to upgrade a classic unit or leave it as designed?
at the end of the day....there are no rules in the world of
making and recording music.....
The Urei LA-4 is a classic unit and provides a sonically
unique compression, which finds it's use on vocals, bass,
and guitars amongst other sources. The lack of attack and
release controls has an upside, freeing the operator to focus
on the fundamentals of the compression ratio and threshold
whilst exploiting the particular attack and release characteristics
of this opto compressor.
So why tamper with this? Well, the LA-4 uses what are, quite
frankly, very crappy op-amps : The RC-4136 is slow...operating
with a lazy slew rate of 1.7 V/uS, this can induce a nasty
distortion at high frequencies and hot levels. On the upside
it has reasonable noise performance but it's overall distortion
figures are a bit rude.
Normally changing op-amps for another type is very straight
forward, giving due consideration for power supply requirements.
Not so for the 4136, the pinout on a 4136 does not conform
to the "standard" that virtually all modern opamps
use for their pin layout ...... you can't just plug another
IC in.
Thank goodness for the likes of Joe Malone from JLM
for his 4136 adapter, which provides a "translation"
header to take your choice of pairs of dual op-amps then plug
it into the 4136 socket.
So what do you get?
The upgrade that I do replaces all 4136 opamps with high
performance opamps in the audio path and fast op-amps in the
control path and also replaces the output driver IC with one
more suited to driving the 330 ohm load that it sees.
I also replace the caps with high grade bipolar electolytics,
pull the opto cells out and select the best one of the pair
for the audio path. ( opto cells do degrade! ) Align the unit,
repair the VU meter mounting...they are always broken!
And back to the studio.....
Here are some shots of the distortion products of an original
LA-4, taken with a 1kHz input at +10dBm, no compression.
the distortion is measuring at ~ 0.45% THD
note the ugly-ness! - cross-over distortion
Here is the same setup on a modified LA-4, this time the
distortion is reading ~ 0.025% THD, almost 26 dB lower!!
this time the distortion products are all low order 3rd and
some 2nd harmonics
the noise floor has also been lowered by ~ 10dB.
Under compression the distortion rises, this is the character
of the opto cell and is pretty much pure 3rd harmonic, typically
at 10dB compression the distortion rises to 0.3%.
Here is a quote from Billy Field regarding
the LA-4 I upgraded for him.
" - the original LA 4's
always had a very good sounding "dynamic" but wouldn't
put through the quality. I think this is so with almost all
UREI solid state gear, although the re-release 1176 is really
good. (Prob. better than the originals - AT LAST somebody
is doing things GREAT ! Good on you, to the young Puttnams.
Anyway, put simply Rob's LA 4 mod is the BEST non valve compressor
I have EVER heard, and by a mile. In fact I'm not sure right
now that it doesn't even beat the genuinely "superb valve
units" I've heard.
It puts through ALL the "crisp, fat sound"... and
handles the "dynamics" PERFECTLY. It's a bit like
an old DBX 160 , it handles the dynamic right (but the 160
NEVER sounded good enough)
On a vocal the meter sit's on zero like TONE and the attack
is beautiful & clean and the decay is just perfect ! AND
you can not hear ANY compression. Wow this is good. It's a
piece of art ! I love it and my thanks to Rob. Now it is possible
to get some good gear. You virtually couldn't get ANY good
gear in the 80's & early 90's.
GO ROB ! "
Billy "Bad Habits" Field - Sydney 2004
|
