LA-4 Help Thread!

[Luny Tune]

Not much can be wrong, RegularJohn.

I'll ask you a couple of questions here:

Did you make the rectifier bridges for the meters yourself or were the meters already fitted with that internally? (The meter will not show VU reading without it so if you have a VU reading and didn't build the rectifier bridges yourself, they're already built in.)

Is the +8 reading correct?

(I suspect you've accidentally wired the 4k7 resistor on the meter switch so it's active on both the +4 and +8, but the 7-8 dB difference shakes that theory a bit, but you should check to see that the switch is wired correctly.)

What meters are you using. I can't make out a brand name on the pics. They look nice though.

 

[herrmann]

And great raww look you have on yours, Hermann!

The "distressor like" pot is very convenient, easy to recall.
Done the mods, and now I'm recalibrating the thing. There's a step I don't really understand : ratio. When increase the input by 20dB, the unit distort !

 

[Rybow]

Do you have your input sensitivity switch set to low or high? If the unit is distorting then that means you are sending in too high of a level. When I calibrated mine, I made sure not to send in any level over -16DBFS with the pad in just to be safe. For the ratio adjustment, I left the pad in, sent in a sine wave at -36dbfs, and then increased it to -16dbfs for the calibration. With the input sensitivity set to low, I think the input clips at around -18dbfs from my limited experimentation.

It looks like I will have to get the VTL5C4/2 after all. I think my LDR's are mismatched as I can't get the unit to calibrate all the way. Still sounds great though! After the mods, the channel is sooooooo smooth. I love how this compressor doesn't trample all over the transients.

 

[Luny Tune]

Look at #9 on page 4 in the complete manual. There's you'll see that +20 dBu (7.75 Vrms) is not enough to overload the input.
But remember to set the Input Range switch in the High position.

BTW, I find that the Input Range switch pads the signal exactly 20 dB so I often use it for adjusting ratio, as a matter of fact.
Switch to High
Send in 0 dB
Adjust Output for a +1 dB reading on the meter in GR mode.
Adjust Threshold until the meter drops 1 dB and now reads 0 dB.
Trim for at +2.5 dB reading on the meter.

 

[herrmann]

Yes I've the switch in the high position.
My input is overloading at about 21dBu. I guess it's normal. Now the thing is that the output isn't increased or far less than expected when adding 20dB on the input.
The setup for 8:1 is correct. I have 2.5dB increase, but for the others values, the unit "overlimit", "oversquashing" (I don't know how to say) the output level.

 

[regularjohn]

Not much can be wrong, RegularJohn.

I'll ask you a couple of questions here:

Did you make the rectifier bridges for the meters yourself or were the meters already fitted with that internally? (The meter will not show VU reading without it so if you have a VU reading and didn't build the rectifier bridges yourself, they're already built in.)

Is the +8 reading correct?

(I suspect you've accidentally wired the 4k7 resistor on the meter switch so it's active on both the +4 and +8, but the 7-8 dB difference shakes that theory a bit, but you should check to see that the switch is wired correctly.)

What meters are you using. I can't make out a brand name on the pics. They look nice though.

Hi Luny,  I didn't build rectifier bridges for the meters as they show level on their own, it's just not accurate.  I didn't include an option for +8 metering, so the 4.7K is not used.  I used DPDT switches, to choose between +4 and GR, so it's not much to mess up in the wiring - but i have double checked everything.  They are very close to accurate in GR mode, btw.

The meters say "sterecorder".  I think they came out of a vintage consumer tape deck or something. 

 

[Luny Tune]

Ahhh, so you're possibly dealing with a meter that does not meet the specs. In that case you could stick a trimmer in replacing the 3K6 resistor between the output and the meter (or if you want a solution that'll save you the trouble replacing that resistor, short the resistor on the component side just, just above IC2, and solder the trimmer on to the meter itself) and trim to the right reading. A 5K trimmer should be enough as you need less resistance.

(Be aware that I haven't tried this myself. The meter is sitting across the actual output and I'm not sure if too low resistance will affect the sound/impedance in some way. I'm pretty sure you'll be ok, though... But just in case; Keep your ears open. If it isn't audible then never mind...)

 

[Luny Tune]

Ok, so you're not getting distortion on the way in. Never mind the way out at this point in the adjustment procedure.

but for the others values, the unit "overlimit", "oversquashing" (I don't know how to say) the output level.

Don't listen, just measure...

If you're certain that something must be wrong, please upload a bit of audio so I can have a listen.

 

[regularjohn]

thanks for the info, i'll try it out!

 

[Child1]

Hi there

Anyone who has an idea on how to make the meter switch in the LA4 a toggle instead of an rotary?
I have been drawing and thinking my brain out, but I cant figure out how to connect it and what kind of toggle switch i should use...?
Any input would be great, but if someone has an idea and would be so kind as to make a drawing I would be extremely happy...!

Kind regards
Mads

 

[benlindell]

Are you looking to just toggle Gain Reduction and +4? If so that's what I did, I may have drawn it out, lemme look.

 

[MrZpliff]

Changed the resistor, trimmer and the optos to the VTL5C4/2's yesterday. Fired it up today and ran some drums thru it
just to check that all was ok, and man does it sound sweet. 

I'll try and calibrate it tonight and see if I can get it to track nicely in stereo mode. I couldn't get that right with the NSL32's.

 

[Luny Tune]

Yeah, it's adifferent beast now, huh? ;D

With regard to the toggle switch (both Child1 and I are danish and discussed it on the danish sister forum groupdiy.dk) the hurdle is that we need a switch that both breaks and makes new connections and we need acess to the wipers/poles. If anyone know about toggle switches like that, please let us know. I can't find any.

 

[ilfungo]

Hi guys
I just finished my fifth LA4 ...
the various projects I have left 2 edcor XSM 10k/10K
may be fine for this project?
There are no changes to make?
how can I use the input transformer?
thanks

 

[gar381]

Hey Luny amd Child 1
Just a regular old toggle...  nothing fancy works fine
for a +4/GR switch.

GARY

 

[benlindell]

Yea i just used a dpdt switch on mine to toggle +4 and GR

 

[Luny Tune]

Yeah yeah, I know. For only GR and +4 (or +8) a dpdt On-On switch will be fine. I'm talking about a switch with all three options.

But who needs a +8 setting today anyway??

 

[MrZpliff]

All the calibration is going rather smooth (I'm using a Digital multi meter and a tone generator in my DAW)...
but now to the stereo Threshold trim:

When the instruction says: "Apply a 200 Hz input signal to the STEREO jack..." does that mean I should  feed it to
the STEREO LINK .. i.e. point 15 on the pcb ?..
... and should I do it at one channel at the time or connect them together (i.e. linked)


Hmm... Should I put R4, the "common mode pot" to zero ?

 

[Luny Tune]

The STEREO JACK is point 15 on the board, yes.

I believe one channel at the time.

No, the common mode rejection trimmer has it's own adjustment procedure in the manual. See the beginning of the procedure. If in doubt, leave it in the middle.

 

[MrZpliff]

OK, got it !

I finally finnished this project, though I forgot to drill the holes for the rack mounting in the frontplate ... but I don't have any space left in my rack anyway, so I'll fix that later. I also got the meter-switch-labels the wrong way - DOH ! +4 is GR and so on... but I'll fix that.

Calibration done, decent stereo-tracking, sounds really good 

I wanted to use the case from an old D2MAC-reciever killed by lightning years ago, so it got a bit crowded...

Big thanks to Luny for this project.

here's some pics: