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LA502 Optical Compressor / Re: LA502 setup question / potential problem

« on: October 28, 2024, 12:10:44 PM »

Hi JPK, 

thanks for the clarification! I have redone the adjustment and get good values across all measurement points. Then it seems everything is ok. 

One remaining question would be the rather sensitive "PEAK" potentiometer. Is there a way to make the side chain less sensitive, so you can cover more poti travel before the compressor kicks in and make compressing percussive instruments with high attack e.g. Fender Rhodes, Clarinet more precise and more subtle (currently it is more like an "on/off" thing...)?

Best
Moritz

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LA502 Optical Compressor / LA502 setup question / potential problem

« on: October 25, 2024, 02:18:37 PM »

Hello,

I have built the first of two LA502s. Everything went smoothly (the manual is very nicely written and well done!). I have followed the setup guide meticulously but ran into a couple of problems or at least things that raise a question on my part. Maybe someone can help?

I insert a sine wave @ 1kHz and 1VAC into the unit. The 1 VAC were measured between Pin 2 of the Soundskulptor extender and and the 0V Pin on the PCB. It follows the input source nicely and I don't know which Pins on the extender I should use for measuring it otherwise. "Gain" and "Peak" potentiometers are fully ccw, all switches are left except for the bypass switch, which is center. Gain still fully cccw, I get 1,42VAC at the output. When I turn the gain all the way up, I get only 19,1VAC. The setup insists it should be at least 20VAC... All other setup steps work as described. The unit itself seems to function nicely, except for a couple of strange things:

• The "Peak" potentiometer reacts very sharply, i.e. a light compression is only achievable between 0 and may 20. This is dependent on the input level (of course) but still a bit odd as the device should handle line level easily, right?
• When Attack is set to 5ms, the LED meter reacts a lot (!) more compared to 10ms and 20ms, i.e. when 20ms is selected the GR LEDs will maybe hit 3dB reduction but when switching to 5ms attack, it will indicate 12dB gain reduction for a very short time. Is this to be expected?

I have scanned through the forum and followed most of the follow up questions for troubleshooting, especially from the troubleshooting guide (https://www.soundskulptor.com/forum/index.php?topic=19.0)

Send a good constant sine signal on input, like 1VAC measured between pins 2&3 of the input XLR. PEAK at 0. Connect the voltmeter black probe to 0V and trace the signal on:
--> I have measured 1VAC between In+ (Pin 10) of the extender and the 0V pin on the PCB (I don't have a suitable XLR cable right now).

R2 right pin: Should be at 0.7VACand follow the input voltage.

--> My measurement follows input voltage nicely but I get a reading of 1,4VAC (exactly double of the target)

REV1.4: R5 right pin should be at 0.5V. 

--> My measurement reads 0.97VAC (also exactly double of the target)

- TP1: should go above 10V when GAIN is at 100.

I measure 1V when fully ccw and 13,62 when gain is at 100

My input/output transformers seem ok:

Input transformer coil resistance:

1-5 in circuit: 2,13kOhm (target: 2 kOhm)

6-10 in circuit: 1,77kOhm (target: 2 kOhm)

Output transformer coil resistance:

1-3 in circuit: 4,9 Ohm (target: 5 Ohm)

6-8 in circuit: 4,9 Ohm (target: 5 Ohm)

9-12 in circuit: 11,3 Ohm (target: 12 Ohm)

13-16 in circuit: 11,6 Ohm (target: 12 Ohm)

Other test questions from various threads were:

• R17 is the correct one (brown, black, black, brown, brown)
• The output voltage across R17 (one probe on each side of R17) is 1,86VAC with gain fully ccw and 26VAC with gain at maximum
• The secondary DC resistance with the module unplugged between fingers 2 and 3 is 23 Ohm (target: 23 Ohm)
• I get 13,6VAC on TP1 against 0V Pin.

Especially the measurements for R2 and R5 (I get double the target voltage) lead me to believe there is a mistake. I have double checked every resistor on the board (pictures attached), they all seem fine. What could be problem and how could I troubleshoot/fix it?

Best

Moritz