Amplifier Circuit Unknown (but working)

Thread Starter

chazincaz

Joined Feb 20, 2023
4
I have made the following circuit on my bread board. When no AC signal is input - the MOSFET turns completely off. But when I add an input - it produces a hefty gain and pleasurable sound for this TIP41C.

The emitter is not hooked up to ground through a resistor (as I have been taught previously) BUT when using a 25V 2500uf cap as an output filter - it seems that all the power is delivered directly to the speaker (8 ohm load) and the MOSFET produces much less heat that in class A mode.

As a guy who works in studios and engineers, I can safely say I am pleased with the sound with minimal further effort yet to be done. Although - I have blown about 5 different sized caps for the output so getting here took a while.

is this just a bad circuit or a type of circuit I'm unaware of?

schematic attached below:

New to this forum so thanks for any advice or correction of post.

TIP41c emmiter no ground .pngPS - when I remove the R2 and bias the transistor up to the source voltage - I produce more gain and volume.
 

DickCappels

Joined Aug 21, 2008
10,128
It is hard to imagine how this produces a signal since the output capacitor SHOULD charge up to the peak output voltage voltage, which only takes a moment. Beyond that time there SHOULD be no output.

That there is an output suggests that some component may be incorrectly connected or defective.
 

Thread Starter

chazincaz

Joined Feb 20, 2023
4
It is hard to imagine how this produces a signal since the output capacitor SHOULD charge up to the peak output voltage voltage, which only takes a moment. Beyond that time there SHOULD be no output.

That there is an output suggests that some component may be incorrectly connected or defective.
thanks for the Reply!
so I swapped out TIP41c and rewired with same result. With a scope I’m only getting an AC signal powering the loudspeaker. Is it possible that there is so much dc current potential from emitter that I’m getting an untraceable amount of reverse leakage? Hence why capacitors so hot? I mean I’m using hefty size and ratings according to data sheet.
Thanks again Dick!
 

Audioguru again

Joined Oct 21, 2019
6,620
The class-A transistor circuit will not work properly with C2 in series with the speaker. You must have had the polarity of C2 backwards for it to do anything and to heat and blow up the capacitor.

Without C2 then the speaker will have DC in it and produce lots of distortion since the heating voice coil will not be centered in the magnetic gap.
The transistor will also produce distortion since it has no negative feedback. All audio amplifiers have negative feedback.
Without C2, the transistor will heat with 10W even when it is not playing sounds.
The transistor is an emitter-follower with a gain of 1 or less, the same output level or less as the input level.

Nobody uses a class-A amplifier anymore.
 

KeithWalker

Joined Jul 10, 2017
3,034
Have you measured the gain or looked at the output wave shape? I think what you are getting at the output is just the input signal, not amplified, after it has passed through C1, the base/emitter diode of the transistor and C2. It is probably distorted too.
 

BobTPH

Joined Jun 5, 2013
8,688
Here is a simulation of your circuit.

The top trace shows the behavior in the first 10 ms. It operates as a class C amplifier.

The bottom trace show the performance over 10 seconds. As the capacitor charges up, the output falls to zero as expected.

If I short the capacitor out, it looks like the top trace but with bigger amplitude. I think this is what you are getting. If that sounds good to you, what can I say?

1677004409482.png
 
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