D-LA2A Support Thread

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

audiophreak

Well-known member
Joined
Oct 3, 2008
Messages
884
Location
Litchfield, CT. USA
Thanks tmuikku ,
                        I did some searching and thinking , still a little confused , but this  what I've got so far :

  Kingston posted this calculator link  - http://www.muzique.com/schem/filter.htm
-  to help me with my NewYorkDave One Bottles.

  Looking at the D-LA2A schematic ,  C112  ( 1n ) is the Cap associated with VR137  , using these values in this calculator -
    1M = 159.2Hz
  800k = 199Hz
  600k = 265.4Hz
  400k = 398.1Hz
  200k = 796.2Hz
  100k = 1,592.4Hz
  50k = 3,184.7Hz
  20k = 7,961.8Hz
  15k = 10,615.7Hz
  10k = 15,092.6

    also from the first page on Limiter Response Calibration  - " 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. "   

  ... does this mean its LESS SENSITIVE to lower frequencies  ?  as the Super Side Chain for the GSSL ? 

  I think whats confusing me is that increasing the resistance Lowers the corner frequency - according to what I've found above - and I could be totaly wrong  ;D
 

Kingston

Well-known member
Joined
Nov 1, 2005
Messages
3,712
Location
Helsinki, Finland
That's because it's not a simple RC filter. It's close, but not a plain RC. See the schematic below for a great drawing of LA2A signal chain. Much easier to follow the signal flow there compared to the usual fifties drawings around.

Notice: the 1meg trimmer is in parallel with the 1nF cap. The R of this RC filter is elsewhere.

1. When the sidechain filter is at max resistance the signal is almost completely going through the 1nF cap: now we have a high pass filter, and no bass is going to the sidechain. Calculate it roughly as a 1nF/47k RC filter. But it's actually a bit more complex than that.

2. Lowest resistance is effectively a bypass. No 1nF cap in the signal chain, no high pass, and all bass is now going to signal chain.

I just went with 0R, 500k and 1meg selectable switch. It works well, the steps are not too coarse and you can easily hear what this does to compression. This feature is not subtle, but I would personally have no use for finer range (like 250k and 750k added). A Linear 1meg pot would also work very well.
 

Attachments

  • LA2A-deconstructed.jpg
    LA2A-deconstructed.jpg
    772.6 KB · Views: 206

audiophreak

Well-known member
Joined
Oct 3, 2008
Messages
884
Location
Litchfield, CT. USA
Thanks so Much Kingston  :)
 
    That makes more sense now  ... I was looking for where the ground connection was following the other D-LA2A schematic as with a plain RC . 

  Thanks again for taking the time ,  I gots me some reading to do  ;D
 

benlindell

Well-known member
Joined
Feb 28, 2009
Messages
631
Location
NYC
The only thing I noticed when measuring my sidechain is that the 1M pot probably shouldn't be linear, at 50% it was effectively identical to full, all the tweaking range was between 0-50% I just ordered a log pot to try out and see if I can get a better range out of it.
 

Kingston

Well-known member
Joined
Nov 1, 2005
Messages
3,712
Location
Helsinki, Finland
My experience is quite different. Feeding it a very bassy drum loop and setting compression so that I get 10dB gain reduction. Then switch to 500k and gain reduction goes to about 7dB. Finally set it to 1meg and it goes to 5dB. That to me is an even spread.
 

bkbirge

Well-known member
Joined
Jan 4, 2010
Messages
135
Location
Houston, TX
re: gain mod question & help! I am having trouble getting the gain mod to work the way I expect so I guess I'm doing it wrong, can anyone help? I've tried 100K log, 100K linear (just for the heck of it) and 25k log pots in the gain along with 47k or 100k resistors, either on the wiper to cable or on one of the legs to cable. Almost no difference that I can tell before gain is way too much. It is usable and always has been but I can't get gain past about 8 o'clock on the dial before it's just too much. I've also done the 12ay7 mod and changed R9/13 to 100k's. Clearly I'm missing something obvious, anyone care to clue me in?
 

MicDaddy

Well-known member
GDIY Supporter
Joined
Jul 11, 2010
Messages
869
Location
Lincoln, Nebraska
Alright so I fired up my unit for the first time last night with no T4Bs, no sparks, no flames.

First off, cheap pots both feel, and sound awful  :(  my gain sounds scratchy and grainy when adjusted.  Need to find a nice pot to swap in.

Both channels noise-floor sit ~-74dB with gain at 12O'Clock, with no signal or load at the input, the noise floor has an ever slight hum, I haven't checked on an analyzer but sounds of 60Hz/harmonics and increases with gain. 

Channel 2, the one nearest the PSU/Toroid side, is a touch noisier.  This is with nothing in the Octal socket on either channel. 

I'm only listening to D-LA2A output-->FF UFX frontpanelpre(0dB gain applied)---> ADC  even with the slight hummy noise-floor, I'm surprised how quiet it is.  At max gain the noise-floor is some -65dB with no load at the input.  I'm not sure how to or if I should attempt to bring noise down, or if I just live with this small bit of hum.  Perhaps when shown a load the signal:noise will be so much that it's non-issue?

At first I had my VU/GR switch wired backwards, which then caused me to turn the zero trimmer too far in attempt to zero the meter, which then knocked out the neon  :eek: .  Once I realized this, the neon came back and the meter now rests at 0 in gain reduction mode.

Waiting on T4 cells so figured I'd come on here and blab some.
 

etiefenthaler

Active member
Joined
Sep 12, 2010
Messages
39
Location
Switzerland
Hi,

Thanks to Volker and  this forum for making this build possible.

My unit is working great. no hum, no smoke, still alive  etc..

Pictures below:

IMG_2212.JPG


IMG_2211.JPG


IMG_2195.JPG


IMG_2197.JPG


IMG_2200.JPG


IMG_2202.JPG


IMG_2203.JPG



One issue I have: While there is no hum when using the alpha pots.
I get hum whith the stepped switches. (picture above)
Could someone please give me an advice how to wire this switch?


Thanks a lot again to Volker and this Forum.

Erich T.


p.s. amplifier without "l"  8)



 

etiefenthaler

Active member
Joined
Sep 12, 2010
Messages
39
Location
Switzerland
Hi,

since I got some pm questions I try to reply here:

Lamps get V from relay 5V. Voltage for meter leds is reduced through resistor to 4V.
regulators heatsink temp is 39° C.

Original Track recorded by college students (not at my studio)
http://www.flutemusic.ch/mp3/vocal%20raw.wav

here through dla2a
http://www.flutemusic.ch/mp3/vocal%20dla2a.wav


Noise:
Setting: no input, both PR and Gain at 12 o clock then gradually raised gain noise at 3 o clock pm
then raised PR to full and raised gain to fully open.

http://www.flutemusic.ch/mp3/dla2a%20noise.wav

Erich
 

etiefenthaler

Active member
Joined
Sep 12, 2010
Messages
39
Location
Switzerland
Hi

I dont  want to be impatient, but i'd like to try the stepped gain pots in my la2a.

Between 1 and 2 stepped values can be meausured. form 0 to 100 kohms.
Between 1 and 3 always 100 kohms.

no connection between brassbackplate and case ground.

thanks

Erich
 

Attachments

  • IMG_22022.JPG
    IMG_22022.JPG
    43.3 KB · Views: 54

MicDaddy

Well-known member
GDIY Supporter
Joined
Jul 11, 2010
Messages
869
Location
Lincoln, Nebraska
:eek: 

OK so my hum problem was user error.  When shown a proper load to the input this thing sounds beautiful!

I was monitoring the output with NO load to the input but when I shorted the input the hum completely went away. 

With proper chain in front of LA2 it sounds very nice, and doesn't take much LA2 gain to match bypass/in.

Just waiting on optos to find out if I screwed something up in the side-chain amp.

I've got Blackburn Mullard 7025 and EH 12BH7A in the line amp, with Tele ECC83 and NOS RCA 6AQ5A in the side-chain amp. 

I can't thank enough everyone in this thread Volker especially for the project, as well as those who posted at Frank's place.  Don't think I'd have made it without reading over and over analyzing the posts in this thread and at Frank's.

I've learned so much from this project, more so than with any other.

Thanks again to all of you posters and again Volker!  Now to get this PM670 rocking  8)

Happy New Year everyone!!    ;D





 

Cardinen

Well-known member
Joined
Feb 11, 2008
Messages
101
Location
Italy
etiefenthaler said:
Hi

I dont  want to be impatient, but i'd like to try the stepped gain pots in my la2a.

Between 1 and 2 stepped values can be meausured. form 0 to 100 kohms.
Between 1 and 3 always 100 kohms.

I've used the same stepped attenuators as yours. Mine is 25K + 75K resistor (too much gain with 100K).
This is the wiring diagram.


 

django

Member
Joined
Jul 13, 2011
Messages
9
Just started up wiring up my LA2A and i'm having some problems wiring up the in / bypass, link / dual and g.r / output switches and i can't find much info on how to wire them up and which way the wires go. Can anyone give me a bit of help with this please?
 

julien31

Well-known member
Joined
Oct 17, 2008
Messages
58
Location
Toulouse,France
I just finished my 2-LA2A and I found that 1 of the output transformer doesn't work (Edcor XSM 10K/600). As it's very long to have 1 more from the US I try to find another supplier. I saw audiomaintenance sells carnhill VTB 2290 (9600:600 gapped) and VTB 2291 (9600:600 ungapped) for a very decent price. Do you think they'll work?
Thanks

Julien
 

Latest posts

Top