Direct Boxes - Yamaha PM-1000 Manual

FOURT2
DIRECT BOXES
The term ''direct box" refers to an adapter which
permits a power amplifier to drive a relatively lower
level input. Direct boxes are most often connected
between the speaker output of electric instrument
amplifiers and the input of a mixer, such as the PM-
1000 channel inputs. By using the amplifier's speaker
output, the reverb, tremolo, brightness, and any other
sound characteristics are conveyed to the mix. The
standard direct box consists of an attenuation pad that
reduces the power, and an impedance matching trans-
former to correctly terminate the mixer's input.
A variation of the direct box just described is not
meant for connection to the amplifier output. Instead,
it contains a "Y"'-adapter that enables it to be inserted
between the instrument and the amplifier. The "Ү"-
adapter taps the instrument output and feeds it
through an impedance matching transformer into
the console.
Rather than building the standard direct box, we
suggest using a combination of a T-pad and a matching
transformer box (described in the following pages). The
T-pad should be placed between the transformer's high
impedance input and the output of the amplifier. Ap-
proximately 20-40dB of padding is needed to prevent
transformer saturation (the pad value depends on
amplifier power). Then, another 20dB (approximately)
of voltage level is lost in the transformer due to the
impedance matching. As a rough guide, the 40dB T-pad
should be used with amplifiers rated from 50 to 200
Watts (continuous sine wave(rms) power), and the
20dB pad should be used for smaller amplifiers. Small
level variations are corrected with the console's input
level switches. If the instrument amplifier does not have
a phone jack speaker output, prepare a cable similar to
Figure D shown on "Connector and Cable" Chart (under
"Connections" in this section), but substitute a pair of
clips for the phone plug. These clips are then attached
to the speaker terminals, and the XLR connector is
brought to the T-pad. If hum occurs, try reversing the
clip leads on the speaker terminals.
The direct box variation can be assembled by utilizing
the impedance matching transformer box, and preparing
a special "Ү"-адартег cable.
RED WIRE
CLIP
CLIP BLACK WIRE
JUMPER
TO SPEAKER TO TPAD
OR AMP OUTPUT
TO INSTRUMENT'S
To
AMPLIFIER INPUT т CENTER CONDUCTOR - XFMR
MALE
; SHIELD :
Ж CENTER CONDUCTOR 1
XLR
Y
р
г
STD PHONE
PLUG
STD PHONE
JACK SHIELD
MATCHING TRANSFORMER BOX
Impedance matching transformers are manufactured
by several firms. A transformer capable of handling
nominal +4dB inputs (with at least +24dB peaks) should
be used. Because there is an apparent loss of about 20dB
in the transformer, due to voltage stepdown, the +4dB
input becomes about -16dBm at the secondary, and the
PM-1000's Input Level switch is then set at -20dB. The
-20 setting is ideal because it bypasses the console's pre-
amplifier, avoids any input attenuation within the con-
sole, and results in the best overall signal-to-noise ratio
and the best headroom.
The transformer should have a primary impedance
of approximately 30,000 ohms, with a secondary im-
pedance of 600 ohms (for high-z microphones, a primary
of 50K ohms and a secondary of 150 ohms is preferable).
A UTC-010 transformer, with the UTC-019 shield, is
ideal for most applications; equivalent transformers
should have similar level handling and impedance char-
acteristics. In-line transformers, such as those manu-
factured by Shure Brothers, may be used (although
suitable cable adapters will be needed).
The transformer should be mounted in a mini box,
wired to XLR connectors with standard cable, and
connected to the auxiliary equipment with one of the
cables previously illustrated.
FEMALE XLR MALE XLR
UTC-010
or equivalent
1
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