Unison Research P70 White Paper - VMAX Services

Unison Research
P70
Fully balanced dual mono
valves Amplifier in Push-Pull
topology
Why a push-pull amplifier
A push-pull topology amplifier is characterised by a power stage in which
active devices pairs work together amplifying separately the positive and the
negative signal half waves. The output transformer, with its particular
structure, reconstructs the amplified signal and makes it available at the
output terminals.
The main benefit of this topology is the possibility to design amplifiers which
have a much higher power than their single-ended counterparts, using the
same power valves in the output stage.
This advantage reflects in a better efficiency, which also means a significant
reduction of the dissipated heat at the same power rate.
Nonetheless the push-pull stages, because of to the simultaneous action of two
different devices, needs a very careful design and set up to avoid the large
distortion that always occurs when the amplification of the two half waves is
not equal.

A bit of history...
In the past Unison Research had designed and produced different push-pull
models, mainly because it was complicated and expensive to obtain large
output power with single-ended stages. Among the others let us mention the
Triode 20, the Power 35 and the early Nimbly.
The P70
Designing the P70 has required about one year of work. The very first
prototype has been developed in 2006, and it immediately showed all the
critical elements related with this kind of design. It took time to find all the
electronic solutions that have lead to the current cirquitry.
The P70 is now an amplifier capable of delivering more than 70 W per
channel into a 6Ohm load, with a THD lower than 0,2% over the whole audio
band. Its performances are comparable to those of a good single ended
amplifier.
Structure of the amplifier
The amplifier is laid out in dual mono configuration. Both the amplifier stages
and the power supply circuits are completely independent.
The signal path in the amplifier is fully balanced from the input through the
output. This means that the signal at the output terminals is balanced with
respect to the amplifier ground.
The P70 has four line inputs. All of them can be balanced or unbalanced, an
external switch allows the selection. The volume potentiometer is a four
sections motorised Alps blue series potentiometer. The selection of the
channel is performed by four high quality relè.

All the operations of the amplifier, included the safety functions, are
controlled by a microprocessor to ensure maximum reliability and intuitive
user interface.
The amplifier itself is constituted by an input stage featuring two double
triodes ECC83 and one double triode ECC82, and a power stage with two
KT88 per channel.
The output transformer has been custom designed by Unison Research and
ensure a bandwidth of more than 30kHz at full power.
The design
During the development of the first prototype, we had the possibility to
observe and analyse how the push-pull power stage requires a driving current
significantly higher and differently shaped than that of a single ended power
stage. Thus we concentrated our efforts in redesigning the circuit so to keep
the output impedance as low as possible.
Another point that has required special care is the polarization of the whole
output stage, which had to be stable enough to sustain the large signal
excursions required to properly drive the stage under the most critical
operating conditions.
The preamplifier is constituted by a double differential pair in an original
configuration. The two differential stages are DC coupled and allow to reach
the needed gain. They are followed by a couple of cathode followers, again
DC coupled, which guarantee the low output impedance required.
The power supply for the whole preamplifier stage has a very high voltage,
around 600V.

The input stage has a local feedback, characterised by different feedback
factors for the bias component and the signal. The two different values are set
keeping into account the different need for precision on the value of the bias
and of the signal.
The importance of a stable controlled bias in the amplifier is due first of all to
the choice of the DC coupling between the stages and also to the need to
allow the same signal excursion along the paths of the balanced signal.
The circuit we have designed amplifies signals up to 200V pp.
Conversely, the feedback factor affecting the signal has been kept to the
lowest possible value.
It worth mentioning that the particular circuit configuration chosen and the
presence of a double differential stage guarantee the possibility to use the
amplifier also with unbalanced signals. The proper connection can easily be
done through the input terminals on the back.
Finally, the signal is feedback in a balanced way to the cathodes of the input
differential stage, as in a traditional triode amplifier.
The power stage is constituted by a couple of KT88 per channel. During the
set up, we have considered that a bias current of about 50mA allows an
adequate trade-off between maximum power, distortion and power
dissipation.
We have also observed how, in most of the traditional push-pull stages
solutions, the level of the bias is hardly constant when the input signal or the
power supply voltage changes, and this makes the tube work, although for
short times, in interdiction or over-bias condition.

Moreover, these schemes often require a manual “tuning” of the bias current
and the balancing of the currents in the two paths that has to be performed
with the amplifier warm and fully working.
To solve this problem, in the P70 a brand new circuit has been designed,
which ensures a constant bias level with respect either to variations of the
input signal or the power supply, or in overload conditions, or to variations of
the tubes characteristics due to ageing.
The correct polarization is reached immediately after switching on, even
though the amplifier has not yet reached the optimal operating temperature.
This circuit continuously verifies the bias current level and the currents in the
push-pull paths, and automatically provides the eventual needed corrections.
The user doesn’t have to manually tune the bias, even after the KT88 pair has
been replaced.
We are considering to have this solution patented.
As far as concerning the power supply, according to the dual mono lay-out
already mentioned, there are two completely independent circuits.
It has a first filtering stage entrusted to a couple of high voltage p
configuration LC filter for the 650V of the power stage supply.
The second filtering stage is entrusted to a couple of high voltage p
configuration RC filters for the 600V of the amplifier stage supply.
The tube heaters power supply has a regulated DC voltage.
Another negative voltage is regulated to provide the bias of the KT88.

Technical data:
Power Output:
70W + 70W RMS
Output Impedance:
6 Ohms (4-8 Ohms)
Input Sensitivity:
500 mV
Input impedance:
47 KOhms
Frequency Response: 10 Hz to 40 Khz (0.5 dB 1W)
Signal/Noisse Ratio:
83 dBA
Total Harmonic Distortion:
0.3% (10W)
Negative Feedback:
12 dB
Damping factor: 8
CMRR:
59 dB
Power Requirements:
105-120/210-240V (50-60 Hz) at rest 360W
Dimension (WxDxH):
460x450x200 mm
Weight:
35 Kg