Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
tances become the dominating noise resistance,
thus the generated noise is much higher. The
preamp can’t tell whether a mic is there or not,
it just amplifies whatever signal is there even if it
is just noise.
Some other pointers may help you get lower
noise from this preamp. Do not leave an external
mic pad on the line then goose up the gain later
to get enough level. You should always use the
strongest mic signal you can get and use the
lowest mic preamp gain possible to keep the level
right. Use the internal 20dB pad only when the
input signal is approaching a line level. You will
know when that is necessary if you switch the
preamp gain all the way down to 21dB and the
meter is still pegging.
Don’t expect to be able to run mic cables directly
along a.c. power cables. You are likely to pick up
dimmer buzz. If you have to get near a.c. power
cables then lay mic cables crossways to minimize
induction. If you get a real pesky hum problem,
try a different microphone, cable, channel, etc.
Nothing beats substitution for tracking down a
bad component.
2.5.4 Insert Jacks (Send - Return)
Located next to the Line Out XLR connector, these
1/4” phone jacks allow you to insert outboard
audio processors between the mic preamp and
the A/D converter as well as between the preamp
and the analog line output. The jacks are normally
looped straight through (“normalled”). With a
stereo plug inserted in the return jack , the signal
will be routed through the outboard equipment.
The Send jack may be used as an auxiliary output
without breaking the normal link, but it is not
fully isolated from the preamp and could cause
noise or distortion problems if not properly used.
The operating level at the Send jack is approxi-
mately 0dBu when the preamp is operated with
typical headroom allowances. It may run hotter if
you are pushing the MicLim heavily.
You should use the input gain controls of your
outboard equipment to match this level rather
than turn down the preamp gain. The return
audio level should be adjusted on the outboard
equipment to match the send level. The front
panel Headroom Meter indicates the level at the
Return. This is also the A/D converter feedpoint,
so the meter always indicates what the A/D con-
verter is receiving, thus also indicating the con-
verter’s digital audio level.
2.6 Mute Jack
Plugging a switch of any sort into this jack allows
you to remotely mute the preamp. Muting
affects not only the line output, but also the digi-
tal audio. When the switch is open, the preamp
is muted. A closed switch unmutes. The front
panel mute button works in parallel with the jack.
Whichever causes a mute takes precedence. The
other cannot cause an unmute.
If no phone plug is inserted, then the jack nor-
mals to a closed condition, giving all mute control
to the front panel button. Connections are to the
tip and sleeve only. You can therefore use either a
mono or stereo phone plug but it must be wired
as mono, i.e., the stereo “ring” is not used or
optionally it can be grounded to the sleeve.
2.7 Impedance Balancing and You
For the technically minded (and maybe just a bit
skeptical) reader, here’s how an impedance bal-
anced output works. Your balanced input stage
looks at the two wires and detects only the poten-
tial (voltage) difference between them. Anything
that is the same on the two wires (for all practi-
cal purposes as seen measuring from ground) is
called a common mode signal and is cancelled
out by the differential amplifier. Figure 2-6 illus-
trates how the hum is induced into both wires
equally and therefore is cancelled out.
Page 2-4
Back Panel Features
Tip = HIGH
Ring = LOW
Send & Return Jack Wiring
Sleeve = GROUND
Figure 2-5 Remote Mute Plug Wiring
Stereo Phone Plug
Open Switch Mutes
Optional
Open Switch Mutes
Mono Phone Plug
