LA3A MODS

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

First, those 2N5117 transistors have to go. For Q1 and 2 I use Motorola MPSA18's.
C15 should be Mica, if it's ceramic, change it to mica or a Wima FKP-2 polyprop cap.
C1 is changed to .47 uf, MKS-2 Wima with a .0047 MKP-2 1kv bypass.
If C2 is a tantalum, leave it. If it's electrolytic, change to 10 uf 50v Panasonic FC.

C14 is either mica or Wima FKP-2, C3 is another Panasonic 10 uf 50v FC. R14 is now 15k, C4 is now 220 pf Wima FKP-2 or a mica. C5 is changed to a 1000 uf 25v Panasonic FM bypassed with a .1 uf 250v MKP-2 Wima. C8 is another 10 uf 50v FC and C9 is a 100 uf 50v FM. C12 is changed to a 2200 uf 35v and C10 is 4700 uf 50 v Nichicon.

Last, change Q5 to a 2N6716 and Q6 to a 2N6728 for a faster sound.

No, these do not sound boxy at all, in fact, they breath.

Jim Williams
Audio Upgrades

These mods have been around longer than dirt. Change the 220k feedback resistor to 15k.
Change the 12 pf feedback cap to 220 pf. Dale RN60's would be a good resistor choice and Wima FKP-2 poly films for the cap. Use cheaper parts for more "vintage flavor".
Replace the 470 uf output cap to a Panasonic FM series.

Jim Williams
Audio Upgrades

The mod [well, the one done to my LA-3's] is changing R14 to 47K from the stock 220K.

hope this helps...

John

The 50/30 dB GAIN switch is an input pad to prevent high output devices from overdriving the input transformer
of the LA-3A. Setting the switch to the 30dB position will attenuate the incoming signal by 20dB. For studio
use, the normal setting is down (30dB) when not using the MOD function. See MOD Switch below for more info.

MOD Gain Switch
The MOD (modification) GAIN switch improves the signal to noise ratio of a “stock” LA-3A. Typically, both the
MOD switch and the 50/30 dB switch are set in the “up” position. This disengages the 20dB input pad and
drops the output amp’s gain by 24 dB. In this configuration, the unit’s gain is +26dB and will be less noisy
than when in normal mode with the 50/30 dB switch at 30. In addition, the MOD switch lowers the threshold
point where compression begins and allows a greater amount of overall compression. Many engineers have
performed this mod on vintage LA-3As. The loss of gain is not a problem in most applications as the LA-3A has
about 50dB gain to begin with. The improvement in signal to noise is usually a desirable trade off. We have
supplied this modification as a switchable feature so you can operate with or with out the modification.


From UA manual