THÖRESS
Full Function Integrated "Super" Preamplifier
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INSTRUCTION MANUAL
Thank you for purchasing the THÖRESS Full Function “Super” Preamplifier. We
believe our versatile preamplifier to offer the highest possible sonic performance from
both the line and the phono section while preserving a state-of-the-art signal-to-noise
ratio. It is a reference device which will easily measure up to the expectations of even the
most critical and experienced music enthusiast or professional user. Please read the
following explanations and instructions carefully to get the most out of your preamplifier.
PHONO INPUTS AND VERSIONS
The "Super" is available in 3 Version differing only in the circuitry of the phono section
to suit particular types of cartridges in terms of input sensitivity (gain) and cartridge load.
Each version provides 3 phono inputs P1, P2 and P3 selectable with the source dial on
the faceplate. Please note, that detailed information concerning cartridges and vinyl
reproduction in general may be found in the manual of our Phono Enhancer. The
Versions are as follows:
Version 1:
This version offers exceptionally high phono stage gain and customized cartridge loading
on P1, P2 and P3 to make these inputs suitable for low-output, low internal resistance
MC cartridges. Cartridge loading is achieved by means of standard axial lead resistors
soldered to tags adjacent to the phono jacks. These load values are factory preset to 100
Ohms, a value that is suitable for most low-output MC cartridges. Other resistor values
can be preset on request, and these values can be changed by the user whenever it is
required.
The cartridge load values preset on the high-gain MC phono inputs P1, P2 and P3 of
version 1 are printed on the rim of the rear panel for reference.
On these phono inputs any dual-coil low-output dedicated mono MC cartridge can be
used without restriction, the Audio Technica AT33-Mono, the Lyra Dorian Mono or Lyra
Helikon Mono and the excellent Dynavector DV-1s-Mono being examples of such high
grade dedicated dual-coil mono cartridges (all of which are omniflexible).
It is possible to mix stereophonic and dual-coil dedicated mono MC cartridges on
any of the high-gain MC phono inputs of version 1 without further adaptation.
However, if a single-coil low-output (dedicated mono) MC cartridge is to be used on
these (stereophonic) phono inputs and it is intended for dual-speaker-mono listening,
precautions must be taken to "double" the mono signal in such way as to avoid hum.
Please see the relevant chapter below for more details on using such cartridges. Examples
of current production single-coil low-output MC cartridges are the dedicated mono
cartridges made in Japan by Miyajima Lab, all models of which employ monoflexible
styluses. A monoflexible stylus has zero vertical compliance and is therefore suitable for
tracking of mono grooves only. If a cartridges with monoflexible stylus is accidentally
used on a stereophonic record it will instantly destroy its grooves as stereo grooves are
modulated laterally and vertically.
Be warned - monoflexible (mono) cartridges can seriously harm stereo grooves !
It should be pointed out, that any omniflexible dedicated (microgroove) mono cartridge is
able to read out stereo grooves without harming the grooves and converts the separated
signals from the two channels (coded into one single groove) back into a consistent mono
signal. Interestingly, the "monophonized" version of a stereophonic record obtained by
tracking with a high grade omniflexible mono cartridge can sound very appealing. On the
other hand, any stereophonic cartridge is able to read out both early and secondgeneration monophonic microgrooves faithfully and will output two identical signals on
each of its channels. Please recal, that more useful information concerning cartridges and
vinyl reproduction in general may be found in the manual of our Phono Enhancer
downloadable from our website.
Version 2:
Here the inputs P1, P2 and P3 are suitable for medium-ouput MC or MI cartridges
(medium phono stage gain). Each input is loaded at 47K.
Version 3:
In this version the phono circuitry is designed to make P1, P2 and P3 suitable for
ordinary "loud" MM or MI cartridges (low phono stage gain) and accordingly are loaded
with 47K. Of course, low-output (low internal resistance) MC cartridges may be used on
any low gain phono input of this version with the aid of an external step-up transformer.
Please note that, it is NOT possible to mix different kinds of cartridges (such as MC
and MM) on the inputs P1, P2 and P3 (of any version) as they all feed the same fixed
phono circuit optimized for one particular type of phono cartridge regarding input
sensitivity (gain) and load requirements.
LINE INPUTS
Besides the phono inputs P1, P2 and P3 our preamplifier offers two inputs CD1 and CD2
for high-output line level sources such as CD/DVD players or sound cards. One
additional line input HP for classical low-output line level sources such as a tape recorder
or a phono stage is provided. If our Full Function Preamplifier is to be used for line
amplification of our Parametric Phono Equalizer (the Enhancer) or another external
phono device, it is advisable to use the input labelled HP (rather than CD1 or CD2) for
optimal sonic results, especially when the device is based on a tube design.
OUTPUTS
To maximize versatility, the preamp includes two paralleled main outputs. Importantly,
the output resistance at the main outputs is low enough to drive long interconnect cables
and power amplifiers with an input impedance as low as 10,000 ohms without
compromising sound quality. It is thus possible to simultaneously drive a power amplifier
AND the solid state power amplifier of an “active” subwoofer without further adaptation.
The paralleled main outputs may also serve for proper signal doubling if a single-coil
(mono) cartridge is used on one of the phono inputs for dual-speaker mono listening in
the way described in the relevant chapter.
Moreover, a monitor output Q for the chosen source is provided. As the output signal of
the internal phono section will appear at the Q-output as soon as one of the phono inputs
P1, P2 or P3 has been switched active, this output allows the internal phono section of
our preamplifier to be used with high impedance line inputs of other preamplifiers.
Again, the output resistance at Q is low enough to drive long interconnect cables without
compromising sound quality. However, if the input resistances of the line input is
considerably lower than 100K it may be necessary to increase the value of the coupling
capacitors at the output of the phono section, if bass response at the very low end of the
audio band is not to be compromised. At this point only a modest capacitor value has
been employed for coupling in order to obtain a certain amount of attenuation of any
subsonic artifacts present in the phono signal, as it may occur with warped records.
If the phono section of the "Super" is to be used with an external line preamplifier
via the Q output - it is advisable to use a high impedance line input for optimal sonic
results.
USING SINGLE-COIL (MONO) CARTRIDGES ON STEREOPHONIC EQUIPMENT
When both inputs of a stereophonic phono preamplifier are wired to a single-coil
generator cartridge in attempt to create signal doubling to allow for dual-speaker mono
reproduction it is impossible to avoid a conductive connection between "left ground" and
"right ground" of the (non-symmetric) phono circuit and, as a result, unwanted ground
loops are formed.
Single-coil (mono) cartridges will cause residual hum due to ground loops when they
are wired to feed both inputs of a stereophonic preamplifier (with non-symmetric
circuitry) in an attempt to double the mono signal at the phono input for dualspeaker-mono listening !
To overcome this problem the signal should be doubled at the point of output rather than
attempting to accomplish this earlier in the signal path. The paralleled main output jacks
can be used to conveniently achieve this aim in the following manner. Only the right
channel of the phono section should be fed by the single-coil generator via a customized
monophonic tonearm cable, whilst the corresponding left input should be short-circuited
directly at the jack with the help of a RCA plug internally soldered to suit this purpose. If
a monophonic tonearm cable is not at hand, a conventional stereophonic tonearm cable
can be used equally well (with both plugs inserted into the respective jacks), in which
case shorting the inactive left input can be conveniently accomplished at the very end of
the left channel lead of the tonearm cable by shorting the "left hot" and "left ground" pin
of the headshell. If the tonearm is equipped with free leads, the desired conductive
connection can be easily formed by taping the lead ends of the respective tonearm leads
firmly together. After these adaptations required to create dedicated single-coil cartridge
input, signal doubling can be obtained at the output by connecting both power amps to
the right channel main output by using the respective paralleled pair of jacks.
Note, a doubled version of the right channel rather than a proper stereophonic
signal will be reproduced by the system in dual-speaker-mono mode if a
stereophonic source is (accidentally) switched active.
ABSOLUTE PHASE OF LINE-OUT SIGNAL
Line amplification in the “Super” is performed by a unique single-stage, zero-feedback
circuit employing one 12SN7GT tube per channel. This topology has several technical
and sonic advantages over a more common multistage circuitry, and yet the line amplifier
is still capable of driving long interconnect cables and/or power amplifiers having an
input-impedance as low as 10,000 ohms without compromising sound quality. As a sideeffect of this puristic concept, the output signal of the line amplifier is 180 degrees out of
phase relative to the input signal. Thus, for a correct absolute phase in the system, the
speakers should be connected to the resective power amplifiers with reversed red-blackpolarity. That is, connect the red binding post of the amplifier to the black binding post of
the speaker, and vice versa. Here we assume that all other components in the setup
process the signal without phase shift and phono cartridges are wired for correct polarity.
For correct overall phase in a setup with the THÖRESS Full Function Preamplifier,
the speakers should be connected to the power amplifiers with reversed
red-black-polarity.
Note, that in our Full Function Preamplifier the volume of each channel is controlled
separately via high-grade rotary potentiometers (custom made to order by ALPS in
Japan) and these parts are carefully matched, to preserve precise channel balance with
respect to the rotation angle (and thus to the scale printed on the faceplate).
INPUT SENSITIVITY AND THE PREAMPLIFIER’S IDLE NOISE
The noise performance of the line section of our preamplifier is such that no audible idle
noise will be present in the setup as long as the power amplifier and the speakers are
suitably matched with respect to input sensitivity and speaker efficiency. We guarantee
that line amplifier idle noise will remain within inaudible limits when using speakers up
to 100dB/W/m efficiency with each of our power amplifier models. But line amplifier
idle noise will unavoidably become audible in cases where the power sensitivity of the
power amplifiers are high (solid state amps) and/or the speakers are exceptionally
efficient (large horn speakers). A worst-case mismatch scenario is to be expected if both
of these conditions were simultaneously met.
In order to keep the line amplifier idle noise within inaudible limits the power
amplifiers and the speakers need to be suitably matched with respect to input
sensitivity and speaker efficiency.
AMPLIFIER PLACEMENT AND HUM
The phono section within the “Super” (located in the right-hand side of the case)
amplifies the MC signal actively (employing one PCC88 and two 12SN7GT tubes),
without the aid of step-up transformers, and is consequently an extremely sensitive
device.
Thanks to an outstanding circuit topology, amplification of the phono signal in our
Full Function Preamp is purely active without the aid of step-up transformers.
Much care has been taken in arranging each aspect of the internal construction, including
wiring techniques, to achieve an extraordinary signal-to-noise ratio, even though the
power transformer is built-in. However, stray electro-magnetic fields produced by the
power transformers of other electronic devices positioned in the vicinity of the
preamplifier may introduce hum into the phono section of the "Super".
Power transformers of other electronic devices positioned in the vicinity of the
preamplifier can seriously deteriorate the signal-to-noise performance by
introducing hum into the phono section. Consequently, the preamplifier needs very
considerate placement for optimal performance !
TURNTABLE DECOUPLING AND LOW-END CUT-OFF FREQUENCY
Coupling capacitor values between the amplifier stages of our Full Function Preamplifier
have been chosen to give a very low overall cut-off frequency so as not to compromise
frequency and phase response at the low end of the audio band, and yet to provide a
certain amount of attenuation of subsonic artifacts. Considering the puristic concept of
the preamplifier circuitry, the use of (steep) active subsonic filters cannot be tolerated.
The same holds for our power amplifiers, where our ultra-high quality output
transformers offer cut-off frequencies as low as 5 Hz or even lower. Since the
preamplifier was designed to accommodate power amplifiers with input impedances as
low as 10,000 ohms, we have employed high-value coupling capacitors (several
microfarads) in the line output circuit, giving a very low cut-off frequency when power
amplifiers with higher input impedances (i.e. tube amplifiers) are used. Therefore, much
care is needed to prevent subsonic artifacts from entering the audio signal when a
turntable is part of the setup. Speakers of the vented type (“bass reflex”) are exceptionally
critical in this regard, as power amplifiers lose “control” over such speakers at
frequencies lower than the port frequency, allowing them to enter into “subsonic
feedback resonance” with the turntable if the later device is poorly decoupled from the
rack and/or there is a mismatch between the tonearm and the phono cartridge (i.e. if the
resonant frequency of this combination is too low). In extreme cases, even the power
amplifier itself may enter into subsonic resonance if it is poorly designed. If such effects
are observed in a setup, lowering the coupling capacitor value in the line section of the
preamplifier may be necessary to help increasing the attenuation of subsonic frequencies.
To accomplish this modification, please contact your distributor or local dealer.
Subsonic artifacts may enter the signal when the turntable is poorly decoupled from
the rack and/or the tonearm and the phono cartridge are not suitably matched.
SETUP
To set up the Preamplifier, proceed as follows:
Do not connect the amplifier to the mains until the following steps have been taken.
1. Remove the top plate from the amplifier case and place all tubes into their sockets very
carefully. Note, the tube heaters of the tubes are connected in series and fed from a
constant current source. Thus, none of the tubes will glow/operate unless all tubes have
been installed into their respective sockets to form a “closed chain.”
Never switch on the preamplifier unless all tubes have been placed into their
sockets. Never pull a tube out of its socket while the preamplifier is switched on.
Take care to tighten the screws properly when closing the amplifier case, so as to ensure
a conductive connection of the plate to the case, as:
Improper grounding of the top (or bottom) plate may result in hum !
2. Switch off all amplifiers and other powered devices which are part of the setup.
3. Make sure that the power switch of the preamplifier is in the "Aus" (off) position and
both volume controls are set to zero.
4. Now bring the preamp into its final position (further advice concerning preamplifier
placement has be given earlier) and connect all the cartridges you want to use to suitable
phono inputs via the respective tonearm cables. Do not forget to connect the ground leads
of each tonearm cable to the ground terminal adjacent to the phono jacks. Next connect
all other line level sources to be used to suitable line inputs. See the relevant chapter for
further details on the line inputs.
If our Full Function Preamplifier is to be used for line amplification of our
Parametric Phono Equalizer or another external phono device, it is advised to use
the HP-input for optimal sonic results !
5. Connect one pair of the main output jacks to the inputs of the power amplifiers. Use
the second pair of main output jacks to connect the preamp to an eventual active
subwoofer, if desired. Please note that proper signal doubling for dual-speaker mono
listening can be achieved at the output in the way described in the relevant chapter if a
single-coil (mono) cartridge is to be used on one of the phono inputs.
6. Now the preamp can be connected to the mains and switched on.
7. Finally the power amp(s) can be switched on, respecting a delay of about one minute.
8. Select the desired tone source with the source dial, adjust the volume (and balance) for
convenient loudness and enjoy the music.
Always switch on the preamplifiers first, then switch on the power amp(s) with a
delay of not less than one minute !
Never switch the preamplifiers on or off when the power amplifier is powered on.
When powering off the system, always switch off the power amplifier(s) first, then
switch off the preamplifier(s) observing a delay of not less than 30 seconds.
TUBE QUALITY AND SIGNAL-TO-NOISE RATIO
The THÖRESS preamplifier is equipped with “new old stock” tubes carefully tested to
meet tight specifications, and hand-picked for low microphony and low noise. The use of
tubes with questionable parameters or of poor quality will lead to inferior sound quality
and/or a decreased signal-to-noise ratio (i.e. increased pink-noise and higher sensitivity to
microphony). In extreme cases, damage within the circuit may occur ! It is therefore
strongly advisable to use only the carefully tested tubes supplied by the manufacturer.
The use of tubes of questionable quality may lead to an inferior signal-to-noise ratio
and degraded sound quality.
When the amplifier is to be equipped with “fresh” tubes, proceed as follows:
1. Carefully clean the tube pins with a dry brush.
2. Spray a few drops of highly viscous oil (such as Ballistol) on a cotton-tip.
3. Then use the tip to apply a thin oil film onto the contact pins.
Tubes handled in this way will move in and out of the sockets more easily and will help
to increase the life expectancy of the tube sockets.
Never switch on the preamplifier until all tubes have been placed into their sockets.
Never pull a tube out of its socket while the preamplifier is switched on.
Never pull a tube out of its socket while it is still hot.
Always remove all tubes from their sockets and put them in their original transport
box before shipping or transporting the amplifier.
FUSE
Our Full Function Preamplifier draws a current of 0.6/0.3 amperes from the 120/230 Volt
mains. It is equipped with exactly one fuse rated at 1/0.5/ Ampere, located in the fuse-
case adjacent to the power inlet. Occasionally, the fuse may blow at the moment the
amplifier is switched on. This is due to the current spike drawn by the power transformer.
Should this problem arise more regularly it may be advisable to use a fuse with slightly
higher current rating. If fuses with larger current ratings still blow regularly, the amplifier
should be returned to the factory for inspection.
A NOTE ON EFFECTIVE RECORD CLEANING
At the THÖRESS factory we clean records by letting them rotate (at 1 rpm) in the tank
of an industrial grade ultrasonic cleaner filled with demineralized water and a few drops
of concentrated dish cleaning liquid for at least half an hour. For best results it is
advisable to use a cleaner with an integrated heater programmed for 45 degrees Celsius.
After this procedure, the records are vacuumed on a conventional record cleaning
machine such as the Nitty Gritty. The gifted DIY hobbyist will be able to build the device
needed for rotating (up to 3) records through the cleaner bath with the aid of a barbecue
motor. Our cleaning methode will be found far superior to any other conventional
cleaning method. Any residual groove noise still experienced with records treated in this
way will be due to imperfections of the grooves and not to groove dirt. Do not hesitate to
email us if you require more details regarding this cleaning method.
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THÖRESS...
A Tribute to Professional Equipment from
the Golden Age of the Electronic Tube !
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