LA-4 Help Thread!

[ilfungo]

thanks Luny
common mode rejection:
what is it?
how to adjust it?

 

[Luny Tune]

See page 2 in the manual for adjustment.

The adjustment ensures maximum cancellation of any common noise on the + and - signals at the point where - is inverted back and put together with + again for it's unbalanced journey through the rest of the circuit. Note which trimmer you're actually adjusting in the schematic. That should make sense of it for you.

 

[SSLtech]

Thanks to Luny Tune, Gustav and everyone in the thread for helping me make this the easiest DIY build I've ever done!

Here's how I made it look so authentic:

http://www.groupdiy.com/index.php?topic=43774.0

Cheers!

Keith

 

[Rybow]

That looks great Keith! How is mister graphic taking it?

 

[SSLtech]

amorris is going to "console" mister graphic!  ;D

 

[okgb]

Keith what version of the opto's did you use ?
there was some discussion here about what sounded
best [ as compared to authentic ? ]

 

[SSLtech]

I used a pair of optos from a T4-B which had a dead EL panel. -Given the ~$100 cost of a new EL panel, this seemed like the most cost-efficient way of 'recycling'.

Works exactly like the two genuine UREI LA-4's which I have here; I was just testing the opto speeds this morning, in fact.

Keith

 

[Rybow]

I used a pair of optos from a T4-B which had a dead EL panel. -Given the ~$100 cost of a new EL panel, this seemed like the most cost-efficient way of 'recycling'.

Works exactly like the two genuine UREI LA-4's which I have here; I was just testing the opto speeds this morning, in fact.

Keith

Nice! Now that's recycling.

 

[isophase]

Hi all,
i'm starting a dual LA4 using a pair of B11148 output transformers, can someone explain why i should use a blocking capacitor with this particular transformer ? i read this somewhere in this thread and i don't understand why it would need a blockning cap ?
how do i calculate the correct value ? whats up with this cap ?
thank you

 

[isophase]

i finished the metal work today, the unit is going to turn out pretty nice with a symmetrical layout.
any hint on the necessary cap while using the B11148 ouput transformer ? i read on the forum that this transformer is "not gapped" and so needs a cap to block DC current from flowing in the primary... i don't see the problem, isn't that the role of the transformer ? any help greatly appreciated.
the unit should pass some audio tonight, if i get off my ass and finish the wiring !
be back soon with some photos

 

[Davo]

A few pages back, Ed Anderson mentions using a 470uf for dc blocking... I'm gonna be using the 11148 as well, please post your results

cheers,
dave

 

[dtrax]

I've been reading through this thread for the past few days; a great wealth of knowledge on the LA4. I've assembled my own bill of materials for this project, and a stereo unit is coming in under $350! Seems like a pretty awesome project from a value/performance perspective.

However, I'm still a little confused in regards to the opto. Is it necessary to DIY it? I assume things like an NSL32 and VTL5C9 are "drop-in" opto cells that can be used w/o DIY'ing, correct? Pardon my ignorance here.

Also, the power toroid is a bit elusive. It seems 15 volt-amp rating isn't too common. Where are you guys in the USA getting 'em? I use Triad Magnetics transformers all the time, but their lowest power rating is 25VA that mouser carries. Probably too much current, even for a stereo unit, huh?

 

[Rybow]

Hey dtrax! You don't need to DIY the opto. I have a channel with the NSL 32, and it works great! There are some notes about a couple of components that need to be changed to get it to calibrate properly. I just dropped the Vactrol VTL 5c4/2 in my other channel. Haven't been able to get to testing it yet though. Anyway, the NSL 32 is quicker than the DIY or 5C4 optos, but I like it.

I used a 30VA 2 x 18V hammond to power my dual mono unit. I think 2 x 18V is the suggested voltage rating for the LA 4 power supply. 25VA will be fine, and ensure that you have a some extra power on tap just in case. Don't want the PS to choke on you during some heavy compression!

It is an awesome comp. Very useful for a variety of sources. Good luck!

 

[dtrax]

Hey dtrax! You don't need to DIY the opto. I have a channel with the NSL 32, and it works great! There are some notes about a couple of components that need to be changed to get it to calibrate properly. I just dropped the Vactrol VTL 5c4/2 in my other channel. Haven't been able to get to testing it yet though. Anyway, the NSL 32 is quicker than the DIY or 5C4 optos, but I like it.

I used a 30VA 2 x 18V hammond to power my dual mono unit. I think 2 x 18V is the suggested voltage rating for the LA 4 power supply. 25VA will be fine, and ensure that you have a some extra power on tap just in case. Don't want the PS to choke on you during some heavy compression!

It is an awesome comp. Very useful for a variety of sources. Good luck!

Thanks for the reply Insomniaclown! Once I'm done with my BoM I'll place an order for some PCBs. LA4's aren't as popular as the LA2/3's, but that's because (in experience anyways) they're just too damn slow. Faster opto's are a must for me. I have a stereo LA3A clone in my rack, so I'd love to have something a bit faster to compliment it.

Please report back if you have the time in regards to the NSL32 vs VTL5C4/2. I plan on buying both, and perhaps wiring both to be switchable for my build. But in the meantime I'd like to know your thoughts and experiences between the two.

And thanks for clearing up the power tranny issue for me. I'm still the "hand holding" stage haha!  8)

Cheers!

 

[isophase]

Hi all,
strange, the PSU L7918 negative regulator just burnt while i was doing some PSU testing... weird thing is that it was working properly since the start of the project, I was testing some resistors for my VU meter LEDs + EAO switch lamps, and when i switched back the unit on the power trafo started vibrating, i switched off immediately unplugged the PSU board, tested the power trafo (which is 2x18V 30VA) all ok, i tripled check the PSU board for some mistakes/solder bridge.. all ok. i reconnect the whole thing, switch ON, no trafo oscillation but the 7918 went in smoke...
i've got the data sheet for my regs and while i was checking the pinouts against the LA4 schematic i found out the pin numbers (on the schematic) did not correspond to the regulators (which are standard fairchild L7818 and L1918) is this normal ? i know that the main ICs pin out on the schematic don't correspond to the TL074 used for this project, is it possible that the L7818 and L7918 regulators pinout changed since 1979 ? thank you for clarifying.
i can verify this with the schematic against the PCB+nomenclature PDF found on the fist post of this thread.

 

[isophase]

ok i've changed both regulators, it seems to run ok now, i must have made a mistake but i don't know what!
back at work...
i got confused when checking the schematic with the data sheet of the 78XX and 79XX regs...
i guess the pin out was different in 1979 then :-\

 

[isophase]

finished the wiring last night and passed some audio, this thing sounds very good!! it sounds soft i love it!
the B11148 output transformers are mounted without the blocking cap as for now, i havn't tryed with a blocking cap yet but since i did not measure any DC voltage at the output, i don't think it needs the blocking cap for this design ? (advise needed)
it uses a paire of VTL 5C4/2 vactrols with 150KOhm R13 resistor, it's not calibrated yet but boy it sounds very good !!
special thank you to Luny tune for this really great project! thank you to Gustav for the boards and thank you to all the forum gurus for great inspiration.
i implemented a dual relay bypass board (proposed by marcfrom here on the forum) and used some EAO switches taken from old 'taken apart' telefunken modules, the metal work was done in my kitchen with a dremel, hand drill and some files
will report back after some tests..
blurry pics:

 

[dirtyhanfri]

Hi

I`m gonna start building a LA-4, I`m waiting for the boards and comps, but I didn`t order a enclosure yet, I`ve looked this box at mouser:

http://es.mouser.com/Search/ProductDetail.aspx?R=CU-3010-Avirtualkey56310000virtualkey563-CU-3010A

Will it be space enough for everything or will it be a bit small? (6x10x3.5 In)

Thanks

 

[gary o]

Quick question the1% 4K99 resistors.......I cant lay my hands on any at the moment.......could I use 5K here and match them up.....thanks

 

[Rybow]

Hi

I`m gonna start building a LA-4, I`m waiting for the boards and comps, but I didn`t order a enclosure yet, I`ve looked this box at mouser:

http://es.mouser.com/Search/ProductDetail.aspx?R=CU-3010-Avirtualkey56310000virtualkey563-CU-3010A

Will it be space enough for everything or will it be a bit small? (6x10x3.5 In)

Thanks

There should be enough space in there for everything. Choose your parts carefully according to their size. You'll probably end up mounting the toroid vertically. The PCB from gustav is about 4" x 6". I'd do a rough layout to make sure everything will fit.

Gary o- At 1% tolerance, you can use 5K resistors since they may vary between 4.95 and 5.05k. Just get a bunch and measure until you find 4 that are close. I actually may have a bunch of 4.99k resistors kicking around that I could pop in the mail for you.