any single 2n3055 audio amp schems out there?

[toprepairman]

Agreed a second '3055 is cheaper than a transformer, but the OP called for just one '3055, and he suggested PA use so reasonable efficiency is also required.

[wg_ski]

If "reasonable efficiency is required", one transistor isn't the way to go. And a little single ended class A amp requires big heavy and expensive compenents to go with that cheapo 3055. If the objective is to build a SE-A amp because you like the particular sonics, that's one thing. Remember, these things are heaters and the transistor is the cheapest part. If you're looking for a cheap easy way to use a 2N3055 to put out a few tens of watts I stand by the original recommendation.

[john_ellis]

Hi
John Linsley Hood considered this with his Class A (search in diyaudio). An emitter (or collector) resistor is only about 12% efficient, so you won't get a lot of power for PA like that. You might get 1W from a stage using an 8 ohm load resistor. 8 ohm speaker, which has to run from 14V (dissipation 12W, effective efficiency 8%). I've used transformers for single device outputs with about 30% efficiency. Yes, the cost is high these days for a transformer compared with a push-pull stage.
I would not change the 2n3904 for a TIP31 driver. Quite often such types of transistor are specified for drivers, but the TIP31's are epi devices with fT's around 3 MHz. I'd recommend a higher frequency driver like BD140 or (in TO39) 2N4036 etc if you like old tin cans.

John

[infinia]

i did some digging around for an audio amp schematic using a single 2n3055 in the output and much to my surprise, i couldn't find any.

there's a reason for that. You could copy some of the most earliest semiconductor designs ie car radio, but they will mostly use a big expensive germanium power PNP and an output transformer. so you'd have to flip it around a bit adapting to a NPN. By the time of 2N3055 device, audio topologies have moved on to class B and AB, first using transformers, then without using quasi PNP w/ either split supplies or DC coupling (err, blocking) caps.

w/o a standard classic output transformer, you are in custom territory. w new class A load lines for yer Vcc Ic.

[RJM1]

The Tip31 was because it has to dissipate about 3W.
john_ellis
You really don’t want to do that. With an ft of .8 MHZ on the 2N3055 if your driver stage can go far beyond this you are just asking for major oscillations.

[toprepairman]

The car radio type circuit is the way to go. You only need to get hold of an old valve radio type output transformer, use the secondary as the collector load, don't use the primary (insulate the primary leads to avoid a belt), and then couple to an 8 or preferably 15 R speaker via a capacitor, 2200uF should be enough.
Regards
Henry.

[john_ellis]

RJM1 -The circuit may already have oscillation problems. The original 2N3055 had 0.8 MHz ft and I suspect one of these WOULD be stable with a high frequency driver.
But the current devices will be closer to 3 MHz. HAving two transistors rolling off at the same frequency may be worse than one while the other maintains a better phase margin. But all this can be optimised.

However, in the circuit shown, the driver needs to be able to hold the gain at higher currents in this configuration - nearly 8A peak current ? will need up to 1A drive. Perhaps an MJ21193 to remain more linear at 8A would be better. Or use a transformer for higher efficiency.

[john_ellis]

Sorry, MJ21194.

[john_ellis]

How about this ...
Needs a transformer capable of 2A/24V with peak current 4A without saturating.
Could be a biggy - say EI150.

[john_ellis]

First pass transformer design:
plates M6 0.35 mm (GOSS)
size EI150 (1.5 inch square section core; =pattern #120; 1.5 inch stack)
Primary: 220 turns 1mm insulated copper wire
Secondary: 220 turns 1mm insulated copper wire
Stacked unidirectional, air gap 1.2mm (use 0.6 mm cardboard spacer)
Inductance ~ 100 mH
Resistance ~ 1 ohm
should not require additional resistor in series.
Probably OK to use bifilar winding method since other approach (1 layer pri-1 layer sec-etc would have almost same capacitance between windings).
Turns ratio 1:1 should give 15W max rms output but good performance at 10W and below. Simulated distortion .01% at 10W, but excluding transformer. Hence need very tight coupling between primary and secondary.
John