LA500 opto comp leveling amp BUILD THREAD 500/51X

[Joe Malone]

Hi Joe, my name is Greg.

I've just built my first LA500 and am experiencing a small problem. Like previous posters my comp works but appears to be suffering from the side chain oscillation problem. Voltage on audio out @ 1kHz is 1.23VAC. GR meter shows about -2dB (I'll let it warm up next time I re calibrate). If I have the input terminated with a muted mic pre, the comp ratio @ 3:1 and I wind up the threshold knob to about 2 o'clock the GR meter moves back to -20dB, then as I increase the threshold knob all the way clockwise the GR meter comes back to -10dB. This is only made worse with the two higher ratio settings and isn't affected at all by the side chain filter freq switch. During this process I can un mute the mic pre and monitor the audio going through the comp with headphones. Apart from the obvious attenuation, everything sounds fine.

As per your earlier advice, I've made sure the 3 wires to the threshold pot are twisted tightly and are as short as possible. I've been over both boards with a magnifying glass and cleaned them with Fluxclene.

How would you suggest I proceed from here?

First make sure you have C8 47pF fitted on the LA500 pcb under the JLM99v opamp.
Second make sure you have C2 100pF fitted on the MAC pcb.
If oscillation is still happening making the meter in GR mode show compression without anything plugged into the input of the LA500 when the threshold is turned up.
Add a 100pF cap at the threshold pot between the centre wiper leg and the 0v ground leg and all oscillation should stop.

[scissor]

...Like previous posters my comp works but appears to be suffering from the side chain oscillation problem....How would you suggest I proceed from here?

First make sure you have C8 47pF fitted on the LA500 pcb under the JLM99v opamp.
Second make sure you have C2 100pF fitted on the MAC pcb.
If oscillation is still happening making the meter in GR mode show compression without anything plugged into the input of the LA500 when the threshold is turned up.
Add a 100pF cap at the threshold pot between the centre wiper leg and the 0v ground leg and all oscillation should stop.

Thanks Joe, first thing I did was add the 100pF to the threshold pot like you suggested. That almost fixed it. So then I replaced C8 on the LA500 board and C2 on the MAC. It looks like C2 on the MAC had a fractured leg all along. Now with that solved the threshold is working nicely but I must have bumped something along the way because the circuit is now very noisy and makes a "Pop" when I switch (the meter switch) between VU / Off / GR.

I've downloaded the schematics and a tech friend is going to try and help me find my error with his scope. Do you have any clues as to what we might look for?

[Joe Malone]

...Like previous posters my comp works but appears to be suffering from the side chain oscillation problem....How would you suggest I proceed from here?

First make sure you have C8 47pF fitted on the LA500 pcb under the JLM99v opamp.
Second make sure you have C2 100pF fitted on the MAC pcb.
If oscillation is still happening making the meter in GR mode show compression without anything plugged into the input of the LA500 when the threshold is turned up.
Add a 100pF cap at the threshold pot between the centre wiper leg and the 0v ground leg and all oscillation should stop.

Thanks Joe, first thing I did was add the 100pF to the threshold pot like you suggested. That almost fixed it. So then I replaced C8 on the LA500 board and C2 on the MAC. It looks like C2 on the MAC had a fractured leg all along. Now with that solved the threshold is working nicely but I must have bumped something along the way because the circuit is now very noisy and makes a "Pop" when I switch (the meter switch) between VU / Off / GR.

I've downloaded the schematics and a tech friend is going to try and help me find my error with his scope. Do you have any clues as to what we might look for?

No idea on that one but check both 10R on the MAC pcb are ok and OPA2604AP has +16v on pin 8 and -16v on pin 4. But look for solder short under MAC pcb probably around C2

[jbjoubaud]

I have a major doubt about what I did with the pot.
The manual said to link 2 of the legs but two pictures below, the legs were all soldered together. I somehow decided to go with route
is it ok ?

[Joe Malone]

I have a major doubt about what I did with the pot.
The manual said to link 2 of the legs but two pictures below, the legs were all soldered together. I somehow decided to go with route
is it ok ?

Yes what you have done in the photo is fine.

[jbjoubaud]

I have a major doubt about what I did with the pot.
The manual said to link 2 of the legs but two pictures below, the legs were all soldered together. I somehow decided to go with route
is it ok ?

Yes what you have done in the photo is fine.

Thank you Joe !

[jbjoubaud]

Hey Joe,

I finally finished building my unit, here are my issues :

Trying to calibrate the VU meter, the trim and balance pot don't do anything.

As far as the controls (threshold, makup...), they do have an impact on what's going on on the meter(in VU and GR), but they don't have any impact on the audio. The audio that comes in is what comes out.
But the transformer (?) "sings/vibrate" more and more as I apply more and more make up gain.

So where do I start ?

[Joe Malone]

Hey Joe,

I finally finished building my unit, here are my issues :

Trying to calibrate the VU meter, the trim and balance pot don't do anything.

As far as the controls (threshold, makup...), they do have an impact on what's going on on the meter(in VU and GR), but they don't have any impact on the audio. The audio that comes in is what comes out.
But the transformer (?) "sings/vibrate" more and more as I apply more and more make up gain.

So where do I start ?

Sounds like the audio path is working but the bypass relay is not activating. Email me some clear well lit photos of the pcbs and cables with the JLM99v removed so I can check everything is in and around the right way. Especially the optos and relay. I need to be able to see the dot on the side of the optos.

[jbjoubaud]

Did you get my email ?

Hey Joe,

I finally finished building my unit, here are my issues :

Trying to calibrate the VU meter, the trim and balance pot don't do anything.

As far as the controls (threshold, makup...), they do have an impact on what's going on on the meter(in VU and GR), but they don't have any impact on the audio. The audio that comes in is what comes out.
But the transformer (?) "sings/vibrate" more and more as I apply more and more make up gain.

So where do I start ?

Sounds like the audio path is working but the bypass relay is not activating. Email me some clear well lit photos of the pcbs and cables with the JLM99v removed so I can check everything is in and around the right way. Especially the optos and relay. I need to be able to see the dot on the side of the optos.

[Joe Malone]

Did you get my email ?

Hey Joe,

I finally finished building my unit, here are my issues :

Trying to calibrate the VU meter, the trim and balance pot don't do anything.

As far as the controls (threshold, makup...), they do have an impact on what's going on on the meter(in VU and GR), but they don't have any impact on the audio. The audio that comes in is what comes out.
But the transformer (?) "sings/vibrate" more and more as I apply more and more make up gain.

So where do I start ?

Sounds like the audio path is working but the bypass relay is not activating. Email me some clear well lit photos of the pcbs and cables with the JLM99v removed so I can check everything is in and around the right way. Especially the optos and relay. I need to be able to see the dot on the side of the optos.

HI finally got back and starting to clear emails.

Your build looks fine except I cannot see if you have fitted the 1k8 resistor for 500 series in stead of the 2k7 next to the centre switch on the MAC PCB?

Can you hear the relay clicking when you switch the GR OFF VU switch?

Does the meter light turn on when you switch to GR or VU?

Does the makeup control turn up the output level when switched to GR or VU?

[jbjoubaud]

Hey Joe,

I finally finished building my unit, here are my issues :

Trying to calibrate the VU meter, the trim and balance pot don't do anything.

As far as the controls (threshold, makup...), they do have an impact on what's going on on the meter(in VU and GR), but they don't have any impact on the audio. The audio that comes in is what comes out.
But the transformer (?) "sings/vibrate" more and more as I apply more and more make up gain.

So where do I start ?

Sounds like the audio path is working but the bypass relay is not activating. Email me some clear well lit photos of the pcbs and cables with the JLM99v removed so I can check everything is in and around the right way. Especially the optos and relay. I need to be able to see the dot on the side of the optos.

[/quote]

HI finally got back and starting to clear emails.

Your build looks fine except I cannot see if you have fitted the 1k8 resistor for 500 series in stead of the 2k7 next to the centre switch on the MAC PCB? Yes, here's a better pic

Can you hear the relay clicking when you switch the GR OFF VU switch? No, the only thing I hear is the switch itself and the needle in the VU meter when it changes position

Does the meter light turn on when you switch to GR or VU? No

Does the makeup control turn up the output level when switched to GR or VU?[/quote] No

[Joe Malone]

If the meter led doesn't light the bypass relay will never operate as the led is in series with the 1k8 and relay?

Swap the blue and yellow wires on the meter to see if that fixes the meter light?

Or if not join the blue and yellow wire together and see if the relay and comp now operates?

[jbjoubaud]

If the meter led doesn't light the bypass relay will never operate as the led is in series with the 1k8 and relay?

Swap the blue and yellow wires on the meter to see if that fixes the meter light? That doesn't change a thing

Or if not join the blue and yellow wire together and see if the relay and comp now operates? Yes, still no light but all the controls are doing what they should

Is it ok to use it this way ?
I calibrated it using the guidelines and got the ground reduction and VU to show similar results at -11, that seams about right ?

[Joe Malone]

If the meter led doesn't light the bypass relay will never operate as the led is in series with the 1k8 and relay?

Swap the blue and yellow wires on the meter to see if that fixes the meter light? That doesn't change a thing

Or if not join the blue and yellow wire together and see if the relay and comp now operates? Yes, still no light but all the controls are doing what they should

Is it ok to use it this way ?
I calibrated it using the guidelines and got the ground reduction and VU to show similar results at -11, that seams about right ?

Yes it means the led was blown at some stage since powerup due to a fault. It is fine to use this way but you will have no meter led to shown that the comp is on. The round part of the meter unclips of the square section and the scale slides up and off to reveal the white 5mm 1000mcd high brightness type led which can be replaced.

Calibration should be done at the -5dB point for VU and GR as explained in the cal procedure. But otherwise it sounds like the comp is working fine now besides the light.

[jbjoubaud]

If the meter led doesn't light the bypass relay will never operate as the led is in series with the 1k8 and relay?

Swap the blue and yellow wires on the meter to see if that fixes the meter light? That doesn't change a thing

Or if not join the blue and yellow wire together and see if the relay and comp now operates? Yes, still no light but all the controls are doing what they should

Is it ok to use it this way ?
I calibrated it using the guidelines and got the ground reduction and VU to show similar results at -11, that seams about right ?

Yes it means the led was blown at some stage since powerup due to a fault. It is fine to use this way but you will have no meter led to shown that the comp is on. The round part of the meter unclips of the square section and the scale slides up and off to reveal the white 5mm 1000mcd high brightness type led which can be replaced.

Calibration should be done at the -5dB point for VU and GR as explained in the cal procedure. But otherwise it sounds like the comp is working fine now besides the light.

Yep I followed the cal procedure, but adjusting the balance trip pot to get the same reading on the VU and GR at lended me at -11, which doesn't make a whole lot of sens to me...
Anyway, thanks very much for your help !