LA3A 500 BUILD

[saxmonster]

HAHAHA OK, there are 2 IN BUTTONS,  I forgot about the main one at the top. 

Ok when the LA3A is "OFF"  The very top button, hahahahah  I get -5DB on the meter with a +4DB signal feeding it.  Sorry about the confusion.  I always forget about the so called POWER BUTTON, hahahah

How would I adjust it so that I get  -4db or should that actually be zero maybe?

 

[kdramsey]

HAHAHA OK, there are 2 IN BUTTONS,  I forgot about the main one at the top. 

Ok when the LA3A is "OFF"  The very top button, hahahahah  I get -5DB on the meter with a +4DB signal feeding it.  Sorry about the confusion.  I always forget about the so called POWER BUTTON, hahahah

How would I adjust it so that I get  -4db or should that actually be zero maybe?

Ok great to know yours is behaving exactly like mine! 

It should read zero with the +4dBU signal applied.  I am not sure if there is an adjustment on the board for that... Maybe that was the discussion in previous posts about R13?  I will need to study the original schematic some more. 

 

[saxmonster]

I guess its time to reread this thread and the schematic.  Back to the drawling board.  Anyone else know why we are not getting 0db on the meter when we feed it that?  I was able to zero out the meter on GR and thats little bottom pot that you access from the front bottom but thats not the answer I guess.

 

[saxmonster]

Ok so after more messing around.

With the LA3A turned off (top button disengaged)

I sent a +4db (1.228V) 1khz signal to the lA3A and adjust RV36 till I got the meter to zero out.

I then put it on and into GR mode and had to finess the RV23 trimmer a hair to zero out the meter again on GR.

I think I am now completely done calibrating.

my TP1 = 6.5 volts as said early and from what the schematic said to do.

Anything else I might need to check?

 

[kdramsey]

Ok so after more messing around.

With the LA3A turned off (top button disengaged)

I sent a +4db (1.228V) 1khz signal to the lA3A and adjust RV36 till I got the meter to zero out.

I then put it on and into GR mode and had to finess the RV23 trimmer a hair to zero out the meter again on GR.

I think I am now completely done calibrating.

my TP1 = 6.5 volts as said early and from what the schematic said to do.

Anything else I might need to check?

Great!  Thanks for figuring that out.  I will check mine and adjust RV36 when I get home this evening. 

 

[saxmonster]

Ok, now I am totally confused.  After doing that I moved around my xlr to 1/4 wires because I have 1/4 balanced inputs on my MOTU 2408MKII unit. 

Turns out the input wire to my LA3A wasn't balanced.  Why does this make a difference? 

I guess I will have to read up on that now, dam.  So much to learn.

I sent a round trip loop out of the out of the 2408MKII back into it with a 1khz sine wave and now both output equals input of channel 5.

Ok so with all balanced wires connected an input equal to outputs I connect to the LA3A  and I get an output of -2.5db back into my daw and that with the LA3A (OFF)

Seems like as the signal runs through the LA3A is looses -2.5db of the signal, no squash or boost are on either.

With a +4db sent out of the 2408 to the LA3A (Off - Disengaged top Button) the meter is at zero but still is -2.5db softer on the output of the LA3A back into the daw.

With the LA3A (on - top button is engaged)  I put the GR on and the meter is at zero.

Everything seems ok just wondering why the -2.5db drop when running it through the LA3A in its (off state - top button disengaged)

 

[saxmonster]

KDramsey, where are you located?

 

[kdramsey]

Ok, now I am totally confused.  After doing that I moved around my xlr to 1/4 wires because I have 1/4 balanced inputs on my MOTU 2408MKII unit. 

Turns out the input wire to my LA3A wasn't balanced.  Why does this make a difference? 

I guess I will have to read up on that now, dam.  So much to learn.

I sent a round trip loop out of the out of the 2408MKII back into it with a 1khz sine wave and now both output equals input of channel 5.

Ok so with all balanced wires connected an input equal to outputs I connect to the LA3A  and I get an output of -2.5db back into my daw and that with the LA3A (OFF)

Seems like as the signal runs through the LA3A is looses -2.5db of the signal, no squash or boost are on either.

With a +4db sent out of the 2408 to the LA3A (Off - Disengaged top Button) the meter is at zero but still is -2.5db softer on the output of the LA3A back into the daw.

With the LA3A (on - top button is engaged)  I put the GR on and the meter is at zero.

Everything seems ok just wondering why the -2.5db drop when running it through the LA3A in its (off state - top button disengaged)

My best guess with the -2.5dB is because of pan law in your DAW.  My 2mix bus is stereo and I see -3dB lower there since the mono LA3A signal gets panned across left and right so it is in the phantom middle. I use ProTools... Search for pan law settings for your particular DAW.

 

[Spino]

ordered all the remainig parts today but mouser is out of 2n3417 transistors. did it happen to anyone else? in this case are you waiting for them to restock those transistors or you went with a different one? or maybe ordered from a different seller 

 

[kdramsey]

ordered all the remainig parts today but mouser is out of 2n3417 transistors. did it happen to anyone else? in this case are you waiting for them to restock those transistors or you went with a different one? or maybe ordered from a different seller 

I got mine from parts-express.com and they are less expensive from there!

 

[kdramsey]

KDramsey, where are you located?

I'm in the USA Virginia

 

[saxmonster]

My best guess with the -2.5dB is because of pan law in your DAW.  My 2mix bus is stereo and I see -3dB lower there since the mono LA3A signal gets panned across left and right so it is in the phantom middle. I use ProTools... Search for pan law settings for your particular DAW.

From what i read it says that DP has a -3.5 pan law that can't be fixed or changed.

But why is it when I run a TRS 1/4 out of channel 5 of my DAW out and then back it to the input of channel 5 I get the same readings.  So my in and out levels of the DAW are equal.

I then put the LA3A into the circuit and I get -2.5 or so DB loss back into my DAW. 

Is this really Pan's Law into effect?

I'm not sending a stereo signal just mono out to mono in.

Im in Central NJ

 

[kdramsey]

But why is it when I run a TRS 1/4 out of channel 5 of my DAW out and then back it to the input of channel 5 I get the same readings.  So my in and out levels of the DAW are equal.

I then put the LA3A into the circuit and I get -2.5 or so DB loss back into my DAW. 

Ok I agree that does not sound like pan law causing the issue.  It would appear that the LA3A has a -2.5dB insertion loss with the top button off.

I am getting mine setup now to adjust RV36 then I will test for the insertion loss on my units.

 

[kdramsey]

I adjusted RV36 and now my meter is on 0VU with 1kHz +4dBU signal applied. 

Tested both of my units and they seem to have between -1 and -2 dB insertion loss in my converter round trip.  So your measurements seem fine.

 

[saxmonster]

I adjusted RV36 and now my meter is on 0VU with 1kHz +4dBU signal applied. 

Tested both of my units and they seem to have between -1 and -2 dB insertion loss in my converter round trip.  So your measurements seem fine.

Very cool!!!!!

yea mine looses about 2.5, why do you think we are loosing a couple of Db?  Why would mine be different than yours?  Are yours slightly different between the two of them.  I wonder if the different voltage at the test point 1 is why?  I have r7 set to give 6.5vac at r18 and ground.  AKA TP1

Just for giggles I put my JLM 500 I have in the circuit and it also looses some Db.  It looks to be about .15 of a Db or so.  Very very little compared to the LA3A.  Maybe I am splitting hairs here now.


Thanks
-Scott

 

[kdramsey]

Very cool!!!!!

yea mine looses about 2.5, why do you think we are loosing a couple of Db?  Why would mine be different than yours?  Are yours slightly different between the two of them.  I wonder if the different voltage at the test point 1 is why?  I have r7 set to give 6.5vac at r18 and ground.  AKA TP1

Just for giggles I put my JLM 500 I have in the circuit and it also looses some Db.  It looks to be about .15 of a Db or so.  Very very little compared to the LA3A.  Maybe I am splitting hairs here now.


Thanks
-Scott

I wouldn't worry much about the insertion loss. A could dB is easily made up for with the boost.

I am still trying to figure out my strange TP1 issue.  Where are you picking up ground when you test r18?  Can you pull out the gain board (the one with r18 on it) and measure the voltage on each of the pins that feed it when you get a chance? 

I don't understand yet how mine is working exactly like yours with such a large difference in bias voltage on r18.  I did find a schematic for an original LA3A and it says to adjust bias for 12 - 13 volts at TP1.  So I need to spend some time working out an actual schematic for the LAZ LA3A since there must be some differences.

 

[saxmonster]

On peter's schem  TP1 is to the left of C5 on the top half of the page about in the center of the right quadrant.  Tp-1 with a line and an arrow pointing toward the intersection of R18 and R19.

Then is says "Adj bias for 6-7 volts at TP-1" which is written near R7 on the top half near the center of the page.

I guess there wasn't room to write it near TP-1

I put the positive of my meter in-between R18 and R19 and then the black part of my meter to the ground wire on my boost pot.  There are many ground spots you could use.

Here is what someone said earlier.

i found it a little confusing to follow the schematic with the daughter boards and the additional, non schematic additions to the circuit, but Peter posted the layouts on page 2 of the thread. you can infer general areas of the schematic if you reference the boards from the layout.
If you look at the schematic is says to ' adjust for bias to between 6-7 volts at 'tp1'. you can see that is between R18 and R19. This is a DC reference, so probe from ground to 'tp1' and adjust until you have the correct voltage. It is paramount to have all boards connected when you do this so you are properly adjusting the bias voltage
Hope this helps
Ian

Here is Peter's schematic

http://lazpro.com/la3adv.jpg

 

[kdramsey]

On peter's schem  TP1 is to the left of C5 on the top half of the page about in the center of the right quadrant.  Tp-1 with a line and an arrow pointing toward the intersection of R18 and R19.

Then is says "Adj bias for 6-7 volts at TP-1" which is written near R7 on the top half near the center of the page.

I guess there wasn't room to write it near TP-1

I put the positive of my meter in-between R18 and R19 and then the black part of my meter to the ground wire on my boost pot.  There are many ground spots you could use.

Here is what someone said earlier.

i found it a little confusing to follow the schematic with the daughter boards and the additional, non schematic additions to the circuit, but Peter posted the layouts on page 2 of the thread. you can infer general areas of the schematic if you reference the boards from the layout.
If you look at the schematic is says to ' adjust for bias to between 6-7 volts at 'tp1'. you can see that is between R18 and R19. This is a DC reference, so probe from ground to 'tp1' and adjust until you have the correct voltage. It is paramount to have all boards connected when you do this so you are properly adjusting the bias voltage
Hope this helps
Ian

Here is Peter's schematic

http://lazpro.com/la3adv.jpg

Thanks for posting that schematic.  I don't know how I missed it... But it solves my issue!  I was using the back half of D4 as my ground reference and on the schematic that is part of the blue line which is -16V and not ground.

Tomorrow I will check my bias voltage with reference to the ground on the boost pot (green line) 

 

[kdramsey]

Ok I just tested TP1 with the ground in the proper place! I used the ground/shield wire on the squash pot and calibrated RV7 until my DMM read 6.5VDC. 

Now I am curious to try out the stereo link feature.  Has anyone successfully done that?

 

[saxmonster]

Can't help you there, I only have one, hahahaha