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API 550 history / tech info 500 Series EQ\'s
Old 27th April 2012
API 550 history / tech info

I have been trying to find some information about the original 550 EQ but not much is available online. I'm interested to know what the production years were for these and also the differences from the 550A? I know the high and low bands have more frequencies on the "A" and there is one less 2520 but I'm curious to know if they use the same transformers and caps etc. I suppose if there was a schematic floating around this would be much help.
Old 27th April 2012
member no 666
Fletcher's Avatar
I don't know [and frankly couldn't care less] about the production years... but you're right on the 550 having fewer frequencies than the 550A... and with the exception of the release with the 550A-1, I'm like 99.99857% sure you're wrong about 550's having only one 2520 op-amp.

I know the 550A-1 [from the "Datatronix" iteration of API] had only one 2520 [and a 5534], but I'm nearly positive the 550 was running 2x 2520's [small shred of doubt only because I don't have one in front of me to confirm... but over the years of being in and around I don't believe I ever saw one that didn't have 2x 2520's].

Old 27th April 2012
I'm curious about the production years because I just got a pair and I wanted to get an idea of how old they were. I'm pretty sure I read that they started making them in 1969 but I have no idea how long they were making them. I'm sure not many people care about that but I do so if anyone has that info I'd much appreciate it. At some point someone had replaced the transformers in the pair I have so I also wanted to find out which model API they came with originally.
Old 27th April 2012
Deleted User
The 550's would have came with 'Studio Systems division', 2520 op amps (i have a bunch of modules).

The 550a had also 2 op amps but has three transistor VF's per band.
Old 28th April 2012
Gear Addict
Avast!'s Avatar
John Klett sez:

API List

Doesn't really answer your questions, but hey! It's gearslutty.
Old 28th April 2012
Lives for gear
cheu78's Avatar
Old 5th May 2012
Gear Head

vintage vs re issue 550a

hi guys i am looking at purchasing a vintage 550a model as i love what they do on vocals, brass, guitars .. in your collective opinions are there major sonic benefits to the vintage model? do the older switches give you trouble ? i would buy it re capped of course. also, if i add a second can i be sure they will match ok in stereo compared to the re issue models ? have looked about but have not found detailed info on this. as i am new to API i appreciate your comments
Old 6th May 2012
Gear Head
klett's Avatar
Originally Posted by Avast! View Post
API List

Doesn't really answer your questions, but hey! It's gearslutty.
yes it is... I have all sorts of lists... people used to call or email me a lot before I put up my curmudgeon barrier and stopped answering the phone. Some would ask about or mention makes and models of things I did not know much about so over time I made note of those things, looked through my files and libraries and made lists and databases to have handy so I could keep the model numbers, products and features straight - because who can remember all this crap - and also to let me know what I may need to gather docs and info for in the future... that api list is one I printed and posted... it's an old one.

on api 550 versions...

some years ago Paul Wolff, while he owned api, wrote up a little history and posted it. Because those older sites were so ephemeral I cut and paste his post into a file and made some edits and additions... most of this is from Paul... all the wrong stuff is from probably from me... in May 2000

Versions of API 550 Equalizer

The api 550 series equalizer, with the exception of the 550A-1, has a family of boost cut curves that are referred to as "Proportional Q". Proportional Q describes a relationship between the energy that is added or removed by boost and cut and the range of frequencies (bandwidth) effected by differing degrees of equalization.

If you look graphically at the the lines representing flat response and the lines in the family of curves describing the equalized boost or cut responses you'll find that the area enclosed by those lines remains reasonably constant despite the boost or cut amount. The enclosed area represents the amount of energy being added in boost or subtracted in cut. With small amounts of boost or cut the equalization curve is low and very wide and, as the amount of boost or cut is increased, the Q increases and the equalized response curve becomes narrow or sharper. The bandwidth and the boost/cut amounts are proportionally related - WIDER at 2dB and NARROWER at 12dB.

The API 550:

Proportional Q Equalizer with three frequency selections on the top band, three frequency selections on the bottom band and five frequency selections in the middle band, four discrete transistor buffers each using two transistors, a two pole filter for the high/low pass switch (different than 550A), the EQ is switched out by tying all the filters together and taking them to ground through the in/out switch, two 2520 opamps, and long frequency knobs.

The API 550A:
Proportional Q equalizer with five frequency selections on the top band, five frequency selections on the bottom band and five frequency selections in the middle band, four discrete transistor buffers each using three transistors, 1x-10x high and low pass filters with same frequencies as the 550 (50 HZ to 15KHz band pass), each band went through it's own in/out switch, two 2520s and short frequency knobs.

The API 550A-1:
Constant Q equalizer... the bandwidth remains constant regardless of boost or cut amount. The 550A-1 was essentially the same circuit as used in the API 560 graphic equalizer but the frequency selections are distributed over three boost cut "bands" having the same frequency center points as the 550A. That's why it sounds so different. This model had an IC opamp based input and filter stages and one 2520 on the output.

The design change has been explained two ways... one explanation was that API simply wanted to lower the cost of making EQ's and the other was that a customer was ordering several large consoles and expressed that he really liked sound of the 560 EQ but wanted the stepped controls and center points of the 550A... since this represented a large sale and involved about a hundred equalizers API made that change and then kept making them that way because they were less costly to replicate. API never checked the Q other than full up at 12 dB, so they never knew that they were different. They actually did not realize that the original 550 was proportional Q. By that time they figured this out 2500 550A-1's were manufactured and sold and everyone was flipping out, mostly not in a good way.

The Datatronix 550A:
Proportional Q equalizer - a re-engineered reissued of the original 550A. After a lot of flak from end users, and specifically Sunset Sound, who paid for the redesign, the proportional Q version of 550A was reworked to make the switches and transistor buffers removable. That is the circuit board you see in some 550A equalizer modules made by Datatronix. Sunset Sound got the rights to the first 500 and then Datatronix could sell them. These were used in the Sunset custom console originally installed in Studio 1. For a time both 550A-1 and the Sunset version 550A equalizers were in manufacture. Then Paul Wolff buys api from Datatronix... (was that 1988? - something like that)

The "Paul" 550A:
Paul Wolff built about 250 of the redesigned 550A's, and ****-canned the 550A-1

The "Paul" 5502:
Two Proportional Q equalizers in a 2RU rack mount package. Paul redesigned the proportional Q 550 circuit into a four band version called the 5502. Eighty-Five of them were manufactured.

The "Paul" 550B:
Proportional Q equalizer - Henry Sanicola of O'Henry Studios asked Paul to build seventy-five four-band 550 type equalizers that would fit in a regular 550. Each of the four bands had seven frequency selections.

Paul "When I released the original 550B, XXXXX XXXXX got ahold of the prototype and did not like it. It turned out that it was oscillating and sounded bad. He, to this day, still thinks they all sound that way, but every one else likes them. I never discontinued the 550A, and after the release of the 550B, up until this day [May 2000], have only sold seven 550A's and over 3000 550B's. Everyone bitches at me about them, but didn't buy the new 550As. Go figure. The 'Experts' like XXXXX XXXXXXX feed the industry about how bad the new stuff is so they can sell old ones and support their uninspired lives."

Paul went on to make the 550-S which, along with the 212-S, would retrofit into Sony MXP-3000 consoles

These became the 550-L and 212-L that were designed for the api Compact Console that was never built but, instead, grew a little to became the Legacy

Paul "You will never be able to "simulate" a 550 Eq in a plug in. It's like a blow up sex toy. It's either real or it's not. If you think you have or can, then you should, because XXX XXX'X XXXX anywayzzz...."

Old 7th May 2012
Gear maniac

Perhaps off topic, but I have a pair of 560s I love as much as life itself. I find them suprisingly useful for a graphic eq. I agree with you as to the 550's being impossible to replicate in a precise manner. But I will say the waves 560 plug was a familiar sound to my ears as outrageous as that sounds. Also thanks for that informative post klett. I've never seen it all written out as such. interesting indeed.
Old 8th May 2012
Lives for gear
Ike Zimbel's Avatar

Interesting that JK's post with the history of various API eq modules (post #9, above), STILL doesn't shed any light on the mysterious "550-M" (not the recent "M"astering version). I had a couple in the shop a while back and have a client with a couple. They appear to be early 70's build, have THREE two-transistor buffers, two 2520's and serial numbers in the 1400's. Sorry, I didn't take note of how many frequency selections each band has.
Old 17th December 2012
Gear addict
skinnypete's Avatar

Is there an issue with API eqs not having an input transformer, or input impedence, or something like that? I have a newer 550B, and I have had noise issues with my setup in the past that I'm trying to solve.

With 550As and 550Bs is there some reason why you should put them directly after a mic pre, instead of post-compression? I thought I read something on GS about this, but I couldn't find the thread.

Sent from my Droid
Old 19th December 2012
Gear addict
skinnypete's Avatar


Sent from my Droid
Old 21st December 2012
Gear addict
skinnypete's Avatar

Old 22nd December 2012
Registered User

hey skinny, you should drop an email on Jeff Stieger, he's the owner of Classic Audio Products of Illinois CAPI, he knows a ton about API, and then do yourself a favor, and buy anything he makes, especially the vp28 2 stage preamp and lc53a eq. you will be blown away.
Old 28th December 2012
Gear addict
skinnypete's Avatar

Originally Posted by Tonycamp View Post
hey skinny, you should drop an email on Jeff Stieger, he's the owner of Classic Audio Products of Illinois CAPI, he knows a ton about API, and then do yourself a favor, and buy anything he makes, especially the vp28 2 stage preamp and lc53a eq. you will be blown away.
Thanks for the tip man. I just might, but then again I probably would end up buying one of those sick looking eqs if I contacted him! Ha

Sent from my Droid
Old 28th December 2012
Deleted User
I think the letter suffixes like 550M or S related to consoles they were put in. Don't take my word for that. But a Sony MXP 3036 console that belongs to a colleague of mine has a few channels with the EQ's swapped for something called API 550S eq's . The faceplate is a different color and length/width than a standard 500 series module (to fit the console). and it has even a slightly different font / font size saying '550S' , which led me to believe that it was made for that particular board ( S = SONY ) ?? Solely theory.
Old 29th December 2012
Gear addict
skinnypete's Avatar

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