Transistor push-pull long-tail power amplifier?

[G6Tanuki]

In these virus-times, my mind has been wandering.

I'm thinking of a 'long-tailed-pair' push-pull power amplifier - see attachment.

R1=R3

R2=R4

R5 is a few Ohms.

18V as the supply.

Using something like OC35 or NKT404 transistors, passing 750mA or so each [good heatsinking needed] I figure that even when limited by the never-more-than-50%-efficient restriction implicit in Class-A operation, it could still be interesting.

The centre-tapped choke in the collectors could be quite small, since the two DC static currents in the windings will produce equal-and-opposite magnetic fields, so the core won't saturate. In an ideal world the capacitor in series with the speaker wouldn't be needed.

Since the common-emitter resistor on the output stage provides loads of negative-feedback, I don't think there's much risk of 'parasitic' oscillations.

Perhaps I should do a low-power trial-run with a couple of AC153 and a 75-Ohm speaker before going for the big stuff?

[Tyso_Bl]

Isn't this really a differential amplifier? from my little knowledge R5 would need to be a constant current source, and the bases would need to be biased exactly the same, as well as both transistors being matched electricaly and thermally. I'm prolly wrong though.

I've had similar ideas, but ended up with four output transistors, in a bridge arrangment, but with one transistor on each side driving the oppposit side. I may still have some scribbles somwhere nearby, looked like it would work, but never got round to soldering it up.

[Radio Wrangler]

There's no emitter to emitter resistors, so the output devices are being run flat out for gain (and distortion!)

David

[Ed_Dinning]

I think a remember a circuit like this with 2N3055's being used as the amplifier for an ice cream van. As RW says, max gain, max distortion

Ed

[G6Tanuki]

Originally Posted by Radio Wrangler

There's no emitter to emitter resistors, so the output devices are being run flat out for gain (and distortion!)

David

Surely the unbypassed common emitter resistor (R5 in my sketch) provides a form of negative feedback?

(And yes I'm all about gain. My ears are made of Lead, not Gold!)

[Jon_G4MDC]

Now we know why Ice Cream van chimes sound the way they do!

[Radio Wrangler]

Originally Posted by G6Tanuki

Surely the unbypassed common emitter resistor (R5 in my sketch) provides a form of negative feedback?

It provides emitter degeneration type feedback for the common-mode (IE bias) current but no feedback for the differential mode signal. Drive the base on the input sife and that transistor's emitter sees the emitter impedance of the other transistor which is acting as an emitter follower for its base bias voltage. So the emitter sees effectively 2*Re.

Lead ears ought to be good. well-damped a million pipe organs can't be wrong. Viva Henry Willis!

David

[Tyso_Bl]

Originally Posted by G6Tanuki

In these virus-times, my mind has been wandering.

I'm thinking of a 'long-tailed-pair' push-pull power amplifier - see attachment.

What about this? this is what I was on about,