other oddities
Post by: Perihelion on September 30, 2026, 10:34:40 PM

Heya,

Just assembled and calibrated my 502T and have a couple questions about things that may or may not be an issue. All test points checked out good, and there were no meter calibration issues. I did the D1-D4 shorting mod and the r16 cap mod.



Quirky thing 1:

The release sounds fast, really fast, like all-buttons-in 1176 fast. The GR meter also shows this. It doesn't exhibit a logarithmic release like you'd think an opto comp would. I can link a video in a follow-up post.



Quirky thing 2:

Switching to 5ms attack from 10ms or 20ms adds about 3-4 dB of gain reduction on a constant 1kHz sine wave input. I understand a faster attack would do more GR on a music signal, but I've never seen that effect on a sine wave.



Quirky thing 3:

A 1.000v 1kHz sine wave at the input comes out at 0.924v on the output, compressor engaged with no GR, no output gain. Bypassing the compressor restores the output to 1.000v.



Quirky thing 4:

There is an audible 14kHz whine coming from the unit itself when powered on. With the cover off I can hear it about a half meter away. I didn't want to get my ear too close but it sounds like it's coming from the center of the board around the tube or opamps.



The unit otherwise operates like it should. All of the pots and switches do what they're supposed to do.



All of this kinda adds up to something shorted in the side chain or maybe a bad U7 or U8, but if you have any advice as to where to look first I'd definitely appreciate it. Thanks!

Title: Re: Release time, and a couple other oddities
Post by: Perihelion on October 01, 2026, 05:07:51 AM

Here is the 502 on a snare channel: 

https://youtube.com/shorts/raAIkmTgPdc?is=uK2wjD3vC5Dufp2W