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LA-2A compressor (re: Trace on EVH in "apology")


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4/27/2000 12:21 AM
CG
LA-2A compressor (re: Trace on EVH in "apology")
Many recording engineers are familiar with these tube compressors (LA-2A). They are about $2,800 now (used or for the reissues) but when you jack them up they break up in a very musical way! Andy Johns used it on the song "Black Dog" from Zeppelin. There was nothing at that time (amp wise) that would get that kind if distortion so he ran JP's guitar signal into the LA-2A and crank it to get the extra gain. Everyone thought it was an amp so you can see how musical it can be when it breaks up. ;-)
 
 
In case anyone's interested in playing with an LA-2A for less than 2800 bux...  
 
The schematic, the original operation manual and sources for the more unusual bits are at http://w3.one.net/~robgrow/circuits/la-2a/la-2a.html  
 
A note on another site points out this error in the schematic:
The LA2A schematic contains an oversight: Pins 3 and 8 of the 12AX7A in V3A and V3B should directly be tied together (that is, above the capacitor and resistor.)
 
 
The re-issue itself has a home page at http://www.uaudio.com/products/LA-2A.html and includes a shot of the guts.  
 
There are some suggestions at the first site for improving the circut with modern parts and techniques (regulated suply, better output stage etc) but it doesn't mention anything about stereo operation. The way I heard it this type of compressor would compress the two channels of a stereo signal inconsistantly due to poor matching of the photo conductive cells. I also heard that modern electro-optical compressors had pretty muched licked this problem. That's all I know. Can anyone tell me more?  
 
-Carl Gigun
 
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4/27/2000 1:02 AM
Ken Gilbert

nice links, carl, thanks.  
 
ken
 
4/27/2000 4:01 PM
Trace

quote:
"but it doesn't mention anything about stereo operation."
 
 
They are mono units. Now $2,800 seems steep for a mono unit but they are magical. They also used Jensen trannys in the originals.  
 
The other widely used compressor is the black faced 1187 and that also rules!!!! I haven't seen any schemtics for it though. I use both the LA-2A and the 1187 all the time and let me say that NOTHING sound like these do.  
 
As a side note my favorite EQ of all time (outboard) is the old Pultecs. They sound like God in a box! (ha, ha)  
 
Trace
 
4/27/2000 6:08 PM
Ivan
Has any one build these things and what are the results?  
 
Ivan
 
4/28/2000 12:21 AM
CG
Re: LA-2A compressor - Pultec and 1176 schems
Well I guess stereo matching won't be an issue then ;-) My goof, I've got a lot of learning to do still, but I sure am enjoying it! Seriously, I did read that it was difficult to create a stereo compressor with this technique, and that this difficulty has been all but solved in similar modern circuts. Maybe this is why the LA-2A was mono?  
 
About the 1187 and the pultec stuff, I haven't seen anywhere handing out free schematics for those (I couldn't even find out whether the 1187 was it's own model, or just a typo), but you can buy/trade for some pretty big name stuff through some schematics collectors. The only link I have is a fella from Vancouver Island, BC which is very near my hometown of Vancouver (though I live and work in the SF Bay area these days). He trades/sells schematics and has some Pultec EQ schematics and the 1176 and 1178 compressors.  
 
that's at http://www.nanaimo.ark.com/~pat/  
 
Quite a lot of stuff there, but I've never ordered/traded for anything from him so I can't comment on reliability etc... I'm still working on my first "from scratch" guitar amp, which will likely be pretty humourous at first. Hopefully it'll at least be unique. And playable. Playable is important :-)  
 
Have fun with the schematics.  
 
-carl gigun
 
4/28/2000 6:48 PM
Trace

quote:
"About the 1187 and the pultec stuff, I haven't seen anywhere handing out free schematics for those (I couldn't even find out whether the 1187 was it's own model, or just a typo)"
 
 
Opsss.....it was supposed to be the 1176. I get finger tied sometimes! (ha, ha) I emailed that gentleman about the schematics. Thanks for the link!  
 
Trace
 
4/28/2000 8:39 PM
Gus

Trace I just recieved a broken 1176(mine to keep) Is this a good unit? looks very well made. However I don't like transistor sockets would I hurt its value If I soldered the transistors in?
 

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