LA500 opto comp leveling amp BUILD THREAD 500/51X

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wthrelfall
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by wthrelfall »

Joe Malone wrote:
wthrelfall wrote:Hi Joe.
So I've just got around to checking that the 2 units are tracking correctly when linked. I decided to re-calibrate first, but came across a strange problem - I'm sending a 1khz tone through my desk, meters are hitting 0db on the desk, patching the compressors to the master bus. I'm unable to get the meters to hit 0VU on the compressors, the pots will only let me get to -2db on both units. So I just increase the signal by 2db so that I can continue calibrating and check the link. Now I have another strange issue.. with the 2 units calibrated exactly (with 5db GR/VU showing and balanced as described in the instructions), it seems that to achieve the same amount of GR, one unit must have the threshold turned up much higher. I've attached a photo showing the 2 units, unlinked, attenuating the same signal by 10db to show the pot positions. When i then link the units, the units jump further apart, ie. the one compressing less compresses even less, the other compresses even more. I've attached another photo showing the difference in GR between the units when linked (right hand unit is the master with the new linking pot).
any thoughts?
OK is the master buss insert fully balanced +4dBM or unbalanced -10. As it should be easy to get meters to 0VU = +4dBM if balanced.

The difference in threshold pots could be down to one threshold pot being linear and the other being log as some kits originally shipped with 100kB linear pots. All pull switch pots are 200k log and need both the front and back half of the pot paralleled to make a 100k log pot.

There is no master compressor when linking old style compressors. You must adjust both comps for the roughly same amount of compression on music before linking the 2 devices. With 1k tone into both comps and 5dB Gain reduction the meters should only deflect away from each other by a 1 or 2 db on each meter when linked which is fine and will not show on music signal. If error is greater first check mechanical -20 point on both meters is set the same with meter front adjustment before calibration then check calibration is not out of adjustment if both are ok then one of the optos may have settled to far out of range.

Since the release of the pull link switch version of the LA500 kit at the end of January this year 2012 we have started adding a 3 digital number to the side of the opto cells when we test them to make stereo pair matching better than it really needs to be. Even if the second unit is ordered at a later date with the 3 digit number we can match units.

Joe, thanks for your response.
Yes, it's an unbalanced insert point, my bad!
so with the compressors properly calibrated to 0vu direct from the main outs of the mixer (for calibration purposes), with 5db GR each and then linked, the comp with the pull switch jumps to -3.5db, the other to -8db. the mechanical -20 points on the meters are the same (can you explain how to do calibrate?).

no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

OK you should have about 4v on the outside terminals of the zero trim pot? As there is only a 2k7 resistor in series with the pot and the 16v rail.


OK the threshold 100k log pot only gives me 26k on ohmeter when leads placed across outside terminals of the pot, regardless of any/all switch settings. I desolder the red wire and get 100k. I did a AC and DC voltage check on the same outside terminals and it's negligible. I think this is what you were asking me to do.

But if I disconnect the mac pcb from the opto comp pcb, I get 96k ohms across the pot. It goes from 26k to 24k when I disconnect the transformer so I guess the issue is in the opto comp pcb?

voltage is negligible on all 3 trim pots. I have tried using the ohmmeter and adjusting the resistance but don't know how far to turn it. the trimmers are 1k and 20k ohms. Is the proper adjustment to turn until it's at it's original spec value? I'm researching this at the moment but still unclear.

How did you set +4dBM (1.23v rms) on output XLR to be 0VU on meter? If this is set wrong (to high level) you will only get a small movement on the GR meter from the zero pot. Also are you using the full 20 turns on the GR zero meter pot ?
All I did was turn the makeup pot CW about half way. Am I using full 20 turns on GR zero meter pot? The makeup log pot appears to have 40 steps, I go all the way CW and CCW and it doesn't move much, GR switch on. When I turn the threshold pot CW the meter needle gets more negative when GR switched on.


Sorry to be such a novice
I'll buy another once I figure this one out.

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Joe Malone
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

Joe, thanks for your response.
Yes, it's an unbalanced insert point, my bad!
so with the compressors properly calibrated to 0vu direct from the main outs of the mixer (for calibration purposes), with 5db GR each and then linked, the comp with the pull switch jumps to -3.5db, the other to -8db. the mechanical -20 points on the meters are the same (can you explain how to do calibrate?).
While that is in spec for old vintage comps linked (with there +/-3dB match) it is out more than the usual optocell range.

Does the GR difference = VU meter difference when switching between GR and VU ?
Can you see the difference on a real music signal when running in the same mono signal into both channels ?
If the -20 mechanical points are both the same and you followed the alignment on the MAC schematic and trimmed to exact meter readings then the differences will be in the opto match between the 2 comps.
If the optocells internal matching is out the easiest thing to do is swap one meter optocell (closest to trim pots) in one LA500 unit with one audio optocell in the other LA500 unit and realign to see if they are a better match.

A good trick (I learned in my telecommunications days) when aligning it is alway good to use one eye only (close the other eye) as it helps with parallax error and force you to move your head to direct in front of each meter.

Let me know what you find.
Joe :-)
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Joe Malone
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

no2 wrote:
OK you should have about 4v on the outside terminals of the zero trim pot? As there is only a 2k7 resistor in series with the pot and the 16v rail.


OK the threshold 100k log pot only gives me 26k on ohmeter when leads placed across outside terminals of the pot, regardless of any/all switch settings. I desolder the red wire and get 100k. I did a AC and DC voltage check on the same outside terminals and it's negligible. I think this is what you were asking me to do.

But if I disconnect the mac pcb from the opto comp pcb, I get 96k ohms across the pot. It goes from 26k to 24k when I disconnect the transformer so I guess the issue is in the opto comp pcb?

voltage is negligible on all 3 trim pots. I have tried using the ohmmeter and adjusting the resistance but don't know how far to turn it. the trimmers are 1k and 20k ohms. Is the proper adjustment to turn until it's at it's original spec value? I'm researching this at the moment but still unclear.

How did you set +4dBM (1.23v rms) on output XLR to be 0VU on meter? If this is set wrong (to high level) you will only get a small movement on the GR meter from the zero pot. Also are you using the full 20 turns on the GR zero meter pot ?
All I did was turn the makeup pot CW about half way. Am I using full 20 turns on GR zero meter pot? The makeup log pot appears to have 40 steps, I go all the way CW and CCW and it doesn't move much, GR switch on. When I turn the threshold pot CW the meter needle gets more negative when GR switched on.

Sorry to be such a novice
I'll buy another once I figure this one out.
First you need to measure +4dBM or 1.23vac across the output XLR pin 2 to pin 3 of the lunchbox or 500 rack and then adjust the Meter 0VU trim pot so the meter with meter switched to VU reads 0VU with the VU trim pot. If this is not done properly you will not be able to get 0 in GR mode.

If you cannot work out how to do the above then while the LA500 is off set the Meter 0VU trimpot to read roughly 1k as shown below. I think once you get this Meter 0VU trim pot correct you will be able to set zero GR and balance.

Do not measure across the threshold pot in ohms but across the outside solder pads of the zero set trim pot with meter on dc volts with the LA500 on should be about 4v as shown below?

Image

Let me know how you get on.
Joe :-)
JLM Audio
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no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

OK adjusting the meter 0vu trim pot has made the makeup gain pot read 0 vu on the analog meter when further CCW than before. So that works.

the lunchbox XLR pin reading is 0.4vac when nothing connected and is 0 vac when la500 is connected.
I think I broke the ZERO GR trim pot because I turned it all the way clockwise and it makes a click.
Also I get no voltage reading from it and no matter how far I turn it in either direction I get the same ohm reading.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

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no2 wrote:OK adjusting the meter 0vu trim pot has made the makeup gain pot read 0 vu on the analog meter when further CCW than before. So that works.[
Ok did you set the pot to 1kohm roughly?
the lunchbox XLR pin reading is 0.4vac when nothing connected and is 0 vac when la500 is connected.
You have to send a 1kHz tone into the input the LA500 plugged into the lunchbox and turned on and adjust the output with the makeup until you get 1.23vac. Make sure you are on AC volts on your multimeter. Follow the MAC schematic PDF alignment procedure step by step.
I think I broke the ZERO GR trim pot because I turned it all the way clockwise and it makes a click.
The pot is not broken. They just click so you know you have reached the end of the 25 turns in one direction. Just wind the pot back the other direction.
Also I get no voltage reading from it and no matter how far I turn it in either direction I get the same ohm reading.
Is the LA500 plugged into the lunchbox rack and power turned on? Make sure you multimeter is on dc volts?
Joe :-)
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no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

OK it's real close. But I cannot get 1.23 vac. I get 2.1 vac at all times on the XLR output.

And I get 3vdc on the zero GR trim pot.

However, I did get both GR and VU to have the exact same readings on the analog meter. So both read 0vu when the tone is on and both thresh and makeup pots are fully CCW.
Also I can switch back and forth and both will read -5dB on the analog meter. Does this mean I am done or do I have to get 1.23 vac and 4vdc readings still?

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

no2 wrote:OK it's real close. But I cannot get 1.23 vac. I get 2.1 vac at all times on the XLR output.
Ok something wrong in the measuring there. So may be easier to plug the LA500 output into your A/D. If you know the input level your A/D clip or hits 0DFS is +18dBM for example. Then when the software meter shows -14dBFS this will equal +4dBM. But the exact level 0VU should be for you system depends on if they are using K20, K14 etc of headroom.
And I get 3vdc on the zero GR trim pot.
Cool perfect.
However, I did get both GR and VU to have the exact same readings on the analog meter. So both read 0vu when the tone is on and both thresh and makeup pots are fully CCW.
Also I can switch back and forth and both will read -5dB on the analog meter. Does this mean I am done or do I have to get 1.23 vac and 4vdc readings still?
Yes the comp is aligned now and should work fine. Your 0VU will not probably be exactly +4dBM but that is ok.
Joe :-)
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no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

Ok something wrong in the measuring there. So may be easier to plug the LA500 output into your A/D. If you know the input level your A/D clip or hits 0DFS is +18dBM for example. Then when the software meter shows -14dBFS this will equal +4dBM. But the exact level 0VU should be for you system depends on if they are using K20, K14 etc of headroom.
I'm getting a reading of 2.6 vac now. Don't know how.

I have the API A2D and the highest I can get that tone generator to hit is -9dBFS on the A2D ADC. That is clipping on my interface software meter (focusrite scarlett 8i6). So the software tone generator produces a signal that goes from the focusrite DAC to la500 to A2D ADC back to interface via SPDIF. Can you say what to do with this system? I want to get it done right.

A2D specs say:
"calibrated for XLR outputs, 0VU=+4dBu"
output clipping level: XLR: better than +28dBm
I don't know if I have K20 or K14. How do I find out?


I'm also getting a little pop that I think goes away when I increase the latency in my interface? Do you know anything about that?

kante1603
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by kante1603 »

Hi Joe,

just a quick question on the schemo in the first post of this thread (LA500 PCB).
The nice yellow connection signs do refer to the card edge connector,no?
If so:Shouldn´t the link pin be #6 instead of #5 (#5 is used for audio gnd on the connector)?
Also the psu 0V at the bottom of the page says "pin 9",which is right for the IDC port .If the sign means the card edge wouldn´t it be better to change it to #13?
Just a thought...

Oh,by the way-have received my pair yesterday and have even built them immediately.Lot of fun doing them,will start calibrating and testing soon (have to hook up a test plug first because I don´t have a rack yet).Will report back then.

Thx Joe,have a nice weekend,

Udo.

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Joe Malone
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

kante1603 wrote:Hi Joe,

just a quick question on the schemo in the first post of this thread (LA500 PCB).
The nice yellow connection signs do refer to the card edge connector,no?
If so:Shouldn´t the link pin be #6 instead of #5 (#5 is used for audio gnd on the connector)?
Also the psu 0V at the bottom of the page says "pin 9",which is right for the IDC port .If the sign means the card edge wouldn´t it be better to change it to #13?
Just a thought...

Oh,by the way-have received my pair yesterday and have even built them immediately.Lot of fun doing them,will start calibrating and testing soon (have to hook up a test plug first because I don´t have a rack yet).Will report back then.

Thx Joe,have a nice weekend,

Udo.
LA500 schematic typos errors fixed.

LA500 PCB schematic
http://www.jlmaudio.com/LA500/LA500%20S ... 120304.pdf
Joe :-)
JLM Audio
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no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

OK I tried my other interface and I get 1.28vac on the output TS cable. I plug that in to DAC and I get -18dBFS on the DAC. So 0 VU = -18dBFS as it should .
On the la500, with both threshold and makeup pots fully CCW the VU meter reads
-20 dB with no tone and it reads 0db with tone.
with both pots fully CCW the GR meter reads 0 with or without tone.

with tone on I give -5dB of compression to GR meter and VU meter reads the same. The makeup gain pot doesn't affect the GR meter it only scales from -20 to 0.
at -5db on the VU meter I can apply makeup gain and send it up past +4dB, scaling from -20 dB to +4dB.

Everything works right except my damn focusrite interface!!!?????

when i switch to the focusrite what was 0 db on the la500 is >+4dB now and what was -18dbFS on the ADC is now -9dBFS. piece of crap.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

no2 wrote:OK I tried my other interface and I get 1.28vac on the output TS cable. I plug that in to DAC and I get -18dBFS on the DAC. So 0 VU = -18dBFS as it should .
On the la500, with both threshold and makeup pots fully CCW the VU meter reads
-20 dB with no tone and it reads 0db with tone.
with both pots fully CCW the GR meter reads 0 with or without tone.

with tone on I give -5dB of compression to GR meter and VU meter reads the same. The makeup gain pot doesn't affect the GR meter it only scales from -20 to 0.
at -5db on the VU meter I can apply makeup gain and send it up past +4dB, scaling from -20 dB to +4dB.

Everything works right except my damn focusrite interface!!!?????

when i switch to the focusrite what was 0 db on the la500 is >+4dB now and what was -18dbFS on the ADC is now -9dBFS. piece of crap.
The 1.23vac should be between pin 2 and 3 off a XLR and tip and ring of a TRS lead. A TS lead would be unbalanced into the LA500 and if the pin 3 of the lunchbox input is not shorted to pin 1 the level will be 6dB low. I presume the focusrite is fully balanced in and out. If the LA500 was adjust to read 0VU instead of +4dBM from the focusrite that should make the 0VU = roughly -14dBFS which is K14 standard. Just depends on which interface you use more as to want to set the LA500 too.

But since you have it aligned I would leave it as it is as once you start using the LA500 on music you will not care what the actual in and out levels are anyway as it has a quiet enough noise floor to handle a huge range of levels without a problem.
Joe :-)
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no2
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by no2 »

Alright, thanks so much Joe!!!

I'll definitely recommend this experience to other audio nerds I run into.

It sounds great too.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Joe Malone »

no2 wrote:Alright, thanks so much Joe!!!

I'll definitely recommend this experience to other audio nerds I run into.

It sounds great too.
8) :D
Joe :-)
JLM Audio
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