LA500A opto comp MAY 2021 BUILD THREAD 500/51X

Support for JLM Audio Kits

Moderator: Joe Malone

User avatar
Joe Malone
Site Admin
Posts: 1801
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Re: LA500A opto comp MAY 2021 BUILD THREAD 500/51X

Post by Joe Malone »

Dolbah wrote:
Sat Nov 06, 2021 8:30 am
I just built two of these units, and all seems fine. But, I am not able to get any reaction on the VU needle when turning the "Meter 0 VU" trim pot.
This applies to both units, I also tried to switch to GR and tested the GR pot, but still no reaction. I also tried putting the jumper over to "match" put I get nothing at all.
Threshold and all other functions react as expected and sound is flowing through.
  • I am sending 1000hz tone from my DAW, into the unit(s) and I have adjusted the input dB so that the LA output is showing exactly 1,23VAC, the needle is beyond 4dB on the VU.
  • All knobs counter clockwise
  • Ratio 3:1
  • Meter: VU
  • HPF: Flat

Any tips on this one?
First are you using the blue 0VU trimpot on SC PCB to set the +4dBu = 0VU? Some people mistakenly use the -20 mechanical set screw on the front of the meter which is only for setting the -20 point when the comp is off. If this set screw is moved it needs to be reset correctly before doing the calibration to the comp.

With 1.23vac on the LA500A output measured between pin 2 & pin 3 of the output XLR on first power up with no trimpots touched the VU meter will always read about -2VU and need to be turned up a couple of dB. If your VU meter reads higher usually you are reading the 1.23vac between pin 2 & pin 1 of the output XLR so really have 2.46vac.

In GR the meter will be off scale and need to be turned down several turns with Zero trimpot.
Joe :-)
JLM Audio
Capturing Audio without Injury

Dolbah
Posts: 2
Joined: Fri Nov 05, 2021 3:28 am

Re: LA500A opto comp MAY 2021 BUILD THREAD 500/51X

Post by Dolbah »

Joe Malone wrote:
Sat Nov 06, 2021 8:52 am
Dolbah wrote:
Sat Nov 06, 2021 8:30 am
I just built two of these units, and all seems fine. But, I am not able to get any reaction on the VU needle when turning the "Meter 0 VU" trim pot.
This applies to both units, I also tried to switch to GR and tested the GR pot, but still no reaction. I also tried putting the jumper over to "match" put I get nothing at all.
Threshold and all other functions react as expected and sound is flowing through.
  • I am sending 1000hz tone from my DAW, into the unit(s) and I have adjusted the input dB so that the LA output is showing exactly 1,23VAC, the needle is beyond 4dB on the VU.
  • All knobs counter clockwise
  • Ratio 3:1
  • Meter: VU
  • HPF: Flat

Any tips on this one?
First are you using the blue 0VU trimpot on SC PCB to set the +4dBu = 0VU? Some people mistakenly use the -20 mechanical set screw on the front of the meter which is only for setting the -20 point when the comp is off. If this set screw is moved it needs to be reset correctly before doing the calibration to the comp.

With 1.23vac on the LA500A output measured between pin 2 & pin 3 of the output XLR on first power up with no trimpots touched the VU meter will always read about -2VU and need to be turned up a couple of dB. If your VU meter reads higher usually you are reading the 1.23vac between pin 2 & pin 1 of the output XLR so really have 2.46vac.

In GR the meter will be off scale and need to be turned down several turns with Zero trimpot.


Yes, I'm on the Blue trimpots ;)

But, yes! I was using wrong pins, looks good now. Thanks!

kellyad33
Posts: 5
Joined: Sun Nov 28, 2021 2:27 pm

Re: LA500A opto comp MAY 2021 BUILD THREAD 500/51X

Post by kellyad33 »

Question about the POT board. When installing the IDC header one of the pics shows a linking wire to a hole from the center pin of the header (basically in the direct center of the board).

I only noticed in it in the picture, not instructions. Is this necessary? (maybe it's an older version?)

User avatar
Joe Malone
Site Admin
Posts: 1801
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Re: LA500A opto comp MAY 2021 BUILD THREAD 500/51X

Post by Joe Malone »

kellyad33 wrote:
Thu Dec 02, 2021 10:15 am
Question about the POT board. When installing the IDC header one of the pics shows a linking wire to a hole from the center pin of the header (basically in the direct center of the board).

I only noticed in it in the picture, not instructions. Is this necessary? (maybe it's an older version?)
That was only for the first batch. All pot PCBs since have the track on the pcb. You can actually see the track on the pcb going to the via pad.

No jumper is needed.
Joe :-)
JLM Audio
Capturing Audio without Injury

Post Reply