Fully balanced dual mono valves amplifier in Push-Pull
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 circuitry.
The P70 is now an amplifier capable of delivering more than 70 W per channel into a 6 Ω load, with Total Harmonic Distortion lower than 0,2% over the whole audio band. Its performances are comparable to those of a good single ended amplifier.
Structure of P70
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 30 kHz at full power.
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 600 V.
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 amplifiers signals up to 200 V p-p. conversely, the feedback factor affecting the signal has been kept to the lowest possible value.
Specifications
Power Output: 70 W +70 W RMS
Output Impedance: 6 Ω (4 - 8 ohms)
Input Sensitivity: 500 mV
Input Impedance: 47 k Ω
Frequency Response: 10 Hz to 40,000 Hz (0,5 dB, 1 W)
Signal-to-Noise Ratio: 83 dB A
Total Harmonic Distortion : 0,3% (10 W)
Negative Feedback: 12 dB
Damping Factor: 8
CMRR: 59 dB
Power Requirements: 105 - 120 / 210 - 240 V , 50/60 Hz at rest 360 W
Dimensions (W x H x D): 460 x 450 x 200 mm
Weight: 35 kg
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