All things LA2a related

[lassoharp]

but i seem to have a bit of a higher noise floor than expected

What figures are you getting?  And how is your noise distributed - getting more 60/120 hum or hiss?

Have you compared your noise floor value to what the published value is for a stock unit?

I can't access the link you posted without registering for tumblr.  If you have a PT inside the case with an A-10 you will be susceptible to picking up hum there within a radius of about 14 in or so.  This can be lessened by rotating the A-10 to a good null point.

 

[rjd2rjd2]

thanks for the help-i havent measured the noise on a bench or anything, just off the cuff "by ear" call on it. i have the A10 on the far side of the case interior from the PS transformer, so there's probably 12-14" btw them. it is mounted to the side of the case on the inside right wall, if looking from the panel side.

is it safe for me to physically hold it and rotate it around? sorry, i don't know what kind of voltage it sees, and want to be careful.

 

[lassoharp]

is it safe for me to physically hold it and rotate it around? sorry, i don't know what kind of voltage it sees, and want to be careful.

Yes it is safe to rotate A-10 - there is no voltage on the input transformer.  If you have some type of spectrum metering on your recording software you can use that to monitor changes in noise/hum level as you rotate.

 

[rjd2rjd2]

SUCCESS! noise floor issue was just due to a mis-monitoring issue in my workbench area. in the studio, the LA2A is nice and loud, and s/n ratio is totally fine.

last question: before i install in the rack permanently, are there any signal path component upgrades i should consider? i've got the $50 mallory 10uf output cap that was suggested in the build notes, but otherwise all the caps are standard stuff from the BOM. 560pf at the freq response cap. anything else that might make a noticeable difference in the signal path? thanks!

 

[dacapitan]

SUCCESS! noise floor issue was just due to a mis-monitoring issue in my workbench area. in the studio, the LA2A is nice and loud, and s/n ratio is totally fine.

last question: before i install in the rack permanently, are there any signal path component upgrades i should consider? i've got the $50 mallory 10uf output cap that was suggested in the build notes, but otherwise all the caps are standard stuff from the BOM. 560pf at the freq response cap. anything else that might make a noticeable difference in the signal path? thanks!

Mundorf caps in the audio path come recommended with this build from what I have found, they are quite pricey but apparently worth the difference - I am still building my unit and have ordered the Mundorf Supreme Silver Oil Caps 10uf (C5) and 0.1uf (C2 and C3) but Mundorf have recently stopped production of the 0.022uf Cap (C1).... so now I need to know what cap would meet the same quality or better the Mundorf Supreme Silver Oil and is available in 0.022uf? any suggestions would be highly appreciated!

 

[Luca]

Hi guys!
I'm finishing building my la2a '65 replica.
There are still two things I couldn't figure out...
Do you know how to choose R7 and R25? I saw on the schematics they are listed as a "range" so I assume I have to choose best value related to the sensivity of my t4b.
What am I going to see in order to find the correct value?
Thank you very much!!!

 

[Deepdark]

Hi guys. I took all measurement and would like to know if they look ok. I foud a voltage chart to compare mine. Just wanna be sure everythingis ok before testing it with my apogee haha. The measurement where made with the T4B installed, UTC Trannys and instead of the adjutable cap, I installed fixed one, around 300K if I remember.. Thanks for your time


directly after the 1n4007 diodes: 370VDC (350V)
directly after the 4.7K/2W: 265VDC (255V)

6AQ5 (V4)

pin2: 7.5V (6,16V)
pin5: 115V (125,7V)
pin6: 137V (104V)

12AX7 (V3)

pin1: 107V (78V)
pin3: 0.75V (0,77V)
pin6: 107V (78V)
pin7: 0V (0V)
pin8: .75V (0,77V)

12BH7 (V2)

pin1: 93V (61V)
pin2: 0V (0V)
pin6: 216V (201V)
pin8: 98V (78V)

12AX7 (V1)

pin1: 90V (132V)
pin3: 1.53V (1,75V)
pin6: 100V (97,5V)

 

[Deepdark]

I was wondering about V1 pin 1 since it is a little higher than the comparaison voltage. Does these measurements look ok?

 

[sr1200]

May want to sub out some resistors in the PS section.  Personally, I wouldn't take that as passing.  I would take a tolerance of maybe 5-10v +/- you're like 30-40+ volts off in some cases.  I do recall having to change 2 resistors for numbers to start getting closer.

 

[Deepdark]

Thanks for the reply. Which resistors are talking about? The first ones after the diodes? Lower them would help getting closer?

 

[Deepdark]

Ok I found the culprit. R21 was 220R instead of 100K.  I'll let you know if it works asap

 

[Deepdark]

Hi again. I put the 100k res. And voltages on V1 and V2 are all in the ballpark. But on V3 and v4, here are my volages:

6aq5:

Pin2: 6,16v
Pin5: 128v (instead of 115v)
Pin6: 108v (instead of 134v)

12ax7 (v3)

Pin1: 84v (instead of 107v)
Pin3: 0,7v
Pin6: 84v (instead of 107v)
Pin7: 0v
Pin8: 0,7v

Are these measurements look ok?  R33. And R35 are ok (220k).

Also, the v4 socket has a center pin. Do I need to ground that pin or I can just don't care of it. Thanks in advance.

 

[sr1200]

I dont recall any center pins.  Check all your grounds and wires against the diagram from recpro audio.  Print it out and go through each part and wire and X them out one by one as you make sure you have it hooked up correctly.  I spent the better part of a week and a half hunting down a single missing ground wire.  As soon as i hooked it up correctly, EVERYTHING fell into place.  Could be something as silly as that.

 

[Deepdark]

Thanks. I already did a complete checkup, highlightening everything on the cayocosta layout. I there was an error in the comparatives values I used. On v4, pin5 is supposed to be around 134v and pin6 around 100v, according to the schematic. So, my v4 is perfect.

 

[Deepdark]

Hi folks. I have a strange issue that I don't undersstand. The voltages from the tubes are all ok, but when I'm passing signal (I tried a 1k tone from Logic and a track of acoustic guitar)  the signal is really low, barely audible. the compression seems to work since I saw the needle moving on gain red. with the acs guitar track. I recheck my wiring, ground, etc. I don'T saw what can causes the output to be so low. I'm using a UTC A10 and UTC A24, and yes, the pin 10 and gnd are tied together and going to the case. I'm at a loss, so if anybody should give me some hint, it would be really nice. Thanks folks

 

[Deepdark]

Forget it folks. Stupid I am  , the TRS jack wasn't completly inserted in the input  :-X ......I hate myself hahaha. Now I have a fully signal passing, great sound.  8)

 

[Deepdark]

Ok folks, I think everything run as expected. Just to be sure everything is ok:

On the gr mode, I saw the needle of the meter moving down when peaks occur, which tells me there is compression applied to the signal (am I alright?), since the more I compress, the more the signal lower and I need to had gain. Is the meter really precise? When the needle hit -10db, is that there is 10db on gain red?

On +4, the needle sit on the left of the meter and move to the right when signal is passing. There is a lot of gain, depending on the input signal. I dont think it is necessary to put the gain pot higher than 12 o'clock. I dont really understand the utility of the meter in this mode, however lolol.

The only thing I would change, is that the v3 voltage on pin 1/6 is only 74v, which is a bit on the low side, so I'll put an 150k res. Instead of the 220k. With a higher voltages, i guess it will result in a greater peak red. Control. I don't know if it's due to my tube (12ax7-eh).

Overall, could someone confirm me that it works correctly, as expected

 

[Deepdark]

Here are 3 files of an electric guitar solo. One with through LA2A and compression, one through LA2A without comp, and one dry, not passing through LA2A

https://soundcloud.com/eric-labrie-composer/guitar-dry-no-comp
https://soundcloud.com/eric-labrie-composer/guitar-la2a-comp
https://soundcloud.com/eric-labrie-composer/guitar-la2a-no-comp

8)

 

[sr1200]

Meter is responding correctly.  Enjoy!

 

[Deepdark]

That's what I do, Enjoying  8) We can really see the compressor works on the files I posted hehe. I'm really happy with how it respond to a lot of source. Next one, maybe a P2P STA Level hehehe