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Transformer coupled transistor amplifier

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Hey guys, long time since i've last been here!

I've recently designed myself a low wattage transformer coupled transistor amplifier out of parts i've got.

I've built it on a breadboard using literally just parts that I could find and at low voltage levels, its quite pleasing to my ear! has a quite sweet sound. I'm using Acoustic Solution Instate 90 MK 2 speakers (Picked em up from a carboot sale,


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Where have you been for 56 years??

Amplifiers have not used push-pull and output transformers since 1955 when Philips showed a portable amplifier with them using germanium transistors.

Your circuit has no negative feedback to reduce distortion so it will sound horrible.

Modern amplifiers use a complementary (NPN and PNP emitter followers) output stage without a transformer.


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Would NFB really add a significant change in sound quality?

Solid state amplifiers have extremely low distortion at any power level becaue their open-loop gain is high and a lot of negative feedback reduces the gain to a useable amount and also reduces the distortion. The negative feedback makes the output have an extremely low impedance (0.04 ohms or less) for good damping of the resonances of a speaker.

I'm using a transformer for the phase splitter now, if I drove that by a transistor, could I add NFB?

A transformer causes phase shifts so only a small amount of negative feedback can be used to avoid oscillation. Your amplifier does not have much gain.

Also, what do think about the zeus amp then? http://www.susan-parker.co.uk/zeus.htm That is a transformer coupled mosfet amp with some quite impressive specification!

It is a class-A heater with low output power. Each Mosfet heats with 25.5W all the time!
The highest output power on the distortion graphs is only 4.2W.
The output transformer is huge, heavy and very expensive.
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Here's a simple push-pull transistor amplifier. It has a power output of just 250mW. With a few modifications and a higher voltage power supply, the power can be increased to 5W with a much lower quiescent current and lower distortion than a class A amplifier with a transformer.

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Hi, thank you for the reply hero :) I am fine with a simple push pull amplifier, even to slightly more complex stages, e.g. replacing the resistors with current sources etc. I was merely practising the concept of a transformer coupled amplifier. I just wanted to know what flaws my circuit had! Thank you for the help anyway! :)

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