550

3 band, 3 frequencies per band, 12 db boost/cut stepped discrete eq with LF and HF shelving on pushbutton
FILTER switch for band-pass in 50-15khz.
Two 2520 opamps
Proportional Q
Unbalanced input, transformer balanced output
Frequency knob was longer than the 550A and has a small black dot instead of a protruding pointer.
Gain strap on pin 11 could be used to adjusting for unity gain. Later models, like the 550A, 550b, and the 560's also have this adjustment on pin 11, but it's not featured on the 554.

All discrete design using two transistor (TIS prefix) voltage followers (VF) per band. The 2520's were Studio System Division and/or Farmingdale types, which were the oldest. Sometimes 2.2M ohms resistors were found around the opamps, trace side, for stability. Red lamp indicator for IN, which would usually burn out and had it's own voltage source from the 15 way connector. I would usually rewire this and put a LED with a voltage drop resistor.

550A

First view:
3 band, 5 frequencies per band, 12 db's stepped boost/cut discrete eq with LF and HF shelving on pushbutton
FILTER switch for band-pass in 50-15khz.
Two 2520 opamps
Unbalanced input, transformer balanced output
Frequency knobs shorter and had a pointer. Came in both blue and clear.

All discrete, like the 550, but using three transistor VF per band. The 2520's were Melville and Huntington types, and slightly improved over the older ones used in the 550. Additional frequencies per band, improved opamp stability, 47 ohm soldered on the trace side output, and LED indicator running off standard rails were other improvements over the 550.

In mid-1980, Kappa Systems Inc took over and licensed Datatronix to manufacture the API line who had been manufacturing the API line anyway by the late 70's. Datatronix changed the circuit board design but kept the electrical discrete design virtually the same. The circuit board had traces on the top and bottom, making it a little more difficult to service, especially around the opamp traces; and the VF amps were on a separate smaller board which plugs into a socket.

The 550A was re-introducted by API in 2004.

550A-1

Same features as the 550A but made to be an economical version in the late 70's. Key differences with the 550A are:

A) Electronically balanced input instead of the unbalanced input of previous three bands. This alone makes some prefer using it when experiencing the 6 db drop with an unbalanced input.

B) Constant Q instead of the Proportional Q

C) Use of integrated circuits (IC) instead of discrete components

According to Al Davis, a designer at API, it should have been called a 551 but readily available 550A faceplates were used instead. One way I can usually identify an A-1without 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. The only thing that bothers me is when someone purchases one and pays the full price of a 550A.

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. 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 the same sound as a 550A due to the Q difference and not quite as full in the low end, in my own tests, but still better than other eq designs of it's time as the popularity of using IC throughout in the late 70's became prevalent and overall quality diminished.

550b

4 band, 7 frequencies per band, 12 db's stepped boost/cut, discrete with LF and HF shelving on toggle switches
Proportional Q
Unbalanced input
uses two 2520 opamps

A well designed and versatile extension to the 550A, but a little different in sound to the 550A due to small changes in the Q. The 2520 opamps were on sockets, making it easy to replace on the two, double-sided circuit boards. The input 2520 can only be replaced with a new type (from 1990 on) because they are a slight bit smaller than the old ones, which do not physically fit due to an adjacent circuit board . Later 550b's used surface-mount VF amps.

553

3 band, single frequency per band, 15 db's boost/cut, discrete
Unbalanced input

The 553 was available in the early to mid 70's, and designed as a low cost module to be used in places where simple equalization was needed. High and Low frequencies were only shelving, and the mid was set at 3kHz, a frequency most sensitive to our hearing to help bring out the presence in music and dialogue. Very handy in broadcast and television for simple sound shaping.

The circuit used one 2520 opamp to drive the output transformer, but like the 550, 550A, and 550b, it had an unbalanced input. It also used two inductors for the low and mid frequencies. The 553 did not have a cover and care must be taken to prevent any damage to the inductors.

554

3 band parametric, sweepable frequency, 15 db's of boost/cut with peak/shelf/bell selection toggle switch on the HF and LF, and bell/peak for the mid band. Electronically balanced input.

From the mid-70's, the 554 filled a need for a sweep-able frequency equalizer, instead of the set frequencies of other models, making it a good "seek and destroy" method of eq'ing. With the peak Q switch, the 554 could be used as a notch filter, making use of the "seek and destroy" method of eq'ing to take care of noise problems like in video editing environments, or for removing a sibilance or resonance. Because the boost/cut switch is stepped, the last two positions go from 10db to 15db, reaching a big step with just one click designed to aid in notch filtering requirements. But this eq also offers bell for one octave bandwidth adjustments.The shelving side of it can also be powerful, helping to shape a high pass or low-pass filter with the amount of cut you need at the frequency that sounds right with the sweep.

The 554 makes use of many dual IC opamps in a state-variable filter design found in many mixers and consoles also employing a sweepable frequency equalizer. There are two 2520's with one in the output stage pushing an output transformer. The electronically balanced input uses a 741 type (MC1439, MC1741), and sometimes an LF series IC is found there although it's a JFET chip. The filter chips are 4558's, which are like dual 741's. Putting in something else, like high-speed JFET type opamps, may make the circuit unstable in certain areas. Because there are traces on the top and bottom of the circuit board, care must be taken in removing any components for replacement to prevent damage to the traces. Any damages must be followed by redundant wires in place of the traces.

A couple of variations of a similar design were also used in the 544P (not to be confused with the 544 input module for the 1604 console) and 954 Programmable Parametric Equalizer. The 544PM was a Manual version.

559

9 band Graphic, -12db cut, +15db boost, Unbalanced input

Manufactered by APSI (Audio Processing Systems, Inc.) and distributed by API (then called Automated Processes Inc.). Unlike the 560, the 559 had an unbalanced input. The filter amps used IC's, but had it's own potted output stage discrete amplifier and an output transformer. The boost/cut sliders were actually mechanically stepped level switch with the numerical reading on the front. The bandwidth, or Q, was tighter than the 560, giving it a more "notch-ey" sound. Like all the other API eq's, it has good noise and headroom specifications that holds well with 600 ohm loads.

560

10 band, 10 ocatave Graphic Equalizer, 12db of boost/cut, Electronically balanced input

Like the 550A-1 and the 554, the input was differential (IC) balanced which made it easy to interface with other gear, even if the source was unbalanced.

The first version with no suffix had an average date code of the components that would place it around the mid to late 1970's. Amplifiers for each stage used socketed IC's, using LF356's and RC4558's, but this model also had a single 2520 opamp in the output stage along with the AP2503 output transformer. Also came in an enclosed chassis with cover.

560A & AT

The 1980's model which had small changes to the slider values, but the biggest difference was - no 2520. Since only output drivers were needed to provide a low impedance output, the 2520 was replaced with two transistors, MPSU06 and MPSU56, which are hard to find today. The output transformer was optional and so the 560A was designated to not have it, and the 560AT came with the transformer. But the faceplates did not have A or AT written on them so you just have to look inside to know, but this shouldn't be hard because these units did not come with covers like the 560 before it. I've done some modifications to these for adding the output transformer and replacing some resistor values to bring things back to unity.

Note - the 560A/AT and earlier 560b's had a direct output on pin 15, which would be designated as the phantom power pin used by microphone preamplifiers in racks and this may cause damage to the unit from the 48V phantom voltage meeting the output of an amplifier or shorting through the primary of the transformer. If you have any of these modules, check if pin 15 is already cut off, which is normally done at the trace just inside the gold pin finger. If not, get someone who knows what to look for if you aren't confident about doing it yourself... and please do a clean job.

560b

10 band Graphic Equalizer, 12db of boost/cut
Electronically balanced input, optional transformer balanced output

From 1987 on to present, these had the same overall design as the old, and the 2520 opamp was brought back in the output stage with a transformer. Later units had surface mount IC but still had holes on the board for putting IC sockets if you needed to put a DIP-8 package. The 550bt was supposed to be the transformer output, and the 550b is the transformerless version but I have never seen a 'b' without transformers.

Not all of the 560x family have the exact same eq curves because slider pot values and cap values/types changed slightly over the years. Someday I will post an Audio Precision graph to illustrate this.

575

This is an oscillator, not an eq, so it doesn't really belong on this list. And the 15 way connector is on the opposite end so it's not the same as the other eq's listed here and can't plug into a standard rack... unless you used the last position in BAE's 11 space which has a cutout for another connector to the right, but you'd have to keep that slot only for the 575.

Another thing... if you look at the 575 faceplate cutout, it's really a 525 compressor faceplate - but upside down.