Revision D of the Urei 1176 range was produced in 1973 and was the first to implement the B11148 transformer, allowing a cleaner and higher gain output. Since each original unit has been circulating studios for over 40 years now, each of them have been aged differently by thorough use or years in storage.
The two Urei 1176 Rev D units that I had access to instantly could be seen to have lived different lives. The faceplate of one unit had a lighter shade of black, as if the colour had been washed out by sun, whereas the other was still jet black. Both units were scattered with assortments of scratches and residue left behind by engineers labelling them with tape. Most notable the second unit had the clear acrylic ring missing from the output knob, making matching the output levels somewhat troublesome. Someone had attempted to resolve this issue by scratching a line into the brushed metal top of the knob. Each unit’s VU meter rested at a different level, indication 0dB of gain reduction, neither of the two on the 0dB mark.
The majority of the tests I had conducted remained on the first of the two units, to keep as many variables as possible consistent. I felt that it may be interesting to find out if over the 40 years, the ageing process had changed the sound of the compressors in a similar way or if so long after production, each unit had developed their own unique characteristics.
In order to test this, I repeated some of the tests I had conducted on the first 1176 again on the second, attempting to keep input level, gain reduction and output level consistent with the first. Attack and release were left set to 7 ensuring both units were operating at their fastest.
From the gain reduction graphs it is visible that the although the shape of the curves are similar, there are subtle but certain differences. To begin with the initial transient burst’s release curve is much more affirmed on one unit. Unit A completed the release in just over 150ms with a steep curve. Whereas Unit B had a much shallower slope that continued to tail off gain reduction over 300ms. The sustained burst showed correlating results, with unit A having the steeper slope and faster release time, around 950ms. Unit B had a very linear slope and drew out the release phase for well over 1500ms.
Based on these results, I think it would be a fair guess that unit A has been in regular use throughout its lifetime, causing its faded faceplate, but fast and responsive transformers to remain responsive. Unit B could have been in storage for a long period of its life, keeping the faceplate in better cosmetic condition, but allowing the transformers to ‘cease up’ and become slower to respond.
It is also clear from the frequency spectrum that the harmonics each unit produces are different throughout the series. The overlapping graphs don’t make identifying the differences terribly clear, but the second unit (green) can be seen to have a much stronger 3rd and 7th harmonic. The first unit, represented in red, showed the majority of other harmonics as stronger.