This test shows how the Urei 1176 colours audio without actually imparting any gain reduction, whilst the attack knob is turned fully counterclockwise to the ‘OFF’ position, bypassing the gain reduction circuit. No parameters such as attack, release or ratio alter the sound produced while the gain reduction circuit is being bypassed.
By comparing the frequency response of the clean sine sweep with the resulting sweep from the 1176, we can see that the clean sweep has been coloured somewhat by the Urei 1176. Although the gain reduction circuitry has been bypassed, the signal still runs through the transformers (Universal Audio, 2009).
In the Urei 1176LN Rev D these were the UA-5002, that were used up until Rev E. Revision F saw the change to the B11148 transformer with higher gain (Universal Audio, 2011).
From the graph, it can be seen that the transformers create third harmonic distortion. On this particular sweep off of 800Hz the series can be seen to ascend 1.6kHz, 2.4kHz, 3.2kHz, 4kHz etc. There is also slight presence peaks within the higher frequencies, visible at 2.5 kHz, 3.5kHz and 5.5kHz.
In the low frequencies narrower bands of resonances can be seen to subtly colour the signal. These occur at 56Hz, 84Hz, 110Hz, 165Hz and 190Hz. The 1176LN is also imparting a subtle low shelf boost on the audio, visible from 50Hz and below.
All tests conducted on the Urei 1176LN Rev D were performed directly though an Apogee Symphony. To minimise signal flow I connected straight from the DAC into the 1176, then straight back into the ADC.
Universal Audio,. (2009). Model 1176LN Solid-State Limiting Amplifier. [Online]. Available at: https://media.uaudio.com/assetlibrary/1/1/1176ln_manual.pdf [Accessed: 29 January 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].