So the "sound" of the LA2A is the LDR???

[mkiijam]

By sound I mean "response" cause for sure the tubes and transformers have the "sound." But, the sought after "response" came from the LDR and not so much the EL panel? Like you could mostly use LED's or any light source really, and if you had the right LDR you would have what %?

 

[dmp]

Yes, I think that is correct - anyway more important than the panel. Lots has been posted about this over the years if you search. I measured various LDRs to make T4Bs.
Start taking some measurements if you want to build a T4B.

 

[mkiijam]

Why did they use EL panels then? Fancy?

 

[CJ]

if you read the spec sheet on the original panels it is pretty elaborate, 8 angstroms of light at 43.87 kilo hz shifted 37.5 degrees off the polar axis,

LSI does military stuff so they have to be able to vouch.

i have built t4's with cells that caused audible distortion and do not ask me how but it was really weird to hear that.

depends on how you use the compressor also. if you have the panel lit up a bit all the time, then the cells never have time to recover. if you have the compressor set to catch a peak now and then, then the recovered cells act very quickly.


my most magical T4 has a 5 dollar alarm clock back light EL panel right before they switched to blue LED's, dang, no more cheap panels, it also used cells from the Radio Shack assortment pack which show up from time to time on evilbay.


there are still clairex cells out there but they are spendy.

 

[Brian Roth]

Wind back the history. I've done a bit of checking.

Photo resistors have been used for decades in "compressors and limiters". The light sources included incandescent and neon as well as probably other sources.

The EL panel used by Lawrence in the original LA-0 (g!) was a unique idea. Look up the patent.

The EL panel had a very fast response time when driven by an audio signal.

Hence the LA-2 and LA-3. The LDR was the determining factor for the attack/release and hence the "sound".

Later models (LA-4???) did use a LED instead of the EL panel.

The EL panel had a unique light output which varied upon the applied audio voltage.

Bri

 

[ruffrecords]

Why did they use EL panels then? Fancy?

Because LEDs had not been invented and they were faster than incandescent bulbs.

Cheers

Ian

 

[user 133392]

One of the things to be aware of when coupling light emitters to LDRs are the differences in optical response curves.

For example: Yellow LEDs coupled to LDRs have the best coupling because most LDRs are tuned for sunlight but will not respond the same as an EL coupled to an LDR where the response peaks are offset from one another. My thinking on this is that it not only affects transfer (coupling) efficiency but maybe attack/release time constants.

Also: When an EL panel is driven directly by AC its' output color shifts with frequency. The ones I tried were more green/yellowish at LF and became bluer as the frequency (and current) increased.

This is an old post and many of the Vactrol links to distributors is broken but it does contain Clairex, Silonex and Vactrol catalogs: Clairex Photocell Application Handbook, Vactrols and Silonex - Pro Audio Design Forum