Analog Obsession
Classic Filters: The UA 550-A and UREI 565 little Dipper
By Will Shanks

UA’s 550-A filter set--among our first products

Lately I've been noticing that filters of the UA/UREI variety are cropping up on eBay quite a bit. I've been interested for some time in finding out more about these, perhaps, less glorified audio tools. I say less glorified because they don't command quite the same high prices as vintage compressors or preamps. In a moment of serendipity, I got a call from someone who wanted to know if I was interested in some old Universal Audio filters. Needless to say, I was!

Model 550-A Filter
The 550-A is among the first products ever made by Universal Audio, and the two I came across are serial numbers 140 and 141, probably dating them some where between 1958 and 1959. The units are completely passive; no AC required! Little documentation can be found on the Web, but I happen to have an old Universal Audio catalog handy at all times...

“One of the coolest nontraditional uses of the Little Dipper is for radical phase sweeps, as the tight Qs and the 20Hz to 20kHz range of the combined filters allow extreme tonality shifts. Both band filters can be used simultaneously for even trippier phase acrobatics.”

"550-A Recording Filter is a variable hig pass and lowpass filter with excellent band restriction over the important ranges required by modern film and disc recording, broadcast and sound reproduction. The 550-A has independent controls for both high and low frequencies, each with six positions. Separate on/off controls are provided for both high and low frequencies. Standard double jacks are furnished on the front panel for input and output connections, or permanent connections may be made to a barrier strip on the rear of the unit."

After putting them on the AP, I found the filters are 18 dB per octave-the highpass filters put a little bump at the cutoff point. All the filters are stepped. Highpass frequencies include 40, 55, 70, 85, 100 and 200 Hz. Lowpass frequencies are 15, 12.5, 10, 8.5, 7 and 5 kHz. As might be expected, the unit's components are simple, and the build quality is phenomenal: PCB boards, caps and chokes, point-to-point wiring, chunky stepped switches, and a robust chassis and faceplate with etched markings. The 550-A filters were sold racked as a single unit, or in pairs. It looks like the units I received are 100% original. With just a little component cleaner for the pots, the units seem to function as well as they did in 1958. What a testament to the UA legacy! A very useful set of filters with a very vintage sound.

UREI 565 Little Dipper
The Little Dipper filter set was introduced at the 1970 AES show in New York, and filled a desperate little niche created in the audio and film production room: the need for the removal of very specific sonic artifacts and anomalies. The Little Dipper has some of the tightest band filters ever made, and is most famous for this incredibly tight 5% Q on its dual-band filters; the Little Dipper can do these tight cuts in a relatively transparent manner. The Little Dipper can also do extremely tight boosts. Six Q settings are available for each band--three for notches, three for peaks. There is also a Range control for the depth of the cut or boost, with coarse and fine controls for frequency selection. In addition to the two band filters, the Little Dipper features 18dB-per-octave high- and lowpass filters, as well as a global Bypass switch. There are two versions: the 565 and 565T. The 565T has an output transformer, allowing balanced output to 600-ohm loads, and it draws an elevated market price.

One of the coolest nontraditional uses of the Little Dipper is for radical phase sweeps, as the tight Qs and the 20Hz to 20kHz range of the combined filters allow extreme tonality shifts. Both band filters can be used simultaneously for even trippier phase acrobatics.

From the original cut sheet:
"While the Little Dipper's versatility makes it an ideal tool for many signal processing tasks, its primary function is to remove undesirable 'coherent' or 'semi-coherent' noise components from audible range recorded tapes, film or records. For example, AC-hum oscillations, heterodyne 'whistles,' etc. can be rendered completely inaudible with no apparent effect on the sound quality of music or voice, because of the fluorescent fixture 'buzz' can be greatly reduced. Sounds of an incoherent nature, such as jet aircraft, amplifier noise, and general background noise, can be effectively reduced using the Low Cutoff and High Cutoff filters with or without the band reject filters. A silent-operating 'bypass' switch is provided on the front panel to add or remove all filter action instantly at will."