As I understand, there are two types of class B push-pull amplifiers. The single supply type requires a phase splitting input transformer and results in a reversal of current during each half cycle. By contrast, the complementary symmetry type uses a dual supply and the current flows in the same direction during both half cycles.

If that is correct so far, do all class B or AB consumer audio devices, including PC sound cards, now use the second type? If not, what would be the exceptions?

 

As I understand, there are two types of class B push-pull amplifiers. The single supply type requires a phase splitting input transformer and results in a reversal of current during each half cycle. By contrast, the complementary symmetry type uses a dual supply and the current flows in the same direction during both half cycles.

If that is correct so far, do all class B or AB consumer audio devices, including PC sound cards, now use the second type? If not, what would be the exceptions?

I do not think that is 100% accurate. I think you can have a class B amplifier, with a single supply, biased at Vcc/2, and AC coupled on the input and output. Long ago with tube amplifiers it was common to use transformer coupling between the stages. I haven't seen that since about 1960.

 

Audio amplifiers are never class-B which produces crossover distortion.
Amplifiers used transformers many years ago, complementary symmetry amplifiers can use a single supply voltage and input and output capacitors or use a dual supply voltage.

Many modern audio amplifiers use class-D switching at a very high frequency for efficiency.

Here is a fairly old class-AB audio amplifier IC using a dual supply voltage and using a single supply voltage:

 

I don’t know what you mean about the current flow. The current to the speakers reverses for both configurations.

Bob

 

The two output transistors are usually complementary emitter-followers and are idling at half the supply voltage for a single supply and at 0V for a dual supply. The NPN pushes the output voltage positively and the PNP pulls the output voltage negatively. They are called push-pull because the signal is AC (Alternating Current). The output voltage and current go positive, negative, positive, negative with the AC signal, which maybe you say is reversing.

 

AG:

I was questioning this statement in the original post:

By contrast, the complementary symmetry type uses a dual supply and the current flows in the same direction during both half cycles.

Bob

 

I think the noob is talking about a "electric geetar" amplifier that rectifies the signal so that the fuzz and distortion are extreme!

 

The fuzz is normally done in the preamp, not at the output stage.

Bob

 

So then, my premise was wrong. All class A/B audio amps apply alternating current.

If I have a stock hi-fi amp is there any way externally to force the current to be monopolar and not alternate between positive and negative? For example by adding a split winding output transformer and/or offset from a separate DC supply?

 

So then, my premise was wrong. All class A/B audio amps apply alternating current.

If I have a stock hi-fi amp is there any way externally to force the current to be monopolar and not alternate between positive and negative? For example by adding a split winding output transformer and/or offset from a separate DC supply?

Why would one ever want to do that?

 

ALL audio amplifiers produce an AC output. DC in a speaker causes its coil to permanently move and bash into the magnet structure. DC also heats the coil.

With a single supply, an output coupling capacitor feeds AC to the speaker but blocks the DC.

 

You could indeed add a DC offset to an AC output from an amp, making an output that was always positive. But what would you do with it then?

Bob

 

You could indeed add a DC offset to an AC output from an amp, making an output that was always positive. But what would you do with it then?

Bob

There is no speaker involved. I am using the hi-fi amp to experimentally drive an electromagnet and do not want the field polarity to reverse under AC. In doing so, the intention would be to not apply any continuous DC. IOW the drive signal should swing between 0 and Vpp out.

I have bench power supplies. How would you recommend adding the positive offset voltage? Is a transformer required?

Can the same be done with a negative offset voltage, using a dual supply, to enable the coil polarity to be switched via the applied signal?

To avoid mishaps, I would appreciate a few sound ideas to try.

 

Where does the input to the amplifier come from? I realize you said this is "experimental" but it would be exceedingly useful to know just what the experiment and its goals are in order to render useful help and not just make guesses based on changing information.

 

Hi-fi amps are designed to amplify music or audio signals. They generally do not have frequency response down to 0Hz. What you need is a power op-amp. For example, Kepco makes power supplies that do this.

 

The amp input is line level audio frequencies from a function generator. The objective is to create a unipolar magnetic field at various frequencies for seed germination exps. There is no DC, as in 0Hz, required.

To the greatest extent possible, I would prefer to use a "standard" home amplifier so other people without construction skills can replicate. As previously mentioned, the question is how to simply (if possible) convert its AC output to unidirectional current.

Thank you for the advice so far.

 

hi SK,
If I read your requirement correctly, you could use a full wave bridge rectifier, to give an unidirectional current.?
E
Added: A demo circuit.

 

Eric: If you don’t mind changing the waveform and doubling the frequency.

You could simply put the audio out in series with an isolated DC supply.

Bob

 

hi Bob,
As you say, there are a number of options, I have posted the basic idea to give the TS food for thought.

Let's see which way he wants to go.

E

 

hi Bob,
As you say, there are a number of options, I have posted the basic idea to give the TS food for thought.

Let's see which way he wants to go.

E

Agree. I think my comment sounded like a criticism of your idea, it was not, I just intended to point out that you get a different output the two methods.

Bob