Handbook for Sound Engineers

296 Chapter 11

11.2.2.3 Line to Microphone Input or Direct Box

Because its high-impedance, unbalanced input accepts
line-level signals and its output drives the low-level,
low-impedance balanced microphone input of a mixing
console, the device shown in Fig. 11-38 is called a
direct box. It is most often driven by an electric guitar,
synthesizer, or other stage instrument. Because it uses a
transformer, it provides ground isolation as well. In this
typical circuit, since the transformer has a 12:1 turns
ratio, the impedance ratio is 144:1. When the micro-
phone input has a typical 1.5 kȍ input impedance, the
input impedance of the direct box is about 200 kȍ. The
transformer shown has a separate Faraday shield for
each winding to minimize capacitively coupled ground
noise.

11.2.2.4 Line Isolation or Hum Eliminators

There are a remarkable number of black boxes on the
market intended to solve ground loop problems. This
includes quite a number of transformer-based boxes.
With rare exception, those boxes contain output trans-
formers. Tests were performed to compare noise rejec-
tion of the original interface to one with an added output
transformer and to one with an added input transformer.
The tests accurately simulated typical real-world equip-
ment, see the definitions at the end of this section.

Fig. 11-39 shows results of CMRR tests on a
balanced interface using the IEC 60268-3 test procedure
discussed in Section 11.3.1.2. This test recognizes that
the impedances of real-world balanced outputs are not
matched with the precision of laboratory equipment.
While the output transformer reduces 60 Hz hum by over
20 dB, it has little effect on buzz artifacts over about
1 kHz. The input transformer increases rejection to over
120 dB at 60 Hz and to almost 90 dB at 3 kHz, where the
human ear is most sensitive to faint sounds.

Fig. 11-40 shows results of ground noise rejection
tests on an unbalanced interface. By definition, there is

0 dB of inherent rejection in an unbalanced interface,

see Chapter 37.While the output transformer reduces

60 Hz hum by about 70 dB, it reduces buzz artifacts

around 3 kHz by only 35 dB. The input transformer

increases rejection to over 100 dB at 60 Hz and to over

65 dB at 3 kHz.

Fig. 11-41 shows results of CMRR tests when an

unbalanced output drives a balanced input. A two-wire

connection of this interface will result in zero rejection,

see Chapter 37. Assuming a three-wire connection, the

í30 dB plot shows how CMRR of typical electroni-

cally-balanced input stages is degraded by the 600ȍ

source imbalance. Again, the output transformer

improves 60 Hz hum by over 20 dB, it has little effect

on buzz artifacts over about 1 kHz. The input trans-

former increases rejection to almost 100 dB at 60 Hz

and to about 65 dB at 3 kHz.

Figure 11-38. A transformer-isolated direct box.

R 1

J 1 6.8 K^7

Line

level

input

Yel

Org

Blk Gnd lift

Whi

1 k 7

Gry

Brn

Red

T 1

JT DB E Miclevel

output

P 1

10 nF

(^32)
1
Figure 11-39. Balanced output to balanced input.
Figure 11-40. Unbalanced output to unbalanced input.

10

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(^140) 20 200 2 k 20 k
Rejection dB Versus Frequency–Hz
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Input
Frequency–Hz
0
20 200 2 k 20 k
Rejection dB versus Frequency–Hz
Output
Input

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Frequency–Hz