increasing headroom on an LA2a New
« on: December 20, 2009, 05:37:53 AM »
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« Last Edit: June 20, 2010, 05:13:52 AM by reanimatorstudio »


Rob Flinn

Re: increasing headroom on an LA2a
« Reply #1 on: December 20, 2009, 06:08:06 AM »
If you use the 250 ohm input winding you will make the la2a more sensitive, because you will increase the ratio of the input transformer & therefore have more gain on the input.  if you want to make it less sensitive on the input you could use a pad before the transformer.

As to your comments regarding the o/p transformer & headroom, I don't think you will get more headroom.  You will get more output level, which may mean the transformer saturates earlier.  Maybe someone else can clarify
regards Rob

shabtek

Re: increasing headroom on an LA2a
« Reply #2 on: December 20, 2009, 09:09:49 AM »
frequency response will most likely change (A24 primary inductance  will be 1/4(?) Z stepdown will be 1/2 (?) . Is it original or clone? If value won't be degraded try altering it. External pads are probably a better idea.
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

Re: increasing headroom on an LA2a
« Reply #3 on: December 20, 2009, 10:18:48 AM »
What problem are you trying to solve, exactly?

The problem with interfacing an LA2A with modern levels is usually too much input, not inadequate output. If you push your output, you won't be able turn your gain knob up past 2 (out of 100) without blowing up the input of the next piece of gear in your chain.

There are a few threads around here that discuss this problem. Solutions include swapping the 12ax7 in V1 with a 12ay7, padding the input, and using a different input transformer with less step up. 

raysolinski

Re: increasing headroom on an LA2a
« Reply #4 on: December 20, 2009, 10:29:51 AM »
More gain? From an LA2a?...WHY? My goodness... my LA when driven with the right amount of signal is tremendously hot!  I never have the gain past 40 or 50 (and my input tranny is 10k-10k, 1:1, not the 1:10 step up on the original)...most people are swapping input transformers and going to 12AY7 tubes to have less gain ;)...The tube output stage needs to see a decent impedance to function well...I don't have any sheets right now for the 12BH7 but 10k seems about the minimum hence a 4 or 5 to 1 step down on the way out so the tube sees 10 or 15K while driving 600 ohms. I am not understanding ..do you want less compression (because of your input sensitivity comment)?...Original ones (and clones true to the design) will do 40 db of gain...If you are compressing 15 db and can't make up that gain something is definately wrong with your unit..or your gain staging!

Cheers,
Ray
"orange whip?..orange whip?...3 orange whips please"

Rob Flinn

Re: increasing headroom on an LA2a
« Reply #5 on: December 20, 2009, 10:38:44 AM »
It's a clone with original UTC's.
Quote
As to your comments regarding the o/p transformer & headroom, I don't think  you will get more headroom.  You will get more output level, which may mean the transformer saturates earlier.

More output= more headroom; You need less makeup for same output so headroom will increase.
It's not the transformer you hear saturating I think when you push it to the limit. I hear tube saturation

True, but, what i meant was the headroom of the amplifier will be the same.  i.e it will still start to distort with the same amount of signal running through it.
« Last Edit: December 20, 2009, 05:39:03 PM by Rob Flinn »
regards Rob

sws2h

Re: increasing headroom on an LA2a
« Reply #6 on: December 20, 2009, 10:58:14 AM »
Here's some mods for a real LA-2A

Replace C1 with a .022 uf MIT PPFX. Replace C2 and C3 with a .1 uf MIT PPMF 600 v. Replace C5 with a Panasonic 22 uf 400 v EB series. Bypass with a MIT .1 uf PPMF 600 volt.
Replace the UTC A-10 with a Jensen JT-11P-1.
Replace the UTC A-24 with a Jensen JT-10K61-1M
Replace the TB-4 opto cell with a fast release version from UREI.
Sell the UTC transformers on EBAY for several hundred dollars. Use the money to pay for these new parts. Be happy.
« Last Edit: December 20, 2009, 11:00:55 AM by sws2h »

Biasrocks

Re: increasing headroom on an LA2a
« Reply #7 on: December 20, 2009, 12:40:40 PM »
I use the LA2a on very hot signals like lead vocal. Input is not the problem, it is just a little inconvenient that's all. But normally my console's fader are some where at -7 dB when I mix. Lead vocals are always more up.

Pad the output with one of these guys.



Or build your own pad.

Mark
http://SharktankPro.com

"I'd rather use an SPX90 than a UA plugin....." Joe Barresi

Re: increasing headroom on an LA2a
« Reply #8 on: December 20, 2009, 01:13:10 PM »
On a studio piece, no reason not to have a fresh strong tube in all stages

kambo

Re: increasing headroom on an LA2a
« Reply #9 on: December 20, 2009, 02:03:55 PM »
where to get those MIT PPFX caps from ?



sws2h

Re: increasing headroom on an LA2a
« Reply #10 on: December 20, 2009, 02:42:00 PM »
partsconnexion.com or Percy Audio. THL Audio also has them, they are in Japan (authorized distributor). You can go to reliablecapacitors.com can look for other dealers.

EmRR

Re: increasing headroom on an LA2a
« Reply #11 on: December 20, 2009, 02:47:18 PM »
On a piece like this, with a big feedback loop, no reason tubes shouldn't last forever with little change in performance, and only show sound and gain differences when absolutely dying.    

A difference in 1st tube type only lessens the gain in the feedback loop; feedback loops like this are self balancing and correcting systems to a high degree.  I don't believe it changes the actual gain of the unit to any real degree.  Less feedback gain, softer clipping sound, different harmonic structure.  

I can't see why anyone would need to pad the output of an LA-2A, unless interfacing a -10 system.  

If you want more output, and usually drive a 10K load, why not use something like an Edcor 15K:10K or 10K:10K for output.  You could easily make it switchable between A-24 and Edcor for a roughly 14 dB change in max output level.  

First version LA-2A had socket for a plug-in input pad.  Put a U pad in front of it designed for 600 ohms, or an H or O if you are driving it with an actual 600 ohm balanced source.    An A-10 can take the average modern level fine, but it can be close enough to the border to start sounding pushed.  The input pad will improve input transformer headroom.  It does nothing to active amp headroom, only changes what the knob position looks like.  Or again, change to a 10K:10K or 15K:15K etc input and reduce input gain by nearly 20 dB.  
Best,

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

kambo

Re: increasing headroom on an LA2a
« Reply #12 on: December 20, 2009, 03:05:32 PM »
thanks for the info sws2h :)

Re: increasing headroom on an LA2a
« Reply #13 on: December 20, 2009, 04:52:16 PM »
Headroom is not gain folks.  And surely padding the output is counter productive for increasing headroom.
You can pick up over 1 dB of headroom in the LA-2a circuit by simply MOVING the supply side of plate resistor R13 (220k) over to the higher voltage side of resistor R16 (68k).  The increased supply ripple at that point is not a factor.  A healthy LA-2a can output about +18 dBm; with the mod described above it can do almost +20 dBm at the same amount of THD.
And don't even think of changing out those UTC's.

raysolinski

Re: increasing headroom on an LA2a
« Reply #14 on: December 20, 2009, 07:48:32 PM »
I am not understanding your gain staging. Are you using the LA2a to cut tracks thru your console on an insert or as an insert on your console during mixdown?  If so, are your inserts balanced? I thought you had mentioned external pres in an earlier post?  Are you going to tape or DAW? What kind of console is it?  Does it have direct outs? Anyone who would tell you to use a step up transformer after the LA has absolutely no idea what he/she is talking about..I don't care how "well known" they are. If your LA is healthy and there is not enough gain there is something else going on...bad cables, impedance mismatching, bad gain staging  etc...I spent this afternoon in the studio using my LA in almost every way imaginable to both analog and DAW and had TOO much gain....Most folks who have had your problem have not been hitting the inputs of the LA hot enough which would have very little to do with your console's faders unless you are feeding the LA from one of your main or buss outputs.

Cheers,
Ray
"orange whip?..orange whip?...3 orange whips please"

CJ

Re: increasing headroom on an LA2a
« Reply #15 on: December 20, 2009, 08:15:52 PM »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

drpat

Re: increasing headroom on an LA2a
« Reply #16 on: December 21, 2009, 03:52:59 AM »
More output= more headroom

Here's some mods for a real LA-2A

[snip]

Replace the UTC A-10 with a Jensen JT-11P-1.
Replace the UTC A-24 with a Jensen JT-10K61-1M

I use the LA2a on very hot signals like lead vocal.

An very experienced producer told me to put a step up transformer after the LA2a.

 :o



Just shoot me!
I use the LA2a on very hot signals like lead vocal.

drpat

Re: increasing headroom on an LA2a
« Reply #17 on: December 21, 2009, 09:40:48 PM »
How well does the Urei LA2A unit work compared to your home made unit?

What operating level are your console inserts working at? If it's -10, prepare to have your peak reduction control higher than you think it should be.

What operating level is your Lynx converter working at?

Have you tried adjusting the V72 without all of the other gear following it (including your LA2A clone)? If your preamp's not set to a known reference, it will be hard to tell how any piece of gear will act when it's plugged into the ass end.

Have you tried the LA2A clone directly after the V72, with nothing after it but the record converters? Just because your Tube-Tech likes your other gear, doesn't mean that everything else will get along with each other.

You gave us examples of how badly your LA2A clone works with poor gain staging... how does it sound when you have the fader at unity gain with proper gain staging leading up to it? Did you try setting it to unity gain by yanking the input on a half normalled patch bay?

I'm still not 100% clear what the actual issue is. Does the unit distort when compressing? Does it distort when not compressing? Does it only distort with the V72 in front of it? Does it not compress enough? Do you feel that you're constantly setting the "GAIN" or "PEAK REDUCTION" controls to their extreme settings to make things work correctly? Does it make a growling noise every time you spit on your dog. Be specific, and you'll avoid all of the willy-nilly advice that people are giving you because they are guessing about what problem you're really having with the unit.
I use the LA2a on very hot signals like lead vocal.

strangeandbouncy

Re: increasing headroom on an LA2a
« Reply #18 on: December 22, 2009, 05:26:11 AM »
What operating level are your Lynxes? I had big trouble at a studio that had an Einstein. Their Protools was set for +18FSD. Board inputs were clipping all over the place. Problem pretty much solved at +15FSD. Are you sure it's not you console inputs that are clipping?
. . . . RUH ROH . . . . .

Kingston

Re: increasing headroom on an LA2a
« Reply #19 on: December 22, 2009, 06:32:08 AM »
I use the LA2a on very hot signals like lead vocal. Input is not the problem, it is just a little inconvenient that's all. But normally my console's fader are some where at -7 dB when I mix. Lead vocals are always more up. If I would put the lead vocal fader on 0dB with my Tubetech CL1B and give accordingly make up on the compressor nothing is wrong. I have more than clean gain free. But on the LA2a, I put the console fader to +7 dB and set the make up gain on the LA2a. If I would do the same way as with the Tubetech I have some distortion. Nice pleasant distortion that's true, but you don't always want that.

Due to language barrier I have trouble understanding what exactly you are doing there. That's a very incoherent explanation. Please be more precise.

Also, dB means absolutely nothing when we don't know the reference, might as well stop telling us what dB you set anything at. Start measuring dBu.

From what I can gather, it looks like you are comparing the TubeTech's max output level (26dBu) with LA2A. That's pointless. LA2A has something like 18dBu clean output before output transformer saturates (of course depending on your choice of transformer). gain stages won't saturate anywhere near this due to the self correcting feedback design. You say you hear "tube saturation". You don't, it's the transformer clipping.

Moving on.

Do you realise both of those max output levels are absolutely ridiculous loud? If you think something should be outputting anything that loud, you need to seriously examine your concepts of gain staging.
« Last Edit: December 22, 2009, 06:50:01 AM by Kingston »


 

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