UA WebZine "Compression Obsession" August 03 | 1176 and LA-2A Revision History

Compression Obsession: 1176 & LA-2A Revision History

We get occasional calls here at Universal Audio with guys wanting to know what era some Universal Audio, UREI or Teletronix piece they bought on the vintage market dates from. So this month we're offering a complete revision history of our two reissues, the 1176LN and the LA-2A.

1176

The first major modification to the 1176 circuit was designed by Brad Plunkett in an effort to reduce noise--hence the birth of the 1176LN; LN stands for low noise. Numerous design changes followed, resulting in at least 13 revisions of the 1176. There were a number of revision letters associated with the different incarnations. Most units include the revision letter with the serial number.

Revision A
- Date: 6/20/67
- Serial Numbers: 101-125
  - Original 1176 designed by Bill Putnam.
  - Used FET as voltage variable resistor to control the gain.
  - Input Stage: 600 ohm input transformer.
  - Used ‘T-pad’ resistive attenuator to drop the input voltage.
  - Signal Preamp: Used FET in a voltage divider configuration for GR.
  - Used FET followed by bipolar as preamp.
  - Output amp: Used essentially the same circuit as in the preamp, but followed it with a 2N3053 operating in class A.
  - Output transformer: Used a UA-5002. This transformer has split secondary, tertiary winding for negative feedback, emitter winding. The design was basically that of the 1108 preamp.
  - Enclosure: Brushed aluminum with blue paint and red power light.
Revision AB
- Date: 11/20/67
- Serial Numbers: 125-216
  - Changed several resistor values in signal pre-amp stages.
    - This improved stability as well as noise.
  - Added bypass cap around the resistor feeding the GR FET.
    - This improved stability.
Revision B
- Date: Not indicated
- Serial Numbers: 217-1078
  - Replaced FETs in the signal pre-amp with bipolar transistor (2N3391A).
  - Tapped off of the emitter of the first transistor in the input stage.
  - This provides a replica of the input voltage to the voltage variable resistor.
    
    Photo taken at the The Plant’s Studio “B” in Sausalito, CA
Revision C (1176 LN)
- Date: 1/9/70
- Serial Numbers: 1079-1238
  - Low noise ‘LN’ circuitry was added in the signal preamp.
  - Reduces DS voltage on the GR FET. Keeps the FET within its linear range.
  - Q-bias pot added which is used to adjust for minimum distortion.
  - The LN Circuitry was built in a module that was encased in epoxy to protect the circuit.
  - This module was mounted on the Rev B circuit board
  - Enclosure: Blackface.
Revision D
- Date: Not indicated
- Serial Numbers: 1239-2331
  - No circuit changes.
  - The LN module was no longer used and the circuit board was redesigned to accommodate the LN circuitry on the main board.
    
    Photo of UA's "Golden" blackface 1176LN, below a current re-issue.
Revision E
- Date: Not indicated
- Serial Numbers: 2332-2611
  - The power transformer was changed to accommodate 220V operation.
  - A switch was added to change between voltages.
Revision F
- Date: 3/15/73
- Serial Numbers: 2611-7052
  - Output amplifier changed to a push-pull configuration from the original class-A design. This provided more output drive, and was based on the 1109 preamp.
  - Changed the metering circuit to use an op-amp.
Revision G
- Date: Not indicated
- Serial Numbers: 7053-7651
  - Removed the input transformer, and replaced it with a differential amplifier.
Revision H
- Date: Not indicated
- Serial Numbers: 7652- 8000+
  - Enclosure: Silver-face with red “Off’ button.
  - This is the only revision with a blue UREI logo.
Universal Audio Re-Issue
- Date: 4/1/2000
- Serial Numbers: 101-current
  - The new Universal Audio’s first product release. Based on revisions “D” and “E”.

LA-2A
Originally patented by Jim Lawrence, the LA-2A was produced by Teletronix in Pasadena, California, which became a division of Babcock Electronics Corp. in 1965. In 1967 Babcock's broadcast division was acquired by Bill Putnam's company, Studio Electronics Corporation, shortly before he changed the company’s name to UREI®. Three different versions were produced under the auspices of these different companies before production was discontinued around 1969-70. In addition to these iterations of the LA-2A, two “re-issues” were put out, one while still under UREI, and one under UREI after being purchased by Harmon/JBL.

Revision 1
- Produced by Teletronix in Pasadena, CA (labeled Los Angeles)
- Gray face paint
- Octal jumper socket located before HA-100X input transformer
- Limit only function.
- All rear panel graphics are hand stenciled or stamped. Units from this era are 1965-66
Revision 2
- Teletronix patent acquired by Studio Electronics Corp. of North Hollywood.
- This iteration had a brushed aluminum face and added a compress/limit switch on the back.
- This revision starts with unit #384.
Revision 3
- The UTC HA-100X input transformer is replaced with A-10 input transformer.
Re-issue 1
- Legend has it that during a cash crunch in the late 70’s, UREI brought back the LA-2A for a run of around of 200 units; A rare specimen, and a return to the grey face front panel.
Re-issue 2
- Another re-issue was released by UREI while under Harmon/JBL.
- This brought the unit once again back to the aluminum silver face look, but this time with the UREI name on the front.
Universal Audio Re-Issue
- The new Universal Audio’s second product release.
- Based on revision 2, with the look of revision 1.
- Released in May of 2000.

Vintage Units at The Plant

On a listening test for UA’s new 2192 at The Plant in Sausalito, Ca, we spotted two early 1176’s that were silver faced with a blue “racing stripe” going vertically down the front of the unit where the meter is. This indicates the units were revisions A or B, although we never found out which revision they actually were. However, we found out that they had long been modified to the Revision “C” Low Noise spec.

In addition to the early silver face units, we also found a couple of blackface units specially modified with side-chain insert points: