0% up to 48 Months on over 110 Brands!

New & Current Vintage King Card Holders

Looking for something that will be a similar fit in your studio? Contact us, chat with us, or call us at 888.653.1184.

API 550 A-1 EQ (Vintage)

Three-band equalizer with pedigree

Product Description

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.

Product Specifications

Condition Used / Vintage
Tube or Solid State Solid State
Channels Single-Channel
Chassis 500 Series
Module Type 500 Series
Module Slot Count 1 Slot
Digital I/O No

Product Reviews

No one has written a review yet. Be the first!

* Required Fields

Share Your Opinion

The best gear advice always comes from those who use it the most. That's where you come in! Let the pro audio world hear your voice with a review of the API 550 A-1 EQ (Vintage) . Once you submit your review, please check your email and verify your address to have it posted.

How do you rate this product?
I would like to thank VK for the wonderful sales assistance, service and prices. This was my first purchase with you and it won't be my last. I only wish I'd discovered you years ago!
Matthew R.
I had two Grammy wins this year - Ray LaMontagne & Carolina Chocolate Drops. Both made with help from Vintage King!
Ryan F.
In almost 20 years in Pro audio buying, never did I receive a unit so well packed and exactly as described. Works flawlessly. The whole process has been so smooth. I will never hesitate to do business with you guys in the future and will recommend you to everyone I know.
Martin C.
This was my first experience buying something from Vintage King (although I surf the "used" categories on the website frequently!), and, despite being on the other side of the world, I had absolutely no trouble and everything went very smoothly.
Phil T.
The speed of delivery exceeded my expectations! I was VERY pleased that my shipment arrived in time for a scheduled vocal tracking session. The packing insured that my order arrived safely. And the sales staff responded quickly to questions regarding the product.
James P.
Thank you so much! Vintage King has been so fair and easy to work with. I can't thank you enough. It feels good to trust such a hefty purchase to folks who are so helpful and knowledgeable. Vintage King is a sound designers dream store.
Noa L.
You guys have excellent customer service and I am excited about the next upgrade I complete on my fifth recording facility. I've been in the studio business for forty plus years. The salesman I worked with at Vintage King was the best of the best. You all rock!
Robby T.
I would recommend Vintage King to any one looking for top-class gear, unbeatable customer service and the best prices on the map!
Jake D.
My experience with Vintage King was excellent. The order was here fast considering it was out of stock. The sales person had SKB send it to me directly and I couldn't have been more pleased.
Michael D.
From the minute I placed my order I was kept well-informed through each step in your process. Even though I was buying a comparatively modest piece of gear, your attention to detail made me feel like I was purchasing a six-figure mixing desk.
Thomas W.
I am a new customer of yours. You guys provided me the absolute best deals in ordering all the equipment to power my brand new recording studio. Your customer service is amazing and i will always deal with you from here on out.
Rob M.
Thanks for your great service and getting this out today. You guys really are the Kings!
Alan B.
Better than expected! Truly impeccable service. Your team was professional, knowledgeable, provided speedy delivery above and beyond standard. Vintage King Audio is definitely my go to for all of my audio and information needs.
James O.