LA2A PCB ver 2.0 build thread
« Reply #260 on: May 26, 2008, 05:03:30 PM »
Hello christoph,

Yes, as you can see on the LA-2A schematic, the "P" point of the "Top Meter Switch" layout on the PCB is connected with R24 (3.9K) directly to the "-" of the VU meter. So we'll always have a 3.9K between each connection (+4, +10, GR) and the VU Meter (that will substitute the 3.6k needed for the Sifam VU).
On the SPDT solution, the point is to connect (P with 2) or (P with 3) double throw of course, so you can choose (P with 2) to read the Gain Reduction or (P with 3) to read the +4 Output. the unique difference to my solution is that I have a third connection that put a 6.8k in series, so you'll be able to read +10 output.
Maybe is better to put some drawing to help to understand...

Cheers  :guinness:  :guinness:  :guinness:

Eddie  :thumb:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #261 on: May 26, 2008, 06:14:33 PM »
Hey all!

Here's a UGLY DRAWING from what I'm talking about... the solution to get (+4, GR, +10).
Please let me know if someone has any Idea about what's wrong with my VU reading... thanksssss
Sorry about the terrible drawing... hehehehe  :wink:

Cheers  :guinness:  :guinness:  :guinness:

Eddie :thumb:
Keep on Rockin' and DIYing!!!!


Help please...
« Reply #262 on: May 28, 2008, 10:05:45 AM »
Well, I realize that what's getting my VU's pointing meter strike down with the right side, is a HUGE noise, hum, whatever  :?
I put a 0dB 1khz sine wav in the input, and the output kind of blowing out my scale... I measure almost 21V of noise at the output, with the gain pot at maximum... the gain's pot at minimum level, the noise still there!
Even with no valves at all, the hummmmmm still there and LOUD... all the ground stuff are tested and is on it´s supposed to be. Well, I actually connect the outpu XLR' pin 1 to the same ground as the pad #3, don't know if makes any difference... I think it don't.
I don't know what's happening, neither where and how do I start to troubleshoot that... the circuit is simple, but I'm a little bit scarred with voltage issues... I have an PC based Oscilloscope, with 10x probes, and I'm affraid to dammage that too with wrong measurements...
Could anyone help a desperate rookie?

Cheer  :guinness:

Eddie  :cry:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #263 on: May 28, 2008, 10:09:53 AM »
I measure almost 21V of noise at the output, with the gain pot at maximum...

I can't really help your troubleshoot the thing besides regular errors (shorts, faulty componentsm etc..) since I'm a DIY rookie myself - but the la-2a has massive gain. I run mine at 9 o'clock maximum (with a lot gain reduction), otherwise the pot needs to be "just" open (7, 8 o'clock) to pass signal at unity gain.

The hum problem is present with the sidechain tubes + t4b removed?



LA2A PCB ver 2.0 build thread
« Reply #264 on: May 28, 2008, 10:32:47 AM »
Hello... yeah, the humm exists even with no tubes fitted... I didn't dare to put the T4B yet... with this problems... no.
But the reduction stage appears to be right, I actually regulate the zero GR's point without no problem, and the is no oscillation, the pointer stays there where It's supposed to be! strange behaviour???
All the Power supply are tested and running perfectly! the voltages (AC 228V, Xformer primary 228V, the high voltage 520V, and the green filament 6.3V) all tested!!!
I'm thinking about in supply a 1khz wav at the input again, and "hear" somehow through the circuit, and try to understand what is wrong...

Well... thanks Christoph anyways

Eddie  :sad:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #265 on: May 28, 2008, 09:58:02 PM »

I'm correcting one thing on my last post... with no tubes, the meter at +4 switch position stay still at -7 on the scale, on the +10 position stays still at -10.

It's supposed to get the meter's pointer full left, isn't it? no tubes.

I tried to fit one tube at a time, and the meter stills on scale's -7 / -10, like with no tubes fitted!!!
BUT, when I fit the 12AH7... the thing occurs! the meter strikes hard to the right side...
Any ideas? help pleeeaaasssseeee

Cheers  :guinness:

Eddie  :roll:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #266 on: May 31, 2008, 02:34:49 PM »
used a radio shack 5 tag soldering post and mounted it to the case.
then soldered the resistors and caps to it .
soldered the 220k resistor to r29 b+ side (not the 300V side)
i felt there was no reason to string 275v accross the case )
attached a wire to the resistor and connected it to the proper place
as shown below.

put the ground to the transformer star gnd.

greg, one question to your hum-solving suggestion: My transformer doesn't have center-tap on the secondaries, could I still connect your circuit to the "fake" 2x100ohm 2W resistor center tap?



Finally It works!!!!
« Reply #267 on: June 02, 2008, 08:56:38 PM »
Hello all,

Finaly I have my unit working! Well, I tried everything, redid the wiring, checked all the soldering and parts values, and the problem was...........
When I desoldered and lifted the Input transformer (Sowter 4383C) that was hooked on the PCB, just to see if I made some mistake in the transformer orientation, I realized checking the transformer, that the transformer capsule is connected with one of the pins... so, shame on me, when I hooked the transformer into the PCB, I went all way down with the transformer's pins through the PCB and the tansformer's enclosure made contact with all the pins in the PCB shorting them all! ***** arrrghhhhhhhh!!!!! somebody whip me!!!!

I should have paied attention to the Gregory's manual, that as you can see in two pictures... the input transformer's capsule doesn't touch the PCB ...

This will avoid hummss, and mistakes like mine! the unit is pretty silent, Take care with the ground stuff.
Cheers  :guinness:

Eddie  :thumb:

Ps.: The +10, GR, +4 solution that I'd described above works fine... if you want, use it with no fear... it's correct... well, at least one thing working since the start  :roll:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #268 on: June 03, 2008, 03:13:05 AM »



12AX7 and 12AX7A differences?
« Reply #269 on: June 04, 2008, 07:19:01 PM »
I just got my Silent-Arts Dual PCb and I am comparing it with my Drip V2.The Drip PCB has the miniature sockets for the 12AX7 whereas the Silent Arts version has the regular size.I was under the impression that the 12AX7A was the same tube but low noise for audio application.Are the tubes pin-to-pin dimensions different?
you're a nut ... this is the internet..we're all nuts


LA2A PCB ver 2.0 build thread
« Reply #270 on: June 04, 2008, 09:28:37 PM »
Thanks Christoph!

It works nice! I'll live some pics later!

Cheers  :guinness:

Eddie  :thumb:
Keep on Rockin' and DIYing!!!!


LA2A PCB ver 2.0 build thread
« Reply #271 on: June 17, 2008, 11:05:57 AM »
Howdy folks, stuffing parts and studying various versions here.  

Notice R25 on Drip 2.0 is labeled 33K, while various schematics list it as variable 18K-100K.  Meant to be trimmed for T4 cell matching.  

Has anyone delved into this at all?  Have any comments on the advantage of trimming versus going with 33K and walking away?  Greg, are your T4's matched during build in such a way that 33K is specific to your version?

Related:  SSLtech posted a method for trimming related comp/limit resistor R7 somewhere in the hundreds of pages, which I wish I could point out easily.  Keith, anything you can add about R25?


-----EDIT---found SSLtech quote--------

You adjust R25 until it matches. Without calibrating R25, they don't track accuratley on ANY LA-2a.

Replace R25 with a trimpot of about 1.5 or 2 times the stated value of R25 then start it halfway and NEVER take it all the way to either extreme...

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.

The only way to get them to track correctly is to MATCH the optos in your T4b. Then you re-adjust R25 and try again.

Read the comments by David Kulka and myself in the T4B threads, and you should be able to find out a lot. T4B matching and behaviour research is something that I've done a LOT of...

If you buy a NOS T4B from David and plug that in, you should find that there is a better match between optos, since they DID select and match the optos. Setting R25 will then give you about the closest match that you can get.

However, if you've not adjusted and set the value of R25, you'll NEVER getting them to track would be like winning the lottery, and if you put a different T4 in there, it wouldn't track again, until you adjusted R25 again.

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


LA2A PCB ver 2.0 build thread
« Reply #272 on: June 17, 2008, 12:38:27 PM »
WHere is DripE?

I have been trying to get a couple T4Bs from him.

yo bro email me.
Tube limiters..yum!


LA2A PCB ver 2.0 build thread
« Reply #273 on: August 18, 2008, 08:27:52 AM »
I've just implemented the center tap mod from drip, but my (very low) hum/buzz is still audible....

any other ideas?



LA2A PCB ver 2.0 build thread
« Reply #274 on: August 21, 2008, 04:52:31 AM »
hi there , lost in research ,

emrr ,

all t4bs from urei/jbl come with a 33k resistor ,

it would be beter to use a pot to find the correct value ,

ive always used 33k and it seems to work perfectly ,

but the correct way , is to hook up a pot insted of the fixed resistor ,

then run a 1k tone through the la2a and also a mixing board with a nice

vu / led meter ,  turn the compression up till it drops the vu meter on the
mixing board say 3 db  , then dial in the pot till the la2a meter reads -3db.

thats the perfect matching .


hi canakin , man im sorry i missed your email ,

hot mail is so awful sometimes ,

i finally got in hundreds of panels , the fiasco with the panel manufacturer
lasted it seems 20 some weeks.

finishing the group buy first , and fed x ing to silent ,

then ill start on the rest of the t4bs ,

should have enough panels to last a while .

sorry for the awful wait .


cristoph did you try the ground lifts on both the input and the output xlrs ?


[email protected]
(not so good with PM's so dont pm...use email)
la2a ver 3 atoms manual :
v72 manual :


LA2A PCB ver 2.0 build thread
« Reply #275 on: October 02, 2008, 10:04:11 AM »
Hi there !

.. it seems to be a very nice DIY project. Does somebody have any audio sample from this compressor, so as to hear how it sounds ? ..  do you have compared it to the original LA2A , if there are lots of diffrence ?

thanks for your light  :grin:
Vive la House, vive le Funk !!


does anybody have v1 drip manual?
« Reply #276 on: October 13, 2008, 03:32:15 PM »
I finally got around to assembling a v1, got the T4b (thanks Greg) and have relatively proper Dc voltages. I did a quick audio test, and nothing yet. out the door for a tour, so won't be able to get back to this for a couple of weeks.
Wondering if anyone has the 1srt version manual. I am trying to sort out a couple of ground issues that don't seem to line up with the schematic.
In particular the neon and R25 shound be sharing a ground with pin 6 of the T4B, but they don't on the board. I see a little bit of oscillating DC around the neon..have installed grid stoppers already, using sowter in and out..

I think if i could look at the manual i could make quick work of it.


LA2A PCB ver 2.0 build thread
« Reply #277 on: October 16, 2008, 03:37:28 AM »



LA2A PCB ver 2.0 build thread
« Reply #278 on: October 16, 2008, 04:11:26 AM »
That hinge can cause a lot of problems.
Once you get it working, you do not need the cover open and closed a million times anyway.
I have had grounding and oscillation issues with that hinged cover, I might tried the closed box next time.

Just C clap it to the rack or use Campy quick release.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page:
Guitar Amps:


LA2A PCB ver 2.0 build thread
« Reply #279 on: October 16, 2008, 04:19:35 AM »
What do you mean by "hinge"? Sorry, my english is not 100% great hehe
Could u explain a bit more please?


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