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Compact Hi-Fi Power Amplifier by Melvin Leibowitz


Compact Hi-Fi Power Amplifier by Melvin Leibowitz - Electronics World, June 1961

This is an OCR scan of the article "Compact Hi-Fi Power Amplifier" by Melvin Leibowitz which appeared in the June 1961 issue of Electronics World. Edits - The schematic has been cleaned up to improve legibility.

Compact Hi-Fi Power Amplifier by Melvin Leibowitz - Electronics World, June 1961

Compact Hi-Fi Power Amplifier

by Melvin Leibowitz / Delaware Electronics Supply Co.

Constructing a 6-watt amplifier in which the output tubes serve as their own phase inverter.

MODERN tubes and components have done much to destroy the old-fashioned idea that high-fidelity amplifiers must be cumbersome and expensive. The unit to be described is a case in point. Excellent performance is obtained although a mere handful of lightweight, inexpensive components are required in the construction. The very simplicity of this circuit is partly responsible for its performance as there is so little to go wrong with it.

The design of this amplifier, as is the case with any amplifier, centers around the output stage. EL84/6BQ5 tubes were selected for the output stage for several reasons. They will produce adequate power at low distortion with only 250 volts on the plates and screens. Plate current requirements are modest. The most important consideration in the use of 6BQ5's was their low drive requirements. Only 22.5 volts grid-to-grid are required to drive the tubes to full output. Compared with the better known 6V6, the 6BQ5 will produce more output with lower distortion, less plate-current consumption, at about one-third less grid drive. This means that if we use the 6BQ5, our power supply may be reduced in size and the voltage amplifier stages preceding the power amplifier stage may be simplified. We have achieved a large part of our goal simply by choosing the correct output tubes. A little unusual circuitry now comes into play to enable us to reach our goal of the "ultimate" in a simple, yet good, amplifier.

Compact Hi-Fi Power Amplifier Schematic

Compact Hi-Fi Power Amplifier Schematic

It will be noticed that there is no phase inverter in the amplifier and at the same time, the hook-up of the output tubes is unconventional. The output tubes act as their own phase inverter. This type of circuit is not new and can be found in some of the audio hand-books. The circuit is not used much despite its advantages of good inherent balance, simplicity, and low drive requirements.

There are those words "low drive" again! Only one tube is driven in this circuit as opposed to two in the normal type of output stage. This means that the required driving voltage is halved which, in this case, now amounts to 11 volts. Almost any tube will supply the required 11 volts when driven from a ceramic phono cartridge, FM tuner, or tape deck so we eliminate the multi-tube input stages found in most amplifiers and end up with a single 6C4. Inasmuch as we do not have a great deal of voltage amplification we do not need a lot of filtering in the power supply, hence almost any filter choke and capacitors will do the job. Since we are not striving for maximum gain in the input stage, the cathode resistor has not been bypassed, thus giving us desirable inverse feedback and saving the cost of the bypass capacitor.

Some further information about the output stage is in order. The plate current of V2 varies in accordance with the signal. The voltage across the cathode resistor, R6 will also vary as a result. Thus there is a component of signal voltage on the cathode of V2. Since the cathode of V3 is in parallel with that of V2, it will also have signal impressed on it and since the grid of V3 is grounded the cathode will swing around the grid which is equivalent to having the grid swing around the cathode as we are accustomed to having it. The cathode resistor must not by bypassed (by a capacitor).

None of the parts in the amplifier are critical,however, you should make every effort to get the output transformer of the impedance and power specified in the parts list. This is an excellent unit, modestly priced, that employs interleaved windings to reduce leakage. Note however that the secondary impedance is 3-4 ohms. For those requiring 4-, 8-, or 16-ohm impedances, the Triad S-31-A may be used instead. All other specifications remain the same.

This amplifier has plenty of high frequency response so tone controls have been kept to a minimum and only a simple treble attenuator is employed. The output power is just over 6 watts before the scope shows any noticeable distortion.

The amplifier is built on a 5-1/2" x 9-1/2" x 1-1/2" chassis. Construction is straight forward and, due to the small number of components involved, should present no problems.

The small size and light weight of the amplifier makes it ideal for use as an all-around hi-fi unit and also for use as a monitoring amplifier in broadcast stations, as a power amplifier for professional tape recorders, and as a second channel stereo amplifier for the many non-professional stereo tape recorders now on the market. The amplifier may be incorporated in the speaker cabinet in this latter instance. Components have been designed out of the amplifier, not the quality or performance

Compact Hi-Fi Power Amplifier Schematic

Compact Hi-Fi Power Amplifier Schematic

6BQ5 / EL84 Compact Hi-Fi Power Amplifier

6BQ5 / EL84 Compact Hi-Fi Power Amplifier

6BQ5 / EL84 Compact Hi-Fi Power Amplifier Bottom

6BQ5 / EL84 Compact Hi-Fi Power Amplifier - Bottom View


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