API 550 A

£0.00 £1,259.00 inc. VAT
£1,049.17 exc. VAT

550A Discrete 3 Band EQ

The 550A remains the standard against which other EQs are measured, and it has played a major role in the recording industry for decades. Still copied but never duplicated, the 550A became API's standard channel module EQ when the company began manufacturing consoles in 1971. With virtually all existing units spoken for, popular demand for this EQ resulted in API finally resuming production in 2004.

The 550A provides reciprocal equalisation at 21 points in 5 steps of boost to a maximum of 12dB of gain at each point. The fifteen equalisation points are divided into three overlapping ranges. The high and low frequency ranges are individually selectable as either peaking or shelving, and a band-pass filter may be inserted independently of all other selected equalisation settings. Frequency ranges and boost/cut are selected by three dual-concentric switches, and a pushbutton "in" switch allows the EQ to be silently introduced to the signal path. A small toggle switch is used to insert the band-pass filter into the 550A.

The combination of Walker's incomparable 2520 op amp and his "Proportional Q" circuitry gives the 550A user an uncomplicated way to generate acoustically superior equalisation.

  • 3 bands of classic API equalization
  • Each band offers 7 API selected frequency centers
  • Reciprocal and repeatable filtering
  • Maximum 12 dB of boost/cut per band
  • EQ Band 1 and 3 offer shelf/peak switching
  • "Proportional Q" narrows filter Q at extremes
  • Traditional API fully discrete circuit design
  • High headroom +30 dB clip level
Input Impedance:
66 kΩ high level
17 kΩ low level
Output Impedance:
Less than 85Ω (transformer isolated)
Nominal Operating Level:
–15 dBm to +12 dBm
Insertion Loss:
None (bridging)
Frequency Response, Unequalized:
±0.5 dB 30 Hz to 20 kHz
Unequalized, output noise of –90 dBm, high level strapping and
–98 dBm, low level strapping, 20 Hz to 20 kHz unweighted
.05% THD max, 30 Hz to 20 kHz @ +4 dBm
.07% THD max, 30 Hz to 20 kHz @ +18 dBm
Clipping Level (3% THD):
+30 dBm
Power Requirement:
±15 volts to ±18 volts @ 70 ma
Dual Concentric Rotary: LF select: 30, 40, 50, 100, 200, 300,
400 Hz and stepped degree of equalization –12dB to +12 dB

Dual Concentric Rotary: MF select: 200, 400, 600, 800, 1500,
3000, 5000 Hz and stepped degree of equalization –12dB to
+12 dB

Dual Concentric Rotary: HF select: 2500, 5000, 7000, 10,000,
12,500, 15,000, 20,000 Hz and stepped degree of equalization
–12 dB to + 12 dB

All equalization is in steps of 2, 4, 6, 9 and 12 dB either plus or minus
Toggle Switch:
50 Hz to 15 kHz band-pass filter In or out
Push Button: LF range Peak or Shelf
Push Button: HF range Peak or Shelf
Push Button: Equalization In or Out
Tally light when In
30 pin PC connector supplied
1.5" X 5.25" X 7.75" Deep
Size (Boxed for Shipping):
4.5" X 6.25" X 10.25"
Actual Weight:
2 lbs.
Shipping Weight:
2.53 lbs.