Hi AAC -

Sorry for the cross-posting, I did ask a similar question on the SDIY mailing list - but got a pretty mixed response, so I thought I'd ask here.

First of all, I'm trying to do something with the Class B Amplifier circuit shared on AAC:
http://www.allaboutcircuits.com/textbook/experiments/chpt-6/class-b-audio-amplifier/

Although, one person commented "I would strongly recommend completely ignoring that circuit. It has far too many problems to list. There are much simpler, better, and understandable circuits around for driving a small speaker. Just delete that page from your screen, erase the circuit from your memory, and have a quiet moment to ponder the nature of the internet...."

Have other people run into trouble with this circuit here?

Essentially, I'd like to combine this amplifier with a pre-amplifier - but I have two big questions.

1. Is it OK to attach a transformer to the output of the Class B amplifier? I know that current is great for driving speakers, but I also have other types of transducers to drive that require different impedances and voltage-hikes, so a transformer seems like the best option. I'm assuming it would still be possible to drive a speaker with a transformer at the output of the Class B.

2. Would it be OK to take the "Class B + Transformer" amplifier and send it to an op-amp based processing circuit? The Class B + Transformer would potentially allow me to step-up some low-signal transducers and route them to audio equipment (like compressors or filters, for example)...I know that I'm potentially raising both the current and the voltage considerably in these situations and I want to make sure I'm not planning anything too dangerous.

Thanks everyone,
Alfie

 

1. You can use a transformer but they tend to be heavy and expensive if they must transfer high power at a low frequency and need to be specifically designed for audio frequencies if higher frequencies must be amplified.
What is the frequency and power range you need?

2. You don't normally process signals once they've gone through a power amplifier.
You use a low power op amp to do amplification if further signal processing if needed.

 

I'm mainly working with 20-20khz, ideally in the 10-watt range. I have some "audio output transformers", but I do expect to lose some of the low and high frequency range. So, as long as its ok to attach to the end of the Class B, I'm happy there.

I think I can try to figure out a way around the rest - but is it unsafe to send power amp-->transformer-->low power op amp, out of curiosity?

 

If you are going to build an amplifier for audio use, DO NOT BUILD THIS ONE!!! (the one in post #1) The crossover distortion is very high with this design. There are a lot of single part amplifiers that do the job much much better. May I suggest that you look at the TDA series of amplifiers. Look at the TDA2003, or the TDA2030. They are dirt cheap and out perform that circuit by a hundredfold.

 

Yes, you can connect the output of an audio amp to a low power op amp as long as the voltage doesn't exceed the op amp ratings.
But why?
The voltage at the input to the audio amp should be sufficient for any further processing you may want to do.

 

Are there any single-part designs that are good for bipolar power supplies? I feel like I've only seen single-supply ones, though I'm not sure why.

Re: audio amp-to-low power - my hope was that the transformer would be a suitable way to deal with some troublesome transducer inputs. I think I'm starting to get a clearer picture of how to tackle this now, though.

 

Generally if you provide us with the actual problem you are trying to solve, not the problem you think you are trying to solve, we can better offer suggestions.

 

Well, there's no particular "problem" - I'm just trying to understand some basic things about amplifiers and I struggle a little bit with the textbooks. I appreciate the help and suggestions a great deal, so thank you.

 

So your talking about "troublesome transducer inputs" was hypothetical?

 

Yeah, sort of hypothetical - I didn't mean to make it the focal point of the question - I was more worried about the fact that someone didn't seem to like the bipolar Class B circuit.

Your statement that the audio should be sufficient at the input op-amp made sense to me. The TL;DR of it is that I found that some microphone elements seem to be more robust as a result of being passed through a transformer input stage (though it's conceivable that if I designed op amp circuitry with a more finely-honed range of input impedances, I might have the similar results) - and I was hoping that since I needed a transformer anyway to couple the power amp output to other transducers (magnetic tape heads, piezo discs, etc..) that I could just come up with a "swiss army knife"-style solution, where everything could be passed through the same signal chain and come out amplified in a few ways, across a few outputs. I'm still pretty bad at this stuff, so I think I have a little trouble communicating the ideas.

 

Well, I built the original circuit (http://www.allaboutcircuits.com/textbook/experiments/chpt-6/class-b-audio-amplifier/) - and perhaps unsurprisingly, it distorts considerably.
Are there any additional changes that can be made? Or suggestions of IC-based analog bipolar audio amplifiers?

 

A.........

 

......... Or suggestions of IC-based analog bipolar audio amplifiers?.......

See post #4