dbx Pro dB12 What Does a Direct Box Do?, Technical Support, Special Functions, Specifications

What Does a Direct Box Do?

First, a direct box converts the audio signal originating from an unbalanced device (like a guitar) to a balanced audio signal for transmission over a longer distance than is recommended for an unbalanced signal to travel. As audio industry people know, the advantage of a balanced signal is realized at the receiving end (audio input) of the balanced cable, typically in the form of a mic pre-amp input on a mixing console. The mic pre input circuitry consists of a “differential amplifier” whose job it is to “accept” the audio portion of input signal and “reject” the noise portion of the signal. The noise portion happens to be roughly equivalent on (or “common to”) both the “+” and “-” balanced conductors in the mic cable, since external electromagnetic fields are induced onto both conductors in a roughly similar fashion. Since the conductors are always in a “twisted pair” configuration, they “see” the same noise field. Thus, this is known as “com- mon-mode noise”. Another type of common-mode noise somewhat evenly-induced onto both conductors is due to the inherent physics of connecting two pieces of equipment together and the resulting flow of electrons due to noise voltages present at each piece of equipment. The ability of a balanced input to reject common-mode noise is measured in dB and called its “common-mode rejection ratio”, or CMRR.

The second function of a direct box is to convert from a high-impedance output – on an instru- ment such as electric guitar with passive pickups, for instance – to a low-impedance output. A cable driven by a high-impedance output is more susceptible to signal loss and noise pickup than a cable driven by a low-impedance output. Thus, using a direct box again improves, for a dif- ferent reason, the ability to run longer distances due to the impedance conversion from high to low-impedance.

Third, at the heart of the direct box is an isolation transformer that allows complete “galvanic” isolation, if desired, between the equipment plugged into either end. This eliminates the hum and buzz present in the system due to ground loops.

Example Setup 2: Direct guitar signal and amped signal on two mixer channels

Guitar	1	2	3	4	5	6
	From		Mixer
	OUTPUT
	on Rear Panel

INPUTTHRU

PAD

0dB

20dB 40dB

dB12 Active Direct Box

Mic

Amplifier

Technical Support

If you require technical support, contact dbx Customer Service. Be prepared to accurately describe the problem. Know the serial number of your unit - this is printed on a sticker attached to the bottom panel. If you have not already taken the time to fill out your warranty registration card and send it in, please do so now.

Before you return a product to the factory for service, we recommend you refer to the manual. Make sure you have correctly followed installation steps and operation procedures. If you are still unable to solve a problem, contact our Customer Service Department at (801) 568-7660 for consultation. If you need to return a product to the factory for service, you MUST first contact Customer Service to obtain a Return Authorization Number.

No returned products will be accepted at the factory without a Return Authorization Number.

Please refer to the Warranty information on the following page, which extends to the first end- user. After expiration of the warranty, a reasonable charge will be made for parts, labor, and pack- ing if you choose to use the factory service facility. In all cases, you are responsible for transpor- tation charges to the factory. dbx will pay return shipping if the unit is still under warranty.

Use the original packing material if it is available. Mark the package with the name of the shipper and with these words in red: DELICATE INSTRUMENT, FRAGILE! Insure the package properly. Ship prepaid, not collect. Do not ship parcel post.

Warranty

This warranty is valid only for the original purchaser and only in the United States.

1.The warranty registration card that accompanies this product must be mailed within 30 days after purchase date to validate this warranty. Proof-of-purchase is considered to be the burden of the consumer.

2.dbx warrants this product, when bought and used solely within the U.S., to be free from de- fects in materials and workmanship under normal use and service.

3.dbx liability under this warranty is limited to repairing or, at our discretion, replacing defective materials that show evidence of defect that are not due to normal wear and tear and/or improper handling by the user, provided the product is returned to dbx WITH RETURN AUTHORIZATION from the factory, where all parts and labor will be covered up to a period of four years. A Return Autho- rization number must first be obtained from dbx by telephone. The company shall not be liable for any consequential damage as a result of the product’s use in any circuit or assembly.

4.dbx reserves the right to make changes in design or make additions to or improvements upon this product without incurring any obligation to install the same additions or improvements on products previously manufactured.

5.The foregoing is in lieu of all other warranties, expressed or implied, and dbx neither assumes nor authorizes any person to assume on its behalf any obligation or liability in connection with the sale of this product. In no event shall dbx or its dealers be liable for special or consequen- tial damages or from any delay in the performance of this warranty due to causes beyond their control.

Example Setup 1: Guitar signal sent to amp and PA/Mixer

Guitar	1	2	3	4	5	6
	From		Mixer
	OUTPUT
	on Rear Panel

INPUT	THRU
	PAD
	0dB
	20dB
	40dB
	dB12
Active Direct Box

Amplifier

Special Functions

Polarity Invert

The polarity function is used in situations where the direct box is used as a splitter, and two “sort-of parallel” signals are fed to two mixer channels to be perhaps panned left and right for a nice stereo sound. Flipping the Polarity Invert function will change the phase relationship between the two signals and will typically result in significantly different-sounding results.

A typical example would be to plug a guitar into the input of the direct box, then feed the signal directly back out the Thru jack and into the guitar amp. Then the guitar amp would either be mic’d and sent to one mixer input channel, or a direct output on the guitar amp would be sent to the mixer channel. The output of the direct box would be fed into a second mixer input chan- nel, and the Polarity Invert switch position would then be selected based on preference on how it sounds. In the situation where the amp is mic’d, the position of the mic away from the guitar amp speaker also affects the exact phase relationship between the two guitar signals as well.

Filter

When a guitar amp is mic’d, not only does the guitar amp speaker not reproduce high frequencies, but the mic typically does not reproduce high frequencies well either at these loud levels. Thus, a mic’d guitar amp has significantly subdued “high-end”. Since the 0 / 20 / 40 dB pad feature allows input connection directly from the speaker output of a guitar amp or power amp* when switched to the 40 dB pad position, the Hi-Cut filter is used in conjunction with the 40 dB pad on an amplifier output signal to roughly duplicate the sound of a mic’d speaker. Otherwise the signal would have much more high frequency content and would sound “tinny” in comparison.

NOTE: The Filter switch only works when the Pad switch is set to 40 dB.

* See the maximum input level specification.

Specifications

Circuit Type:	Active
Input:
Number of Connectors:	1
Connection:	1/4” TS (Tip Hot, Sleeve Ground)
Type:	Unbalanced, RF Filtered
Attenuation Pad:	Switchable: 0, 20, 40 dB
Filter:	Switchable: Low Pass @ 6 kHz (40 dB pad position only)
Max Input Level (0 dB Pad):	+10 dBu typical @ 1 kHz with <1% THD
(20 dB Pad):	+30 dBu typical @ 1 kHz with <1% THD
(40 dB Pad):	+33 dBu
Input Impedance (0dB):	1 MΩ
(-20dB):	65 kΩ
(-40dB):	70 kΩ
Outputs:
Number of Connectors:	2
Main Output:	Male XLR, Balanced, Pin 2 hot
THRU Output:	1/4” Unbalanced TS (Tip Hot, Sleeve Ground)
Main Output Impedance:	600 Ω typical, balanced
Main Output CMRR (60 Hz):	106 dB typical @ 60 Hz
(1 kHz):	123 dB typical @ 1 kHz
(10 kHz)	108 dB typical @ 10 kHz
Output Polarity:	Switchable: Normal / Invert
Performance:
Bandwidth:	20 Hz to 20 kHz +0/-2 dB typical with 600 Ω load
	20 Hz to 20 kHz +0/-1 dB typical with 2 kΩ or higher load
Frequency Response:	16 Hz to 30 kHz, -3 dB, with 600 Ω Load
	10 Hz to 70 kHz, -3 dB, with 2 kΩ or higher load
Phase Deviation (Input ð Output):	+40 deg @ 20 Hz, -20 deg @ 20 kHz into 2 kΩ load
Insertion Loss:	1 dB typical
Harmonic Distortion (THD+N):	0.03% typical @ 50 Hz, 0 dBu
	0.003% typical @ 1 kHz, 0 dBu
	0.005%typical @ 10 kHz, 0 dBu
	-112 dBu, 22 Hz to 22 kHz, unweighted
Noise Floor:	122 dB, 22 Hz to 22 kHz, unweighted
Dynamic Range:	+48 V Phantom Power
Power Supply:
	< 8 mA
Voltage:	Switchable: Ground / Lift
Current:
Ground:	5.82” X 5.44” X 2.20”
Physical:
	1.5 lbs. (0.68 kg)
Size (L x W x H):	Metal Casting
Weight:	Powder Coat
Construction:
Finish:

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For Countries not mentioned above, please contact your local authorities for a correct method of disposal.

By doing so you will ensure that your disposed product undergoes the necessary treatment, recovery and recycling and thus prevent potential negative effects on the environment and human health.