Fig. 39A - Pads Constructed in Mini-Boxes.
above +30dBm, use 1 watt, low inductance resistors.
10% tolerance is acceptable for most pads.
Fig. 39B - Pad Constructed in Switchcraft Model S3FM
It is possible to construct a pad within an XLR con-
nector, but the extremely tight fit can adversely affect
reliability. The Switchcraft model S3FM is a tube with a
male A3M (XLR) at one end, and a female A3F (XLR)
at the other end. Pads using 1/4 watt resistors can be
constructed inside this device. Cover the entire pad with
insulation tubing before final assembly into the S3FM.
A "mini-box" fitted with male and female XLR con-
nectors is an easy to build, rugged housing for a pad.
Use stranded wire for best results.
Illustrated are three typical pad construction tech-
niques. For most applications, it will be sufficient to
construct only a few types of pads: 20dB, 24dB, and
40dB pads cover almost any requirement. Consult
Figures 37, 38 and 39 for schematic, construction and
resistor value information.
Transformers
Audio transformers (as distinguished from power
supply transformers, RF transformers or other trans-
formers) are primarily used for ground isolation,
impedance matching and level matching. The following
paragraphs detail several applications of audio trans-
formers at low signal levels. Speaker-level transformers
are discussed on Page SEVEN 6; the Appendix gives
further details on transformer operation.
Matching Transformer Box:
Impedance matching transformers can be used to
connect a high impedance source to a low impedance
load, or vice-versa (see Page SIX 5 for a discussion of
matching versus bridging inputs). The box shown below
may be used to run a 600-ohm balanced or floating line
to the P-2200 input, or it may be used between any
600-ohm source and high impedance input. Use a trans-
former capable of handling nominal +4dB (1.23V)
inputs with at least +24dB (12.3V) peak capability.
The transformer should be mounted in a mini-box,
wired to the XLR connectors with stranded wire, and
connected to the auxiliary equipment with one of the
cables previously illustrated. In line transformers, such
as those manufactured by Shure Brothers, Sescom, and
others may be used, with suitable adapters.
Fig. 40 - Matching Transformer Box
Step Up Transformer Box
The step up transformer box illustrated here is
similar to a pair of matching transformer boxes. This
configuration provides voltage step-up for optimum
drive levels when connecting the output of a low
impedance, low level source, such as the headphone
output of a mixer, to the two inputs of the P-2200. It