DISCOVERY: Why modern LA2A clones seem to have too much gain(??)

[rainton]

Hey folks,

since my group buys for original-style LA2A chassis (http://groupdiy.com/index.php?topic=59538.0) I obviously spent quite some time exploring these simple, but wonderful sounding compressors.

In the past I always used PEC pots for GAIN and PEAK REDUCTION, because that's what UA uses for their reissues.
I often stumbled over threads or posts where people were complaining about their DIY LA2As having so much gain they could hardly turn up the GAIN knob...

...with my PEC pots I also found the LA2As have tons of gain but it was ok and workable - mostly I had the GAIN in the 20s to 30s - sometimes even more.

Then recently I built another unit - with original components (UTC trannys, NOS caps, etc) BUT I used Apha pots instead and noticed the unit had way more gain - or at least turning up the GAIN would give me a lot more gain much earlier.
Since this unit was - apart from the pots for GAIN and PEAK REDUCTION - pretty close to my other one in terms of components I figured it has to be the pots.

I actually just went for the Alphas this time because I always found the PEC pots extremely expensive, scratchy and not feeling very good when turning the knobs. The Alphas feel much nicer, but their log curve doesn't work for an LA2A AT ALL!
The same goes for the compression using the Alphas - way more compression way earlier than I was used to with the PEC pots.

I repeatedly read on the web that the even though PEC took the design from the old Allen Bradley pots  (that were used in the vintage LA2As and pretty much any design from that era), the PECs are far from the build quality of the ABs.
And that's what I always thought in any studio I worked that had reissue LA2As - the GAIN knob was ALWAYS scratchy. The vintage units NEVER (and the gain behavior of the vintage units was obviously different, but since I was mostly on the other side in the live room when working in studios that had vintage units - it was not that obvious to me - scratchy pots - yes very obvious  ;D)

I found  some NOS  100K Allen & Bradley audio pots in a bulk for much less than the PECs go for and measured their resistance at about a half turn and compared them with the PECs and the Alphas and what I found explained EVERYTHING in terms of the  strange gain behavior of modern LA2As

I measured between the 2 pins so the DMM shows 0 Ohm when the pot is turned fully CCW and turned  up the pot half way
(yes this is not very exact, but enough as you will see):



Alpha pot:  18K Ohms 
PEC: 14 K Ohms
Allen & Bradley: 2K Ohms!!

I measured 5 or 6 of the Allen & Bradley's and they all show kind of the same within 10% tolerance.
So if you deduct the above measurements from the 100K you get the resistance the pot delivers in the LA2A circuit's gain position with the knob turned up half way.
This is a huuuuuge difference!!

I now installed the NOS Allen & Bradley pots in both of my units and discovered:

- both units now have about the same gain behavior
- both units now have about the same peak reduction behavior
- not scratchy at all (compared to the PECS) and feel much nicer than the PECs as well
- unity gain is now somewhere around 50-60 (compared to  somewhere around 10 with the Alphas)

Maybe someone else has found  this before - please let me know and post the link to it - then I'll erase my post...
...if not this will change the discussion about the gain behavior of DIY LA2As  ;D

Martin

 

[Gambaholic]

Just curious if your AB's were made in the U.S. Or made in Mexico?
Does anyone here have a U.S. And Mexican made AB they can compare and report back?

 

[emrr]

That will indeed make a big difference in perception, and isn't it funny that it is indeed all perception since total gain is the same.  The same situation exists with certain vintage guitar amp volume pots, and there are custom guitar amp pots sold to address the perception. 

 

[john12ax7]

Interesting that they made a pot with 2% log behavior. 15% is usually the standard as you see in the other pots.

Do you ever need to turn the pot all the way up? If not then maybe using a normal log pot of 20k or 25k might work for people who can't get NOS ones.

 

[[silent:arts]]

Do you ever need to turn the pot all the way up? If not then maybe using a normal log pot of 20k or 25k might work for people who can't get NOS ones.

This is what I (and many D-LA2A builders) do. 25k pot with a 75k resistor.

 

[Winston OBoogie]

That will indeed make a big difference in perception, and isn't it funny that it is indeed all perception since total gain is the same.  The same situation exists with certain vintage guitar amp volume pots, and there are custom guitar amp pots sold to address the perception.

While at Universal I remember a couple of complaints regarding the 2-610 having slightly different gains for channel A & B,  the issue being the tolerance on the PEC pots for the output level, gain itself being identical.
Every one I measured myself was within a dB throughout the range, but I guess a few were a bit off.

 

[emrr]

Do you ever need to turn the pot all the way up? If not then maybe using a normal log pot of 20k or 25k might work for people who can't get NOS ones.

This is what I (and many D-LA2A builders) do. 25k pot with a 75k resistor.

12 dB pad according to a quick calc?  But that's different and alters total gain, if I read correctly.  Going there, another option with the 'corrrect' transformer is taking the secondary parallel rather than series.  Or off the CT.  Or using a different lower gain transformer.  Or padding the original on the pri.  Etc

 

[DerEber]

12 dB pad according to a quick calc?  But that's different and alters total gain, if I read correctly.  Going there, another option with the 'corrrect' transformer is taking the secondary parallel rather than series.  Or off the CT.  Or using a different lower gain transformer.  Or padding the original on the pri.  Etc

I thought gain reduction is dependent on raw Input level before the gain pot? So changing IP-Transformer will change overall behavior, isn't it?

 

[emrr]

It's dependent on the setting of the peak reduction control, unless that's already maxed out, which I doubt. 
Otherwise this is about the knob looking right, rather than any actual gain change.  If there's excess gain, you can change that with little problem.  The Jensen transformer mod plan is old and established, and it's a 1:1 input, so close to 20dB less max system gain.    Mastering Lab used LA-2A's without input or output transformers at all. 

 

[Gambaholic]

...I found  some NOS  100K Allen & Bradley audio pots in a bulk for much less than the PECs go for and measured their resistance at about a half turn and compared them with the PECs and the Alphas and what I found explained EVERYTHING in terms of the  strange gain behavior of modern LA2As...

Hey Martin,
Do you know what the tolerance is on your AB's? In Jahnsen's book he says the originals were JAIN048P104RA. The R there indicates "modified logarithmic 20% tolerance". I'm wondering because I've found some with an AA designation (rather than RA) which indicates a "modified log 10% tolerance". I know that the PEC version used in the UA Reissue are 10% tolerance. How does this affect the behavior?

 

[emrr]

Almost not. 

 

[Gambaholic]

Almost not.

I appreciate the reply, that's good to know.  Do you think the difference in behavior between the PEC and AB pots is attributed to the 'modified taper'?  Allen Bradley spec's show A and R indicate a 'modified taper' while D and K indicate an 'exact taper'.

 

[emrr]

it's entirely the taper.

 

[hugo]

Just for info.. Would a 10k or 20k resistor on the front of the alpha 100k log make the job ?

 

[athnony]

Just for info.. Would a 10k or 20k resistor on the front of the alpha 100k log make the job ?

I like to think I'm a modern day Frankenstein because I can resurrect a 6 year old thread, but maybe it's worth putting it out there that I used a 100k lin pot with a 2k fixed resistor between GND and wiper to modify the taper. Seems to work great.

Not sure if others have done something similar in their builds... Amplified Parts has a nice little tool for calculating taper here.

 

[Jeff5]

I like to think I'm a modern day Frankenstein because I can resurrect a 6 year old thread, but maybe it's worth putting it out there that I used a 100k lin pot with a 2k fixed resistor between GND and wiper to modify the taper. Seems to work great.

Not sure if others have done something similar in their builds... Amplified Parts has a nice little tool for calculating taper here.

Thanks for that link. It is helpful for me.