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The dbx 540 Tube Preamp with Optical Compressor is a Class-A amplifier that provides a transformerless high-voltage signal path. Not only does the preamp boost your mic and instrument levels via an interchangeable tube, but the 540 also offers adjustable compression, a selectable in-line feature with which dbx aims to provide true optical compression characteristics.
With the preamp's gain knob, you can boost your mic level inputs from 10 to 60 dB and your instrument inputs by up to 45 dB. By changing out the 540's 12AX7 electron tube and experimenting with modern and/or vintage tubes, you can customize your sound. Also, dbx adds a low-noise and low-distortion op amp to aid in delivering clean audio.
The 540 has a TRS input with an optional -20 dB pad, 48 V phantom power, and polarity invert, all via mini-toggle switches. Also, with its low cut filter at 80 Hz, you can omit unwanted low frequencies to clean up your input.
The switchable on/off optical compressor provides adjustable attack times from 1 to 100 ms, and release times from 0.1 to 3 sec. You can adjust your compression in the signal path via its level knob. When the compressor is in use, the on-board dual-function VU meter, with peak indicator, automatically changes from displaying master level output from -20 dBu to +3 dBu, to showing the same range of gain reduction. dbx aims to deliver true optical compression with the 540 by attempting to emulate an opto's slow attacks, 2-stage release times, and minimal fluctuations in volume.
Lastly, the multifunctional dbx 540 is a modular and portable one-slot unit compatible with a variety of rack-mounted and standalone power units from dbx, and is compatible with all other manufacturers' 500 series products.
Suitable for use in all 500-series-compatible systems
Class-A transformless signal path
Adjustable optical compressor
Switchable phantom power
Switchable -20 dB pad
Dual-function VU meter: master volume / gain reduction
Input gain control
Output volume control
In the Box
dbx 500 Series 540 Tube Mic Preamp with Optical Compressor
Limited 2-Year Warranty
Table of Contents
Mic/line TRS balanced
Microphone Input Gain
10 - 60 dB
+48 V switchable
1/4" jack unbalanced
XLR from back panel
XLR and 1/4" jack (back panel)
Line in: 10 Hz to 40 kHz within 1 dB
Mic in: 10 Hz to 40 kHz within 2.5 dB
Ratio 1:1 to 10:1
Attack 1 - 100 ms
Release 0.1 - 3 sec
Shelf at 80 Hz
Slope: 16 dB
-20 to +3 dBu
Total Harmonic Distortion (THD)
0.05 % (unweighted)
Signal to Noise
500 series type required
dbx 500 Series 540 Tube Mic Preamp with Optical Compressor Review