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A SRPP-Input Amplifier For Headphones and Loudspeakers

A SRPP-Input Amplifier For Headphones and Loudspeakers

The circuit topology is a SRPP (Series Regulated Push-Pull) input stage which is AC-coupled to a Parallel-Triode Cathode Bias output stage.
To check the circuit gain, I injected a 1kHz sine wave at 1V p/p into the grid of the lower V1. I measured 12V p/p at the grid of V2, and 110V p/p at the anode of V2. The output stage runs at 32mA, 234V, i.e. 7.5 Watts. Class A amplifiers are typically about 20% efficient, so it would be reasonable to expect about 1.5 Watts output.

The amp was designed to work with 8 ohm loudspeakers. Nearly all loudspeaker drive units I have encountered have a nominal impedance of 8 ohms – of course this varies with frequency by a few ohms. When driving headphones, the audio output transformer must be shunted with a 10-ohm resistor (Ro) to present the correct load to the transformer secondary. The shunt allows higher impedance headphones to be used. Remove the shunt (add a switch to take it out of the circuit) when driving loudspeakers.

A wide range of headphone impedances can be used with this amplifier because of its low impedance output. A pair of 60 ohm headphones will present 1/((1/10)+(1/60))= 8.57 ohm load with 60 ohm headphones, while the amp will see a 1/((1/10+(1/300)) = 9.68 ohm load with 300 ohm headphones. I know it isn’t efficient from a dedicated headphone amplifier perspective, where the output transformer would have a secondary matched to the headphone impedance. However, with the 10-ohm shunt, there is still plenty of power.

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