LA500 opto comp leveling amp BUILD THREAD 500/51X

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

Post by thestaylor »

kante1603 wrote:
thestaylor wrote:Hi friends.

I'm new to electronics so bare with me. I've built an LA-500 and the VU meter keeps drifting off after I calibrate it to 1.23 vac. On GR mode, the meter drifts above 0VU after I've calibrated it. And on VU mode the meter always sits between -10 & -7. After the most recent calibration, the meter changed after I swapped another module in my 500 rack.

Thanks in advance for your help,

Sam
Hello Sam,

hope I can help.
If the meter moves in vu mode when you apply some music to it and it does audible compression when in gr mode the circuit seems to work o.k.
There seem to be two things that were not done properly:

1.: Let the circuit heat up for a while,say 20 to 30 minutes otherwise the meter will drift off.Then do the zero dB adjustment to your prefered refernce level,normally +4dB.Seriously,when I switch on my rack all of my eight compressors' vu zero readings are too high in cold state.After some minutes they all fall back to zero spot on precisely!

2.:After having done the full adjustment for gr it seems that the balance procedure wasn't done correct.Please refer again to the procedure on page 1 in this thread.The goal is to get the needle sit in the same spot in both vu and gr mode when toogling between them.This will be somewhere on the scale,not precisely at zero or -5......
It's very important not to alter any of the other switches or the pots or your generator output level while doing it.

Best regards,

Udo.
Thanks Udo.

I re-calibrated after letting it warm up. The meter now sits better on 0VU in GR mode, but when off or in VU mode (no input signal), the meter still sits constant between -7 & -10. Also, when compressing and in GR mode, the meter doesn't show reduction past -10.

The circuit sounds fine when compressing.

I appreciate your help!

Sam

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

Post by Joe Malone »


Thanks Udo.

I re-calibrated after letting it warm up. The meter now sits better on 0VU in GR mode, but when off or in VU mode (no input signal), the meter still sits constant between -7 & -10. Also, when compressing and in GR mode, the meter doesn't show reduction past -10.

The circuit sounds fine when compressing.

I appreciate your help!

Sam
HI Sam
If the module changes when another module is added to the rack then it would indicate the power rails of the rack are moving under the load or that another module in the rack is linked to the comp link pin 6. So check the voltage of the power rails are correct with different combos of modules.

Does the meter go back to -20 when the rack has its power turned off? If so there sounds like there is a short between some components on the VU GR switch.

With 1.23vac on the output of the LA500 can you set 0VU on the meter in VU mode?

Otherwise email me some photos of wiring to pots and meter and ribbon cables while the module is together and also some clear IN FOCUS well lit large photos of the top and bottom of both PCB's removed from the module so I can check them over for problems for you.
Joe :-)
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thestaylor
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by thestaylor »

Joe Malone wrote:

HI Sam
If the module changes when another module is added to the rack then it would indicate the power rails of the rack are moving under the load or that another module in the rack is linked to the comp link pin 6. So check the voltage of the power rails are correct with different combos of modules.

Does the meter go back to -20 when the rack has its power turned off? If so there sounds like there is a short between some components on the VU GR switch.

With 1.23vac on the output of the LA500 can you set 0VU on the meter in VU mode?

Otherwise email me some photos of wiring to pots and meter and ribbon cables while the module is together and also some clear IN FOCUS well lit large photos of the top and bottom of both PCB's removed from the module so I can check them over for problems for you.
Thanks, Joe.

The meter does not go back to -20 when rack is off. Stays between -7 & -10. And I can set 0VU on the meter in VU mode.

Many thanks,

Sam

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

Post by Joe Malone »



The meter does not go back to -20 when rack is off. Stays between -7 & -10. And I can set 0VU on the meter in VU mode.

Many thanks,

Sam
To set the meter to is mechanical lowest point use the large black plastic screw on the front at the bottom of the meter to adjust.

Have you done this? Can you set -20 with this adjustment.

Meter must have its mechanical minimum set before alignment procedure is done.
Joe :-)
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Donny
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Donny »

Hi Joe! Just finished my LA500 build; what a blast! I could use some help though.

When calibrating, the Ground and Zero trimpots aren't changing resistance; the VU meter trimpot seems to be adjusting okay though.

Here's what I've done:

1. I have a mobile set up, so the best thing I was able to figure out to do was to run a test signal through my little Mackie board and feed that line out to the LA500. Seems to capture the signal just fine.
2. I have begun by tweaking the VU trim and it has moved the needle closer to where I'd like to have it set.
3. I have tested both the Ground and Zero trimpots with a DMM but only to see that no matter how many times I turn the screw, nothing changes. :?:

I have done builds before (plenty of opamps for fun; some CAPI stuff), but I admittedly haven't used a trimpot in any of the units I've put together. I have tested the trimpots in the PCB on the outside pins with a DMM but they just stay stagnant at 1k. Any thoughts on where I should start?

Thanks much!!
Donny

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

Post by Joe Malone »

Donny wrote:Hi Joe! Just finished my LA500 build; what a blast! I could use some help though.

When calibrating, the Ground and Zero trimpots aren't changing resistance; the VU meter trimpot seems to be adjusting okay though.

Here's what I've done:

1. I have a mobile set up, so the best thing I was able to figure out to do was to run a test signal through my little Mackie board and feed that line out to the LA500. Seems to capture the signal just fine.
2. I have begun by tweaking the VU trim and it has moved the needle closer to where I'd like to have it set.
3. I have tested both the Ground and Zero trimpots with a DMM but only to see that no matter how many times I turn the screw, nothing changes. :?:

I have done builds before (plenty of opamps for fun; some CAPI stuff), but I admittedly haven't used a trimpot in any of the units I've put together. I have tested the trimpots in the PCB on the outside pins with a DMM but they just stay stagnant at 1k. Any thoughts on where I should start?

Thanks much!!
Donny
The outside pins should stay at 1k as that is the ends of the trimpot and will never move. The centre wiper pin should come set to roughly half way when the trim pot is new. So 500R from the centre pin to each end pins except for the VU meter trim which has one outside leg shorted to the centre leg on the PCB. This should measure 500R when new.

To get the VU meter to 0 with +4dBM (1.23vac RMS) at the output of the LA500 the trim pot will usually have to be wound down to about 250ohm. These trim pots are 25 turns from end to end and will click when they reach the end of there range in either direction.

If you melt a corner on the trim pots accidentally with the soldering iron this is enough to jam the internal worm drive and stop the pot from working.

If you are still stuck email me some photos of the top and bottom of the MAC pcb from me to check over.
Joe :-)
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Donny
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Donny »

Joe Malone wrote: The outside pins should stay at 1k as that is the ends of the trimpot and will never move. The centre wiper pin should come set to roughly half way when the trim pot is new. So 500R from the centre pin to each end pins except for the VU meter trim which has one outside leg shorted to the centre leg on the PCB. This should measure 500R when new.

To get the VU meter to 0 with +4dBM (1.23vac RMS) at the output of the LA500 the trim pot will usually have to be wound down to about 250ohm. These trim pots are 25 turns from end to end and will click when they reach the end of there range in either direction.

If you melt a corner on the trim pots accidentally with the soldering iron this is enough to jam the internal worm drive and stop the pot from working.

If you are still stuck email me some photos of the top and bottom of the MAC pcb from me to check over.
:mrgreen: I feel silly because I just figured this out myself! Honestly, I think I misunderstood something I came across on this thread; I am thrilled to have a better understanding of trimpot resistors now.

I have successfully adjusted and am getting proper reading.

Thanks again, Joe! Cheers from Washington, DC!

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

Post by Jerome »

Hi all,

I've finished my 2 la500, was a very easy and excitement project! I'm a newbie in DIY, and speak french... (So if i could do it...) :D

I've just finished the calibrating process and one is just perfect, everything's ok, sounds very good, but the second one doesn't respond to the 3:1 ratio at all, slightly at 5:1 and seem normal at 10:1, i mean when i push the threshold in 3:1 ratio, nothings happen...

Could you please tell me in which direction i must Check? Seems completely identical to the other.

When i Calibrated the units (atempting to for the second one), i took some snapshots of the frequency response of the units in action, the harmonics are slightly different between the units, is this a normal behaviour or is this relates to my problem?
I attached the pics. Taken from 1khz at -9 dB fs.

Many thanks. Best regards, jerome.
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

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Jerome wrote:Hi all,

I've finished my 2 la500, was a very easy and excitement project! I'm a newbie in DIY, and speak french... (So if i could do it...) :D

I've just finished the calibrating process and one is just perfect, everything's ok, sounds very good, but the second one doesn't respond to the 3:1 ratio at all, slightly at 5:1 and seem normal at 10:1, i mean when i push the threshold in 3:1 ratio, nothings happen...

Could you please tell me in which direction i must Check? Seems completely identical to the other.

When i Calibrated the units (atempting to for the second one), i took some snapshots of the frequency response of the units in action, the harmonics are slightly different between the units, is this a normal behaviour or is this relates to my problem?
I attached the pics. Taken from 1khz at -9 dB fs.

Many thanks. Best regards, jerome.
Sounds like a solder short.

Is the level in the bypass(LA off) and when switched to GR or VU on with makeup & threshold at minimum the same?

Otherwise as always email me some large clear in focus well lit photos of the whole LA500 so I can see wiring and top and bottom of MAC and LA500 PCB's so I can check them over for you.
Joe :-)
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Jerome »

Hi Joe thanks for your response.

For the la 500 that is working i have a slight increase in power when engaged, and perfectly the same between gr and vu.
For -9dbfs i have -12,46 dbfs on bypass and -12,02 dbfs on vu and gr. Since i have this same lost of 3db in my chain with a purple audio comp, i think it's normal.

For the non working one, i have for this same -9dbfs sending a return of -12,53 dbfs bypassed, and -12,44dbfs on gain reduction and vu, seems normal here.

I will email you some photos of the non working unit.

Thank you very much for your help and your time. Best regards, jerome.

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

Post by Jerome »

And with the non working one, when the unit is set to sc flat, gr, 3:1, and cranking up the threshold, there's no audible compression, and the meter doesn't move but there's a big increase in harmonics.

Thank you. Jérôme.

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

Post by Jerome »

[Joe] You are sure the relay is clicking in and you are not in permanent
bypass? The makeup turns up the output level in GR or VU?

Yes sure. 10-1 ratio works well and yes the makeup turns up the output level in gr and vu


[Joe] Very strange as the ratios are only set on the MAC PCB. Have you tried
swapping ribbon cables to see if it is a faulty ribbon cable between the MAC
and LA500 PCB?

Yes I swapped and works well, no faulty ribbon cable between mac and la500

Could it be a component? And how to know which one ? After swapping the pcb
i checked the good la 500 and reheated many pads as well since it was worst
than the wrong unit, works well before and after (but with a peace of mind
!). So i think more and more at a bad component but don't know how to find
it...
[Joe] Check the MAC pcb has +16v @ pin 8 of the IC and -16v at the pin 4 of
the IC?

Don't know how to do that... Ok i'll assume it's with a multimeter... But
do i must power the unit in the rack to have volts in the circuit? Just dont want to mess up more...
And do i plug the ic ribbon câble with the transformer in?


So problem is ratio non working at 3/1 and 5/1 but works at 10/1 and the
problem is on the la 500 pcb solely. Any thoughts?

[joe] I will have a look at the photos and see if I can spot anything.

Thank you very much!!!

Thanks

Joe Malone

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

Post by Joe Malone »

Since you get unity gain the main LA500 PCB is working as is the audio path into and out of the MAC PCB.

The MAC PCB is the only PCB in control of the ratios.So for the ratios to be affected by the LA500 it would need to be something like one power rail not getting to the MAC PCB so the side chain isn't working properly and could explain the ratios not working properly.

Measuring the voltage on the MAC PCB from 0v to pin 8 of the opamp is 16v and pin 4 of the opamp is -16v would check this. You would need a extender cable to test this usually or solder 2 wires on to these pins so you can measure the volts outside the rack while the unit is powered.

If a power rail is missing on the MAC pcb it would usually be a solder joint or broken track usually next to a solder pad or a broken plated hole in a pad on the LA500 pcb if your testing is correct.
Joe :-)
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Jerome
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Post by Jerome »

I have +14.95 between pin 2 and pin 4 and -14.95 between pin 2 and 8 of the idc on the mac pcb

Thank you Joe

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

Post by Joe Malone »

Jerome wrote:I have +14.95 between pin 2 and pin 4 and -14.95 between pin 2 and 8 of the idc on the mac pcb

Thank you Joe
OK that is correct.
Best thing is going to be to align both comps with the good LA500 PCB and then set both comps to 3:1 and same setting except leave the threshold down on the fault one and link the 2 LA500's (make sure pin 6 on back of rack is linked) and see if the faulty comp under the control of the good comp compressors in 3:1.

The two comps may be a few dB out from each other compression wise. Just want to see if the faulty comp compressors under the control of the good comp as this may give a clue as this is making no sense at all.
Joe :-)
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