Vac cpa 1 mk3 owners manual
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Page 1
CPA1 Mk III & CLA1 Mk III Instructions
Please read carefully before installing
The VAC CPA1/CLA1 Mk III Preamplifiers are designed for the most exacting applications. The two
models are similar except for the omission of the phonograph preamplifier in the CLA1. All audio
circuits are Class A triode vacuum tube types. In addition, all direct audio signal paths are hard-
wired for best performance.
General Precautions
1. DO NOT OPEN POWER SUPPLY OR PREAMPLIFIER CHASSIS - THERE ARE NO
USER SERVICEABLE PARTS INSIDE. DO NOT REMOVE TUBE CAGE UNTIL UNIT HAS
BEEN SWITCHED OFF FOR 15 MINUTES. TUBES BECOME HOT IN USE. DO NOT
EXPOSE EITHER CHASSIS TO MOISTURE. DO NOT PLUG INTO AN AC SOURCE
UNTIL ALL CONNECTIONS ARE COMPLETED.
2. Complete all connections before connecting the AC cord to the wall outlet, particularly the
6 pin cable from the power supply to the preamplifier chassis. Similarly, always remove the
AC power cord before detaching the 6 pin DC power cable. Failure to observe this
sequence will result in damage to the power supply.
3. At turn on, all volume controls should be full counter-clockwise and the mode switch should
be set to MUTE. Allow 1 minute for warm up before selecting STEREO.
4. For protection ofyour speakers and power amplifier, always switch preamp to MUTE before
switching the power supply off or on.
5. Allow the system to sit switched off for at least 10 minutes before disconnecting the preamp
from its power supply.
Operation - Line Stage Section
The line stage can be operated in two basic modes, Active and Passive-Buffered. The active mode
may be of high gain or moderate gain. These three options are selected by a rotary switch on the
top of the chassis on the right side. Mute the preamp before operating this switch.
In the Passive-Buffered mode the amplifying triode (V4) is bypassed, and the controls are fed
directly to a Class A impedance translating Class A triode tube (V5). Since the output is buffered,
there are no gross cable interaction problems ofthe type encountered with passive control centers;
since there is no gain stage, this mode has unity gain. The VAC Passive-Buffered mode is
capable of driving long cable runs and the preamplifier may be located some distance from the
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power amplifiers.
Since the Passive-Buffered mode has zero gain, you may need to turn the volume controls well
clockwise...the exact position depends on the output level from your source, and the sensitivity of
your power amplifiers and loudspeakers. Many audiophiles have the belief that it is never good to
run volume controls wide open. With a circuit of this type, however, it is actually preferable to do
so. Don't worry about where the control is set, just be concerned with how it sounds. The Passive-
Buffered mode does not invert absolute phase with respect to the line inputs.
The Active modes add a Class A amplifying triode tube to each channel, and causes the main
outputs to have inverted absolute polarity with respect to the line inputs. To correct for this you may
invert the polarity between your power amplifier and loudspeakers. In most systems, the Active
mode retrieves more ambient and reverb information.
Operation - Phono Stage
Begin by selecting the appropriate phono stage gain using the control on the top of the chassis at
the right rear. The high gain position is appropriate for most moving coil cartridges. The medium
gain mode is appropriate for some high output moving coils and moderate output moving magnets.
The low gain position is for use with very high output cartridges. If in doubt, feel free to experiment
and use the settings that sound best. Mute the preamp before switching gain.
The phono stage delivers an inverted signal with respect to line inputs. Therefore you may wish to
invert the connections at your phono cartridge if its design permits.
Cartridge load impedance and capacitance are adjusted by two rotary switches on the back of the
unit. Start at 47k ohms and 150 pf. Listen for a bit, and then reduce the impedance to 22k.
Continue until you find the best impedance setting. Next, try raising and lowering the capacitance.
When you have found the best capacitance setting, listen to a few other impedance settings to
verify your earlier choice.
In general, higher capacitances smooth high frequency resonances. Lower impedances have a
similar effect, and can also improve subjective detail.
Often the load parameters you choose will differ from those recommended by the cartridge
manufacturer. This may happen because of dynamic load characteristics, which are typically better
behaved with vacuum tube circuits. Use the settings that sound best.
The available load parameters are approximately:
Impedance Capacitance
47,000 ohms 0 picofarads
22,000 100
10,000 120
1,000 150
100 180
220