LA500 opto comp leveling amp BUILD THREAD 500/51X

[Joe Malone]

Hi Joe,

just wanted to show my little progress of my Compressor-Rack for lead-vocals on my next musical.
Building was as easy as can be-again!No issues at all,amazing.....
I want to advise something concerning trimming the meters:When doing it please let the units heat up for some minutes,the meters will slightly drift otherwise.And repeat the procedure 2 or 3 times,you´ll have them spot-on then.Good idea to keep one eye closed when staring at the needle!
Here´s a pic of my "sixpack",and I think I´ll fill up the gap under the 2 blank panels early next year (Yes,I love these guys!).

Merry Christmas to you all and a happy and healthy new year,

Cheers,

Udo

P.S.:Good idea "sealing" the tiny transformer wires (that's hot glue,no?).

HI Udo
Thanks for the great review and photo (just added it to the JLM Audio facebook page here

Yes warming up for even 20 minutes before final cal is a good idea. Thanks for answering questions for others Udo

[Joe Malone]

Hi Joe and Guys,

I am in the process of building my LA500 but a little bug popped up. I have finished the Red Pcb and was looking over my solder points and notice that there is barley not even a hairs width between the one leg of R25 and C8. I can hardly see any read and when I check continuity between the solder blobs there is continuity. Is this true should there be continuity between a leg of R25 and a leg of C8? According the the schematic one leg of each goes to ground and the others are connected to an IC or some thing down the line but the other leg of each connects to that same line running over to something else. I just don't want anything to blow up when I first fire it up. it does seem like the gold contact of the two are touching but I should have checked it out before I soldered.

Thanks in advance,
-Scott

R25 and C8 join at there closest points by the track under the PCB which goes to +in. So continuity is normal.

[Joe Malone]

Thats what I thought, thanks.

Calibration went pretty easy. I measured the 1k tone out of my digital performer that goes out my 2408MKII interface and with the fader at 0 it measured 1.9VAC so I lower the fader to -3.8 to get the volts down to 1.23 on the MOTU2408MKII output. I then sent it into the LA500. It seemed that I had to turn the VU pot a bunch inorder for it to read 0. Everything else was pretty easy to align. I then ran a prerecorded sax track with no compression on it or eq into the LA500 and the GR meter setting was working great and moving around alot with the different ratio and moving the knobs but when i flip the switch to the VU meter setting it was hardly moving. I then pumped some commerical cd tracks throught the la500 and saw how the vu meter responded to that and made sure the highpass filters worked ok. I just have to check the output of the la500 when the 1k tone goes through it with nothing compressing and make sure i am getting 1.23vac out of it.
Anything else I should do? Gonna look at really calibrating my system output to make sure when the fade it at zero it puts out 1.23vac.


Thanks

Measuring 1.23vac at the output of the LA500 is the best point and adjusting the VU for 0dB as that is where the meter is monitoring from in VU mode.

With no compression and no makeup gain the input and output will both be close to 1.23vac depending on tolerance of several parts in the circuit so could be roughly +/-1dB different from in to out.

[Joe Malone]

Finished my pair of LA500 comps... very nice.


After reading this thread, here is how I calibrated. Is this the best way?

Using Nuendo, I created a sine wave file that goes to absolute zero (max amplitude). Fed that playback through my MOTU 24I/O. I used my multimeter, attached to the hot/cold pins until I got 1.23v A/C on it. This became my reference (which is substantially less than max absolute amplitude). Then, feeding that into the LA-500, I did the alignment procedure described in this thread.

Is this the best way to align them?

Nice photo

As I just now mentioned in the post above the 0dB on the VU is best set by measuring the output of the LA500 but at worst should only be +/-1dB different t the input level.

[Joe Malone]

I had emailed Joe with some questions regarding the LA500 kit I’m working on and he asked me to bring the discussion in here so here goes!

I’m a beginner and this is only my third or fourth DIY project. Having completed the kit, I put it into my 500 series rack and fed a 0dB, 1kHz tone into the LA500. I’m using Avid’s signal generator in pro tools, and a Saffire Pro24 DSP for my audio interface. I’ve got the LA500 set to the settings described in the calibration steps of the build thread. When I try to adjust the Meter 0VU trim pot to bring the meter to read 0 in the VU metering setting the trim pot seems to have no effect on the position of the needle in the meter. I’ve tried endlessly to adjust the trim pot with no change in needle position. I’m able to set and adjust the meter when it’s in “Meter GR” just fine, and the balance trim pot also seems to work when metering gain reduction. I’m not sure if this behavior is due to problems in my soldering or if the Meter 0VU trim pot could be a faulty component. Anyways, here are answers to questions that Joe asked in response to my original email along with pictures of the PCB’s + components.

Thanks in advanced for any help!

Q: Does the meter actually move down hard under -20 when you switch to
VU or is it just the same VU reading as in the off position? If it move down
hard when switched to VU you have some wires reversed to the meter.
A: It is the same VU reading as in the off position. If does not move past -20.

Q: Are you going to run this in 51X or 500 mode? You have the 2k7 on the MAC PCB so I presume 51X as this should be 1k8 for 500.
A: I have since clipped the 2k7 and will be using it in a 500 series rack.

Q: Can you hear the compressor working when listening to the output?
A: Yes, the compressor is able to attenuate the signal if I turn the threshold knob up.

Q: Does the makeup control add gain to the output?
A: Yes, the makeup control seems to be functioning normally as is able to add gain to the output.

OK if the GR meter mode work the VU adjust pot must work or not be open circuit or the GR meter mode would not work as well.
Wind the 20k pot down until it measures about 270ohm measured on its outside legs of the pot this will be very close to the full 25 turns.
Can you get the pot down to measure 270ohm?
Measure 1.23v on the output and see if you can see a reading on the VU Meter?

[kante1603]

Hello Joe,

the caps C3 & C4 (3uf3) on the hpf on the mac pcb are shown as bipolar types in the schematics.
My supplied caps are polarized,not bipolar.
Can I use them with minus connecting to 0v/ground?( because of normally being bipolar they aren't marked on the silkscreen).
Just to be save......


Thanks in advance,

best regards from munich,germany,

Udo.

Edit:Mail including pic sent.

[Joe Malone]

Hello Joe,

the caps C3 & C4 (3uf3) on the hpf on the mac pcb are shown as bipolar types in the schematics.
My supplied caps are polarized,not bipolar.
Can I use them with minus connecting to 0v/ground?( because of normally being bipolar they aren't marked on the silkscreen).
Just to be save......


Thanks in advance,

best regards from munich,germany,

Udo.

Edit:Mail including pic sent.

Yes latest mac kits come with two polarised 3u3 caps which can still go in either way around as there is no voltage across them in the MAC schematic.

Udo send me a photo when you have all 8 of your LA500 kits going in your rack

[kante1603]

My dynamics -"octopus"

image.jpg

Best,

Udo.

[Joe Malone]

Thanks Udo Cool a Octopus of LA500s

[drewallsbrook]

I have two LA500 units that i built. The build was easy enough except for a few missing parts. (Makeup gain knob nut and bolt)

I had the hardest time calibrating these units the first time so I ignored them for a while. I sat down to do it again today.

I have a few remaining issues.

On both units I was able to use VU Trim to 0 out the meter. However, I wasn't able to dial in the GR trim pot all the way up to 0db. It seemed to be stuck around -2.5 and then i just got clicks on the pot. I tried changing stuff around with the blend knob (middle trim pot) but that didn't do it. (i tried a lot of variations)

I was able to have the VU and GR both read -5db and that was representing accurately on one unit in Pro Tools and the other unit was -6db when both GR and VU read -5db. Both units were lined up between VU and GR at -5db but any other number and they weren't accurate according to Pro Tools meters.

I also have a 107hz hum on both units. It get's 3db louder when the VU lamp is on.

Any help?

Drew
www.thepeoplesmusic.me

[Joe Malone]

On both units I was able to use VU Trim to 0 out the meter. However, I wasn't able to dial in the GR trim pot all the way up to 0db. It seemed to be stuck around -2.5 and then i just got clicks on the pot. I tried changing stuff around with the blend knob (middle trim pot) but that didn't do it. (i tried a lot of variations)

If GR cannot zero with the threshold fully down then the VU trim has not been set properly to 0VU = +4dBM (1.23vac) at the output of the LA500. Or a wrong value resistor has been fitted tot he MAC PCB or there is a short pulling the power rails down.

I was able to have the VU and GR both read -5db and that was representing accurately on one unit in Pro Tools and the other unit was -6db when both GR and VU read -5db. Both units were lined up between VU and GR at -5db but any other number and they weren't accurate according to Pro Tools meters.

If you cannot zero GR there is no point going any further with alignment until you fixed the reason why.

I also have a 107hz hum on both units. It get's 3db louder when the VU lamp is on.

When the VU lamp is off there is a relay hard bypassing the balanced in to balanced out. So none of the LA500 circuit is in circuit so the hum cannot be from the LA500.
If the LA500 only hummed while switched in circuit then it would be due to the power rails from whatever rack and power supply you were using. Or a short in the LA500 dropping the power rails out of regulation.

[taliska]

Hi.

I'm finally getting around to building my LA500 (been busy with some other kits and stuff), and have noticed that I'm missing a single 5k1 resistor in the LA500 bundle. Not a problem as I'm about to put an order in with mouser for some other projects, but just wanted to check the correct specs for the replacement I buy (obviously want to match the original specification so that if/when I buy a second LA500 kit at a later date, I know they are as closely matched as possible).

So, what kind of replacement resistors should I get? (composition, tolerance, manufacturer, etc)

Looking forward to getting it going...cheers,

Kaz

[Joe Malone]

Hi.

I'm finally getting around to building my LA500 (been busy with some other kits and stuff), and have noticed that I'm missing a single 5k1 resistor in the LA500 bundle. Not a problem as I'm about to put an order in with mouser for some other projects, but just wanted to check the correct specs for the replacement I buy (obviously want to match the original specification so that if/when I buy a second LA500 kit at a later date, I know they are as closely matched as possible).

So, what kind of replacement resistors should I get? (composition, tolerance, manufacturer, etc)

Looking forward to getting it going...cheers,

Kaz

HI Kaz
Yes found out a few days ago that the latest batch of LA500 main pcb kit is missing one 5k1 resistor. Any 1/4 or 1/2watt Metal Film will work fine in there.

Any of the 3 Mouser numbers below are correct. Or I can ship some out to you today? Sorry again for the stuff up.

http://au.mouser.com/ProductDetail/Yage ... WLdo2cA%3d

http://au.mouser.com/ProductDetail/Vish ... MitDsTE%3d

http://au.mouser.com/ProductDetail/Vish ... o2FfPVU%3d

[taliska]

HI Kaz
Yes found out a few days ago that the latest batch of LA500 main pcb kit is missing one 5k1 resistor. Any 1/4 or 1/2watt Metal Film will work fine in there.

Any of the 3 Mouser numbers below are correct. Or I can ship some out to you today? Sorry again for the stuff up.

http://au.mouser.com/ProductDetail/Yage ... WLdo2cA%3d

http://au.mouser.com/ProductDetail/Vish ... MitDsTE%3d

http://au.mouser.com/ProductDetail/Vish ... o2FfPVU%3d

Great, thanks for that Joe, and no worries about the error...these things happen!

I'm putting in an order for a bunch of components for some studer 169k5 modules I'm assembling soon anyway, so it's easier for me to just order the extra resistor with that I think.

One final question: There are some hand-written numbers on the side of the two optos in the mac kit. Do I need to keep a record of them if I plan to get a second module for stereo linking at a later date or is it the just the transformer that matters?

Cheers,

Kaz

[Joe Malone]

HI Kaz
Yes found out a few days ago that the latest batch of LA500 main pcb kit is missing one 5k1 resistor. Any 1/4 or 1/2watt Metal Film will work fine in there.

Any of the 3 Mouser numbers below are correct. Or I can ship some out to you today? Sorry again for the stuff up.

http://au.mouser.com/ProductDetail/Yage ... WLdo2cA%3d

http://au.mouser.com/ProductDetail/Vish ... MitDsTE%3d

http://au.mouser.com/ProductDetail/Vish ... o2FfPVU%3d

Great, thanks for that Joe, and no worries about the error...these things happen!

I'm putting in an order for a bunch of components for some studer 169k5 modules I'm assembling soon anyway, so it's easier for me to just order the extra resistor with that I think.

One final question: There are some hand-written numbers on the side of the two optos in the mac kit. Do I need to keep a record of them if I plan to get a second module for stereo linking at a later date or is it the just the transformer that matters?

Cheers,

Kaz

No those numbers are relevant to the testing only at that time. To match a pair of LA500 it is best to order them together or let us know at the time so we can keep a match set for the unit you have for a month or so.

If ordering a second LA500 at a later date let us know in comments when ordering that you are trying to match a existing LA500. Also you need to let us know if you have a green or red output transformer.

You can remove a opto from your original LA500 comp U1 position to U2 in the new comp and split the new optos from the 2nd LA500 into both U1 positions as we will super match this new audio pair for you so you end up with a matched pair.