Clean used example of this unit. Formerly used in Studio A at Looking Glass Studio in NYC, where the likes of David Glass, David Bowie and David Byrne recorded.
Avedis describes the 550A-1 best: Same features as the 550A but made to be an economical version in the late 70s. Key differences with the 550A are:
- Electronically balanced input in contrast to the unbalanced input of previous three band eq's. This alone makes some prefer using it when experiencing the 6 db drop when interfacing electronically balanced output to an unbalanced input.
- Constant Q instead of the proportional Q of the 550A
- Use of integrated circuits (IC)
- Uses one 2520 opamp in the output stage
According to Al Davis, a designer at API, it should have been called a 551 but readily available old stock 550A faceplates were used instead. One way I can usually identify an A-1, without looking inside, is the rubbery feel of the rotary switches instead of the snap you feel with a 550A. 550A-1's were sometimes found on ebay and sold as 550A's by sellers who may or may not have known what it was because often the serial number sticker said "550A" without a mention of A-1, and sometimes there was no serial number sticker.
Looking at the inside, there is one 2520 driving the output transformer with a feed-forward design using a NPN transistor. The other amps are IC's with an LF356 balancing the input and a TL074CN quad amp powering the bands and filter. The front rotary switches plugged into the circuit board with connectors.
Contrary to what internet loudmouths may say, the 550A-1 was not a bad module, it sounds very similar to a 550A with 2 or 4 db and becomes more noticeable when pushed to 9 or 12 db, which is probably where most people would boost or cut if something was wrong. Overall, it was a good design with good noise performance and had the same headroom as any 550A due to the 2520 and output transformer in the output stage. It is not exactly a 550A with the Q difference and not quite as full in the low end, in my own tests, but still better than other eq designs of its time, as the popularity of using IC throughout in the late 70s became prevalent and overall quality diminished.