Audio Note Kit4 Push Pull Amplifier Integrated Push-Pull Hi-Fi Amplifier - 10W per channel

The KIT 4 is our introduction to Valve Amplifier Kit building.

The circuit and power supply are mounted on one single printed circuit board, with two OTO - type push-pull output transformers, all mounted in a small P1SE type chassis, covering everything, so nobody will be able to see that you have succumb to the lure of the valve amplifier, which is sweeping the world. The circuit consists of two 6V6GT tetrodes running in Push-Pull class A, yielding about 10 watts, driven by a 6SN7GT and a ECC83 input stage. Easy to build, even for the beginner.

Kit4 is a stereo 10 Watt per channel push pull amplifier using selected valves and components it a tried and tested configuration. The valve complement consists of one ECC83 (12AX7) low noise double triode , one 6SN7 double triode and two 6V6 beam tetrodes per channel . These valves were chosen primarily for their sound quality but also for their availability and low replacement cost . The 6V6 although less well known than its big brother the 6L6 is one of the sweetest sounding valves around. The 6SN7 and ECC83 work together to extract the maximum musicality from the output valves

The audio circuit is straightforward : the ECC83 is used as the input stage and phase splitter, one section recieves the incoming signal and amplifies it by approx 40 times. The second section of the ECC83 takes its input from the potential divider connected between the anodes of the two sections and generates the phase inverter signal.Local feedback keeps the “in-phase” and “phase-inverted” signals exactly in balance, this type of phase splitter is known as the floating paraphase or see saw circuit and was chosen for its sound quality and ability to accomodate the variations caused by the ageing of the valves

The following stage is a resistor –capacitor coupled Anode follower 6SN7, this stage amplifies the signal again and drives the output valves, push pull pentode connected 6V6’s These are coupled to an in house designed AudioNote ouput transformer(as used in the highly acclaimed OTO amplifier) with 4 and 8 ohm secondary tappings

The circuit incorporates approx 11db of overall feedback to control the dynamic behaviour of the amplifier and to allow it to drive complex loads. The feedback is taken from the output transformers secondary back to the input valves cathode. Compensation components are the 560pf capacitor across the feedback resistor and the 820pf cap across the input valves anode resistor, these ensure stability.

The power supply is again conventional , using solid state rectification and a high grade capacitor input filter with further filtering and decoupling to eliminate hum.

The sensitive input stage is DC heated to reduce hum problems , whilst the drvier(6SN7) stage and output tubes are AC heated.

The whole amplifier is housed in a non magnetic aluminium chassis and finished with 2.5 mm thick acrylic front and back plates.

All-in-all I have to say that the building of the Kit4 was very entertaining. The only negative thing I can say about it is that the job was over to fast.

I have now used the Kit4 for about a month, and I’m very satisfied. The sound is more than I’d hoped for. Its capability to reproduce details and the essence in the music is beyond anything I’ve ever heard in this price range. I could write much more about the sound, using all the fancy "Stereo magazine" words, but it would all just end up in the following sentence: It sounds great, and the value for money is very high with this kit.

Frank Fossheim, Norway