Preamplifiers

Preamplifiers handle low level signals coming directly from source players. Most preamps nowadays are line preamps, with inputs for CD, Tape, Tuner. Phono preamps have much higher gain for low level signals from phono cartridges, and also provide RIAA equalization.

All preamps use low noise, small signal triodes or pentodes. All circuits operate under Class A bias. The purer triodes, such as the 6SN7 and 6SL7, offer a cleaner, more dynamic sound.

Amplifiers

One of the major distinguishing features of valve amplifiers from transistor amplifiers, is the use of an audio output transformer. Power valves have a high output impedance, and cannot drive low impedance speakers directly. The output transformer serves as an impedance matching component, allowing efficient transfer of power to the speakers. The output transformer also becomes a critical component in determining the sound quality of the amplifier.

As far as valve power amplifiers go, there are two basic circuit designs:

Single Ended

A single ended amplifier uses a single valve to amplify the complete music signal. Both the positive- and negative-going parts of the signal are handled by the same device. The valve has to be biased to a high current to handle the full current swing of the music signal. This is Class A operation.

In these amplifiers, the output transformer sees the full DC bias of the valve. The transformer has to be large to prevent saturation of its core, and this also limits the amount of power that it can transfer.

Single ended amplifiers usually use power triodes, and have low power output (in the range of 5-10 watts). Since triodes produce lower distortion, of low-order harmonics, these amplifiers have a very musically satisfying sound. They may be limited in the reproduction of the frequency extremes.

Single ended amplifiers are best used with high-efficiency speakers.

Push-Pull

Push-pull designs came about with the quest for more power. Here, a pair of valves reproduce the opposite halves of the music signal. The signal is then combined in the output transformer. To do that, the input signal is split into opposite phases through a phase-splitter and fed to the pair of power valves.

The output stage can be biased to Class A or Class AB operation. This determines the amount of overlap in the handling of the signal between the output valves. Because the valves are 'on' only part of the time, they can handle more power and stay within their rated dissipation. Another innovative feature of the push-pull design is that the DC currents are made to flow in opposite directions in the output transformer, so that their magnetic fields cancel. With no danger of saturation in the transformer, higher output power is possible.

Push-pull amplifiers cancel even-order harmonics, reducing the overall distortion. These designs use triodes, tetrodes, and pentodes, in various output configurations. They are the most universal of valve amplifiers.