User Manual

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II
CIRCUIT DESCRIPTION
The signal first enters the input trans-
former. The primary of the transformer
has two windings that are either con-
nected in series or in parallell which
results in an input impedance of either
1200 Ohms or 300 Ohms in MIC mode. A
resistive balanced -30dB pad is inserted
before the transformer in LINE mode.
The transformer is followed by two input gain stages. For gains up
to 50dB, only one of them is being used. For gains above 50dB, the
second gain stage is inserted in the signal path. Both gain stages
uses only three transistors each.
The signal then goes to the insert jack and from there to the
output level potentiometer and then to the output stage. This stage
again only uses three transistors, the last one in the chain is a
hefty 2N3055 power transistor run in class-A mode, driving the
output transformer.
So, all in all, the complete signal chain only contains a maximum
of nine active elements. Compare that to the big number of tran-
sistors that are usually used in one single integrated circuit!
MODERN VERSUS OLD
It is true that there are some great IC´s available today that
achieves very low levels of static and dynamic distortion.The
simple circuits that the PRE-73 MKIII uses, and even more so the
transformers, cannot match the low distortion specifications of
modern IC´s.
It is the distortion components that imparts a sound character
to the audio signal and, if the distortion components are of the
right type, this is a good thing since it makes the recorded voice
or instrument sound “better”, more musical, more pleasing to
the ear. This is one reason why vintage style units are so popular
today.
This is not to suggest that modern, transparent sounding audio
circuits is a bad thing, sometimes they are prefered over colored
ones. It´s all about taste and it depends on the genre. For most
modern music styles, color and character is definitely a good
thing.
And doesn´t it feel good to use audio components built according
to the old, minimalistic approach where one can follow the signal
from one discrete component to another?
USING THE PRE-73 MKIII
Using a preamplifier is not rocket science. Here are some points
though to help you getting the maximum out of the PRE-73 MKIII:
- Connect the cable from the power supply to the AC 24V connec-
tor at the back of the PRE-73 MKIII. Power on the unit with the
POWER switch on the front panel.
- Connect your Mic or Line input source to the input XLR/TRS
combo jack on the back panel.
- Select MIC or LINE mode by the MIC/LINE switch.
- Engage one of the positions of the High Pass filter if you want
to roll off the lower frequency range. Set the switch in the center
position to remove the high pass filter from the signal path.
- Engage one of the two positions of the AIR eq boost if you want
to add some level in the upper frequency range. The center of the
boost is at around 30 kHz. The switch center position = OFF.
- If you want the smallest amount of coloration, always set the
OUTPUT level potentiometer at or close to maximum and adjust
the output level with the stepped GAIN switch.
- If you want more character, turn the OUTPUT level potentio-
meter counterclock-wise and increase the gain with the GAIN
switch. This will drive the input gain stage(s) harder and provoke
more character from them.
- You can also overdrive the output stage and the output trans-
former for even more character, but you will then usually need a
level control after the PRE-73 MKIII in order to reduce the level to
the appropiate one. This level control can be a passive damping
device (like the Shure A15AS XLR switchable pad) or an input level
control in the unit following the PRE-73 MkIII. The Shure A15AS
is also useful for reducing the ouput level if the PRE-73 MKIII is
connected to a unit that has a standard operating level of -10 dBu.
The standard operating level of the PRE-73 MKIII is +4 dBu, the
output level into a 600 ohm load will be about 1,23V when the “0”
VU LED is lit.
- Instruments can be connected to the TRS input at the front which
has an input impedance of about 100 kohm. Press the DI switch to
engage this input. A source at the back can remain connected.
- Engage the +48V phantom power for any mic that needs it. It is a
good procedure to always disengage the phantom power and wait
for about 10 seconds before unplugging the mic.
- When the LOW-Z switch is engaged, the input impedance of the
input in MIC mode drops from 1200 Ohms to 300 Ohms. This will
change the tone of most mics and will give you one more sound-
shaping option. It also increases the level, which is normal.
- The phase switch simply reverses the phase by reversing the
wires from the secondary winding of the output transformer.
Reversing the phase of the signal is useful on a number of occa-
sions, one example is phase reversing the the lower mic of a
snare drum to make it sum in phase with the upper mic.
- There is an unbalanced Insert jack located at the back panel
where you can insert Equalizers and other external effect units
that has an operating level of about -10 dbu to -18dBu.
Send is on “tip” and return on “ring”.
- The output transformer used in the PRE-73 MKIII is made for
having an ideal load of about 600 ohm. The input impedance of
most modern units is 10 kohm or more. The PRE-73 MKIII there-
fore has a 600 ohm output termination resistor that is engaged
by the jumper (JP1) located just behind the XLR output jack. The
termination reistor will lower the ouput level slightly and make
the frequency response flatter in the upper range, it will also roll
off the upper frequency range earlier, Remove the jumper if the
PRE-73 MKIII feed a unit with a 600 ohm imput impedance of if you
want to add som level in the upper range,
WARRANTY
The PRE-73 MKIII is built to last. But as in any electronic device,
components can break down.
There is a 1,0A, slow blow fuse located inside the unit. If the unit
dies, please check this fues. If it has blown, replace it with a new
one. You can also try with another 24V AC adaptor if you have one
available.
If this doesn´t help, or if the unit has another problem, it will need
repair and you should then contact the reseller where you bought
the unit.
The warranty period is decided by the Distributor for your country.
The Distributor will support Golden Age Project resellers and end
users with repairs and spare parts.
REGISTRATION
You are welcome to register your unit at our website:
www.goldenageproject.com
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I would like to thank you for chosing the PRE-73 MKIII!
I hope it will serve you well and that it will help you in
making many great sounding recordings.
Yours,
Bo Medin
Vintage character for modern ideas!