API 5500 User manual

5500 Dual Equalizer
OPERATOR’S MANUAL
Revised 2024-01-17
Written for Automated Processes, Incorporated
by Daniel Pfeifer

Automated Processes, Inc.
8301 Patuxent Range Road!
Jessup, MD 20794 USA
301-776-7879
http://www.apiaudio.com
©2024 Automated Processes, Inc.
All rights reserved under International
and Pan-American Copyright Conventions.
API®and the double-spear logo are registered trademarks of Automated Processes, Inc.
No part of this publication may be reproduced in any form or by any means, whether
mechanical or electronic, without the written permission of Automated Processes, Inc.
As research and development is a continual process, API reserves the right to change the
features and specifications described herein without notice or obligation. API cannot be held
responsible for any loss or damage arising directly or indirectly from any error or omission
in this manual.
PLEASE READ ALL INSTRUCTIONS, PAY SPECIAL HEED TO SAFETY WARNINGS.

Thank You for purchasing API's 5500 Dual Equalizer!
Whether you're seeking a truly superior analog sound, or simply want to be part of the more
than 55-year tradition of API excellence, we appreciate your purchase.
API takes great pride - and great care - in the hand assembly of all our professional audio
products. Above all, we appreciate our loyal users, who have made the Company what it
is today.
Larry Droppa
President

5500 Dual Equalizer API
3
Table of Contents
About this Manual............................................................................. 4
Important Safety Instructions .......................................................... 5
1.0 Overview .................................................................................... 6
1.1 History of the API Equalizer............................................................6
1.2 All-Discreet Design .........................................................................6
1.3 Proportional Q Circuitry ..................................................................6
1.4 Fully Reciprocal Operation ..............................................................6
1.5 Range Control ................................................................................7
1.6 Power Mute and Bypass..................................................................7
2.0 Front Panel Controls ................................................................... 8
2.1 Bypass ...........................................................................................8
2.2 Channel Controls ............................................................................8
2.2.1 IN ............................................................................................ 8
2.2.2 Four-Band Equalization................................................................ 8
2.2.3 Frequency Selection ....................................................................8
2.2.3.1 Peaking and Shelving ...................................................................... 8
2.2.4 Boost and Cut ............................................................................9
2.2.4.1 Range ........................................................................................... 9
3.0 Rear Panel Connections ............................................................ 11
3.1 Balanced INPUT and OUTPUT Connections ....................................11
3.2 AC Power, Voltage Selection, and Grounding .................................11
Appendix ........................................................................................ 12
A1 5500 Technical Specifications........................................................12
A2 5500 Transformer Option ..............................................................12
A3 5500 Recall Sheet .........................................................................13
A4 5500 Housekeeping.......................................................................13
A5 API Limited Warranty and Service Information ..............................14

5500 Dual Equalizer API
4
About this Manual
This manual covers the user operation of the API 5500 Dual Equalizer.
Most sections are useful to all users and it’s recommended that the manual be read
thoroughly before purchase and delivery. Experienced engineers may get up and running
quickly, but if you are new to API equalizers, it’s recommended that you read the entire
manual at least once and keep it handy while recording, editing, and mixing.
Manual Revision History: YYYY-MM-DD format
Original: 2009-06-29
Previous: 2017-03-01
Revised: 2024-01-17
Dan Pfeifer is Professor Emeritus
of Recording Industry at
Middle Tennessee State University

5500 Dual Equalizer API
5
Important Safety Instructions
1. Please read these instructions
2. Keep this information in a safe place
3. Do not use this device near water
4. Clean only with a dry cloth
5. Do not block any ventilation openings
6. Do not install near any heat sources such as radiators, heat registers, stoves, or
other devices that produce heat
7. Do not defeat the safety purpose of the polarized or grounding type AC plug
8. Protect the AC power cord from being walked on or pinched
9. Use only attachments/accessories specified by the manufacturer
10. Unplug this device during lightning storms or when unused for long periods of time
11. Refer all service to qualified personnel
ATTENTION: Exposure to extremely high noise levels may cause
permanent hearing loss or damage. Individuals vary considerably
in susceptibility to noise-induced hearing loss, but nearly everyone
will lose some hearing if exposed to sufficiently intense noise (this
may include music) for a period of time. Be safe.
WARNING – To reduce the risk
of fire or electric shock, do not
expose this apparatus to rain
or moisture.

5500 Dual Equalizer API
6
1.0 Overview
1.1 History of the API Equalizer
The lineage of the 5500 circuit can be traced back to the original 550 equalizer designed by
Saul Walker, the founder of Automated Processes, Inc. That three band EQ was originally
used in the earliest API consoles built in the 1960s. Engineers were immediately captivated
by its fully reciprocal boost/cut characteristic and its proportional Q design, one of API’s
many industry firsts.
The sound of the 550, 550A, and 550B console equalizers was so highly prized by engineers
that many purchased API equalizers separately or took apart old consoles in order to install
the equalizers into a hand-made chassis. This allowed them to have that same API console
sound available no matter where they were working. Today, API manufactures the six and
eight-space Lunchbox and the 10-space VPR rack, making it easier for engineers to bring
the API sound to all of their recordings.
The 550 was designed as a console equalizer which, due to the architecture of the recording
console, uses an unbalanced input. Also, the ergonomics of a console dictates that controls
take up little space, sometimes limiting the number of functions that can be included in a
design. The 5500 is specifically designed to continue the tradition of API’s precision
equalization. It features a balanced input, a true straight-wire bypass, an integrated power
supply with noiseless muting, and a range control that expands its versatility to mastering
applications.
1.2 All-Discreet Design
Like all API products, the 5500 contains no integrated circuits in its signal path. The gain
comes from two hand-built 2520 operational amplifiers in each channel and the balanced
input is handled by a 2510 discrete operational amplifier. Besides being a key component of
the API sound, the 2520 coupled with the API 2503 output transformer is capable of
delivering +32dBu before clipping. With this much headroom, it is unlikely that the 5500
can be driven to distortion.
1.3 Proportional Q Circuitry
The Twin “T” topology is found in many equalizer circuits, but the legendary Saul Walker
included a novel Proportional Q section that makes it truly musical. Proportional Q works by
spreading the equalizer’s action over a broad bandwidth at low degrees of boost or cut. This
provides delicate shading and subtle coloring of the signal’s tone, without leaving the
impression that signal processing was applied. As the amplitude is advanced to more
extreme settings, the equalizer’s bandwidth narrows to provide the user with surgical
precision. This allows the equalizer to be useful in removing undesirable noises or radically
emphasizing an aspect of an instrument’s character, such as its attack.
The additional circuitry necessary to provide proportional Q is not included in most other
equalizers because it adds nothing to the spec sheet. Listening to musicality in the signal is
the only way to appreciate the benefits of the added cost and complexity required for
Proportional Q design.
1.4 Fully Reciprocal Operation
Before the 1960s, few equalizers had the same characteristics in boost mode as in cut
mode. (In fact, many earlier designs did not have both boost and cut to begin with.) Today,
it is common for equalizers to be reciprocal, but again, Saul Walkers design is special. When
switching an API EQ from boost to cut, the exact same components are used, reorganized
around the amplifier for the appropriate function. This guarantees 100% reciprocity. The

5500 Dual Equalizer API
7
benefit becomes apparent when a track is re-equalized to remove the previously applied
equalization. The engineer can be confident that, because of the 5500’s fully reciprocal
design, a truly flat frequency response will be restored.
1.5 Range Control
Each channel of the 5500s four-band EQs features a separate Range Control, which allows
the users to choose from several API module boost and cut specifications, detailed below:
Boost / Cut
Application
API Module Reference
+/- 2dB
Standard Equalization
550B
+/- 1dB
Higher Resolution Equalization
550D
+/- ½dB
Mastering Equalization
550M
The range of the amplitude controls can be reduced to .5 or .25 of their stated scale, with
the Q altered identically as before. Range Control provides a means of adjusting the tonal
balance with finer resolution in an even gentler manner. This should be especially useful for
complex program as contained in stems or submixes, and is ideally suited for mastering
purposes. The combination of reduced range equalization coupled with APIs characteristic
“warmth” can be useful in helping breathing life to the sterile sound often found in digital
recordings.
1.6 Power Mute and Bypass
The 5500 features a true hard-wire bypass. In this mode, the output connector is wired
directly to the input. Shortly after power is first applied, or immediately after it is lost, a
special circuit forces this mode, so signal is never lost and power thumps are never heard.
The BYPASS button illuminates in red when the 5500 is in bypass mode, whenever either
the special circuit or the operator put it into bypass.

5500 Dual Equalizer API
8
2.0 Front Panel Controls
2.1 Bypass
BYPASS: When BYPASS is engaged, the unit enters a true hard-wire bypass,
the output is connected directly to the input.
• There are no electronics in the signal path when BYPASS is engaged
• Illuminates in red when engaged
2.2 Channel Controls
The 5500 is a two-channel unit that can be used as two separate mono EQs or in stereo.
Each channel has an identical set of controls, all of which will be outlined in the following
sections.
2.2.1 IN
IN: When IN is engaged, the equalizer is fully operational.
• When it is “out” (not engaged) there is no equalization applied
• IN differs from BYPASS in that the electronics and output transformer
are still in the signal path
• Illuminates in blue when engaged
2.2.2 Four-Band Equalization
The 5500 Dual Equalizer offers four overlapping bands of equalization that enables
exception control over tonal shaping: LO, LO-MID, HI-MID, and HI.
2.2.3 Frequency Selection
Each band equipped with seven (7) fixed frequency selections:
LO: 30Hz, 40Hz, 50Hz, 100Hz, 200Hz, 300Hz, 400Hz
LO-MID: 75Hz, 150Hz, 180Hz, 240Hz, 500Hz, 700Hz, 1kHz
HI-MID: 800Hz, 1.5kHz, 3kHz, 5kHz, 8kHz, 10kHz, 12.5kHz
HI: 2.5kHz, 5kHz, 7kHz, 10kHz, 12.5kHz, 15kHz, 20kHz
2.2.3.1 Peaking and Shelving
All equalizer sections have peaking (also called “bell” in the UK) response characteristics.
While the central frequency of that boost (or cut) is the one engraved on the panel,
there is of course amplitude variation that occurs to a lesser extent on nearby
frequencies. This is termed the Q where higher Q’s have less effect on adjacent

5500 Dual Equalizer API
9
frequencies. As mentioned before, the API equalizers have a Proportional Q that gets
larger as more amplitude change is selected.
On the LO and HI bands there is an option to select shelving response. When in shelving
mode, the equalizer reaches the indicated amplitude at the indicated frequency and
remains with that amplitude change for all frequencies to the edge of the audio
spectrum.
LO SHELF: When LO SHELF is engaged, the low-frequency band changes
from a peaking to a shelving EQ
Peaking is the default mode
Illuminates in blue when engaged
HI SHELF: When HI SHELF is engaged, the high-frequency band changes
from a peaking to a shelving EQ
Peaking is the default mode
Illuminates in blue when engaged
For frequencies towards the center of the spectrum the change is gradual, but the slope
differs slightly from the peaking mode. This can have important effects on the midrange
character of the sound, but that is not the typical reason to select shelving. The two
principal reasons for using a shelving response is either to eliminate noises like rumble
that are outside the frequency range of the desired sound, or to give a general “tilt” to
the tonal balance towards treble or bass. The API 5500 accomplishes both of these tasks
without sounding resonant.
2.2.4 Boost and Cut
Each band equipped with eleven (11) fixed boost and cut selections:
• -12dB
• -9dB
• -6dB
• -4dB
• -2dB
• 0dB
• +2dB
• +4dB
• +6dB
• +9dB
• +12dB
2.2.4.1 Range
The RANGE control is a unique and powerful addition to the traditional 550 circuit. It is a
formal implementation of a popular factory modification of the 550 EQ. The RANGE of
each channel can be set independently.
In x1 mode, the equalizer behaves
identically to the classic 550A and B.
In x.5 and x.25 modes, the amplitude
scale as shown on the front panel is
reduced proportionally.

5500 Dual Equalizer API
10
True to the design of the 550, the Q is reduced in an equivalent fashion, meaning that if
6dB of boost is selected in the x1 RANGE, you will get the same amount of boost and an
identical shape of the EQ curve by selecting 12dB in the .5 RANGE. The classic subtlety
of the 550 when used at low amplitude settings is enhanced by the ability to perform
micro-adjustments of tonal color.
Mastering engineers also prefer an equalizer that has exactly repeatable switch settings
and the capability of truly reciprocal operation, features that are common to all API
equalizers.

5500 Dual Equalizer API
11
3.0 Rear Panel Connections
3.1 Balanced INPUT and OUTPUT Connections
Each channel is equipped with a set of balanced, line level audio input and output
connectors.
The audio INPUT is a female, 3-pin, XLR connector that feeds an active balanced circuit.
There is a switching 1/4” TRS (tip, ring, sleeve) connector that interrupts any signal that is
present on the input XLR. It is balanced and can be driven from either balanced or
unbalanced sources. A positive signal on the tip will deliver a positive signal on pin 2 of the
output XLR. Using the 1/4” input does not bypass any internal circuitry and does not change
gain or operating level.
The audio OUTPUT is a male, 3-pin, XLR connector and is driven from a transformer coupled
output. The OUTPUT can drive any load from 600 ohms or greater to full output capability.
The polarity is such that there is no change from input to output, so it is suitable in studios
using either pin 2 or pin 3 as the “hot” connection.
3.2 AC Power, Voltage Selection, and Grounding
For line voltages from 100v to 120v, set the switch to 115v. When set to the 115v position
use a 500mA GMA fuse with Slo-Blo characteristics.
The 230v position is good for all line voltages from 200-240 volts. At the 230v setting use a
250mA Slo-Blo fuse.
IMPORTANT NOTE: It is necessary to change the fuse when changing the supply voltage.
A chasis ground connection is provided for technical grounding purposes.

5500 Dual Equalizer API
12
Appendix
A1 5500 Technical Specifications
Rear Connectors: XLR & ¼” TRS Input, XLR Output
Input Impedance: 20K Ohms
Balanced Output Impedance: 75 Ohms, Transformer Coupled, Balanced
Maximum Input Level: +32dBu
Maximum output Level: +32dBu
Frequency Response: +/- 1dB from 20Hz to 30kHz
Signal to Noise Ratio: -105dB
Power Requirement: 18 Watts
Size: 19” x 1.75” (1U) x 11.25” deep
Size (boxed for shipping): 23.25” x 6.5” x 16”
Actual Weight: 10 lbs.
Shipping Weight: 14 lbs.
Filter Frequency Centers: LO: 30, 40, 50, 100, 200, 300, 400Hz
LO-MID: 75, 150, 180, 240, 500, 700, 1kHz
HI-MID: 800, 1.5k, 3k, 5k, 8k, 10k, 12.5kHz
HI: 2.5k, 5k, 7k, 10k, 12.5k, 15k, 20kHz
Filter Boost/Cut Steps: +/- 2, 4, 6, 9, 12dB
Range Multiplier: x1, x0.5, x0.25
A2 5500 Transformer Option
There is a field upgradable option for a transformer-coupled input. The discrete balanced
input circuitry is bypassed using this transformer. It uses the highest quality line input
transformer, which can provide low distortion and wide bandwidth when fed from
conventional professional gear. Other brands of audio equipment will encounter clipping at
around +24dBu, but another API device could distort any input transformer since it is
capable of an undistorted +30dBu output.
Since API equipment is capable of signal voltages that are typically twice as large as most
professional equipment, it is possible that the input transformer will experience some
distortion in the low frequencies when fed signals beyond +24dBu.

5500 Dual Equalizer API
13
A3 5500 Recall Sheet
A4 5500 Housekeeping
Here is a great place to write down the serial number of your 5500 and your
purchase date, just in case you need that information someday in the future.
API 5500 serial number ________________
Purchase date: ____/_____/______

5500 Dual Equalizer API
14
A5 API Limited Warranty and Service Information
a) Warranty Information: API products carry a one year factory service and five year
parts warranty from date of purchase. API (Automated Processes, Incorporated)
does not cover claims for damage due to alteration and/or abuse. This warranty is
limited to failures during normal use, which are due to defects in material or
workmanship. If any defects are found in the materials or workmanship, or if the
product fails to function properly during the applicable warranty period, API, at its
option, will repair or replace the product.
b) PLEASE NOTE: The design or quality of any non-authorized third party service or
vendor is beyond the control of API. Therefore, service or modification of any API
unit except by an authorized API representative may VOID this warranty.
c) API reserves the right to inspect any products that may be the subject of any
warranty claims before repair or replacement is carried out. Final determination of
warranty coverage lies solely with API.
d) This warranty is extended to the original purchaser and to anyone who may
subsequently purchase this product within the applicable warranty period. Proof of
purchase may be required.
e) For questions regarding operation, interfacing or service of your API
product, please contact your API dealer from whom you purchased the unit.
Many times, your authorized API dealer is the fastest and most cost-
effective way to maintain and service your product.
f) The below steps are the best way to initiate the repair process or to submit a parts
order request:
Repair procedure:
1. Fill out a Return Authorization (RA) form at service.apiaudio.com.
2. Wait to receive an e-mail from API Audio with an RA#.
3. Use the API original box to package the unit. Write the RA# large and legibly
on the box (if the RA# is not clearly visible on the box, the unit may be
rejected by our receiving department)
4. Include copy of the RA form with the unit.
5. Ship the product freight prepaid to:
API SERVICE DEPARTMENT
8301 Patuxent Range Road – Ste A1
Jessup, MD 20794
Parts Order procedure:
1. Fill out online PO form (for parts and parts numbers not listed online please
fill out PO form with your name, e-mail, contact phone and shipping address
and describe the part you need).
2. Submit online PO form.
3. API will e-mail you back with part numbers and procedure how to order/pay.
g) This is your sole warranty. API does not authorize any third party, including any
dealer or sales representative, to assume liability on behalf of API or to make any
warranty for API.
h) THE WARRANTY GIVEN ON THIS PAGE IS THE SOLE WARRANTY GIVEN BY API AND
IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESS AND IMPLIED, INCLUDING THE
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
THE WARRANTY GIVEN ON THIS PAGE SHALL BE STRICTLY LIMITED IN DURATION
TO FIVE (5) YEARS FROM THE DATE OF THE ORIGINAL PURCHASE FROM API OR AN
AUTHORIZED API DEALER. UPON EXPIRATION OF THE APPLICABLE WARRANTY
PERIOD API SHALL HAVE NO FURTHER WARRANTY OBLIGATION OF ANY KIND. API
SHALL NOT BE LIABLE FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL
DAMAGES THAT MAY RESULT FROM ANY DEFECT IN THE API PRODUCT OR ANY
WARRANTY CLAIM.
i) This warranty provides specific legal rights and you may have other rights, which
vary from state to state.

8301 Patuxent Range Road!
Jessup, MD 20794 USA
301-776-7879
http://www.apiaudio.com