Amplifier MA-5

5 W Hi-Fi Mono Amplifier, Model MA-5

A low-priced general-purpose Hi-fidelity amplifier based on the popular S-33 for those who do not require a stereophonic system.

Incorporating separate bass and treble controls, gram, and radio inputs. Suitable for most crystal pick-ups.

A printed circuit simplifies construction.

Kit £10.19.6
Assembled £15.10.0

Circuit description.

The first valve is a triode-pentode (ECF80) with the triode section forming the tone control stage using a negative feedback loop.

The high-pass or ‘rumble’ filter is also incorporated in this stage, which is followed by the pentode section and then the ultra-linear output stage comprising two EL84 pentodes in parallel.

Negative feedback is applied from the transformer secondary to the cathode of the ECF80 with appropriate phase correction. The output transformer is a sectionalised type having a high inductance and low leakage reactance and is specially designed to operate with a high DC current.

The rectifier is a heavy-duty type (EZ81) and the unusually generous smoothing capacity of no less than 200 µF contributes to the very low hum.

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