THÖRESS EHT mono – Hybrid Triode . Mono . Power Amplifier
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“EHT Monoblock”
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P R O D U C T D E S C R I P T I O N
THÖRESS is pleased to announce the introduction of the Hybrid Triode Mono
Amplifier (EHT Monoblock), our latest achievement. The most powerful amplifier
model of our product line provides 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 critically low impedance which may present a challenge for
our all-tube single-ended triode amplifier models. While offering the same level of
ultimate sonic excellence – yet without involving exotic and costly power tubes!
The Amplifier is built with meticulous hand construction using our proven point-to-
point 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. The EHT Mono Amplifier is an
ultimate component in every sense and the perfect complement to our Dual
Function Amplifier or Full Function Preamplifier. Particularly when combined with
our under-hung voice-coil 1D66 or horn based 2CD12 loudspeaker the sonic
presentation of the EHT Mono Blocks is of the utmost refinement and will therefor
certainly meet the expectations of even the most critical and experienced music
lover.
EHT 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,
single-ended hybrid triode). Deliberately ignoring the fact that many music lovers
persistently believe in the myth that balanced technology (and the associated XLR
connectors and cable configuration) is generally superior over 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 entitled
THÖRESS Behind the Curtain, available on request.The EHT topology consists of a single-ended triode gain stage (6J5GT or 7A4 vacuum
tube) followed by a unity gain MOSFET output stage also operated in single-ended
(class-A) mode with high idle current. In order to obtain the relatively high power
output with lowest possible distortion it proved advantageous to place an additional
(very powerful) unity-gain vacuum tube buffer (12HG7 power pentode operated in
triode mode) in between the triode gain stage and the output stage. The 6J5GT/7A4
tube is a medium-gain octal/loctal base triode with fairly low transconductance from
the early times of tube electronics, whereas the 12GN7 is a rather modern (all-glass)
high transconductance power tube with high gain capabilities, 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 schematics.
Basic analysis easily shows that…
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 an all-tube amplification principle, without compromising
sound quality. Simply, because in the former case the typical vacuum tube
impedance matching output arrangement (power tube driven output transformer
with high step-down ratio) can be omitted. Therefore, the driver voltage swing
required for a specific power output is up to 10 times (!!!) lower for the EHT
configuration compared to an all-tube topology, resulting in a proportional reduction
of non-linear distortion. This crucial insight led us to the design of the EHT Mono
Amplifier (and the EHT Integrated Amplifier).
The EHT topology 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, the EHT concept 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.
STABILITY
The EHT Mono 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 in 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.
GAIN & BASS BOOST SELECTORThe EHT Mono Amplifier allows for on-the-fly adjustment of the gain (input
sensitivity) in 7dB-steps by means of a 6-position rotary switch on the rear panel
near the input jack. The (voltage) gain is
4dB (1.5-times) , 11dB (3.5-times), 18dB (8-times) and 25dB (20-times)
when the switch rests in position 1, 2, 3 and 4 respectively (neutral tonality (flat
frequency response)). Position 5 and 6 are assigned with bass boost presets.
Summarizing, the mapping between selector positions, gain and sound effects is
given by the chart below.
pos1: 4dB gain (1.5-times).
pos2: 11dB gain (3.5-times).
pos3: 18dB gain (8-times).
pos4: 25dB (20-times), maximum gain.
pos5: bass boost style-1.
pos6: bass boost style-2.
The gain selector enables the user to match the gain of the power amplifier with the
gain of the line device with respect to the efficiency of the loudspeaker. Optimal gain
matching between these components is highly desirable for two reasons. At first, in
a matched scenario the idle noise of the preamplifier remains inaudible even in the
context of highly efficient loudspeakers. Secondly, the volume control knob can be
operated at a higher angle of rotation and such allows for conveniently fine volume
adjustment. Optimal gain matching is attained when the volume control on the line
device in average rests near the middle position for a convenient listening loudness.
The bass boost functions can be applied for a variety of applications. For equalizing
loudspeaker components with slim bass response or limited bass extension. For
adjusting the bass response of the system to the acoustic conditions of the listening
environment (small loudspeakers in large rooms!). For restoring tonal imperfections
of the listening program (often given by a lack of bass extension). Or simply for
switching to a full-bodied presentation if one is in the mood for listening to music in
a non-neutral, enriched and laid back way. Notably, the bass boost functions act in a
much more subtle way than common bass boost facilities and do not rely on clumsy
and sound degrading conventional tone control circuitry. Both functions are realized
by interposing solely one additional capacitor (per channel) to the neutral mode
circuit!
MAINS TRANSFORMER
The EHT Mono Amplifier is equipped with a proprietary mains transformer produced
in-house to ensure the highest possible quality and reliability. This part features ahighly elaborated dual-coil winding structure on a large tape wound cut core (C-core)
and 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 easily built transformers
for all kinds of mains voltages on demand, for example 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 (6J5GT or 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 zero-feedback EHT schematic: single-ended triode gain stage followed by
a unity-gain vacuum tube buffer driving a unity-gain single-ended MOSFET
output stage operated (class-A) at high idle current.
• Total avoidance of sound degrading protection circuitry.
• On-the-fly selectable gain via 6-position rotary switch on the rear panel, 2 of
which positions are assigned with subtle bass boost presets.
• High grade electrolytic capacitors made in Germany in the power supply.
• Low noise dual-coil C-Core mains transformer made in-house for 230Vac (115Vac
via jumper setting), 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.
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THÖRESS…
A Tribute to Professional Audio Components
from the Golden Age of the Electronic Tube !
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