D-LA2A Support Thread

[jplebre]

I'll try and power on the T4B I made with the EL panels from Frank at eletrochronic this weekend on next weeken and will let you know
you can build the 2 T4b for a third of a price of one T4B if it works correctly

 

[delaymix]

Hi, I'm building this compressor, but I'm having some problems.

In the left channel I have some oscillation when the input level is at maximum.
I changed the tubes by other different and I have also changed the tubes from one channel to another and the problem remains the same.
The right channel works perfectly, no noise, no vibrations, everything works fine (this channel is the one that is physically closer to the power transformer). I think the first tube circuit is overly sensitive to gain, so I'm trying to solve this, but do not know how to deal with this problem. All voltages are correct, all the pieces are well. Any idea how can solve this problem?

Another thing that worries me a lot is that the input level control responds too quickly, meaning that just give a little input gain almost to the very high signal level. If anyone has any suggestions are welcome. Thanks.

 

[desol]

There's a few different solutions for the high makeup gain throughout the thread. As far as the oscillations, how are the transformers placed in the enclosure?

 

[Biasrocks]

In the left channel I have some oscillation when the input level is at maximum.

The right channel works perfectly, no noise, no vibrations, everything works fine (this channel is the one that is physically closer to the power transformer). I think the first tube circuit is overly sensitive to gain, so I'm trying to solve this, but do not know how to deal with this problem. All voltages are correct, all the pieces are well. Any idea how can solve this problem?

Another thing that worries me a lot is that the input level control responds too quickly, meaning that just give a little input gain almost to the very high signal level. If anyone has any suggestions are welcome. Thanks.

Easy, don't turn it up more than you need to.

You can pad the input.

When I have the input padded on my stock LA2A the gain rarely goes beyond the half way point for line level sources, without the pad you don't get past 1/4 up.

Regards,
Mark

 

[delaymix]

I have read the thread about seventeen times ... but I have not found anything like that, so I will have to read again ...

As am using Sowter transformers 4383 for input and 8940 for the output. I think they are reasonably well placed in the back of the box, but I tried to get them out of the case and the oscillation problem persists with the input level to the maximum in the left channel.

[quote author=Biasrocks]

Easy, don't turn it up more than you need to.

You can pad the input.

When I have the input padded on my stock LA2A the gain rarely goes beyond the half way point for line level sources, without the pad you don't get past 1/4 up.
[/quote]

I appreciate the answer, but wanted to be more empirical. The right channel is working properly. There no oscillation. I understand that I will not introduce a signal with a high input level as ever, but I like to get the best technical conditions for the two channels, without noise or oscillation, even with the input level to full.

In another vein, I've never worked with a LA-2A original so I do not know always whether to use an attenuator at the input to process line signals. Do you want to say that this is one of the solutions to obtain a more reasonable way in the input level pot? Thanks again for your help.

 

[desol]

Look for the 'gain mod' in the thread with the search. There's 2 or 3 different approaches.
Maybe Kingston will chime in with some advice.

I used the 25k/75k change and the 12ay7 on 1st stage...(with adjusted plate resistors).
I seem to get quite a bit more swing on the makeup.

 

[jplebre]

stupid question of the day - I'm having trouble controlling the meter on O/p
And I just went back to the schemo and can't find R24 (3.3k) on the PCB.

Do I need to add this out of the board?

If so, do I add it so both O/p and GR signals go through it on the negative side, or only O/p?

Thanks

 

[delaymix]

R24 can be seen in the original scheme whose value is indicated as 3K9. Obviously this mounted outside pcb, the circuit in series with the meter.

 

[jplebre]

Yeah I was about to post that I found a 2010 post saying External resistor for Rx24
That will explain why I had mismatching in gain reading

Any reports from ppl that swapped from edcor 600:16k (i/p) to the sowters 1009 "La2a replacement) or sowter 4383?
Did it colour the sound in any way or saturate or help with the gain? (eg. bring +4dBu from "10" to something more manageable like 20?

I don't want to do the mod just yet as I want to use it as a preamp. I've heard some cool stuff done this way.

EDIT: stupid sentence structure that didn't make sense to myself.

 

[chris77]

Hey folks,

Quick question regarding stereo link ,up untill now i have been using the D-La2a for mono ,when linked do i use one or both peak reduction pots ?


cheers

 

[jplebre]

both. You need to match them

But the calibration procedure is done with only one peak reduction knob - left channel.

 

[audiophreak]

ok ... I know this is soooo simple and i may be wayy over thinking this .. but ,

" Somewhere in there will be a setting which makes the meter agree when switching between GR and +4, after setting a tone to 0VU output in +4 mode with no GR present. Increase the GR and set it so that it matches at around -5dB (5dB gain reduction). It will be maybe a little off at -3dB, maybe a little off at -8dB... a little off all over the scale, but I find that matching the meter at-5dB is usually an excellent compromise." 

  Am I just doing this with the front panel control knobs ??  where & when am I actually adjusting R25 ??? 

  just cant figure out the steps ..,.  :'(

 

[jplebre]

Hey

Yes you use the front panel. Inject a +4dBu signal and set the gain so you get 0vu (+4dBu=0vu). No peak reduction.
swap to GR, adjust "zero adjust" for 0vu. raise peak until you read 3dB or 5dB of reduction (depending on your meter - as it said 5dB is usually a good compromise) and swap it back to 0vu and see if it matches (signal should have gone down as well.

If they are not matching, replace R25, turn peak reduction off, start over: zero adjust for 0vu in GR mode, raise peak reduction until you read 5dB compression, check o/p to see if it matches.

Just leave the gain static and just play with Peak reduction at each step.

set gain w meter on VU (o/p +4) then:
Zero adjust (meter=gr) with no peak reduction > Peak reduction until GR=-5dB > check o/p for consistency > adjust R25 > repeat

 

[audiophreak]

Thanks jplebre !! 

    just finished calibration  ;D ;D ;D  , stereo adjust tomorrow - Pix to come soon  ;D ;D ;D

 

[audiophreak]

Hi All,
        Well.. after two years I finally finished my D-LA2A ... and it sounds WONDERFUL !!!!!   

    Its kind of a "Russian" D-LA2A  ala Kingston style with some Russian PIO caps. Tele ECC83's , GE 6AQ5A's and RCA 12BH7A's , Sowter I/O transformers , Rondo PT, Kenetek T4B's  and of course [silent:arts] beautifully done PCB.

  Thanks to [silent:arts] , Kingston,  jplebre and all who post and make these wonderful projects available ... and the people that trouble shoot them with/for people like me.

    ... I have learned so much ... and have so much more to learn ...

Thanks everyone !!

EDIT :    I used SS over braid on the mains wiring to the power switch on the front panel connected to the star ground as another member did for the "Kingston-duct"  ;D 

... here is a link to more pix  - https://goo.gl/photos/hHc82KsZmSgu3i1c6

ps.  noticed that the 6AQ5a's get REAL HOT  - and the Lytic caps next to them are getting hot from the tubes, not sure if this is something I should be concerned about, just thought I'd mention it... maybe a tube heat sink of sorts ?? 

 

[Uhl]

Hi,

I have just finished my LA2A and while measuring for shorts I noticed that I had a short in the HV rail at one side. I always can measure 0 Ohm between C171/C172 and ground. So I was thinking, maybe it is a bad cap. So i took them out and still I'm getting 0 Ohm. It also doesn't matter if tubes are installed or not. I have the white boards. Has anybody experienced something like this? Is there something I have missed?

Thanks

 

[[silent:arts]]

On both sides?
No problems reported yet with any board.

Check your Multimeter Batteries!

 

[Uhl]

No just on one side. The other side is working fine. I just powered it up, secondary voltages seem fine but one this side no voltage read and after 2-3 seconds it smells like a short to ground.
I'm a bit confused, what could it be if not the caps?

 

[Uhl]

Ah stupid. I have found it R136 had just a small connection to R135. Now it seems to be fine. Voltages are okay. I have to make the 5V connection to the VU meters and then calibration time.

 

[rob_gould]

Hi Silent Arts,

I have a load of this single strand wire which of course twists together and holds its shape beautifully which I'd like to use for the heater wiring...

http://www.rapidonline.com/Cables-Connectors/Rapid-Equipment-Wire-Single-Core-1-0-6-Red-Reel-of-100m-01-0335

It's rated 1kV RMS @ 1.8A.  is this underrated for the heater circuit?  I've seen 2A noted as the max current for this circuit elsewhere in the thread, but of course the voltage of 6.3V is very low compared to the high voltage rating of my wire...

Cheers

Rob