Monday, August 26th, 2019

Review of the Universal Audio 1176LN Classic Limiting Amplifier

By editorFebruary 8, 2015


A while back I was in the studio doing some compressor comparisons and I had the opportunity to have an in depth look at the Universal Audio 1176LN Compressor (official title ‘classic limiting amplifier’).  Whilst there have been numerous reviews of the this great compressor, I’m going to focus some of this review on the design elements of the 1176 which give it its unique sound.


The 1176 probably sits in the top three most popular classic compressors in history along with the Teletronix LA-2A Electro-Optical compressor and the SSL G Series stereo bus compressor.  It’s no surprise (and a blessing) that all three are still manufactured in various formats (both hardware and software) today.

Back in 1968, when the original 1176 was first launched, designer Bill Putnam already had a long track record of innovation within the recording industry, both as an audio equipment designer and as a recording engineer and studio owner, having founded one of the first independent recording studios, Universal Recording in Chicago.

In the early ‘60’s the availability of solid state transistors began to offer an alternative to valves in the design of audio gear.  In the UK, Rupert Neve was beginning design of his first solid state mixing console and in the US, Bill Putnam began to redesign some of his original tube audio preamps and compressors to make use of these recently invented transistors and in the case of the 1176, the, then unique, application of a FET (Field Effect Transistor) as the compressor gain controlling element.  There is a good tech article about FETs in wikipedia here.

To me, what sets the 1176 apart, is that it achieves such a wide palette of sound coloration from a non-valve design, plus an (almost) unbelievably fast attack time of 20us.

The 1176 design has undergone a series of revisions throughout its long production life.  Universal Audio have a good summary here but the essence of what makes it such a great compressor has thankfully been preserved to this day whether you purchase it from Universal Audio as a piece of analogue hardware or as a digital plug-in.

At this point I had better not assume that all readers are familiar with the 1176, so in brief …

The 1176LN in a mono compressor housed in a 2RU x 19” steel enclosure.  The audio path has transformer balanced I/O with a FET gain control element and a fabulous class-A output stage.

Rear panel showing barrier strip and stereo interconnect
Rear panel showing barrier strip and stereo interconnect

Audio connection to the unit is by XLRs plus a very retro barrier strip.  There is also an additional connector which allows two 1176’s to be coupled for stereo operation via an external stereo adaptor unit.

The 1176 offers very fast attack times from 20us to 800us (0.8ms)  Bearing in mind that many compressors have a fastest attack time of 1ms then you get some idea of just how fast the 1176’s attack times are.  According to Universal Audio, this feature of the 1176 provided a difficult challenge for their digital emulation, because at a sampling rate of 44.1kHz, 20uS is less than one sample period, so not a lot of time for a digital processing system to react audio levels over the threshold without introducing an unacceptable degree of delay through the digital audio processing path.

The 1176LN release time is adjustable from 50 milliseconds to 1100 milliseconds (1.1 seconds) and employs a programme dependent release envelope which has become quite a common feature in many more recent compressor designs.  This adaptive release envelope allows the compressor to recover more quickly from short duration level excursions above compressor threshold whilst offering a more relaxed recovery time from sustained periods of compression.  This release envelope works in tandem with the adjustable release time setting.  In operation, it’s an important element of the 1176’s unique sound.

Compressor threshold is fixed so you use the 1176’s input gain control to drive the audio into compression.  When I say fixed, I mean it has no user adjustable setting but in reality the design automatically changes the threshold according to the ratio setting.  Higher ratios mean slightly higher thresholds.  This allows the user to change ratios without a sudden large jump in gain reduction which is a very sensible design feature.  The Maselec MLA-2 mastering compressor uses a similar technique of ‘fixed’ nominal threshold which varies with ratio setting (but with a very different sound signature to be sure).

Post compression gain make-up is generous with 50db of gain available.  Both input and output gain controls are smooth to operate and have wonderful large control knobs which make it really easy to set and reset session settings.

Compression ratio is switchable between 4:1, 8:1, 12:1, 20:1 plus a special ‘all button’ mode which we’ll come back to in a little while.

The wonderful large illuminated VU meter can be switched to read either gain reduction or output level with the 0VU mark switchable to either +4 or +8dBm.  A fourth button in the same group allows the unit to be switched on or off.

Mains power is applied via a standard IEC connector and units can be externally switched between 115/120 and 230/240VAC supplies.

When I first unpacked the 1176 in the studio and patched it into a desk insert, first impressions were dominated by the enormous VU meter and small number of control knobs and switches.  This compressor has an entirely welcoming layout and I found it much easier to use than the cluttered layouts of many modern designs.

Unlike many compressors, you only have to put the 1176 in circuit to get an immediate and beneficial effect on the source you are processing.  Even with no gain reduction taking place, the combined audio signature of the transformers and the classic class-A circuitry have a significant impact which brings both warmth and body to the audio.  The introduction of different ratios and levels of gain reduction brings a very wide and flexible range of effects on top of its ‘day job’ of compressing the audio.

We had a lot of fun experimenting with the 1176 across a range of material. It can be very effective with most sound sources, certainly as a vocal compressor and as we found during our drums tracking session, with percussion.  We took the opportunity to put up an additional mono overhead mic and track this through the 1176.  Some audio clips are available at the end of the review.

I was curious to try and better understand what was happening ‘under the bonnet’ so I hooked up the 1176 to my audio measuring system and began to delve into the audio specs in some detail.  Here’s what I found …

With 0dBu applied to the audio input of the 1176; input and output gain controls both set to ‘24’, and no gain reduction taking place, I measured the distortion across the audio band as follows;

20Hz : 0.64%

1kHz : 0.47%

10kHz : 0.45%

Then I passed the 1176’s output through a spectrum analyser and found that distortion was dominated by 2nd and 3rd harmonic products, both measuring around -55db (down from the fundamental) with the higher harmonic products mostly down in the noise except at very low frequencies where some 4th and 5th orders were visible on the analyser display.

I think it is this combination of dominant 2nd and 3rd harmonics (and the lack of higher order harmonics) which give the 1176 such a great ‘straight through’ sound.  You will certainly measure similar distortion characteristics within valve based audio gear.

Increasing the output gain to 20dB (still no gain reduction) caused a modest increase in LF distortion (probably due to increased levels through the output transformer) but I didn’t find this altered the audible tone much.  For the record, the distortion measurements at +20dBu output level were;

20Hz : 0.8%

1kHz : 0.43%

10kHz : 0.40%

Once you start introducing the 1176’s gain reduction into the mix, audio measurements begin to get a lot more complex but I did my best to pick out some of the elements which contribute to the famous 1176 sound.

First thing of note is that the 1176 exhibits some small frequency response changes when gain reducing.  I measured this with 6dB of gain reduction across the different ratio settings.

Above 5kHz there is a very mild HF boost which maxes out at around +0.2dB with +6dB of GR and a ratio of 8:1.  Whilst a HF boost of 0.2dB doesn’t seem that much, it’s more important to note that the 1176 maintains its HF response even when working at high ratios.  This, in part, explains why so many users and reviewers comment about the 1176’s ability to compress without losing the vitality of the original source.  The SSL G-series stereo buss compressor has a similar but even more pronounced HF lift when the compressor ratio is increased.

Below 5kHz I found that the 1176 showed LF boost up to around +1dB at 20Hz, the amount dependent on the ratio and degree of GR, even more when the ‘all button’ mode was applied.

It’s probably impossible to say all these years later, if these subtle frequency effects were an intended part of the design but they certainly contribute to the 1176’s unique sound.

1176 interior showing the input transformer and the rear of the barrier strip
1176 interior showing the input transformer and the rear of the barrier strip

I then ran a series of distortion measurements with the 1176 across the range of switchable ratios with various amounts of gain reduction applied.  I found three prominent distortion characteristics:

  1. There is a significant increase in distortion even when the 1176 is working at its lowest 4:1 ratio and modest amounts of gain reduction.
  2. Distortion increases as audio frequency deceases.
  3. 4th and 5th order harmonic distortion components are much more evident when gain reduction is applied, presumably due to the FET passing current.

As examples of what I measured;

1176 set at 4:1 ratio and 6dB of gain reduction

2nd harmonic  -40db (20Hz)   -50db (1kHz)   -52db (5kHz)

3rd harmonic   -20db (20Hz)   -48db (1kHz)   -54db (5kHz)

4th harmonic   -45db (20Hz)   -64db (1kHz)   -75db (5kHz)

5th harmonic   -33db (20Hz)   -57db (1kHz)   -70db (5kHz)

Bearing in mind that the 5th harmonic of 5kHz is being rolled off by the overall frequency response of the 1176 you can see the significant dominance of odd harmonics within the distortion.  On the scope you can see this quite clearly as the test sine wave begins to get soft clipped as ratio and GR levels are increased.

The ‘all button’ ratio mode of the 1176LN is made possible due to the mechanical nature of the four mechanically interlocking Ratio buttons which can be pressed in at the same time.  It feels a bit strange to do first time but you soon get hooked!

According to the 1176 user manual, in ‘all button’ mode, distortion increases radically due to a lag time on the attack of initial transients, an effect which Universal Audio describe as a “reverse look-ahead”. The ratio goes to somewhere between 12:1 and 20:1, and the bias points change all over the circuit, which has an impact on changing the attack and release times as well. According to UA, it is this constantly shifting compression curve that provides the unique ‘all button’ overdriven tone.

Back to what I actually measured : Whilst the predominance of high distortion at lower frequencies gets more and more pronounced as you increase the 1176 ratio, the ‘all button’ mode is quite different.  Here I measured peak distortion levels between 150 to 300Hz which then reduce both above and below this frequency reference point.  Note the dominance of the odd harmonics which contribute much to the ‘all button’ sound.  Again I’ve set these figures out in a table below.

1176 set at ‘all button’ ratio and 3dB of gain reduction

2nd harmonic  -55db (20Hz)   -43db (250Hz)   -53db (1kHz)   -66db (5kHz)

3rd harmonic   -49db (20Hz)   -29db (250Hz)   -33db (1kHz)   -46db (5kHz)

4th harmonic   -60db (20Hz)   -52db (250Hz)   -59db (1kHz)   -55db (5kHz)

5th harmonic   -53db (20Hz)   -35db (250Hz)    -41db (1kHz)   -55db (5kHz)

Turning to the exceptionally fast attack times available in the 1176, I set the attack time to the fastest setting of 20uS, ratio at 20:1 and applied a digitally constructed ‘fast edge’ test waveform to the 1176.  This test signal level was set to rise almost instantaneously from zero to 6dB over the 1176’s compressor threshold.  This is the most severe test I know of to check the real attack time of a compressor.  In the case of the 1176, it was able to gain reduce the applied signal to within 90% of its final ‘steady state’ compressed level within 20us.  I have not measured any other compressor able to match this attack speed.  Indeed I know of only one analogue limiter which can match this speed but it uses predictive soft clipping to achieve this kind of attack speed.

One aspect of the 1176’s amazing attack time, which seems counterintuitive, is how can the 1176 achieve such a fast attack time when the compressor side chain is sampling the audio after the FET gain control element?  Surely it is already too late to catch the ‘over’ at this point in the internal audio circuitry?

Part of the answer lies in the different side-chain gain structures of feedforward and feedback compressors.  If you want to looks at the mathematics of each design then I recommend a read of the excellent THAT Corporation design note here. For the rest of you, the best way I can describe the difference if that feedback based sidechains have a significant advantage in speed due to their gain structure.  The actual ‘real time’ it takes the audio to pass through an analogue circuit is so small that it makes no practical difference (in this respect only) whether the sidechain is derived at the input or output of the compressor.

1176 photos 007
1176 interior showing mains transformer and rear of VU meter and adjacent switch banks

In terms of physical construction, I popped the lid off the 1176’s case to have a look at the hand wiring and the three transformers (two audio and one mains supply).  It  really is vintage with hand wiring to all of the front and rear controls and interconnect, which I guess is what ‘vintage’ is all about.  The main steel chassis measured at 1.5mm thick with the top and bottom panels made of 1mm steel (or whatever the US imperial equivalents are) so you have robust case protecting the electronics both from physical knocks and from external noise interference.


There was quite a bit of flex on the main printed circuit board which worried me a bit and before I sent the unit back, I tightened all of the screws holding the pcb to the chassis.  To be fair I think my loan unit had been round the world a few times.

I also measured and heard low levels of mains hum which were a bit higher than I would prefer on a piece of studio equipment, (noise excluding hum was a respectable -84dBu).  As I write this I’ve e-mailed UA to get the hum specification just in case it was a one off issue with the demo unit.

These minor issues pale into insignificance when you begin to work with the range of tonal palette available in this unique and simple to use compressor.  It’s not at all surprising that that the Universal Audio UAD Powered Plug-Ins platform plus 1176 plug-in has become such a popular combination, with the advantages of multiple units and accurate stereo matching available to the user at moderate cost.

Would I buy a hardware unit even if I owned the plug-in version?  yes I plan to.

Would I buy two hardware versions? yes I hope to.

Would I buy more than two?  I haven’t got the space or the money, but the thought of a whole stack of 1176’s with examples of all the different revisions is a wonderful thing to imagine.

Browse here for the official product specifications and here if you want to read the excellent user manual.

Sound samples follow.  We set the 1176 for maximum sonic impact but be reassured it can compress in a much more subtle manner  (thanks to Factory Street Studios for hosting the session and the excellent drumming!)

Unprocessed Full Drum Kit

1176 Compressed Full Drum Kit

Unprocessed Snare Drum

1176 Compressed Snare Drum


Robert Campbell
February 2015

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