THÖRESS EHT MONO Single-Ended Hybrid Triode Power Amplifier

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Pris er for et sæt monoblokke!

THÖRESS is pleased to launch the Hybrid Triode Mono Amplifier (EHT Mono
Block), our latest achievement. The most powerful amplifier model of our product
line delivers 30, 40 or 50 watts of output power into a 8, 6 or 4 ohm load
respectively, combined with very low output resistance (high damping factor). Such
providing excellent drive capability for loudspeakers with medium to low efficiency
or with critical impedance behavior which may present a challenge for our all-tube
single-ended triode amplifier models. While offering the same level sonic excellence
without involving exotic and costly power tubes.
The sonic presentation of the EHT Monoblock is of the utmost refinement and easily
measures up to the highest expectations of even the most critical and experienced
music lover. And of course it is a perfect complement to our DFAmp or FFPre.
BUILT
The amplifier is built with meticulous hand construction using our proven point-topoint wiring techniques, whereas much care has been taken in arranging each aspect
of the internal construction to ensure low noise performance, ease of service and
the highest reliability for many years to come.

TOPOLOGY
The amplifier is based on a unique minimalist SINGLE-ENDED ZERO-FEEDBACK
vacuum tube MOSFET hybrid topology which we call EHT Topology (E/intakt H/ybrid
T/riode). Decidedly ignoring the fact that many music lovers still persistently believe
in the myth that balanced technology (and the associated cable configuration with
XLR connectors) is generally superior to single-ended concepts. Readers who want to
learn more about our view on balanced techniques and our general design approach
are encouraged to read the paper (THÖRESS - Behind the Curtain) downloadable on
the ABOUT button of our website.

STABILITY
The Amplifier operates with impeccable thermal stability and is fully short-proof at
the output without the aid of any kind of (sound degrading) protection circuitry.
Moreover, a switch-on delay module (another protective feature often found on
commonly constructed solid state or hybrid amplifiers) is also omitted because no
disturbing noise or harmful signal is developed at the output when the amplifier is
powered on or off.

EHT PRINCIPLE
The EHT topology consists of a SINGLE-ENDED TRIODE GAIN STAGE followed by a
current buffer in form of a unity gain MOSFET OUTPUT STAGE, also operated in
single-ended (class-A) mode with high idle current. It allows to fully exploit and mate
the most desirable characteristics of both vacuum tubes and solid state devices
determined by their very nature. Despite its elegance and extremely high sonic
capability, it has never been utilized in a serial audio product before, as far as we
know. Probably on account of its low power efficiency and because its
implementation is rather tricky.
The EHT topology represents the PUREST form of all possible single-ended triode
power amplifier configurations!
Specifically it can be shown that a higher output power can be more easily obtained
with the EHT than with the all-tube amplification principle, without compromising
sound quality. Due the fact that the typical all-tube impedance matching output
arrangement consisting of a power tube loaded by an iron-cored output transformer
with high step-down ratio can be omitted. As a matter of consequence, the voltage
swing required from the driver for a specific power output is up to 10 times (!!!)
lower for the EHT configuration compared to an all-tube circuit. Resulting in a
proportional reduction of non-linear distortion as a quite substantial benefit.

GAIN & BASS BOOST SELECTOR
The EHT Mono Amplifier allows for on-the-fly adjustment of the voltage gain of the
amplifier by means of a 6-position rotary switch on the rear panel adjacent to the
input jack. When the gain switch rest in positions 5 or 6 the mid-band gain is 10dB
(3-times) whereas the bass response of the amplifier is mildly boosted. The mapping
between selector positions, gain and sound effects is described by the chart below.
Position 1 2 3 4 5 6
Gain 4dB, (1.5x) 11dB, 3.5x 18dB, 8x 26dB, 20x 10dB, B+ 10dB, B++

Notably, the bass boost functions do not rely on clumsy and sound degrading
conventional tone control circuitry. They are realized in a smart way by interposing
solely one additional capacitor (per channel) to circuit.
The gain selector allows the user to match the gain of the power amplifier to the
gain of the line device with respect to the efficiency of the loudspeaker. Optimal gain
matching between these components is desirable in order to avoid idle noise issues
and to allow for conveniently fine volume adjustment.
The bass boost functions are usefull for a variety of applications...Equalizing
loudspeakers with slim bass response or limited bass extension. Adjusting the low
frequency response of the system to the acoustical conditions of the listening
environment (small loudspeakers in large rooms!). Restoring tonal imperfections of
the listening program, often given by a lack of bass extension. Or simply for
activating a more full-bodied presentation in case the user is in the mood for
listening to music in an enriched and laid back way.

TUBES
In order to obtain the relatively high power output with our EHT Monoblock with
lowest possible distortion it proved advantageous to extend the basic EHT schematic
by adding a powerful unity-gain vacuum tube buffer (12HG7 power tube operated in
triode mode) inbetween the triode gain stage (7A4 triode) and the MOSFET output
stage.
The 7A4 tube is an Loctal-base single-triode from the famous ?J5/?SN7/?A4 family of
tubes with fairly low transconductance developed in the early times of tube
electronics. Whereas the 12HG7 is a fairly late (all-glass) development with high gain
and very high transconductance, making it an ideal choice for buffer applications.
Both tubes are unarguably among the most linear amplification devices ever
developed in the history of electronic technology and as such are a perfect choice in
the context of minimalist zero-feedback circuit design.

MAINS TRANSFORMER
The EHT Mono Amplifier is equipped with a mains transformer produced in-house
featuring a dual-coil winding structure over a large tape wound cut core (C-core),
which has been optimized for low body noise emission and low leakage.
Nevertheless, it is mounted to the amplifier chassis via isolation elements in order to
eliminate even the slightest interference of residual transformer vibrations with the
circuit. Since the mains transformer is produced in-house we can readily customize
the amplifier for all kinds of mains voltages, such as 230Vac, 100Vac (Japan), 120Vac
(USA, Canada), 220Vac (South Korea, China, Thailand, Indonesia), 240Vac (UK) or 245
Vac (Australia).

F E A T U R E O V E R V I E W
• Vacuum Tube MOSFET Hybrid Power Amplifier offering 30, 40 or 50 watts
of output power into a 8, 6 or 4 ohm loudspeaker load respectively,
utilizing two vacuum tubes (7A4 and 1x12HG7).
• Ultimate sonic excellence on par with no-compromise all-tube single-ended
triode amplifiers, without involving exotic and costly power tubes.
• Excellent driving capabilities for critical loudspeaker loads due to very low output
impedance (high damping factor).
• Unique minimalist EHT schematic.
• Total avoidance of sound degrading protection circuitry.
• On-the-fly adjustable gain via 6-position rotary switch on the rear panel
entangled with bass boost facilities implemented without clumsy and sound
degrading conventional tone control circuitry.
• High grade electrolytic capacitors made in Germany in the power supply.
• Low noise dual-coil C-core mains transformer made in-house, for for 230Vac,
100Vac (Japan), 120Vac (USA, Canada), 220Vac (South Korea, China, Thailand,
Indonesia), 240Vac (UK) or 245 Vac (Australia).
• Full hand construction, point-to-point wiring throughout.
• Nonmagnetic aluminum casework, front and rear panel with anodized printing,
powder-coated lids.
• Dimensions 434x454x184 mm, 184=20 (feet)+134+30 (tubes over case),
454=434+20 (speaker binding posts), weight 15Kg.
• Dimensions of the shipping crate: 650 x 650 x 350 mm.