207d
instruction Manual
Aphex Systems Ltd. Model 207D
Page 24
207d
2-ch tube mic preamp with adc
Page 25
Aphex Systems Ltd. Model 207D
Appendices
Avoid Transformers
The use of balancing transformers is an option, but
you will invariably lose audio quality due to trans-
former limitations. Try everything else first.
Appendix C: Proper Wiring Techniques
A true balanced line should be used wherever your
equipment allows. Use “twisted pair” shielded cable.
For unbalanced wiring you should use high grade,
low capacitance shielded wire for best results. If you
have an unbalanced output but have a balanced
input, the “pseudo-balanced” configuration may help
deal with ground loop hum. This method and others
are illustrated in Table 2.
CONNECTOR WIRING STANDARDS
The 3 pin XLR, 1/4” (63.5 mm) TS mono phone and
the 1/4” (63.5 mm) TRS stereo phone are the most
commonly used line level connectors in pro audio.
Less common is the use of the “RCA” phono jack,
which is essentially a consumer type connector. The
XLR and the TRS are three conductor and are used for
balanced connections. The TS and the RCA are two
conductor and are used for unbalanced connections.
In addition to the three main contacts on an XLR
there is also a grounding lug contact. This lug is con-
nected to the connector’s case (shell). In all Aphex
products audio ground and chassis ground are one
and the same. Aphex products that use XLR con-
nectors tie Pin I to the XLR case automatically.
There-
fore it is not necessary to use the XLR case-ground lug.
This also makes possible the use of XLR ground drop
adapters (see Note 3).
TABLE 1:
The wiring convention shown is now stan-
dardized in 17 countries including the USA. Please
note that any equipment that still uses Pin 3 as posi-
tive on XLR connectors is not adhering to the stan-
dard.
THE PIN 1 DILEMMA AND HOW IT AFFECTS CABLE
SHIELD CONNECTIONS
The three main contacts on an XLR (or TRS) and
the accepted wiring assignments shown below are
only part of the picture. The standard for terminat-
ing ground is Pin 1 (Sleeve). But which ground? It
could be connected to audio signal ground or chassis
ground depending on the method of grounding used
by the equipment manufacturer. In all Aphex prod-
ucts audio ground and chassis ground are one and
the same at all I/O jacks. This is just good, common
sense engineering practice (which is what you would
expect from us, course). Unfortunately, many prod-
ucts are designed so that the noisy currents from
the shield drain into signal ground instead of chas-
sis ground. This practice creates a real hum and
noise problem for end-users. The appropriate overall
grounding scheme of an audio system would be a lot
easier to predict without this problem.
The standard balanced line wiring recommendation
from Aphex Engineering is this: In the majority of
cases maximum noise rejection occurs when the
shield is connected to the input ground only (espe-
cially in locations with high levels of RFI). That means
the sending end shield should be left disconnected.
However, if you already have cables with the shield
connected at both ends, go ahead and try them
out. If you are connecting a fairly simple audio
system it may be fine as is.
A word on optional shield connections:
Connect-
ing the cable shield of a balanced line at both ends
creates unnecessary ground loops which may carry
noise and hum currents that can be amplified. Con-
necting the shield only at the sending end (instead
of the receiving end) may exaggerate common mode
noises at the receiving input stage. It can actually
increase RFI and noise more than having no shield
at all. Because of the “Pin I Dilemma” (mentioned
above) you may be forced, in some situations, to
experiment with how the cable shield is connected
to ground to eliminate a pesky hum or radio interfer-
ence problem. It might be good to try XLR ground
drop adapters (see Note 3) as a method of trying
these conflicting methods out and being able to
change easily if necessary.
IMPEDANCE
Regardless of inaccuracies, it has become more or
less standard over the years to refer to balanced