LA-4 Help Thread!

[sage]

Just wanted to post to say we got started here, the PCBs are stuffed, the meters and XLR are on the panels already as I am using the box of the MCI JH-110...

I mounted the Pots - realized immediately that the post I have ordered are small - not the switch pots but the 10k are - so if you are taking my BOM from Digikey you might wanna change to larger pots - it works fine with me as it fits the holes where the selector switches were in the MCI panel -

I have ordered cinemags and they are being built as we speak...

Gonna post some pics soon -

sage

 

[ilfungo]

HI
I made ​​some experiments with different values ​​of R13,
Do not feel different but I see the VU meter change ....
Thanks ???

 

[ilfungo]

???

 

[sage]

Ciao Il Fungo

why did you feel the need to play with R13? I am building the dual mono and if there's something that needs to be changed would be nice to know now...

sage

 

[ilfungo]

From Assembly manual - R13 changes the speed of attack and release.
I have 2  Urei LA4 Silver and noticed that they have a slower attack and release.....

 

[sage]

Ah ok, before I will fire this up I will get my friend Gary who is the historic Tech at Henson (A&M) studios to look it over and do the calibration/settings with me, they have a bunch of LA-4 there so if I find any valuable information I will gladly share them.

sage

 

[shot]

Just wanted to confirm my assumptions...

If I feed the external signal from somewhere else (let's say its on its separate xlr/trs connector) into a sidechain link point, it would be possible to have it working with some extra gear as a sidechain. Right?

XLR input to some simple opamp debalancing signal and feeding it to a switch and into point 15 on pcb

true or false?

why not?

 

[ilfungo]

Help!!!
I have a strange problem ...
one  LA4 when I exceed a certain compression, crackles... (unusual attack).
The other with maximum compression does not do so.
What can be?
Thanks

 

[living sounds]

Just discovered this project - looks great! And can't believe how cheap it is.

Just a few questions:

1. Why wasn't there a dual op amp used for IC2?
2. The two 6,8uf caps mounted in series - any reason not to use just a single film cap here?
3. Why would anyone use a VTL5C4/2 - it costs almost 5 times as much as a couple Silonex NSL 6910?
4. Is there a self etch version of the PCB?


Thanks!

 

[sage]

Hello there

I am almost - i overlooked the jumpers so i had to put them all in today... all done but i have a question about the SIDECHAIN...
on a previous post someone wrote about the possibility of adding an XLR input for side chain operation - then he put a drawing of insertion on point 15 on the PCB. though I thought that was the stereo link switch...

Can anyone elaborate on that please - I think this would be very useful to inert a sidechain input but if so how do I do it ?

thank you

sage

 

[shot]

That was me...
I asked as an idea if it is possible or not.
Just wondering because sidechain would be very cool feature.

Any thoughts on this?

 

[sage]

I think that from the schematics there is a rca input for sidechain so this should be totally an option, in fact I would love myself more info on that to see if anyone has done it and how...

 

[sage]

actually.... just got this from my tech friend who knows his craft:

Adding a side-chain, is not that easy.  Like they say, it should be buffered.  You should probably have a separate control to control the side-chain input level.  But we could try to just connect it at the stereo input and see if it works for you.  Read the following link:
http://recording.org/pro-audio-gear/3062-compressor-side-chain-inputs.html

 

[sage]

Hi there

so we finished the units here but major distortion... so I went back and saw that I made a mistake with the Trimmers... they are all 1/4 watts so I shall replace them with 1/2 watts

THIS MEANS THAT MY BOM from Digikey that I posted is NO LONGER A GOOD LIST - is it removable from this thread ?

I would not want other people to buy the wrong parts...

i also noticed that some other resistors I put on are 1 w and 2 w - could those be a problem or as long as theu are above 1/2 w I am fine?

THANK YOU

 

[shot]

Hi!

I have two questions related to part values.

1. I can't get 1N4003 diodes anywhere in town. Is it okay to use 1N4007 instead? It's the same product family but with higher max voltage.

2. I can't get 5 picoF capacitor! Is it okay to use 4.7 picoF instead? It is used around opamp and I guess it should be okay, but i'm hesitating so I guess it's better to ask someone who knows better.

thanks!

Luka

 

[Sredna]

1. yes

2. yes

 

[shot]

Thanks Sredna!

 

[Harpo]

2. I can't get 5 picoF capacitor! Is it okay to use 4.7 picoF instead? It is used around opamp and I guess it should be okay, but i'm hesitating so I guess it's better to ask someone who knows better.

Leave it out (5pF or 4.7pF wouldn't keep the TL074 within its GBW margin or help oscillation) or use a more useful value.
This opamp gain stage in its Lo setting (R7 shorted out) has a voltage gain of 10 (+20dB) and a noise gain of 11. The TL074 has a min.slew rate of 8V/usec and a GBW of 3MHz. Running the numbers and assuming +4dBu input level, this opamp can do 8*3000000/(2*PI()*1.74Vpk*((33K+3.3K)/3.3K)) = 199.567kHz=~200kHz. This C2 better would be at least 1/(2*pi()*33000*200000)=2.411E-11=24.11pF in order to keep the opamp in its comfort zone.
Maybe the '2' from the C2 designator wasn't ment to be there but in front of the 5pF for a 25pF value. A 27pF c0g/np0 would be the next avail. more common part value.
As always YMMV.

 

[Sredna]

Thanks Harpo!

On the original schematic it's indeed C2 = 5pF, maybe because they used the RC4136 wich has a  lower slewrate (1.7 V/us).

Time to go shopping.... 

 

[shredup]

The LA-IV.pdf has been updated.

The BOM now focuses on the diy way to make the optos and NSL 32 is mentioned as an optional replacements.

I've added tips and comments.

Oh and I removed the extra 47Ω from the BOM...

Hi
In the LA-IV.pdf I've downloaded from the first post, there are no NSL 32 mentioned...
So I think I havn't the last version of this PDF.
Does someone have it, please?
Thanx