4
5
UNPACKING & INSTALLING
SERVICING
The user should not attempt to service the P331 Tube Loading Amplifier.
NEVER REMOVE THE P331’S TOP COVER.
Potentially lethal voltages are
present inside the unit.
It should only be opened by qualified service
personnel. To reduce the risk of fire or electrical shock, do not expose to rain
or moisture, or operate it where it will be exposed to liquid. Refer all servicing,
or any questions about servicing, to Whitestone Audio Instruments, LLC.
UNPACKING
Make sure to carefully inspect the P331’s carton for any signs of shipping damage. If you find damage
from shipping, please contact your dealer right away for further instructions. We strongly recommend
you not discard the P331’s carton and custom foam insert. These cartons were specifically designed
to protect the P331 during shipping and will ensure your P331 will remain protected if you ever need
to send in for service.
The carton contains the following items:
1. P331 Tube Loading Amplifier
2. Three prong IEC power cord
3. This manual
MOUNTING
The P331 uses two EIA standard rack spaces and can be mounted in any standard 19” (482.6 mm)
rack. If the P331 is used in a mobile rack or road case, the rear of the chassis should be supported to
help prevent against damage from mechanical shock and vibration. Please handle your P331 with
loving care and it should deliver many years (or many decades) of high performance, high fidelity
audio processing.
VENTILATION
It is important to adequately ventilate your P331 as it does contain two 6SN7GT vacuum tubes and
high voltage power supply which produce heat. The side panel and top panel vents should never be
blocked. We suggest leaving at least one rack space above and below the unit and use a rack spacer
with ventilation such as the optional Whitestone VP-1RU matching vent panel.
POWER REQUIREMENTS
The P331 is shipped with a 3-prong IEC power inlet
module and cord and has a custom-designed universal
power supply that can operate in all countries around
the world. When purchased new the power inlet will be
set for the appropriate voltage. This is indicated in the
little red window on the inlet module. Double-check the
inlet is displaying the correct voltage for your country
before powering up. Operating voltage can be changed
by simply removing and rotating the fuse holder 180˚
within the inlet module (Fig. 1). Please contact us for
additional information or if you have any questions.
Input power range is 115V to 230V at 50/60 Hz. To check
or replace the fuses, make sure the unit is unplugged.
Fuses = T4A L 250VAC SLO-BLO
Never operate the P331 with the ground on the power cord defeated.
(Fig. 1)
Thank you for deciding to add the Whitestone Audio Instruments P331 Tube Loading Amplifier to your
audio production tool set.
The P331 had its genesis when my wife (and multi-Grammy® nominated mastering engineer) Kim
Rosen was looking for a custom piece of analog gear for her new mastering room. Something unique.
We ended up brainstorming on an idea for a versatile tube stage for her analog chain. Much of the
way Kim works is exploiting the sound of the input and output gain stages of some of her favorite
equalizers and compressors with actual processing bypassed. Just the subtle enhancement some of
these analog gain circuits provide often times is just the thing to add a bit of “magic”. The goal for the
P331 was to capture this kind of enhancement potential in a line amplifier that delivers the natural
clarity of a well-designed tube circuit. At the same time, we wanted to bring fully switched controls
that introduce harmonics and other carefully implemented, and repeatable, non-linearities to subtly
enhance the signal without harming the recording’s original intent. We really had no idea what we
were getting ourselves in to!
For the next five years we worked with our electrical engineer, (who’s other position of employment
involves projects he doesn’t have clearance to discuss) on how to implement the features we
envisioned...circuit design, prototypes...add features, scrap features, new prototypes, listen...tweak...
build revisions...listen some more...tweak some more...over and over. We’re so happy we didn’t throw
in the towel (we nearly did on several occasions) because we feel what we’ve ultimately created is a
truly unique, truly exceptional processing device.
We wanted the P331 to have just a few great sounding and useful features. Chief among them being its
Class A fully-differential (balanced) tube amplifier designed around 6SN7GT dual triode vacuum tubes.
This tube was introduced in 1939 and while the 6SN7GT is somewhat rare in pro audio applications, it is
relied upon in the hi-fi market for its inherent “clean and silky” characteristics in audiophile preamps.
The tubes are still in production and can be purchased new without hassle. With the LOADING circuit,
we give you the ability to control where the signal hits these tube along its response curve. There’ll be
a bit more detail on this in the Operation section of this manual.
Taking mixes to new levels without harming what’s already there is what the world’s best mastering
engineers dedicate their life’s work to achieving. When processing on your mix-buss, in your mastering
chain or even enhancing individual tracks...subtlety more often than not is the name of the game.
With that in mind we set upon designing features into the P331 that never sound overly processed,
unnatural or heavy-handed. From the gentle parallel LIFT filter to the optional output transformers
and clean solid-state output gain, all aspects of the P331 were designed to provide a tool which helps
achieve that elusive element, that “something special” which is difficult to describe but unmistakable
when heard.
We believe a great sounding recording is the sum of an accumulation of subtleties. With the Whitestone
P331 Tube Loading Amplifier, we’ve worked to put a palette of these subtleties at your fingertips.
Dave Rosen
Co-Founder/President
INTRODUCTION
Summary of Contents for P331
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