550A Original hardware was made beginning in the late 1960s, and hand-wired. Besides the 3 bands, there is also a switchable wide bandpass filter (cutting below 50Hz and above 15kHz).
Re-issues were made by A*I starting in 2004, including a Saul Walker (the designer) limited edition with a different faceplate, signature and also hand-wired internals.
The modern production 550A is not hand-wired. AFAIK Alex sampled a vintage unit.
550B The 550B was introduced later and featured different frequency selections, a 4th filter band, and did away with the 'FLTR' bandpass button.
Like the 550A, gain is still in 2dB steps from 0 to +/-12dB.
5500 The 5500 is a newer product based on the 550B, with identical frequency selection.
So how is it different? Well, there are 2 units rackmounted together for stereo use, both with a gain multiplier (x0.25, x0.5, x1.0) for more subtle applications like mastering. The smallest boost or cut with a stock 550 is +/-2dB. Using the x0.25 multiplier, you can cut/boost +/-0.5dB.
Sound differences The 550A sounds a bit smoother to me, and with a slightly stronger color. The 550B or 5500 both sound a bit tighter or more focused and precise. The differences are most notable comparing the high and low bands.
Speaking from comparing the hardware, the 550 (no A or B) and 550A both sound a bit dirty or creamy, but the units here are vintage and haven't been serviced in many years. Very sweet top end.
@noorway - I've only spent a few minutes with 5D so far but it seems the same for me until I lower the ceiling. Then the threshold begins to do something. I'm looking forward to trying this out more in Reaper w/Parallel settings.
Nebula is based on the n3 engine: prog rate can go faster, which reduces artifacts, but not faster than 400 microseconds or use safetly more than 1.5 milliseconds of lookahead.
So when AlexB says to set lookahead to 6 ms it is of no use?
BTW, I can't get the latest A5D lib to work, threshold does no compression whatsoever. Other comps works as expected. Got latest Nebula server, running Logic Pro X.
for what I know it leads to crashes, or it is limited automatically by the engine, depending on the version.It is not something we can fix in n3 1.5 is a consequence of the global compatibility setting for handling all presets. Even if you load titanium in nebula it will be limited. Check ahead, it can disable compression due to internnal crashes. Difficoult to say.
With the 525 HW, a popular approach is to have ceiling at the halfway point (10), or all the way up (20) - then find the sweetspot for compression using the input gain/threshold. Sometimes the sweetspot is quite narrow on this unit, so take your time and make small gain adjustments.
Try compression mode (2:1) and use either the fastest release (100ms) working just a bit, or the slowest (2s) digging deeper. These settings can sound great on guitars, keys, bass, and synths.