D-LA2A Support Thread

[flyweight]

the 9V is to be converted to 5v for the relays.

 

[flyweight]

Hi Guys,

A bit of a stupid question but how do I wire the stereo switch?
I saw this:


So I just have a single wire going from bot side to the switch and the connector is jumpered.
Should the black wires be connected from side to side?

If I do not install the Zener and use the 5v rails instead is that ok? I actually blew one bulb trying to wire it.

My unit is not compressing at all.
Only the amp part works and the right channel has a 50hz humm and some hiss. If it touch the shield of gain or peak reduction pot it changes.
Switching the tubes does not change it.

They did not have molex connectors so everything is soldered.

At first my unit would not pass sound.
Again I was stupid as I did not connect the ground wires.
I now made a star ground point with a bolt in the back of the chassis.
I was so happy that sound passed through.

Now only I need to get it compressing.
And calibrate it before adding some negative feedback resistors to the rotary switches.
What a big project for a Noob like me

 

[flyweight]

Ok I found me answerer. omg I am stupid:

https://groupdiy.com/threads/d-la2a-support-thread.32677/post-507151
Lets fix it and hopefully it will start to compress.

 

[flyweight]

Ok compression not working still.
I have removed the wrongly installed Stereo link wire.
R37 is installed as 1 meg pot. I i turn it all the way up and adjust the stereo adjust trim the meters start to move slightly.

So next step is to wire the Stereo link correctly and check the wire of the R37 / R137 pots.

 

[[silent:arts]]

So I just have a single wire going from bot side to the switch and the connector is jumpered.
Should the black wires be connected from side to side?

The black wire is the screen of the shielded cable. It is only connected to the PCB.

If I do not install the Zener and use the 5v rails instead is that ok? I actually blew one bulb trying to wire it.

??? You can't use 5V DC instead of a Neon Lamp or 62V zener diode ...

Only the amp part works and the right channel has a 50hz humm and some hiss. If it touch the shield of gain or peak reduction pot it changes.

Do I see a PSU mains switch in the middle of the front panel?
If so, the hum has to be expected.

They did not have molex connectors so everything is soldered.

This is totally fine.

 

[flyweight]

Hi Volker,

Thanks a lot for your reply. As you can see I am quite a noob.
I removed the switch from the front panel but the hum did not disappear. ( moved the wire far away from the board and panel.
Does that mean that the hum might have another cause? could this switch on the front work at all without hum?

I have some 5-6 VDC bulbs. I mean I can wire those to the 5v line to use as a backlight?
Why wouldn't that be possible?

 

[[silent:arts]]

Sure you can wire the backlight of your meters to the 5V PSU.
Since you wrote "If I do not install the Zener and use the 5v rails instead is that ok? I actually blew one bulb trying to wire it." and didn't mention any meter I thought you were talking about the Neon bulb / Zener diode on the PCB.

 

[flyweight]

Ok thanks a lot.

I haven't calibrated the unit yet, It looks compression is actually working. Even thought I have not yet the correct value in R125 - R225.
The main issue is still the hum.

It is only on the right side (tranformer side)
I removed the power switch from the front, no change
I moved around the output transformer, no change
I swapped V201 and V202 with the other channel.
The other channel is dead quite and working well.
Noise flloor at -94.5 db
If i engange the right channel nose starts a -74db. (that is measured using the Ableton spectrommeter, so its just an indication)

The cable going to the gain knob seems to have the biggest impact. If i touch it on the rubber the hum goes




Honestly my cabeling is a bit messie.
So I guess I have to redo it.

 

[tedsorvino]

Can someone make certain that these are the correct pinouts for sowters and edcors please (the ones mentioned in a couple of posts in the thred don’t seem correct to me)

For input:

Sowter 4383 - Edcor xsm 10k/600. (Reversed)

Green - Pin5. In+
Brown. - Pin 8 In-
Pink - Pin 6 C. T.
Blue. - Pin 1. Out +
Grey. - Pin 4. Out -
Black. - Pin 2. C.T.

For output:

Sowter 8940. - Edcor. Xsm. 10k/600

Green. - Pin 1. In +
Yellow. - Pin 4. In-
Pink - Pin 5 Out +
Grey - Pin 8. Out -
Black. - Pin 2. C. T.


I followed these schematics -pinout diagrams.

https://www.sowter.co.uk/schematics/la2adiy.jpghttps://www.don-audio.com/Edcor-XSM10K-600
Thanks

 

[innercityman]

Hi all,

maybe Volker or someone else could easily explain this behaviour I've encountered with the D-LA2A I've built few months ago. Unit is working fine, sounding extraordinary but I get a little issue... When powering up the unit, both vu meter needles are perfectly aligned on zero when vu meter switch on GR position, but little by little, the needle of channel A moves slowly to the left and after maybe one hour, it shows about minus 0,5 db. Don't have that on channel B.

Thanks for your advices

 

[tedsorvino]

Hi again.
Now i’m nearly set. Transformers work as they should, input ones take the input signal much higher, Output ones take it down. Slight moves of the gain pot make the signal way too loud above 9 o’clock. With a sine wave of 1k at 4 dbm in order to have unison level at the output (measured with my oscilloscope) the vu meter shows -3 db(gain mode). When i crank the peak reduction pot it goes down to around - 6 on the vu meter. Which means a gain reduction of 3 db max.
Considering how loud the signal i’m feeding the unit with, the compression is not what would be expected.
Any ideas what goes wrong?
(tubes work well - i mean both sides show the same results, i use igs t4bx, and i haven’t made any calibration on trimpots yet but no trim pot is fully turned either side).

 

[dmp]

When powering up the unit, both vu meter needles are perfectly aligned on zero when vu meter switch on GR position, but little by little, the needle of channel A moves slowly to the left and after maybe one hour, it shows about minus 0,5 db. Don't have that on channel B.

As it warms up the operating point changes. First try swapping the tubes to see if the issue follows the tubes. If the issue does follow the tubes, narrow it down to a single tube, by continuing to make swaps. Most likely a tube in the sidechain, maybe the NE-2? (unless you used a zener diode)
If it doesn't follow the tubes, then you have some other heat sensitive component. You might carefully try using a heat gun to find it.
This is common issue on compressors - for instance 1176 typically need to be trimmed for GR after being on for an hour - but the fact that it is only on one channel means you can probably eliminate it. Building them with the GR trimmer accessible from a hole in the front panel is typical.

 

[dmp]

Considering how loud the signal i’m feeding the unit with, the compression is not what would be expected.
Any ideas what goes wrong?

What transformers did you use? What T4B?
Did you do a mod to the pots or are they A100K?

 

[dmp]

The cable going to the gain knob seems to have the biggest impact. If i touch it on the rubber the hum goes

Yes, shielded neat cabling is important in a LA2A. The shield of the cable should be connected at chassis ground on one end.
Also when running cable to the front panel switches be cautious of using twisted pair cable as it adds a capacitance across the pot or switch. Better to use single wire shielded cable.
If you used AC heaters make sure the heater wiring is neat (I don't remember how the heaters are done for this build off hand)

 

[tedsorvino]

Custom transormers 600 : 10k and 10k : 600. Igs t4b. A100 k pots, no mods ( but still…mods usually are done after we have built a “properly” functioning standard unit, i guess).
Thanks for the input Dmp, but i seriously doubt that my problem is cabling. Shielding is used to avoid hum and noise, twisting is a good practice to be done to every pair. I’m not talking about noise or quality of signal. I’m talking about db gain reduction quantity. Compression quantity (gain reduction) is my problem and not sound quality. Can noise influence that? I’m not sure.

 

[dmp]

Hi again.
Now i’m nearly set. Transformers work as they should, input ones take the input signal much higher, Output ones take it down. Slight moves of the gain pot make the signal way too loud above 9 o’clock. With a sine wave of 1k at 4 dbm in order to have unison level at the output (measured with my oscilloscope) the vu meter shows -3 db(gain mode). When i crank the peak reduction pot it goes down to around - 6 on the vu meter. Which means a gain reduction of 3 db max.
Considering how loud the signal i’m feeding the unit with, the compression is not what would be expected.
Any ideas what goes wrong?
(tubes work well - i mean both sides show the same results, i use igs t4bx, and i haven’t made any calibration on trimpots yet but no trim pot is fully turned either side).

I'm not sure I understand - but with a 4:1 input transformer and peak reduction turned up you should be smashing the signal, more than -6 dB GR. Although with peak reduction turned down, you should not be seeing -3 dB on GR. So I'm not sure about your description. But it could be that the meter is not showing the GR correctly. You should look at calibrating the unit and checking the GR with the scope.
I built mine with 1:1 line input transformers, others mod the gain pot to reduce the max level, as a stock LA2A can go very high. So if you don't like the high gain behavior look at the mod options.

 

[tedsorvino]

Even -6db Gr is way too modest at that kind of level ( -25db would be more like it).
On the other hand the output waveform on the oscilloscope has a shape difference to the input one but no real reduction in size. So it’s not the vu meter showing something really wrong (even if it’s not calibrated for gain reduction readings, but in the simple level measuring mode, things are obvious anyways).

 

[dmp]

Thanks for the input Dmp, but i seriously doubt that my problem is cabling. Shielding is used to avoid hum and noise, twisting is a good practice to be done to every pair. I’m not talking about noise or quality of signal. I’m talking about db gain reduction quantity. Compression quantity (gain reduction) is my problem and not sound quality. Can noise influence that? I’m not sure.

Look at who I am replying to in that post (not you)

 

[dmp]

i haven’t made any calibration on trimpots yet but no trim pot is fully turned either side).

Turn up the stereo adj all the way. If that is turned down you will have very little GR

 

[tedsorvino]

Turn up the stereo adj all the way. If that is turned down you will have very little GR

Sorry for assuming your post was for me. I’v already tried adjusting the trimpo. Not working. Of course i’ll try again.
Actually what i realise by watching the calibration video is that i have no peak reduction showing, even if on the oscilloscope things work as they are showed in the gain level section (with peak reduction of -3db added).
So something goes wrong around the peak reduction circuit on both sides. Or i should work it a bit more carefully, or read more in the thread. Thanks for the help so far anyways, Dmp.