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SIMPLY HEADPHONE
New Unison SH
The Unison Research SH is a class A, single-ended headphone amplifier. It is made with a tube
circuit with only two stages (both operating in triode mode) and preceded by a passive volume
control. The tradition and reputation of Unison Research single-ended amplifiers is well-known
throughout the world. A simple circuit, it can also be (and indeed, it normally is) fairly linear and
equipped with modest frequency extension. The SH is designed to obtain the substantial
reciprocal compensation of the nonlinearity of the individual stages, for an output voltage of up
to more than a third of the maximum--which corresponds to a listening volume already quite
high. Achieving the same result with solid-state circuitry is extremely difficult, especially in a
simple circuit structure. The output stages of the SH use pentodes (the well-known EL84) used in
triode mode. It is a choice that might surprise some people, since an EL84 can operate in single-
ended amplifiers to produce up to 4.3 watts, but Unison Research has two reasons for doing so.
The result is distortion typically below 0.15%, despite the feedback output / input (which does
not include the output transformer) that is equal to just 16 dB. This “residue” of distortion is also
formed only by the second harmonic and (in smaller quantities) from the third harmonic to about
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0.6V (headphone output to low sensitivity, 33 ohm load) and is completely constant with
frequency. This determines an absolutely coherent tonal quality while listening; all the
components of the music program are delivered with
the same "character" of sound. First, this allows the
tubes to operate at a power much less than maximum
(about 20%), extending the life of the tube significantly.
Second, the excess power obtained in this way (the
power of useful output is equal to almost one watt per
channel) can be used to drive a particular type of
output transformer which possesses excellent electrical
and sound qualities—the only defect is that it has to
present dissipative losses that are not negligible.
Performance
The SH has two outputs, connected in parallel, and has a switch to adjust the amplification to
the type of headphones used. There are large two types of headphones: high sensitivity and low
impedance (16 to 40 ohms, on average about 30 ohms) and low sensitivity and high impedance
(typical values between 60 and 150 ohms). The position of the selector dedicated to the
headphones to "high sensitivity" reduces the output voltage by a factor of 3, but in practice is
mainly to optimize the listening to low volume and medium with any type of headphone, further
reducing the noise level—the one weighted "A" is typically worth about 18 microvolt of output
at "Low sensitivity" and 6
m i c r o v o l t s a t " H i g h
sensitiv i ty. " Th e o u tp ut
impedance is set at 3 ohms
in one, 1 ohm in the other),
because in reality setting the
SH at “Low sensitivity” is
quite adequate to drive
headphones with 30 ohms
impedance or less.
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Construction
Great care was given to the layout and components. A design with very low negative feedback
must strive for the highest intrinsic performance that can be achieved, because the negative
feedback can add a beauty to music that might not be present in nature. So there is little need
to ensure the excellence of the basic parameters (linearity, noise etc.) with the use of negative
feedback. When it comes to the subject of noise, Unison Research has adopted electronically
stabilized power supplies for a "multistellar" strategy for the paths of mass: the return currents of
each stage independently converge towards the point reference potential-free, eliminating
mutual interference and the risk of picking up spurious signals.
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Unison Research Internal USB DAC
The Unison Research Internal USB DAC, used on the T25 and other amplifiers produced by the
Italian company, is a last generation asynchronous USB DAC compliant to USB 2.0 Audio Class.
This means that no driver is required when connecting the DAC to a Mac or a Linux-based
computer, to ensure ease of use and painless migration to more recent operating system
releases.
Its latest generation ESS Sabre conversion DAC, with the patented jitter elimination circuit,
allows for great sonic performance. Low USB transfer jitter is also ensured by the galvanic
isolation between the “dirty” power domain area of the USB interface and the “clean” power
domain area of the conversion IC. The low phase noise clock also reside in the “clean” area, to
avoid performance reduction due to supply noise.
USB DAC SPECIFICATION
Asynchronous USB interface compliant to USB 2.0 Audio Class
Accepts PCM up to 384kHz and DSD up to 128x
Resolution up to 32 bits
Last generation ESS Sabre DAC with jitter eliminator circuit
Galvanic isolation between USB interface and conversion circuit
Low phase noise clocks which reside in the “clean” supply domain to preserve low jitter
performance
110dB SNR (@ 1kHz, “A” weighted, 192kHZ)
0.008% THD+N (@ 1kHz, -3dBFS)
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Technical Specification
Max output power (THD 1%) : Output! "High sensitvity"
0.5 V on 150 ohm, 0.45 V on 33 ohm, 0.4 V on 10 ohm
!!!!!!!!!!!!!! Output "Low sensitvity"
1.4 V on 150 ohm, 1.1 V on 33 ohm, 0.6 V on 10 ohm
Frequency response : -0.1 dB @ 10Hz / 3-dB @ 50 kHz
(out "Low sensitvity" on 33 ohm)
Input impedance : 20 kohm // 100 pF sensitvity 0.67 V:
High sensitvity, ( 10 ohm load ) 0.33 V out
!!!!!!!!!!!!!! Low sensitvity, (33 ohm load ) 1 V out
Input stage: Pure Classe A, ECC83/12AX7
Output stage : Classe A, EL84 triode mode
Input: 1 RCA line , 1 USB
Output : 2 in parallel
Feedback: 16 dB
Output impedance: 6.9 ohm
DF: 3 on 20 ohm load
THD: < 0.2% @ 1kHz, 0.5V,
<0.8% to 1 V (out "Low sens." on 33 ohm)
Power consumption: 45 W
Dimensions: 22.5 cm x 12.6 cm x 32.2 cm
Weight: 6 kg
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