LA-2a T4b Comparison

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igs

igs

Well-known member
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Apr 18, 2009
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Europe
Comparison of T4B photocells by various producers.


2La_obraz1200.jpg



Recently I had an opportunity to gather together a few T4B photocells used in the legendary TELETRONIX LA2A. Basing on the IGS 2LA replica I compared the T4B opto-cuplers. The T4B photocell is the single most important element responsible for the compressor’s sound. Every T4B circuit is composed of an electroluminescent panel (EL panel) and 2 photoresistors. One of them reduces gain and the other influences the meter. The setting for the compressor are: GAIN 60, PEAK REDUCTION 100.

Read more:

http://www.igsaudio.pl/index.php?option=com_content&task=view&id=51&lang=english


IGS_2LA.jpg

_______________
www.igsaudio.com
 
I find these results kind of shocking, the Drip cell appears to have really poor performance in these tests.

I'd like to see at the very least a full range signal test, perhaps pink/white noise signal with lower levels of reduction.

I generally run my LA2 with no more than 10db of reduction and rarely do I run 1K test tones unless I'm calibrating levels.

Do you sell the TB4 cells individually? On your site it appears that you're only selling completed units.

Mark
 
the t4bx is listed under products->others but not in the price list.
I would definitely be interested in a european source for the t4b.
 
FWIW, I've just checked with my NOS UREI optos in a DLA2A and they have the same release characteristics and a bit of wobble on the start of the test like graph says, though the attack is faster, more like the igs version.
 
Im not sure what I should be looking for in the graph?

Your model seems to flat line the signal, while others put in some flavor in compression response?

Which unit is supposedly producing better compression from the units you have tested and why?

You would know sonically since you are doing the test.
 
Ptownkid said:
I have to be honest here...these test results seem a little suspect. I am however interested in your t4b if they are actually accurate...
Click to expand...

I just did a test on my Drip T4B equipped LA2a with around 10db of gain reduction in compress mode and got similar results to what was posted, although with not as much wobble after the level changes.

Mark
 
That IGS one looks awful... the threshold is so far off that it's not even comparable... It's simply over-squashing everything.

I've got a heap of T4's here of different vintages, and the measurement data is just too deep for me to BEGIN collating here, what with variations in the optos and variations in the EL panels.

01-Cluster.jpg


I need to read more about the comparison conditions, but that last cell behaves NOTHING like the others, nor any of the ones which I've got here.

Keith
 
These test results are of course 100% authentic. I’m glad to see that the subject is met with your interest, this will surely encourage me to perform more detailed tests in the near future.
It is important for me to clarify the aim of this test, so we don’t have any misunderstandings. All photocells were operating at maximum compressor peak reduction (100) not because that is how you use them, but to show their full potential. In some cases you may need this potential.
For those interested in the photocell I have updated my website accordingly.
 
I'd be interested to see comparisons with the peak reduction at a more normal setting, say around 30 or 40.
Honestly, I've never used an LA2A with the peak reduction at 100 so it's not really a meaningful comparison to me.
And is this limit or compress mode? As the 2.7K resistor will affect how hard the T4 is hit with signal.

That's a really good price, by the way.  What do you charge for shipping to the US?
 
tomcat said:
Drip slow or fast tested?
Click to expand...

My Drip opto is the slow version and corresponded fairly closely to the results posted.

The problem I'm seeing is in the quicker attack times and how the release tends to waver on the Drip version
where you don't see that on the UA/Urei version.

Here's the result of my own test with my LA2A, you can see that the wavering is much less but the attack and release times still seems quick, this is the slow version.

DripOptoTest.bmp


And a post from Greg which basically says that the attack/release characteristics are fast on both versions. Although there's no comparison with the Urei version.

Re: Fast and slow T4B?
drip t4's are universal , both fast and slow will work with all brands of la2a .

both the slow and the fast have very quick attack times .

but the fast IS a bit quicker on the attack and the release .

g.
Click to expand...

http://dripelectronics.com/community/showthread.php?tid=153&pid=2312#pid2312

Mark
 
I have an apple and you a pea...

Come on at least compare the slow drip ... this one should be closer to the ureis and co.

Beside that .. if you manage to build good tb4s ... i need 2 normal speed ones ;)

Lets see...
 
tomcat said:
I have an apple and you a pea...

Come on at least compare the slow drip ... this one should be closer to the ureis and co.

Beside that .. if you manage to build good tb4s ... i need 2 normal speed ones ;)

Lets see...
Click to expand...

I agree that the original tests should be comparing apples to apples and makes the Drip look very bad.

My test using a slow version of Greg's T4B was done with a much more common peak reduction setting (30)
resulting in 7db of reduction for the first sample.

Greg's slow version T4B still has faster attack/release characteristics when compared to the Urei/UA versions, even though
it's marketed as an exact copy.

Mark
 
igs said:
My Drip is fast.
Click to expand...

Whooops!

It even sounds completely different to the classic T4B (ie. slow drip). And needs compensation because it does less gain reduction on same settings as the classic/slow version.

Biasrocks said:
Greg's slow version T4B still has faster attack/release characteristics when compared to the Urei/UA versions, even though
it's marketed as an exact copy.
Click to expand...

But you don't know how igs set up his sidechain highpass filter. You would have to match yours exactly to get comparable results. My guess is you have set up quite small amount of (or no) high pass, and igs has a moderate amount dialed in.

...one of those features every LA2A should really have on the front panel.
 
Kingston said:
Biasrocks said:
Greg's slow version T4B still has faster attack/release characteristics when compared to the Urei/UA versions, even though
it's marketed as an exact copy.
Click to expand...

But you don't know how igs set up his sidechain highpass filter. You would have to match yours exactly to get comparable results. My guess is you have set up quite small amount of (or no) high pass, and igs has a moderate amount dialed in.

...one of those features every LA2A should really have on the front panel.
Click to expand...

You're quite right, my highpass is set to the flattest position.

Mark
 
Comparison of T4B photocells by various producers. Part 2.

Seeing the interest in my previous test results I decided to follow up. This time I managed to get a hold of the original UA Teletronix LA-2A and as well as the IGS Audio 2LA. The UA device is equipped with the Teletronix photocell while the IGS 2LA with the T4bX.

2La_odzadu2.jpg


Read more...
http://www.igsaudio.pl/index.php?option=com_content&task=view&id=51&lang=english
_______________
www.igsaudio.com
 

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