Compression Study #3 – Urei 1176LN Ratio

The 1176 compressor is famous for its selection of gain reduction ratios. They are selectable by pressing in one of four vertically positioned rectangular buttons, with a satisfyingly tactile mechanical click. The ratios available are 4:1, 8:1, 12:1 and 20:1 on all revisions up until Universal Audio’s anniversary edition which added 2:1 instead of 12:1 (Universal Audio, 2011). Iconically, the 1176 introduced a way to apply all four gain reduction ratios simultaneously by pressing all buttons at the same time, and was aptly named ‘all buttons in‘. The characteristics of this were not subtle and created a hugely distorted pumping and breathing effect that became hugely popular. Some engineers have created excellent results by experimenting with applying two of the gain reduction circuits, for example – Tom Elmhirst uses the 4:1 and 8:1 ratios for bass to add FET distortion (Senior, 2009). The 1176 ratio not only altered the amount of gain reduction of audio above the threshold, but the envelope of gain reduction and how soft or hard the knee of the threshold was (Urei Inc., 1989).

In this post I will be looking further at the distortion characteristics that the different ratios have on audio. I have conducted these tests by inputting a 1kHz sine burst into the 1176LN RevD, generating -10dB of gain reduction. This process was repeated for each ratio, including ‘all buttons in’.

4:1 (Green) –  A clear harmonic series is visible created from the fundamental at 1kHz. Eleven harmonics can be clearly seen above the noise, progressively getting quieter with the odd order harmonics much stronger than the even order.
Urei Rt4

8:1 (Blue) – The harmonics follow the same pattern as the 4:1 ratio, however the harmonic series falls off on a less steep slope, producing stronger harmonics (above the 8th) up to the 19th. The 3rd harmonic is 5dB quieter than the 4:1 ratio also.

Urei Rt8

12:1 (Orange) – Within the tests that I conducted I found that the 12:1 ratio distorted more asymmetrically then the previous ratios. The odd order harmonics had a boost of around 2dB, whereas the even order were considerably louder at around +4dB. At the higher end of the harmonic series (above 12th) the harmonics remained present up until the 20kHz cutoff of the sample rate of the burst tone and session. 

Urei Rt12

20:1 (Red) – This ratio follows the same patten as the 12:1 ratio in terms of asymmetrical distortion, and continues the correlation of boosting the higher harmonics. A considerable boost is visible after the 10th.

Urei Rt20

All Buttons In (Purple) – Whilst all gain reduction circuits are enabled the odd order harmonics are the most prominent again, with the 3rd, 5th, 7th and 9th boosted by around 5dB. All even order harmonics received a cut of 2dB. 

Urei RtAll

Find full quality images of the graphs here


References:

Senior, M. (2009). Classic Compressors. Soundonsound.com. [Online]. Available at: http://www.soundonsound.com/sos/sep09/articles/classiccompressors.htm [Accessed: 2 February 2016].

Universal Audio. (2011). 1176 and LA-2A Hardware Revision History. [Online]. Available at: http://www.uaudio.com/blog/1176-la2a-hardware-revision-history [Accessed: 29 January 2016].

Urei Inc. (1989). Model 1176LN Peak Limiter. [Online]. Available at: http://www.jblproservice.com/pdf/vintage%20jbl-urei%20electronics/urei-1176lnmanual.pdf [Accessed: 2 February 2016].

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