207d
instruction Manual
Aphex Systems Ltd. Model 207D
Page 10
207d
2-ch tube mic preamp with adc
Page 11
Aphex Systems Ltd. Model 207D
4.1 SPLIT INDEPENDENT PREAMP SECTIONS
The Model 207D is a two channel unit. It contains
two separate, independent, preamplifier channels.
Each channel operates as a mic preamp or an instru-
ment preamp. Plugging into the instrument jack
automatically disables the mic input and substitutes
the instrument input.
4.2 USING THE INSTRUMENT INPUT
This input is designed to work with most standard
instrument pickups, both active and passive. Active
pickups have an amplifier built into or attached to
the instrument or instrument strap. These are identi-
fied by running on a battery. Active pickups generally
have a higher output level than passives, but not
always. Passive pickups couple directly through the
instrument cord to the music amplifier or preamp.
Magnetic pickups are exclusively associated with steel
stringed instruments like basses and guitars. These
pickups are wound with coils of wire that generate
an output voltage when the strings interfere with
a magnetic field set up by an internal permanent
magnet.
Acoustic instruments that don’t use steel strings are
usually amplified from piezoelectric pickups placed
on the instrument where vibrations will twist or bend
the piezo crystal to generate a voltage output.
90% of all piezos in use are amplified, but passive
pickups are available. Passive piezos cannot usually
be connected directly to a music amplifier with good
results. The sound will be very thin and weak. They
require a special input stage that provides extra gain
and an extremely high impedance like 10 megohms
or more. Magnetic passive pickups and most active
pickups (piezo and otherwise) are most suited to a
1 megohm impedance at the amplifier input. Since
we wanted to design the Model 207D to meet the
widest requirements, we gave it a 1 megohm input
4.0 Using the 207D
FRONT PANEL VIEW
impedance. Therefore, we cannot recommend it for
any type of passive piezoelectric pickup.
That said, let’s look at this input a little further. The
INSTRUMENT jack is located on the front panel where
it can be accessed by musicians most easily. The front
end of this preamp is a low noise FET amplifier stage
so you should notice this preamp is unusually quiet.
However, because of the high impedance nature of
an instrument pickup, the noise level of the INSTRU-
MENT preamp will be higher than the mic preamp
for the same amounts of gain taken. This is a law of
physics and not a design fault. To help mitigate the
noise problem when the INSTRUMENT is not going
to be used, we designed a way to defeat the INSTRU-
MENT input and its associated noise.
TIP:
You will reduce the INSTRUMENT preamp noise
by running the volume on the instrument higher and
running the 207D’s GAIN lower.
4.3 USING THE MICROPHONE INPUT
The Model 207D is perfect for all types of micro-
phones, either powered or not. We encourage you to
try every mic you own with the 207D.
Many features of the 207D are standard with all
professional preamps - polarity - pad - etc. Some
features are unique and we hope you will fully exploit
them.
All controls for the MIC preamp are on the front
panel. You won’t have to reach around back for the
phantom power switch or pad. Please read the rest of
this section to pick up any tips we may have for you.
4.3.1 USING PHANTOM POWER
Active microphones that take power through the
standard mic cable fall into a class called “phantom
powered” mics. The power is called “phantom”
because it rides the mic cable invisibly, without