D-LA2A Support Thread

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electrisizer

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Dec 26, 2008
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172
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germany near frankfurt
hey there im in troubleshooting process but need some help

as described before theres nearly no compression (about poor 0.5 db) at max limit resp. + peak red.

i figured out that theres no difference if the drip T4 is build in or not  :eek: its the same on both channels...
the VU needle jerks sometimes a very little bit when i crank the PR pot up thats all.

i hope to isolate the problem with it. gain stage is working really good...

even if im not a beatle:
Help! I need somebody.
Help! Not just anybody.
Help! You know, I need someone, help!
 

toffifee

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Jun 8, 2004
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123
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Münster; Germany
Which ratios do those RK310 have? If they are near 1:1 or even 2:1  (and you wired them as step down), it will be hard to get compression at all. So first try to crank up the level of your input source, and adjust the "stereo adj" trimpots so that the 6AQ5 gets max level.
Could be that you have a functioning unit, but not enough gain into the sidechain.
 

electrisizer

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Dec 26, 2008
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172
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germany near frankfurt
hey thank you!!!

thats it!

the stereo adj was the problem  ;D  i crank it up -> now it works with nice full scale compression...

i tested the unit this morning. the noise floor is around -75-80db thats really good.
like the others i have too much gain. even if i use the grid stoppers i have oscillation when i crank gain pot to much...


theres a question left:

do i still need the 3k9 resistor for the VU? (like it wa said at nrgrecording)


 

Randyman...

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Nov 7, 2010
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Texas
So, since most everyone seems to "get by" w/o the Arco Trimmer Caps and w/o trimmer resistors for the T4B's (33K IIRC) - do you reckon I should just scrap them and see what happens w/o them (used fixed value components)?  I'm having a hard time figuring out how to mount the Arco Trimmer Caps, if I'd need to use shielded wire from PCB to Trimmer Cap, and then how in the heck to properly calibrate them once they are wired up.  I saw a small blurb on how to get the 33K T4B resistor spec'd, but I can't find anything concrete about the trimmer caps.

My Chassis should be here any day now, and the PCB's are stuffed with the exception of the Trimmer Caps.  Since they obviously had a reason for calibrating the original units, I can't help but assume these Arco Trimmer Caps and trimmer resistors for the T4B's should be installed and calibrated to have an "in spec" unit?  I have 4 of the Arco's staring me in the face (seem to be hard to find abroad) - so I'd like to use them.

Thanks for any further info.  I'm hoping to get this done very soon (once the chassis arrives), and I have a project that would benefit immensely with this compressor squashing away "in spec"  :D !

Thanks!  8)
 

drgrateful

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Mar 31, 2010
Messages
51
Location
Italy
I just finished my D-LA2A!  ;D

Everything seems to be working fine, but I can't figure out how to do the stereo adjustment. Maybe it's something simple that I don't understand (I am not an expert by any means) and hopefully someone will be able to help me out. I will detail the steps I followed (essentially, I followed the manual), hoping they will be useful to someone else in this thread:

1) I sent a 700Hz, 1.5Vpp sine wave to both channels.
2) I adjusted the two gain controls for equal output (by checking the VU meters), with peak reduction fully counterclockwise.
3) I made sure that RV103 and RV203 ("stereo adj") were set to  fully counterclockwise (I can here that click).
4) I placed the VU meter selectors in "gain reduction" position.
5) I advanced the gain reduction control of the left channel until about 5 db of reduction were shown on the meters.
6) As expected, the gain reduction was not the same for both channels. Then, I noted for which channel there was more reduction (for me it was the right channel) and turned clockwise RVX03 for that channel  (for me it was RV203), until both meters showed equal reduction.

Now, I would expect that the output levels for the two channels should be the same. For me, the are way different!
If the input levels are the same, and the gain reduction shown by the VU meters is the same, why are the output levels so different?  ??? ???

Thanks for any hint!!!  :)
 

thestudio

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Jul 17, 2004
Messages
351
Seems to be a diffence is price in terms of power transformers Antek totals up to $44 but I see that there's one offered for $66? What's everyone using, prefered? 

 

horvitz

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Apr 3, 2010
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432
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Beverly, MA
I just placed an order for this one:

http://www.antekinc.com/details.php?p=671

What are you looking at for $66?  As far as I can tell, there are only two choices for 250V.  The one with the single secondaries is $29 (and out of stock), the one with the doubles which I bought is $34.50.  Haven't actually received mine yet though so I can't comment on it.
 

Biasrocks

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Windsor, On, Canada
Randyman... said:
My Chassis should be here any day now, and the PCB's are stuffed with the exception of the Trimmer Caps.  Since they obviously had a reason for calibrating the original units, I can't help but assume these Arco Trimmer Caps and trimmer resistors for the T4B's should be installed and calibrated to have an "in spec" unit?  I have 4 of the Arco's staring me in the face (seem to be hard to find abroad) - so I'd like to use them.

Thanks for any further info.  I'm hoping to get this done very soon (once the chassis arrives), and I have a project that would benefit immensely with this compressor squashing away "in spec"  :D !

Thanks!  8)

Haven't tried it yet, but will because I've got a few Arco's sitting in front of me as well, from what I understand at least one of the trimmers affect the overall frequency response of the limiter (C4), the other is there for the sidechain eq iirc. Try using your measurement software and adjusting one of them for the flatest response without gain reduction. The other one you could probably get away with a fixed (recommended) value.

More info here

http://www.groupdiy.com/index.php?topic=254.msg2750#msg2750

and here

http://www.groupdiy.com/index.php?topic=3278.msg145013#msg145013

Mark
 

Randyman...

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Nov 7, 2010
Messages
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Location
Texas
Wow - Good info there - thanks Mark!  Do you know if the Edcor transformers in Azone's BOM will "want" to be flat to 20K, or do they have some natural roll-off that shouldn't be over-compensated for in the primary frequency-response trimmer-cap calibration?  I liked the thought of switchable values in the sidechain - maybe once I get it all tweaked out I can select some fixed values that sound good and go from there.  Or optimize one sidechain for Vox and one for Room or Kick?

Also - How did you mount the Arco Trimmer Caps?  I'm assuming you ran some wires from the PCB pads and mounted them off-board somewhere?  Do these wires need to be shielded?  Does anyone have any pics of Arco Trimmer Caps installed in a D-LA2A build by chance?  I'm still a little unsure how I'm going to place/mount/wire them in my build.

Still waiting on customs to get me my chassis!!!  :D  The tracking info shows it's been sitting still in Customs for close to a month now.  Customs must be a very laid-back "get to it when you can" type job  ::)

Thanks again - Hope to have this baby up and running soon for an important project (important to me  ;) )

8)
 

benlindell

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Feb 28, 2009
Messages
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NYC
horvitz said:
I just placed an order for this one:

http://www.antekinc.com/details.php?p=671

What are you looking at for $66?  As far as I can tell, there are only two choices for 250V.  The one with the single secondaries is $29 (and out of stock), the one with the doubles which I bought is $34.50.  Haven't actually received mine yet though so I can't comment on it.

Thats the one I got, it works great, I added a little 5Volt one with it for the relays.
 

Kingston

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Helsinki, Finland
I won't comment on mounting the Arco caps because that part takes only a little bit of imagination, but a word of advice on them nevertheless:

C4 acts very much like a high shelf filter above something like 10khz. It's in the feedback loop of the make up gain amp, and is used to set a flat response with your choice of output transformer. But this cap is such a central part of the amp it has a great effect on sound. And those Arcos are Sh*te! They do horrible things to the top end. I ended up getting a selection of silver mica caps in the range of the arco (something like 150-500pF if I remember correctly), then tested for the one that gave the flattest frequency response. (the socket idea above is great!)

But also remember that this horrible Arco is the LA2A sound like your grandma used to make. Great for those period perfect clones.

The similar Arco in the sidechain has barely any effect on sound. I had heard that it might have some effect on the compressors high frequency response like a "de-esser", but no matter what material I tried the trimmer had no effect on anything. You can even leave it out, or just use the recommended value. Won't make a difference. The very first LA2A's did not have it either.

The 1meg high-pass filter trimmer in the sidechain on the other hand, that belongs to the front panel of every LA2A. rotary as a 0-250k-500k-750k-1M selector would also work great, maybe on-off-on switch with settings of 0-500k-1M. This controls how much the compressor bites your bass.
 

pacemaker

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Jun 8, 2006
Messages
585
Location
France
mmm nice idea for the sidechain filter Kingston !
is that R37 on the L2A2 schematics ?
I also came across a CJ suggestion in the original LA2A thread
"If you really wanna have some fun, put a pot in place of R11 68 k feedback resistor"
My D-LA2A is in use now so i can't test this "mod"
and i'm trying to understand which effect it will have on sound,
add some nice harmonic distortion ? increase gain ?
do you ahve any idea kingston ?
 

Kingston

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Nov 1, 2005
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Helsinki, Finland
Yes, R37 is the high-pass filter. Has rather extreme effect on sound so like I said, put it on the front panel.

R11 is the global feedback resistor. LA2A make up gain amp is absolutely swimming in negative feedback with that 68k. It's a very linear amp, complete devoid of any recognizable tube distortion. You won't know it's tubes until someone tells you because the excess feedback removes all distortion.

R11 as a pot is basically an extra gain control. Try 50k in series with a 470k pot maybe. It will give you a lot of extra gain (and control), and you will be able make those 12AX7's sound distorted by removing feedback. But 12AX7 distortion sounds like crap and now you have a stupid amount of gain, so you probably don't want that. Harmonic distortion, yes, but not nice.

Solution: replace 12AX7 with 12AY7. You will lose some gain, but will be able to take it back by removing some of the excess feedback. Now the distortion sounds much better.

and NOW you have a tube amp!
 

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