La 2 limiter home brew question 3

[John Fisher]

OK! After asking a few other questions (possibly like throwing a rock at a bee hive) I had one more thing I was wondering about. So far with my ideas in question 1 and 2, I have not taken out any of the tubes in the signal path so that lovely tube sound is still there, but why not ditch the 2 tubes used for the optical driver and just stick another op amp there? After all, it will serve the same purpose and we are still not effecting the actual signal path. The LA 3 and 4 are all solid state. I have never tried or heard them but the thought of all solid state to me is like wearing synthetic underwear, but if I was able to make these changes effectively that would eliminate a huge cost and reduce a lot of space. My goal is to build a couple if these for my live shows so reducing the size is an attractive factor.
Thanks in advance for any comments

[km6xz]

I sure would not put that kind of gear on the road, not as consistent and repeatable as other topographies.
The LA-2a has a unique sound because of the non-linear response of the T4 opto attenuator, its attach and curve relies on the slow response to the electro-luminescent panel and CS photo resistors. The LA-4 used LEDs as a variable light source so attack is determined by external drive circuit. They are not that similar.
It the circuit has any tubes in it, it is better to use tube panel driver because the T4 requires a lot of output swing. Their light output is greatly determined by frequency and duty cycle of the drive signal. If you have real T4 modules, the heart of the LA-4 sound you might as well make it all tube since you need the high voltage anyway. Modern compressor/limiters are much more flexible and suited to road use since the gain cells are pretty transparent which means the characteristics are mostly determined by the side change characteristics with any time constants and frequency sensitivity you might want. I used the LA-2s a lot in the day, they were easy to get a good sound in the studio but they were not terrible versatile. They were the main levelers in the rack but when a critical application where more control was needed, they were bypassed.
Considering the state of concert sound, what specific characteristic are you wanting from them for stage use? If you are using them in the stage instrument rack, you know it is all going to be compressed heavily again in the FOH, not for sound but for reducing the dynamic range, decreasing average to peak level ratios to make it sound louder while staying under the IDP(Instantaneous Destruct Power) of the speakers

[John Fisher]

I sure would not put that kind of gear on the road, not as consistent and repeatable as other topographies.
The LA-2a has a unique sound because of the non-linear response of the T4 opto attenuator, its attach and curve relies on the slow response to the electro-luminescent panel and CS photo resistors. The LA-4 used LEDs as a variable light source so attack is determined by external drive circuit. They are not that similar.
It the circuit has any tubes in it, it is better to use tube panel driver because the T4 requires a lot of output swing. Their light output is greatly determined by frequency and duty cycle of the drive signal. If you have real T4 modules, the heart of the LA-4 sound you might as well make it all tube since you need the high voltage anyway. Modern compressor/limiters are much more flexible and suited to road use since the gain cells are pretty transparent which means the characteristics are mostly determined by the side change characteristics with any time constants and frequency sensitivity you might want. I used the LA-2s a lot in the day, they were easy to get a good sound in the studio but they were not terrible versatile. They were the main levelers in the rack but when a critical application where more control was needed, they were bypassed.
Considering the state of concert sound, what specific characteristic are you wanting from them for stage use? If you are using them in the stage instrument rack, you know it is all going to be compressed heavily again in the FOH, not for sound but for reducing the dynamic range, decreasing average to peak level ratios to make it sound louder while staying under the IDP(Instantaneous Destruct Power) of the speakers

Great response to my post! I assumed that the LA 2 had a fairly lineal response but apparently it doesn't. I realize that to drive the T4 opto attenuator I would need tubes with high voltages but my main idea in this case was to use LEDs which would require less voltage and also a reduction in space using solid state. I also realize that the the LA 2 is very simple or limited in its use as a flexible compressor to where there are many flexible ones on the market. When recording vocals or acoustic guitar in the studio I just simply loved the sound of my LA 2 clone. Very simple to use and it just makes everything sound great. I thought that if I can make up something similar for the road (2 units or 2 in one) I can achieve the same bliss. I also thought that if I can make a couple of these short cuts I can possibly come up with something that is good in sound but more compact. I figured I can drive a led with a solid state circuit while still going through the tubes in the in the signal path. I guess I will just have to try it to see if it sounds good.
The other questions I had were, is it possible to eliminate the input and output trannys using IC's to get comparable results. What does a transformer do that an IC can't except in some cases limit the frequency response if the transformer is not that good.
Thanks!

[km6xz]

The reason the LA-2 has a unique sound and is so forgiving in set up is the combination of the non-linear, frequency dependent response of the EL panel, the old CS photo-resistor and tube driver running into a varying load Z of the EL. The rest of the the audio change is nothing unique. Putting input and output transformers in the chain impacts the sound also. It was designed for terminated output and low z input so like gear was in the early 60s when it came out(when everything was matched for power transfer not voltage as now with bridging input, dbm instead of dbv) so just that aspect has an impact when put into a bridging modern system of very low drive Z and very high input Z.
The reality is that thousands have tried to create a solid state version but they just do not have the character of the LA-2. If you wanted to make a low weight and small size unit, make a copy of the driver and attenuator sections including the T4 and use the whole audio path out of a couple opamps since the only attenuation action if the passive resistor inside the T4(actually there are 3 CS photocells inside a T4 which also drives the meter).
A lot of people liked the LA-2 for the reason you said, simple to set and forget on a session, very forgiving and it gave a fatter, fuller sound for instruments or vocals without much fuss, but it also sounded a little darker which for some vocals and instruments, was exactly what was needed for a warmer sound. Its biggest effect was heard on instruments that have a wider DR and low average to peak levels like overheads, female vocals, anything acoustic, cymbals, etc, but worked fine on crunch guitar and drums as well but not as dramatic of tone character enhancement. I doubt I ever recorded a female singer without a LA-2 as primary dynamics unit. There were outtakes still floating around of vocals on some Fleetwood Mac "Rumors" album with LA-2's and the same tracks with FairChild 670 and LA-3 we did as experiments in getting the sonic "formula" for the album and it shows off why it became the secret weapon of many a producer.
If you want the closest to LA-2 sound and simplicity, get a small, solid-state LA-3. It is as close as anything has gotten to the sound since if used a T4 also. It is small, light and roadworthy. Copying the circuit would be much easier. The LA-4 is readily available but some say a step backwards, it is more versatile but un-inspired class AB circuit just does not have the magic of the pure class A LA-2 and solid state LA-3. If you are into copying things, there are several close enough kits out there and also for the 1176, another item of legends. You can buy a LA-4 for $500 used.
I have used a lot of compressors and limiters and the LA-2a is by far the simplest to get positive results without a lot of tweaking on more types of program material for more types of indented results. The 1176 is second. I doubt I ever did a record that did not use either or both of those units. My old studio had 14 LA-2a's and 12 1176's so about everything done had those two someplace in the process, either tracks or stereo bus for mix.
Another favorite is very small, cheap and cool, the RNC by FMR, a couple hundred $ and it is a simple, little dynamic range modifier that sounds really good, almost transparent on vocals and acoustic instruments, yet A/B'ing between in and out, you really hear a pleasant difference.
Another favorite for more radical change is the Distressor. The SSL 4000G series consoles had a great compressor that is available as a stand-alone. There are lots of units out there all have some limits and some optimum conditions for use. Specs mean nothing in compressors, just results.
I could write all day about compressors, they are an essential tool of recording and performance and the least understood and least used by players, who tend to think they need special amps to get the sound they are seeking to emulate from their favorite guitarist. The reality is that whatever the amp the star is using they want to copy, the amp is the least influence on the sound they hear from stage or recording. But that futile quest drives a lot of amp sales, magic transformer and capacitor sales. People used to buy records to get the sounds they wanted to hear and now they buy an endless series of gear trying to emulate what they admire on stage or recordings. If they thought more like a producer and less as a fan, they will find that the frustration level would lower right away, and for less money.