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The Universal Audio / Urei 1176 Compressor

Introducing the "True Peak Limiter"

The Universal Audio 1176 Compressor/Limiter was first introduced in 1968, created by Bill Putnam Sr. as the solid state successor to his tube-based 176 limiting amplifier. The 1176 uses a Field Effect Transistor, or FET, as a voltage divider to accomplish the task of compression, in conjunction with I/O transformers and a Class A line level amplifier. The 1176 was meant to be a "true peak limiter" with a very fast attack of up to 20 microseconds, and lots of gain for a compressor: up to 45 dB!

Bill Putnam Sr., creator of the 1176 compressor

Evolving A Classic Design

The design of the unit went through numerous changes, taking a while to hit its stride. The originals were somewhat noisy, prompting Brad Plunkett of Urei to design the Low Noise circuit that gave the unit its "LN" designation. Improvements continued to be made over the years, with the result that there are 9 major revisions of the vintage 1176, given the letters A, A/B, B, C, D, E, F, G, H . The substance of these revisions is detailed below, gleaned from notes by Barry Rudolph, the Universal Audio website, and research on units sold by Vintage King.

Producer, engineer, mixer Michael Brauer

“What's not to like? This is the workhorse of the industry. The most powerful compressor of them all. You can use it on any instrument and it'll do the job of making the drums sound punchy, the vocal sound stressed, urgent and contained if you choose, the bass pop, the drum room explodes, the guitars drive, the piano harmonics shimmer. It's very colored in sound and doesn't have the high bandwidth sound of the newer compressors but who cares. When I go into a studio to mix a record without my racks, I make sure there are 4 of these in the room.”

Michael Brauer

Urei / Universal Audio 1176 faceplate
Urei / Universal Audio 1176 rear panel

A Guide To Urei/Universal Audio 1176 Revisions

Based on the look of their faceplates, Revisions A, A/B, and B are all nicknamed the "Bluestripe," since they all featured a section of blue paint around the VU meter. Revisions C through G all have the nickname "Blackface" for their black anodized aluminum, while the final Revision H is known as the "Silverface", for its natural brushed aluminum. Revisions C, D and E (the first three with the LN circuit) are all very similar, and are generally the most revered. The modern reissue of the 1176 is based primarily on these versions.

Despite the many design changes, all 1176 units have their basic features in common:

Input level control

Output level control

Threshold: dependent on input and ratio levels

Attack time: variable 20 microseconds to 800 microseconds

Recovery time: variable 50 milliseconds to 1.1 seconds

Compression Ratio: 4:1, 8:1, 12:1, 20:1, "All-in"

VU Meter

Meter select: Off, Gain Reduction, Output Level at +4 dBm or +8 dBm

Barrier strip I/O and 1176 SA stereo-link RCA jack on rear

In essence, there have been only four basic versions of the 1176:

The original Bluestripes A, A/B and B
These are the edgiest versions, with plenty of noise and distortion to provide a distinct color to any signal.

The low-noise Blackfaces C, D, and E
These have the classic character and lower noise floor that is the heart of the 1176 sound. Modern versions hearken back to this era.

The push-pull, higher output Blackface F.
This version continued the classic sound, but changed the output transformer for considerably higher gain.

The op-amp input Blackface G and Silver-face H
The final refinement was to eliminate the input transformer in favor of an op-amp, giving a cleaner sound. One-third of all vintage 1176 units are of this type.

The Original Bluestripes: Rev. A, A/B & C

Revision A

Serial Numbers 101-125
June 1967 to November 1967

The original 1968 design by Bill Putnam used an input transformer at the industry standard 600 ohms resistance (Peerless, later UTC), a T-pad resistive attenuator to drop the input voltage, a FET acting as a voltage variable resistor to control gain, followed by a bipolar transistor as a preamp (the famous 1108 circuit with a Darlington pair). The output amp circuit was similar to the preamp, but followed by a 2N3053 bipolar transistor operating in Class A. The output matching transformer at 600 ohms was a UA-5002 (later improved to UA5002A) with a split secondary, a tertiary winding for negative feedback to the final line output circuit, and a separate emitter winding.

Covering up all this electronic wizardry was a brushed aluminum faceplate with blue paint around the Weston meter. The rest of the design features two black plastic control knobs with silver tops and clear plastic collars, two smaller black and silver knobs, eight black plastic pushbuttons and a red power indicator light. All the revisions to follow would maintain the same controls, but the red power light would disappear when the black faceplate was introduced.

Revision A/B

Serial Numbers 125-216
November 1967

A few short months after the initial version, an intermediate step was taken to make some small improvements to the circuit and components. Resistor values were changed in the signal pre-amp stages, and bypass capacitors were added around the resistor feeding the gain reduction FET. These changes improved noise and stability. All cosmetics remained the same.

Revision B

Serial Numbers 217-1078
January 1968 to January 1970

The next year saw some more changes to the preamplifier circuit. The FETs were replaced by a 2N3391A bipolar transistor. Also, a tap off the emitter of the first transistor gave feedback of the input voltage back to the gain reduction FET. All cosmetics remained the same. Occasionally, the letter "B" appears after the serial number. Note that some revision B units appear to have been later upgraded by Urei to the Low Noise spec, as indicated by the lower serial number with a replacement black faceplate.

The Low-Noise Blackfaces: Rev. C, D and E

Revision C

Serial Numbers 1079-1238
January 1970

In 1970, two major and two minor changes were implemented. Low Noise circuitry was added to the preamp stage in order to reduce the drain-to-source voltage of the gain reduction FET, keeping it within its linear range. This circuit was sealed in epoxy to keep it secret while a patent was pending. Also, a Q-bias pot was added to the feedback circuit of the audio FET, to minimize distortion. Both of these mods were mounted onto an otherwise normal Revision B circuit board.

The important addition of the Low Noise circuit changed the model designation from 1176 to 1176LN, which was prominently displayed on the new black anodized aluminum faceplate. The blue stripe disappeared and the UREI logo was placed over the meter, which was changed from a Weston to a Modutec with two overhead bulbs. This was a major facelift for the model that would become the basis of all future 1176 versions.

Revision D

Serial Numbers 1239-2140 (approx.)
January 1970 to January 1973

The next revision did not change the circuitry, but the new LN circuit and Q-bias pot of the previous revision were now integrated into a new circuit board instead of being soldered on as an afterthought.

Revision E

Serial Numbers 2141-2611 (approx.)
January 1973 to March 1973

Another minor revision in the early 70s was the addition of a power transformer to switch between 110v and 220v. Thus, revisions C, D, and E are all virtually identical. The earliest units had the transformer switch to the left side of the fuse on the rear panel. By about #2300, the switch was moved to the right of the fuse, with the power cord being moved directly underneath the fuse. There are more of this revision (and less of rev D) than was previously thought.

The Push-Pull, Higher Output Blackface: Rev. F

Revision F

Serial Numbers 2611-7052
March 1973 onward

In 1973, some major changes were implemented. UA had developed a new preamplifier, the 1109 model, and this was the basis for changing the output amplifier from a Class A design (the 1108 model) to a Class A/B push-pull configuration (the 1109) providing more output current. In conjunction with this, the UA5002A output transformer was replaced by the B11148 transformer (used in the LA-3A), providing 12 dB of gain. The meter drive circuit was also changed, replacing the discrete circuit of balanced bridge transistors with an operational amplifier, to simplify meter calibration.

Around unit #6950, the Urei plant moved from North Hollywood to Sun Valley, California. Many units still bore the old serial number stickers with the former street address, as the chassis built in Hollywood were still being used until the move was finally complete.

The Op-Amp Input Blackface and Silverface: Rev. G and H

Revision G

Serial Numbers 7053-7651

The final major change to the circuitry occurred in Revision G. The input transformer was removed and replaced with a differential input op amp stage. All Revision Gs were first produced in the new factory, and it is possible that the change was planned to coincide with the move and/or commenced when the stock of old parts had been used up.

Revision H

Serial Numbers 7652-12200+

The final revision made no changes to the circuitry, only the cosmetics and the layout. The faceplate was changed from black anodized to natural brushed aluminum. The Universal Audio silkscreened logo was removed and replaced with a blue Urei logo on a raised badge, (sometimes only a black printed Urei logo is used). The VU meter was changed to a Modutec "light box" type, using two internal lights, and the Off button for the meter was changed from black to red plastic.

On the rear panel were some key safety improvements: the fuse holder went from round to square and the voltage switch was moved to be above the new IEC cable input, replacing the attached power cord that had been a literal fixture of the device since the very first model.

At approximately serial number 10750, Harman Electronics took over Urei, and the familiar black and silver Urei sticker on the rear was changed to a Harman sticker. No changes were made to the circuitry, and another couple thousand units were manufactured before the brand folded.

The Universal Audio 1176 Reissue

Revision UA

Serial Numbers 101-current

In 2000, the sons of Bill Putnam Sr. resurrected Universal Audio to make authentic reissues of the legendary gear from Urei. Their first release was the 1176LN, based primarily on revisions C, D and E of the most famous FET compressor in history.

When it comes to modern versions of the 1176, the first place to start is with the Universal Audio 1176 LN. After relaunching the brand in 2000, the design team at Universal Audio took cues from the Blackface 1176 LN, including the C, D and E revisions of the famous compressor. This new 1176 LN stays true to the original designs of Bill Putnam and introduces the compressor to a whole new generation of users.

Aside from the Universal Audio reproduction, several other brands have worked to create their own versions of the 1176. In the world of analog hardware, the Purple MC77 and Retro Instruments 176 are among some of the most widely used in the world of the studio. For those working in the box, look no further than the Waves CLA-76 and Universal Audio's Classic Limiter Collection, both of which expertly recreate the compressor in digital form.

If you're a guitarist looking to add a high-quality compressor to your pedalboard, the answer to your search is the Origin Effects Cali-76 Compact. Origin Effects has managed to squeeze studio hardware components into a stompbox size pedal to offer a classic vintage-sounding compressor.