Stereo solid-state hi-fi integrated amplifier Puresound M845 black

Overview

An amplifier capable of reproducing the full range of instrumental tones with extraordinary dynamic contrasts, energy and colors. When paired with the appropriate speakers, the 845 tubes are capable of unparalleled reproduction of all types of music.

The M845 mono amplifier uses the 845 power triode in the output stage (single-ended configuration). Operating in pure Class A and capable of delivering 27 W output, it is perfectly suited to medium or high efficiency speakers.

Originally, the 845 triode was developed by RCA in 1931 as a radio transmitter and audio amplification tube. The anode is constructed of graphite to allow for high power dissipation. It features an extremely linear transfer curve. In recent years, renewed interest in this type of tube has led to their remanufacture by many factories in China.

Implementing the 845 in a suitable circuit to take full advantage of its characteristics is no small feat. The filament dissipates considerable power and is also in the signal path, which is why powering it correctly is a very critical part of the design. In addition, the tube operates at very high voltages; the choice of suitable filtering components in the power supply is therefore essential. The required current must be supplied instantaneously at all frequencies of the bandwidth. In the M845, this power supply includes a hybrid Graetz bridge for initial rectification followed by a cell using capacitors and a coil capable of satisfying the high flow rates required.

Although all these elements, power supply design, choice of circuit topology and components, are of paramount importance in the construction of a tube amplifier, the quality of the output transformer also plays a very important role. The high impedance of the primary, the wide transformation ratio, the need to ensure a high level of winding insulation, all conspire against the transformer designer and the quest for wide bandwidth with high power transmission capability.

The M845 uses a transformer specifically designed by PureSound after numerous tests, testing different core materials and different winding techniques before choosing the final configuration.

The other difficulty in using the 845 is the realization of an input stage capable of instantly responding to the very high current demands required for the tube’s full power. The M845 incorporates a driver stage based on a 6SN7 triode coupled to a specially designed and built interstage transformer. The amplifier input is directly coupled to this stage, without a coupling capacitor.