I´ve been searching info about this mods and I thought maybe it could be useful for someone. I see there´s some info at the forum already.
LA3A Mods
The original LA 3A (and LA 2A) were designed as broadcast compressors. The gain structure and facilities
of the unit were arranged for that purpose. These units have found wide spread use in the recording studio,
and as such require modification to better be utilized in the studio environment..
1) Relocate Compress/Limit switch to front panel For convenience.
2) Add Link Switch to front panel. For convenience.
3) Add HF Boost switch to front panel
This facility was originally used for broadcast transmission systems utilizing a pre-emphasis
audio curve. This facility was designed to keep these EQed transitions from overloading in the HF region.
Though not designed as such, in recording studios this function is used similar to a deesser, making the compression circuits more sensitive to HF (esses) signals.
4) Set HF boost rear panel control to MAX. FULLY CCW.
5) Relocate Power Switch to front panel.
6) Add BYPASS switch to front panel. Install termination at BYPASS switch (see below)
7) Align LINK facility.
8) Set rear panel gain switch to "50".
This removes the PAD in front of the input transformer. Making better signal to noise ratio.
9) Add 15k resistor in parallel with R14 (220k). equivalent resistance = 13.5k
This reduces the gain taken in the front end by leaving the gain switch in the "50" position.
Both these together also put the GAIN and PEAK REDUCTION controls in a better range for interface in the recording studio.
REMEMBER... Even if you don’t do all the other mods, Terminate the output transformer with 600 ohms
(620 is ok) to prevent frequency response and transient ringing anomalies .. This of course assumes that
the circuit that the LA3A will be feeding is NON terminating... i.e High Input Impedance (most circuits
after 1970 are).
Ed Evans
Circa 1991, update 2003
