Aphex 1788A Premium Eight Chan Mic Pre - Manual (Page 46)

Page 46
MICROPHONE PREAMPLIFIER
Page 47
Instruction Manual
APPENDIX F
NOTES
The following notes are referenced in the text and in Table 2 on the preceding pages.
Note 1: ADDITIONAL READING SUGGESTIONS: Sound System Engineering by Don Davis and Carolynn Davis (Howard W.
Sams and Co.), Handbook for Sound Engineers (The New Audio Cyclopedia) Edited by Glen Ballou (Howard W. Sams and
Co.) and Sound Reinforcement Handbook by Gary Davis and Ralph Jones (Hal Leonard Publishing Corp.).
For more information on the “Pin 1 Dilemma” see the June 1995 issue of the Journal of the AES (Vol.43/No.6, Audio Engi-
neering Society, New York). This issue is dedicated to “Shields & Grounds.”
Note 2: TABLE 2: LEVEL CHARACTERISTICS (unity gain verses 6dB loss)
Diagrams 1, 2 and 4 through 8: The effective interface gain will remain at OdB for all of the interfaces shown.
Diagram 3 Balanced to Unbalanced: This configuration should be used with equipment incorporating a conventional active
balanced output stage. Most non-Aphex equipment uses this kind of output stage because of its simplicity and low cost.
Therefore, you will find yourself using diagram 3 fairly often when interfacing typical equipment together. In this case, the
interface gain will be 50% down, giving a 6dB loss of level. This is because each output driver has fixed gain and supplies
only half the balanced output amplitude. You can usually compensate for the loss by adjusting the output level or input
level settings on the associated equipment.
Note 3: XLR GROUND DROP ADAPTERS A word on using XLR ground drop adapters (and mic cables): A secondary advan-
tage to not using the XLR case lug as a ground is the ability to use barrel style XLR “ground drop” adapters (hard-wired
female to male XLR adapters with Pin 1 disconnected). These can be used on one side of the cable to disconnect the ground
as discussed earlier. If the case lug is tied to the cable’s shield (with or without Pin 1) the cable will still be grounded to the
equipment’s ground though the XLR ground drop’s case. XLR ground drops are useful especially for live sound situations
where the same stock of XLR cable may be used for line level patching and/or for patching microphones. The reason is
that XLR cables used for microphones should never have the shield disconnected at one end. This is a safety issue for the
performing artist - microphones should always be grounded!
Note 4: If you have an unbalanced output but have a balanced input, the “pseudo-balanced” configuration may help deal
with ground loop hum. However, it is possible to wire this as an unbalanced interface. To wire unbalanced - use a coax cable
with a single conductor and a shield, wire as follows: Output (From) - connect the ground contact to the cable shield, con-
nect the positive contact to the positive conductor; Input (To) - connect the ground and the negative contacts to the cable
shield, connect the positive contact to the positive conductor. Please note that this in fact occurs by default when plugging
an unbalanced cable with a 1/4” TS (Tip-Sleeve) phone plug into a TRS (Tip-Ring-Sleeve) phone jack.
Note 5: Some electronically balanced output circuits from other manufacturers exhibit problems when the negative output
amplifier is shorted to ground to in order to drive an unbalanced load. The grounded output amplifier pumps current onto
the ground which may cause distortion or cause the amplifier to fail. When wiring this type of output be careful to leave
the negative contact unterminated.
Note 6: Shield Option: Connect receiving end only. See text preceding Table 2 titled - THE PIN 1 DILEMMA AND HOW IT
AFFECTS CABLE SHIELD CONNECTIONS
Note 7: Single-Ended, Impedance-Balanced
Note 8: It is possible to wire this as an unbalanced interface. To wire unbalanced - use a coax cable with a single conduc-
tor and a shield, wire as follows: Output (From) - connect the ground and the negative contacts to the cable shield, con-
nect the positive contact to the positive conductor; Input (To) - connect the ground contact to the cable shield, connect
the positive contact to the positive conductor.