All things LA2a related

[hodad]

I think I've got it figured out--user ignorance. I didn't realize that the signal level to the GR circuit is controlled by the level of your input signal. The Gaiin control has no effect on that part of the circuit, does it?

Anyhow, I boosted the input signal & started to show reduction on the VU. I still have some little tweaks to do, but I'm up & running.

 

[AnalogPackrat]

That is correct. Signal in, transformer, gain reduction, makeup gain is the signal path. Effective threshold is fixed and therefore determined by input signal level only. So crank up the preamp that's driving the LA-2a to "lower" the threshold.

Squash on you crazy pumpkin,
A P

 

[pH]

Transformer question here: I have a Triad A-9J input transformer 600/85000. Also have a Triad output A-55J 15000/600. Anyone know if these would work in an LA-2A?

Sorry, no photos. Found a pic of the A-9J on the 'bay:

http://cgi.cafr.ebay.ca/2-TRIAD-INPUT-TRANSFORMERS-MODEL-A-9J_W0QQitemZ160148606881QQihZ006QQcategoryZ67815QQcmdZViewItem

 

[CJ]

if thats all you got, and you are broke, by all means.
you can always pad the input xfmr if the permalloy craps out into that pleasant mush.
I need to add to my Triad web catalog.

 

[hodad]

Some while back rafafredd mentioned to me that the A9J isn't made for terribly high input levels. There may be a thread here somewhere where he quotes the specs for it, but I can't find it. At any rate, it's probably better suited to other purposes--if you've got options in the matter.

 

[pH]

Nah, don't have to use 'em for this. I acquired a pair of each a while back from another DIYer as part of some unfinished tube pre that I thought about cannibalizing if they would make a good fit. Maybe they would be better as a colored mic pre?

I guess I'll be ordering my Sowters now... :thumb:

 

[CJ]

yes, colored pre.
tub in.
low noise = ef86 family wired for triode first stage, parafeed second.......no wait, thats a v72...

 

[Bluzzi]

So I built my first LA2A with UTC transformers Input and output.

For my second one I've decided to try the Sowter for input. The UTC seems a little too much gain and I think the Sowter is a little tamer.

I did a search on Sowter and LA2A looking for any tips as to what the wiring differences are.

I found the grid stopper mod with 1K resistor. Is there anything else?

I haven't gone through this thread yet and if none of you answer I guess I will but it is getting huge and I've gone through it twice in the past for other research.

So if anyone has any tips that could save me scanning 100s of pages, please don't be shy. :grin:

jim

 

[CJ]

start reading, pal.

you gotta pay your dues, if you wanna sing the bluz...


i never did like it that we never figured out why sowter went weird and utc didn't.
maybe the extra shielding, takes in eddy currents, lowers high end, which means less osc.
correct?

 

[Bluzzi]

Yar a kruel man...but I'll walk the plank now.... a pirate never forgets...

Anyone else have pity on me.....113 pages!

Thanks anyway.

jim

 

[CJ]

it goes pretty quick

the voltage chart link is in the bottom of this post if you dont want to look for that..

 

[Bluzzi]

Its cool CJ, found what I needed.

The grid stoppers are only needed if I use Sowter on the output. I'll only try on the input if at all.

I'm going to try the A24 backward to get a 1:5. My only complaint with the LA2A is its really high gain.

Even with the 12AY7 in there I can't turn my Gain past 9pm without distorting. Otherwise it sounds great, just really touchy on that input. hoping the 1:5 will give a bit more leaway.

jim

 

[hodad]

Bluzzi
I'm with you on the gain thing, but with a lower ratio transformer aren't you going to need to have hotter inputs in order to get the compression right? I'm actually working on a drip la2a as well & was debating on whether to go with a lower input ratio or not, so I'm kind of in the same boat you are.

 

[[silent:arts]]

I changed the gain pot to a 22K log in series with a 75K resistor to get an easier to adjust gain range (still 26dB gain - enough for a compressor / limiter).

 

[emrr]

[quote author="hodad"]Bluzzi
I'm with you on the gain thing, but with a lower ratio transformer aren't you going to need to have hotter inputs in order to get the compression right? I'm actually working on a drip la2a as well & was debating on whether to go with a lower input ratio or not, so I'm kind of in the same boat you are.[/quote]

Look at the original specs; it's meant for down to around -20dbm input, which is mic level. Orig input iron is 20db of gain.

 

[CJ]

Evrybody tests the LA2 with a hot cd line in.
It's just easier to hook up, instaed of digging for a mic, a cable, a good pre, trying to sing and turn knobs is lame, you just patch in from your cd player's rca's out, no wonder the vu pegs, eh?

With all that gain, you can stick an sm 57 on an acoustic, (heaven forbib), run it into a 72/la2, and still be able to get a healthy line out signal.

So when you switch to like a V72 -LA2a Signal Chain, the gain is actually welcome again.

I used expensive Alps 100K pots, so I go not feel bad about using them, thats why they are there.

The only thing that would bug me is a loss of high end when the poot is turned down to 2 or 3.

 

[Bluzzi]

[quote author="hodad"]Bluzzi
I'm with you on the gain thing, but with a lower ratio transformer aren't you going to need to have hotter inputs in order to get the compression right? I'm actually working on a drip la2a as well & was debating on whether to go with a lower input ratio or not, so I'm kind of in the same boat you are.[/quote]

That is the problem to start off. The level coming in is too high for the input. Its way too sensitive.

But we'll only know when we try. The Sowter is almost 1:10 as well so I'll try the backwards A-24 (because I have 3 of them sitting there) first and see what happens.

jim

 

[CJ]

Remember that early LA2's had that plug in pad..


ah yes!...now I remember.......

Maybe the engineer knew his bacon.

A good engineer will put 10 times the control he needs on a pot, just so he does not have to keep f'n with it the rest of its life, once its set, its good for 20 years, regardless of any component changes, drifting, this is called the Monte Carlo side of engineering.

I have learned the hard way about trying to get too fine with trim pots.
Best to have too much control and a touchy pot, rather than not enuff control.

Umm, try 12AY7, 12AT7, 12AV7 (the NYD tube, hmm never tried the one bottle tube in an LA2, somebody should get on that right away...

and change the 220k on v1 to 100 k.

 

[Bluzzi]

CJ - I've already taken your advice on the 12AY7 on my first LA2A and I will do same for this one.

I've noted the resistor change and will try as well.

Thanks, or in your case to go with your new Avatar - sknaht

jim

 

[CJ]

that cannabis creative, thank you for that, don't deport mr seed.