You are here


Universal Audio 1176LN Limiting Amplifier By Hugh Robjohns
Published June 2001


More than 30 years after the original 1176 was introduced, it has been put back into production by the designer's sons. Hugh Robjohns investigates the latest and most authentic reproduction of this classic 1960s compressor.

It was in 1966, when even I was still too young to know or care, that Bill Putnam first started work designing his 1176 limiting amplifier. Evolved from the popular Universal Audio 175 and 176 valve limiters, the 1176 retained many of their proven qualities, whilst combining it with the latest solid‑state amplification derived from the company's successful 1108 preamplifier.

Inside A Classic Design

While the 1176LN's metering can be switched to show either the amount of gain reduction or the output level, the latter increases the level of noise and so would not normally be used after levels have been set.While the 1176LN's metering can be switched to show either the amount of gain reduction or the output level, the latter increases the level of noise and so would not normally be used after levels have been set.

The Field Effect Transistor (FET) had only recently become available and Putnam found a way of using this new device as a voltage‑controlled variable resistor, and thus as the gain‑controlling element of a compressor. The fundamental problem facing Putnam was how to keep the device operating within its linear region, in order to keep distortion sufficiently low. After much experimentation he eventually hit upon the simple and elegant idea of using the FET as the bottom half of a voltage divider circuit, across which the audio signal was applied. This voltage‑controlled attenuator he then placed ahead of a solid‑state preamplifier stage and line driver, and derived its control voltage from a relatively conventional level‑sensing circuit monitoring the output.

During this period, professional audio equipment employed 600Ω input impedances and consequently demanded a lot of signal current. As a result, output stages had to be pretty beefy and Putnam addressed this with a Class A amplifier based around a special custom‑wound output transformer, the performance of which became crucial to the sound of the 1176. This transformer performed two functions simultaneously: it converted between the unbalanced internal circuitry of the limiting amplifier and the balanced external connections; and it also provided the correct impedance‑matching for the 600Ω line.

However, Putnam knew that transformers are notorious for introducing distortion, so he used additional sets of secondary and tertiary windings to provide feedback signals — a practice already commonly employed in valve amplifiers. In this way the transformer was enclosed within the negative feedback of the output amplifier and its non‑linearities were compensated and corrected for automatically, producing very low output distortion.

Putnam's original design of the Universal Audio (later Urei) 1176 Limiting Amplifier was launched in 1967 and did very well for itself, but in 1970 it received a significant update largely designed by an engineer called Brad Plunkett. His modifications improved the noise performance tremendously and resulted in the 1176LN ('Low Noise'). There have been numerous revisions and updates since (see 'Design History' box), but the D and E 'black‑face' LN versions are widely believed to be the best‑sounding models, and it's these that have been incorporated into the 1176LN which is under review here.

The pages of Sound On Sound have already seen one reincarnation of the 1176, the Purple Audio MC76 which was reviewed back in November 1999. However, the thing that makes this Universal Audio model so special is that every effort has been made to ensure that it is an exact, authentic replica in every possible detail. With the exception of changes made to comply with CE legislation, no attempt has been made to modernise the design of the Universal Audio 1176LN. Universal Audio claim that their 1176LN re‑issue is as faithful to the original, in terms of components and construction, as you can get. Internal construction is, as might be expected, on a par with the originals, showing the same hand‑wiring and classic 1970's approach to circuit board layout: most of the critical active devices are mounted in sockets, all components are identified, and both the top and bottom casing panels can be removed for easy maintenance.

Guided Tour


The front panel of the strong 2U steel chassis contains a pair of large input and output level controls, both scaled from infinity up to zero. Between these is an unlabelled hole which provides access to a calibration trimmer for the meter zero position (when in gain‑reduction mode). Next, a smaller pair of knobs determine the Attack and Release time constants — both are scaled from one to seven, though the faster settings are obtained at the higher numbers, the opposite of what you'd normally expect. The attack times range from 800µS to a very fast 20µS, and the release times span 1.1S to 50mS. A back‑stop switch on the Attack control provides a bypass condition.

Two columns of four interlocking buttons are arranged either side of the illuminated VU meter. The left column determines the compression ratio with options for 4:1, 8:1, 12:1, or 20:1. An unintended, but popular, aspect of the 1176 is that pressing combinations of buttons simultaneously, you can create a number of not entirely predictable compression effects. Although there is no physical threshold control on the 1176LN, the threshold setting is varied internally depending on the selected ratio — the higher the ratio, the higher the threshold — and this is very logical and practical. For example, the specifications suggest the internal threshold is ‑25dBm for a 20:1 ratio, falling to ‑32dBm at a 4:1 ratio. (Incidentally, the use of the 'dBm' for the relative levels here is correct, as the unit is intended to interface with 600Ω impedances. In practical terms, the dBm value equates directly with modern dBu terminology.)

The column of switches to the right of the meter provide a variety of house‑keeping functions. Any of the top three of these will switch the unit on, while the bottom one powers it down. The top button switches the meter to show the amount of gain reduction while the central two buttons display the output level referenced either to +8dBm or +4dBm levels. Since the latter two modes can introduce a little more distortion, they are not normally used except when setting up.

The rear panel is a replica of the original 1176, complete with terminal‑strip connections. The only concession to modern technology is the inclusion of an IEC mains inlet, with integral fuse holder and mains voltage selector. XLR sockets are provided for the main audio input and output — there's only one of each, as this is a strictly mono unit. The seven‑way terminal strip can be used to make hard‑wired audio connections (in parallel with the XLRs) with solder‑tags or bare wires, and the metering signal is also available to allow a remote meter to be connected, if required.

Finally, there is a phono socket labelled 1176SA, which is used to link 1176LNs for stereo operation and this has always been the weakest aspect of the 1176 design, to my mind. No simple cable connection between two units here, but a special 1176SA adaptor module, and battery‑powered at that! What's more, you have to manually align the FET bias voltages using a trimmer on the adaptor, if you want to get a pair of units working reliably together in stereo.

Blast From The Past


Connecting the 1176LN is straightforward enough, though the approach to setting up the controls is a little unusual when compared to modern devices, albeit perfectly logical, simple and, above all, fast. The procedure is to set the input and output controls to somewhere around their mid‑positions and then punch in the required compression ratio. The output level control can then be set to bring the music peaks to the required level, and the amount of 'squash' can be determined by adjusting the input level control — turning it up forces more input signal above the fixed threshold and into the compression region. Finally, the attack and release times can be set to suit the material being processed.

The 1176LN is judged by many to be unsurpassed as a vocal compressor, and I would certainly agree that it can be extremely effective. It can be surprisingly transparent when used fairly gently on a 4:1 ratio, a setting whose warm, valve‑like quality can be sublime on softer voices. Yet it can also accommodate the raunchiest hard compression demands too, which can be fantastic on strong, belted‑out rock vocals.

The original was often also celebrated as a compressor for bass, and I certainly found the re‑issue's compression to cope wonderfully with the wildest excesses of electric or acoustic string basses, without changing the inherent sound or losing the essence of the player's dynamics. In fact, this unit is versatile enough to be used in a variety of applications, and can even work wonders to tighten up individual drum tracks, especially as it is fast enough to be able to control peak levels well.

The 1176LN always had a slightly bright character — more of a subtle spectral tilt than an obvious high‑frequency lift — which is reproduced faithfully here and which generally helps tracks to cut through in the mix without you needing to reach for the EQ. This element of the unit's sound is one of the more significant reasons why it has enjoyed the success it has over the last thirty years.

I was only supplied with a single unit for this review, so I can't specifically comment on how easy (or otherwise) it is to link two of the reissued models for stereo operation. However, most engineers generally steer clear of this kind of application with the 1176LN, preferring to use it only on mono sources. It will be reassuring to some, though, that the option is there if they have the tenacity to pursue it.

Perhaps because of the pioneering era from which this product descends, the new 1176LN immediately seems to impart a classiness and 'rightness' to any sound which passes through it. What exactly this 'rightness' is proves hard to quantify, but few current dynamics processors have this same character while being so quick and easy to control. No wonder then that so many original units are still in everyday use after so long. Anyone who has used an 1176LN will probably have fallen in love with it, and will also be glad to hear that this Universal Audio reincarnation is every bit as good as the original — hardly a surprise given that it's design is almost exactly the same as the original, in every critical way.

The only real down side with the 1176LN is its price, which seems rather steep, in the UK, at about a thousand pounds over that of the Purple Audio MC76 replica. Without having the two side by side, I wouldn't like to say which sounds the best, but the MC76 certainly received a lot of praise when it was launched a couple of years ago. Clearly, the most significant element in making a decision here is whether you would be happy with the slightly updated purple copy, or would rather have the (admittedly) classier looking 'authentic replica'. Check them out side by side if you can — but be prepared to part with your money one way or the other!

Design History


Following its introduction in 1967, the 1176 limiting amplifier has seen many revisions and updates through its long life. The original version was denoted the 1176A, but was revised to the model AB only a few months later, with improvements in stability and slightly reduced noise. The following year saw the model B, with further minor changes to the preamplifier circuit. These models all featured a brushed aluminium faceplate with blue control legends.

The 1176 first adopted the black faceplate in September 1970 for the 1176LN model C, with new low‑noise circuitry designed by Brad Plunkett. This circuitry was originally encased within an epoxy module, but a subsequent redesign fully integrated these improvements with the main PCB, producing the model D. The model E was introduced in the early 1970s and was the first to accommodate European 220V mains power with a voltage selector on the rear panel.

Of all the revisions, model D or model E variants are most often favoured. However, another significant redesign occurred in 1973. The model F output stage was modified to provide higher output current capability by using a push‑pull circuit design borrowed from Universal Audio's new 1109 preamplifier. This new output stage replaced the original Class A circuit borrowed from the 1108 preamp. The meter drive circuit was also updated, with an operational amplifier instead of the previous discrete circuit.

The classic transformer front end of the 1176 met its demise with the model G, in which an electronically balanced input stage replaced it. The final update, the model H, simply marked a return to a silver faceplate.

The Designer: Bill Putnam

Bill Putnam (front): audio designer, engineer and entrepreneur.Bill Putnam (front): audio designer, engineer and entrepreneur.

Born in 1920 in Illinois, Putnam grew up in an era of early radio broadcasting and crude home‑built electronics, and by the age of 16 was already constructing his own audio equipment. During the War he served with the US Army as a radio engineer and immediately afterwards started his own recording studio back in Illinois, the Universal Recording Corporation. At that time, recording studios had to build most of their own equipment and Putnam created the Universal Audio manufacturing company to build and market his designs.

The company relocated to Chicago in 1947, and it was here that Putnam used artificial reverberation for the first time on a pop record. The reverb in question was created using the tiled toilets at the studio, and graced The Harmonicats 'Peg‑O‑My‑Heart', which sold over 1.4m copies. However, this was by no means Putnam's only recording innovation: he also made the first use of tape echoes, he made the first vocal booth, and performed some of the earliest experiments in eight‑track recording.

His studio became much sought after, recording the likes of Muddy Waters, Bo Diddley, Chuck Berry, Tommy Dorsey, Count Basie, Dizzy Gillespie, Ella Fitzgerald, Sarah Vaughan, Nat King Cole and Duke Ellington, amongst many others. Needless to say, Putnam had a hand in the engineering and production of many of these records, and even wrote some of the songs and lyrics as well!

By 1955 Universal Recording was probably the largest and most advanced independent recording facility in America, but Putnam sold his interest in it in 1957 to set up the new United Recording Corporation on Sunset Boulevard in Hollywood, a facility which eventually expanded to three studios, a mixdown room, three mastering rooms, and a small manufacturing plant. Once again, the list of both staff and clients quickly grew to read like a Who's Who of the music industry: Ray Charles' classic 'I Can't Stop Loving You' was recorded there, as well as Sinatra's 'It Was A Very Good Year' and The Mamas & The Papas' 'California Dreaming'. These studio facilities still exist today — as Cello Studios and Ocean Way Recording — and are considered to be some of the best‑sounding rooms ever built.

The Universal Audio equipment manufacturing business continued to thrive, changing its name to Urei. The patent rights were acquired for the LA2A leveling amplifier, which was marketed under the Teletronix division of Urei, and the 1108 FET preamp and the 813 range of monitor speakers were designed and built. Putnam continued to innovate in California, building the first modern‑style control rooms, introducing the first console design with equalisers in every channel and developing the feedback equaliser.

In the late 1950s Putnam could see the potential for stereo even though the record companies were not yet interested. He decided to record everything in both stereo and mono simultaneously, using separate control rooms, partly to gain experience in this new format and partly for shrewd business reasons. Inevitably, when stereo became a big hit in America in around 1961, none of the record companies had any stereo material in their catalogue, but Putnam had amassed an enormous collection of already successful material over a period of about two and a half years. However, rather than simply selling the tapes, Putnam struck a fantastic deal where the record companies repaid all the studio time for the entire two and a half year period when he had been tying up two control rooms — this being at a time when the studio was phenomenally busy and was doing about $200,000 worth of work a month (roughly equivalent to $1 million a month at today's prices)!

The sixties saw yet more experimentation from Putnam, who custom‑made his own 30ips tape machines in order to make recordings for some of the super hi‑fi labels of the time, such as the Mercury Living Presence series. Moreover, realising that retaining the quality of these recordings when cutting to disc was impossible, he also designed his own half‑speed disc mastering system in order to get the high frequencies onto his records.

Bill Putnam died in 1989, but his company Universal Audio lives on, under the careful management of his two sons, marketing re‑issues of his classic designs as well as developing software plug‑ins including digital versions of the celebrated LA2A and 1176LN processors.


  • An authentic replica.
  • The classic becomes easy to get hold of, at last.
  • Sounds great on almost everything.


  • Expensive.
  • Requires a slightly different working approach than with modern compressors.
  • Setting up stereo linking is Elm Street territory, and is best avoided!


By far the most authentic replica of this classic limiting amplifier to date, built by the sons of the original inventor. Every rack should have at least one of these fabulous processors, which seem able to impart a special magic to almost everything passing through them.