Purple Audio MC76

A modern-day recreation of the classic UREI 1176LN compressor?

Review by Dave Martin

There's something strikingly familiar about the Purple Audio MC76. It's not the color of the front panel (though I did have a tie-dyed T-shirt in 1974 that was exactly that color, and there was a teacher whose neck would turn a similar shade when some of us came to class a few minutes late...), and it's not the model number (though if you were awake the day your math teacher discussed Roman numerals you'll figure it out).

But really it's quite simple. A couple of years ago (the MC76 was introduced at the 1997 AES convention in New York) the price of vintage gear was skyrocketing: a UREI 1176LN was selling for $2500 - $3500... when they were available at all. So Andrew Roberts and John Klett formed Purple Audio to design and build their own version of the 1176.

Since there were many versions of the 1176, from the original tube-powered 176 through the blue stripe, black-faced and silver-faced models... as well as different revisions (A through F), Roberts and Klett based the MC76 on the E revision of the 1176LN, which most compressor connoisseurs feel to be the best sounding version of the original.

The MC76 boasts two large Input and Output controls, and two smaller knobs that control attack and release times. The unit can be bypassed by turning the attack control counterclockwise. A row of buttons to the left of the large VU meter offers compression ratios of 4:1, 8:1, 12:1, and 20:1. The buttons on the right control the meter, showing either gain reduction or input levels calibrated to +8 dBM or +4 dBm. Power implementation is complete, with an on/off switch and an fused IEC detachable AC connector with switchable voltage selection. There's also a lug to facilitate chassis ground.

The MC76 is not a complete clone of the UREI-it uses XLRs for I/O (with the pin out configuration stenciled the between the connectors) rather than the barrier strips common in the original units. The MC76 is also easier to link than its predecessor; the UREI required an 1176SA (stereo adapter), which some engineers claim were problematic at the best of times.

The MC76 has link circuitry built in, with two 1/4" jacks, along with offset adjustment and a phase switch on the back of the box. This circuitry is powered with an AA battery.

The MC76 has transformer-coupled inputs and outputs, discrete transistor electronics, and a Class A output amplifier. Attack times are adjustable from 20 to 800 microseconds, release times from 50 milliseconds to 1.1 seconds, and the MC76 is capable of up to 45 dB of gain (with no limiting).

The manual is quite readable and it offers information applicable to all levels of users, from the simple explanation of compression and limiting aimed at the novice to quite detailed instructions on how to service the unit (for techs and repair people). Besides, how can you not like operating instructions that end with "7) Experiment"? (And speaking of experiments, both the 2-button and the 4-button trick work. If you don't know what I mean, forget I ever said it....)

How do they sound?

Quite good. If you're used to the distinctive sound of an 1176, you won't find the Purple to be a great surprise, though to my ears the MC76 is a little bit brighter and may have a little bit more of an edge than the originals.

While the MC76 doesn't sound exactly like any of the original 1176s that I've played with, it still sounds like an 1176. The sonic differences between an 1176 and the MC76 are pretty subtle, and both the monitoring environment and the signal passed through the box can change your perception of these differences.

It's also true that every original 1176LN sounds different, for a variety of reasons. Electronic components change differently as they age; units that have been well maintained-having dried-out capacitors replaced, for example-will sound better; and it wasn't uncommon for a studio or broadcast facility to modify electronic equipment to suit the tastes of the engineers who used it. So it's quite possible that the Purples sound like brand new 1176s.

Regardless, the MC76-and for that matter the 1176-is a little edgier than a comparably priced a tube limiter, and that's not a bad thing; I've used that edginess to make a dark vocal sound out in a track a little and to help keep a kick drum at the front of a mix. In fact, I've used my Purples pretty much anywhere I need to keep a track big while controlling the dynamics.

It's safe to say that I haven't done a mix in a year and a half without my pair of Purples somewhere in the processing. I've tried them on all sorts of instruments and voices, and they work well everywhere. If you're in the market for a the sound of a vintage piece of gear but don't want the hassles, the Purple Audio MC 76 is an excellent choice.



Purple Audio MC76 - Sound on Sound 1999

Classic studio signal processors such as the vintage Urei 1176 limiting amplifier impart a unique sound while keeping levels under control, but are rare and expensive. Purple Audio have an answer in the form of the MC76 -- an old idea revisited. Hugh Robjohns does the time warp again...

Some of the most useful pieces of equipment in any studio -- after the microphones, console, monitors and recorders, of course -- are the dynamics processors. Virtually every audio manufacturer in the world has at least one compressor/limiter in its product listing and there are many, many fine dynamics processors out there. Yet I would suggest that every professional studio worth its title is likely to have at least one classic Urei 1176LN limiting amplifier in its rack, as well as some hi-tech devices of the modern age. These ubiquitous devices have remained firm favourites with recording engineers across the world because of their ability to enhance almost any signal source in a fascinating variety of ways, ranging from subtle and delicate to hard and raunchy. They also happen to have a fantastically simple, but very effective, user interface which encourages the user to listen rather than just dial in the numbers.


Unfortunately, when Urei was taken over by JBL, and eventually entered the huge Harman audio manufacturing empire, the 1176 ceased production after many years of gradual evolution. The most commonly found version these days is probably the black-faced solid-state 1176LN, which used an FET as the active level-controlling device. However, the earliest and rarest version (the 176) employed valves and had silver faceplates with blue markings around the meter. The last examples of the line reverted to a silver panel, and were designated as 1176LN-F, the final letter denoting the design revision.


Pre-owned Ureis of any vintage can command substantial prices these days, so a lot of people were very pleased when Purple Audio set themselves the challenge of revisiting this particular piece of audio history. Their MC76 is based very closely on the 1176LN-E variant, which the company claims to have been the best-sounding version. The product name is as much a reinvention as the machine itself because, depending on who you believe, the letters either denote 'Mono Compressor' or provide the missing two digits from the original Urei device name -- but in Roman form!

The Ins And Outs
Intentionally, the MC76 looks (apart from the striking purple paint job) extremely similar to the Urei original. Even the Purple logo bears a strong resemblance to the old Urei badge. The 2U rackmount box is about eight inches deep and weighs roughly 5.5kg.

On the left of the front panel are two large knobs for setting input and output levels respectively, whilst the smaller attack and release time controls are arranged vertically in the centre of the machine. Grouped around the large illuminated VU meter are two rows of four chunky push-buttons, the set on the left selecting the compression ratio and those on the right determining the metering mode. Between the input and output controls a small hole in the panel provides access to a trimmer for calibrating the zero point of the gain-reduction meter.

The original 1176 could fairly be described as a 'quirky' device to use. It simply doesn't operate in quite the way that you might anticipate if you have experience of virtually any other dynamics processor -- and yes, the MC76 retains all of its 'lovable' features! There is no threshold control, for example -- the input level control determines how much of

  Double Trouble: Linking Two MC76s  
  The linking procedure involves delving around the back of the machine to plug a standard jack-jack cable between the 'offset' jack of one unit and the 'direct' jack of the other. Assuming both units are switched out of circuit and are without input signals at this stage, the gain-reduction meters will probably take off in opposite directions from their resting positions at the zero mark, because the FET biasing voltages are not yet matched. The next step is to recalibrate this biasing voltage (on the machine connected with the 'offset' socket) using a trimmer pot behind a small hole in the rear panel. This process is very difficult, as you can't easily see what you're doing to the meter displays when crouched at the back of a 19-inch rack. Once the biasing has been balanced and the gain-reduction meters have been guided back to their zero points, the units can be used in their stereo-linked mode.But there are a couple of things to remember. The simple circuit design means that the attack and release time constants interact across the two machines and are effectively paralleled, so that the fastest attack time is slowed to double the original value. The best way to use the machines in this linked condition is to match ratio, output-level and input-level settings on both machines, and to position the attack and release controls of one unit fully anti-clockwise (slowest position). This effectively makes these controls redundant, and the controls on the other machine then determine the time constants for both. Gain reduction should track fairly accurately over the first 10dB or so, but is likely to mistrack beyond that because of variations in the transconductance values of the FETs in the two units.  

the signal is driven above the internally fixed threshold, and therefore how much of the signal is compressed. The attack and release controls provide faster settings at the clockwise end of the rotation -- the opposite to the vast majority of dynamics units -- and the first time I used an 1176 I couldn't find the bypass switch, because it's hidden away as a backstop switch at the slow end of the attack control!


For the record, the attack time is variable between 20 and 800 microseconds, and release between 5 milliseconds and 1.1 seconds. The four ratio buttons are marked as 20:1, 12:1, 8:1 and 4:1, although they can be used in combination, with sometimes effective, if rather unpredictable results. Although there is no dedicated threshold control, the actual threshold point is sensibly related to the selected ratio, such that the higher the ratio, the higher the threshold. For example, at 20:1 the threshold is set to -24dBu, while a more gentle ratio of 4:1 starts with a lower threshold of -30dBu. Setting the unit up thus requires you to select the desired ratio, set the peak output level with the relevant control, and then adjust the input level to provide the required degree of 'squash'.

The four buttons to the right of the meter are configured in an equally unusual way. The bottom button switches the unit off, while pressing any of the other three powers the machine. The top button switches the VU meter to display the amount of gain reduction occurring; this is the normal operating mode. The two middle buttons show the output level from the machine, referenced to either +4 or +8dBu, although these hang the meter directly across the output without the aid of a buffer circuit, so can add distortion to the signal. These options are really only intended for initial setting of the machine's gain structure.

Few engineers would ever bother to use multiple linked Ureis for stereo or multi-channel working, simply because of the impractical method required to link the machines. A special interface box (the 1176SA) was needed. This featured an FET biasing battery, and the control voltages from each unit had to be trimmed and matched by hand. The MC76 improves this situation slightly, in that the interface unit has been built in, although there is still a need to match and trim the control voltages via a rear-panel trimmer and associated DC bias polarity switch. You wouldn't necessarily know it, but there's a lithium battery (AA-sized) tucked away inside the machine to provide the necessary biasing too. I recall a discussion in late 1997 (shortly after the MC76 was launched) regarding whether this consumable item could be replaced with a permanent biasing supply, courtesy of an extra tapping on the mains transformer, but unfortunately it appears that this idea has not been put into practice.

The MC76's rear panel reveals further improvements over the 1176, in the adoption of proper transformer-balanced and floating XLR connectors for audio input and output, and quarter-inch jack sockets for stereo linking. The original used terrible solder-tag connections on screw blocks for the audio I/O, and phonos for the linking facilities. The remaining connectors on the rear panel are an IEC mains connector, with integral fuse holder and voltage selector, plus a pair of sturdy binding-posts to link signal ground and mains earth.

Internally, the MC76 retains much of the electronic design approach of the 1176LN-E on which it is based, even down to identical component types in many instances. The balanced constant-impedance input attenuator is the same A
llen & Bradley device used in the original 1176, for example, as is the UTC input transformer which follows. However, there are also a number of small but significant improvements, including tweaks to the overall circuit design and improvements to the circuit board layout. Alternative components have also been used in a few areas where there were advantages in consistency or sourcing, including a custom-made output transformer. Structurally, the Purple Audio unit is also much stronger and generally better built than the old Ureis.

The input impedance is constant at a lowish 600(omega) (at all gain settings), and a signal level of up to +30dBu can be tolerated. Maximum through-gain is an impressive 45dB, and the Class-A output stage can drive a 600(omega) load to +24dBu or a high-impedance bridging load to +30dBu. Despite the transformers on input and output, the machine delivers an overall frequency response quoted as 15Hz-80kHz, +/-1 dB. Distortion is specified as less than 0.5 percent during limiting (assuming sensible attack times) and the signal-to-noise ratio as better than 81dB (unweighted).

In Use
It has been a while since I used an 1176 in earnest, but the MC76 seems to behave very much as I remember the original, even down to the wacky results obtained by pressing multiple ratio buttons. Other reviewers have suggested

"The original 1176 could fairly be described as a 'quirky' device to use -- and yes, the MC76 retains all of its 'lovable' features!"

that the MC76 sounds brighter than the original Ureis, and although I can't comment on this, as I was unable to make a direct comparison, I pass on the comment for your information.


The MC76 certainly imparts an almost valve-like warming effect to signals with just the gentlest amounts of compression, and it works particularly well on strong vocals, most drum tracks and, especially, bass guitars -- just as the original did. At low ratios, with only about 4dB of gain reduction, the unit is virtually transparent in the way it processes the dynamic range of a signal, yet it still seems to be able to enhance the sound passing through it in an almost intangible, yet desirable way. With harder compression it produces a more raunchy, busy sound, sometimes tending towards a buzzy harshness which I don't remember from the original.

Unfortunately, linking two units together is almost as much of a pain as it was with the original Ureis, I'm sorry to say (see the 'Double Trouble' box for details), and I'm not sure many engineers would be bothered to try it. The MC76, like its antecedents, is a wonderful mono compressor but a finicky and largely impractical stereo one!

To sum up, the MC76 is a lovely machine with, as far as I can tell, all the desirable attributes of the classic 1176s in a slightly 'ruggedised' and modestly improved package. Easy to use, great sounding and offering worthwhile value for money, this device is a certain winner. 


Scroll to Top