D-LA2A Support Thread

[chrisenglish]

Hi,
here is my DLA2A! Frontplate is my own Design, all Metalwork by myself. Looks a little bit like a 670 Comp, i like this Millitary Look. Compressor works fine and sounds great, thanks for this beautiful Projekt.

with a right click and open in new tap you can see the pictures better!

Best
eric

What does R37 do?

 

[saxmonster]

Limiter Response trimmer, some put it on the front panel.

"Limiter Response Calibration (RV137/RV237)
The LA-2A was designed for use in broadcast applications. The audio signal in FM broadcasting undergoes pre-emphasis and results in a 17 dB boost at 15 KHz. Due to this increase in signal level, transmitters are subject to over-modulation. The LA-2A provides a control (R37) which controls the amount of high-frequency compression.
This potentiometer is factory set for a “flat” side-chain response (clockwise). Increasing the resistance of this potentiometer by turning it counter clockwise will result in compression which is increasingly more sensitive to the higher frequencies."

 

[Ericbazaar]

Hi,
here is my DLA2A! Frontplate is my own Design, all Metalwork by myself. Looks a little bit like a 670 Comp, i like this Millitary Look. Compressor works fine and sounds great, thanks for this beautiful Projekt.

with a right click and open in new tap you can see the pictures better!

Best
eric

What does R37 do?

try out the LA2A plugin from Waves, this have a r37 screw on the front. for me is this a killer feature! 8)

 

[Barton]

Could someone please verify that I have the Rondo primary wiring correct in the attached diagram  for 120v?  I'd rather not burn the house down experimenting.

 

[[silent:arts]]

The Transformer is wired fine for 120V.
Change the L and N connection!
The IEC contains a fuse? Otherwise add one between L and the switch.

 

[Pandaeus]

Hello Volker, hi all!

First of all, thanks for the marvelous pcb and all the  valuable information found here.
We recently finished building  the dual la-2a.  My companion did all the technical stuff while I did the funding and act as the musician thus doing the testing and  QA.

We calibrated the unit and everything looks quite good at the moment.
I am using a test tone generator for measuring the frequency curve and with the gain reduction set to zero, the frequency curve is nearly linear within the bounds of 20-20k.

Now I'm facing a strange issue that I'm quite sure it needs to be addressed:

Starting with a sine tone at 1k ,I set the gain reduction to -5 db. When increasing the frequency to 4k, the gain reduction increases to -7db max and after that, while sweeping up to 15k it decreases again to -4db.
From what I read in the Universal Audio manual, the gain reduction should be quite linear when r37 is set to flat operation.

How is it possible that the frequency curve is shaped like a band-pass around 4k when compressing/limiting the signal? As far as I understood, r37 acts as a high shelf filter for the side chain circuit. So even if the resistor has a wrong value and it's not in "flat mode", how can the peak reduction max occur at around 4k?

Any ideas where to start looking?  Is C4 and r37 a good point to start?

Ideas would be highly appreciated.

Cheers,
Pandaeus

 

[djfatum]

Another LA2A built
This is an amazing project 8) 8) 8)

 

[offthewallstudio]

What happened to the build thread from S.A. first build?
I just looked for it a couple weeks ago referencing the PCB I have from that buy.

there never was a build thread except the one at Franks place.

Anybody have a BOM for the Dual LA-2A for the european customers including where to buy tubes and T4B cells ??
And also the right type of tubes

here you find some links for the caps I used, and a BOM bernbrue once did.
T4Bs:
http://www.dripelectronics.com/t4b.html
http://www.studioelectronics.biz/sunshop/index.php?l=product_detail&p=1142

hello Volker.  I am done population with the pcb board.  Does anyone know the value of the knobs and for gain and out put ext? I am starting to do all the wiring.  Any tips would be greatly appreciated.  Thanks!!!!!!!!!

 

[offthewallstudio]

How do i find the values of the volume knobs?

 

[offthewallstudio]

Please someone i need help with the wiring part. Any color codes or any help with values on switches anything will help!

 

[saxmonster]

Check out post #720 in this build guide.

http://groupdiy.com/index.php?topic=32677.msg560982#msg560982

its a pdf of the switch wiring

the gain (can also add a resistor which is what i did to get more control when using the gain knob)  and peak reduction pots are in the bom

41 MOUSER.COM ALPHA 100KA POTENTIOMETER RV101,RV102,RV201,RV202 31VJ501-F 4 $1.40 $5.60


its all here just have to dig.

 

[kosi]

Hi there,
I'm in the finishing stages of my build,  just waiting on the T4B from Kenetek.

My problem:  voltages are  odd
I have on the right side of R29 350 Volts on the left side 320 Volts. So all following voltages are way too high. (both sides)
But
On my Neons, I only have 51,30 Volts, but they are glowing nicely...
I measured this without tubes and T4B.

Any ideas what to do are highly appreciated.

Andreas

 

[rock soderstrom]

So all following voltages are way too high. (both sides)
But
On my Neons, I only have 51,30 Volts, but they are glowing nicely...
I measured this without tubes and T4B

Hi Andreas, this is quite normal. Without the tubes, there is no load on your power supply, so the voltages are way to high! You have to measure with the tubes installed. Grüße 

 

[Zander]

I've read somewhere that the T4B's should be replaced every two years or so. Is this correct?


Zander

 

[[silent:arts]]

I've read somewhere that the T4B's should be replaced every two years or so. Is this correct?

Why? I don't.

 

[Zander]

Why? I don't.



Excellent.

 

[nielsk]

Having an odd issue with a fresh DLA@A build, running out of things to try.
Unit functions perfectly, signal quality is excellent, but there is a buzz in the audio that interacts with the gain pot. It is pretty loud with the gain low, decreasing to a total absence (null?) at around 3:00, the coming back in to all the way up.
Shorting the input makes no difference, tapping audio before the output tranny makes no difference.
Tried it first with the Edcore P/T on the back of the unit, then then with an outboard all DC power supply, then a bench supply, no change.
No hum at all, audio path is extremely clean when at the null point.
I have built at least 6 LA2s, point to point,  a Drip, and so on. Never encountered this issue before.
All components are correct, I have tried different tubes...
Any ideas?

 

[rock soderstrom]

This sounds like oscillation to me.  The LA2A is infamous for its huge gain. Check this thread, there are modifications for this issue. For instance, you can change your first tube to a low gain version or you can do the gain pot mod. Its all in this thread... 8)

PS: Check your wiring first especially around the input transformer and the first tube.

 

[nielsk]

all wiring is shielded, gain pot is 50k with 50k resistor (tried with 100k pot, just changed the null point)
The load is constant no mater what value pot used as there must be an associated value resistor on the pot input to make the overall load 100k... the 100k just provides limited useful working range.
12AX7 input tube.
Maybe I'll try the 12ay7 mod, I've tried everything else I can think of!

Update: Figured it out, even though the heaters are running off DC adding a 100 ohm resistor to audio power ground eliminated the buzz... odd as the heater DC measures very clean, but whatever, it worked

 

[kosi]

So all following voltages are way too high. (both sides)
But
On my Neons, I only have 51,30 Volts, but they are glowing nicely...
I measured this without tubes and T4B

Hi Andreas, this is quite normal. Without the tubes, there is no load on your power supply, so the voltages are way to high! You have to measure with the tubes installed. Grüße 

Thanks a lot, this was quite helpful.
I installed the tubes and  everything  is  fine.
And yesternight I got a mail from Bill Jones from Kenetek, that my matched pair of T4Bs are ready !!!

excited  8)