1176 and LA-2A Hardware Revision History

We get occasional calls here at Universal Audio from people wanting to know what era some Universal Audio, UREI or Teletronix piece they bought on the vintage market dates from. So in this post, we're offering a complete revision history of the world's two most popular compressors, the 1176 Limiting Amplifier and the Teletronix LA-2A Leveling Amplifier. We also get calls wondering if we do repairs on vintage equipment; sorry, we don’t. However, we do highly recommend David Kulka at studioelectronics.biz for all 1176 and LA-2A repairs, or any other vintage UA/UREI or Teletronix equipment you may have. David is the best in the business.

1176 Classic Limiting Amplifier

While the 1176 has seen a huge number of revisions in its history, the most significant revision to the 1176 circuit was designed by UREI engineer Brad Plunkett in an effort to reduce noise — hence the birth of the 1176LN (LN stands for low noise) at revision C. Numerous design changes followed, resulting in at least 13 revisions and variations on the 1176. Most units include the revision letter with the serial number. Revisions C, D and E (the first three with the LN circuit) are all very similar, and are historically the most beloved. The current Universal Audio’s 1176LN reissue is built from that circuit, as are the 1176 limiter in our 6176 Vintage Channel Strip.

Revision A (AKA “Bluestripe”)

Design Date: 6/20/67

Serial Numbers: 101-125

Faceplate is brushed aluminum with blue paint and red power light

Original 1176 designed by Bill Putnam for broadcast use, class A design

Signal Preamp: Used FET (field effect transistor) in a voltage divider configuration for gain reduction, a technological first

Used FET as voltage variable resistor to control the gain

Used ‘T-pad’ resistive attenuator to drop the input voltage

Input Stage used a 600 ohm input transformer

Used FET followed by bipolar transistor 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 was the UA-5002; this transformer has split secondary, tertiary winding for negative feedback, emitter winding; the design was basically that of the 1108 preamp

A Weston meter is used

Revision AB (AKA “Bluestripe”)

Design Date: 11/20/67

Serial Numbers: 125-216

Changed several resistor values in signal pre-amp stages; this improved stability as well as noise

Circuit added bypass cap around the resistor feeding the gain reduction FET to improve stability

Revision B (AKA “Bluestripe”)

Design 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

Revision C (1176 LN) (AKA “Blackface”)

Design Date: 1/9/70

Serial Numbers: 1079-1238

Faceplate is anodized black

Low noise ‘LN’ circuitry was added in the signal preamp; this reduces DS voltage on the gain reduction FET and keeps the FET within its linear range

Modification to program dependency tuning

Trimpot added to feedback circuit of audio FET, to minimize distortion

The LN circuitry was built in a module that was encased in epoxy to protect the circuit

A Modutec VU meter is used, with two lamps mounted above

Revision D (AKA “Blackface”)

Design Date: Not indicated

Serial Numbers: 1239-2331

Adds "UREI" branding to existing Universal Audio branding

The LN module was no longer used and the circuit board was redesigned to accommodate the LN circuitry on the main board

Revision E (AKA “Blackface”)

Design Date: Not indicated

Serial Numbers: 2332-2611

A switch was added to change between voltages

Revision F (AKA “Blackface”)

Design Date: 3/15/73

Serial Numbers: 2611-7052

Output amplifier changed to a push-pull (class-AB) configuration from the original class-A design

Output transformer is changed to the B11148 type with 12 dB of gain (same transformer as the LA-3A)

Push-pull design provided more output drive, and was based on the 1109 preamp

Changed the metering circuit to an op-amp following the meter FET, instead of balanced bridge transistors

Revision G (AKA “Blackface”)

Design Date: Not indicated

Serial Numbers: 7053-7651

Removed the input transformer, and replaced it with a differential input op amp stage

Removal of program dependency

Revision H (AKA “Silverface”)

Design Date: Not indicated

Serial Numbers: 7652- 8000+

Faceplate is brushed aluminum with red “Off’ button; this is the only revision with a blue UREI logo

Removes Universal Audio branding

VUi meter is changed to Modutec "light box" type, with two internal lamps

Universal Audio Reissue 1176LN

Design Date: 4/1/2000

Serial Numbers: 101-1959

The new Universal Audio’s first product release
Based on the most beloved revisions C, D and E — most similar to E, due to 110/220 switchable power transformer

Serial Numbers: 1960-2946

When Clarostat T-pad input attenuator became unavailable:
Changed from 600ohm T-pad to 1Kohm log taper input attenuator (same configuration in 6176 and 2-1176)
Changed 012 Magnetika to Cinemag input transformer (same configuration in 6176 and 2-1176)

When original 600ohm T-pad input attenuator became available again under Precision Electronics:
Resumed production with T-pad attenuator
Resumed production with 012 Magnetika input transformer

Revision AE "Anniversary Edition"

Design Date: 6/04/08

Serial Numbers: 001-500

Faceplate is black anodized with blue paint

Unique Rev. A/E “hot-rod-mod" features, cosmetics and sonic signature

Return to program dependency of legendary “Bluestripe” Rev. A 1176
New, gentler compression ratio with 2:1, 4:1, 8:1 and classic 20:1 for hard limiting

Expanded range of “multi-button” compression and classic “all-button” mode with 2:1 ratio

Click-stop “Slo” 10 ms Attack setting adds increased versatility for new uses

Limited-Edition of only 500 units worldwide

Note: Other variations of the 1176 exist, such as the 1178 stereo limiter and the 1176/LA-3A FET/optical dual-detector hybrid UREI Modulimiter.

LA-2A Classic Leveling Amplifier

Patented by Jim Lawrence, the LA-2A optical Leveling Amplifier was first produced by Teletronix in Pasadena, California. Earlier variations such as the very rare LA-1 and LA-2 still exist in private collections, but the LA-2A was the improved design that caught on. Teletronix later 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 Harman/JBL.

Revision 1 (AKA “Greyface”): Faceplate is painted grey

First compressor to use of photo resistors and luminescent panels to achieve gain reduction - the T4A

Produced by Teletronix in Pasadena, CA (sometimes labeled Los Angeles, sometimes labeled Sunnyvale)

Octal jumper socket located before the UTC HA-100X input transformer for pad/pre-emphasis modules

Later units included a variable pre-emphasis filter, R37

All rear panel graphics are hand stenciled or stamped

Some units offer a fixed Limit function while others feature a Limit/Compress switch on the back
Revision 2 (AKA “Silverface”): Faceplate is brushed aluminum

Teletronix patent acquired by Putnam's Studio Electronics Corp. of North Hollywood

This iteration had an added a compress/limit switch on the back
Input transformer changed to UTC A-10

Limit/Compress switch on the back
This revision starts with unit #384
Reissue 1: In the late 70's, UREI brought back the LA-2A for a run of around 200 units

Return of the greyface front panel

The T4A is replaced with a T4B

A safety switch is added so power shuts off when front panel is opened
Reissue 2: In 1985, 235 units were re-issued by UREI while under Harmon/JBL

This brought the unit once again back to the aluminum silverface look, but this time with the UREI name on the front
Universal Audio Reissue Teletronix^® LA-2A: Based on Revision 1

Released in May of 2000

Moved the Limit/Compress switch to front panel

Removal of user accessibility via front panel thumbscrews