View attachment 372291these are the resistor values I calculated which do amplify the signal but I am doubtful because the values are too small.

I didn't calculate, but this C2 seems too small for a 8 ohm speaker at 20 Hz, for example.
Now, calculating, C=10/(2*pi*f*R)=~10 mF
That's why your gain is so low, all the signal is on C2.
Then you can apply the factor to the resistors blindly and it won't decrease the gain, hopefully.

 

I have opened and investigated a couple of HIGHER POWERED (more than 30 watts) auto style amplifiers AND CERTAINLY they DO INCLUDE VOLTAGE BOOSTING!!
MrC is totally correct!!
The reasoning for operation at the higher voltages are several, but the simplest part is that high power at low voltage demands high current, and that gets incredibly difficult ar 12 volts. Aside from being difficult, it would also require impedance matching transformers that are BIG, and HEAVY, and EXPENSIVE.

Look at the circuit for any good quality sound system and the voltages are a lot higher than 12 volts, often closer to 100 volts, always above 50 volts. (or +/- 45 volts for a few 1965 systems.)

Hi,

Yes and it is easy to tell how much power you can pull from 12v into a 4 or 8 Ohm speaker with single output or bridge outputs.

If I was going to design a high power audio amp for a car at 12vdc, I would seriously consider using a voltage converter to get a higher voltage. Doubling the voltage gives us 4 times the power as I am sure you know, and that is a big advantage if going for seriously high power. An output transformer is much more difficult to work with and is probably old school now.

I'd be very careful about using the LM358 for audio work though, even though it can be done. There are at least three pitfalls:
1. Output of the LM358 has to be biased into class A operation or some serious crossover distortion can result.
2. Bandwidth is only 20kHz with a gain of 50.
3. Slew rate may affect the output unless the output peak to peak voltage is low like 2v or so

As long as the output is biased as above it could be ok for this amplifier because it looks like the opamp output will be low level, but a bandwidth of 20kHz might start cutting some of the highs (not always undesirable).

 

Hi, im currently designing a class AB amplifier to drive two 8 ohm speakers, but I am not able to understand the concept of impedance matching and input/output impedance. My block diagram flows from an input signal(cassette player) -> preamp and tone control-> AB amp->speakers. From my understanding, some kind of impedance must match the speakers impedance to avoid signal distortion. Any advice would be greatly appreciated.

Hi,

As others have mentioned, you don't have to 'match' the output to the speaker, it just has to be low enough to not waste too much power in just driving the speaker. If the speaker is 8 Ohms and the output resistance is around 1 Ohm it might not be too bad, but usually it can be even smaller when using decent transistors. If this is for a low power amplifier then it may not matter as much, but still better to get it down.
Also as others have mentioned, the input impedance should be higher than the output impedance of the previous stage.

To shed some light on input and output impedance itself, if you have an output impedance of say 10 Ohms with no load you measure 10 volts output, then if you load the output with the same resistance (10 Ohms) the output voltage will drop by 1/2, so it will go down to 5v. You can sometimes test for this but sometimes you can't. In a spice environment it may also be a little difficult with an AC amplifier because you can't just tack the load resistor right on the output of the transistor itself, it has to go through a capacitance, and that capacitance has to be quite high in value. Knowing this test however does provide some insight as to what output impedance means.
Input impedance is the same really. If you load the previous stage with your circuit and the previous stage has 100 Ohms output resistance and your circuit has 100 Ohms input impedance, then when you connect your circuit to the previous stage the voltage out of the previous stage will drop by 1/2 also.

Matching impedances is often used for RF circuits where it matters more. For an audio amplifier, you want the maximum output voltage swing to be as high as possible with the given speaker connected properly. That's unless of course you are not going for a maximum power output design, and of course the actual speaker to be used can handle the full power output without burning out.

 

Bridge Configuration
Gv = 2 x R1 / R2
BW ~ 1MHz (100kHz with 150pF || R1)
Pout = 0.5W @ 8 Ohm
THD = -83dB (0.007%) @ 1kHz

 

Bridge Configuration
Gv = 2 x R1 / R2
BW ~ 1MHz (100kHz with 150pF || R1)
Pout = 0.5W @ 8 Ohm
THD = -83dB (0.007%) @ 1kHz

View attachment 372317

I bet that he can't use integrated circuits, as he's learning biasing, amplification, and other things.
Few mins before I read your circuit, I realized I have gone through this subject in college and what we did was what you did in this circuit: invert the phase (180°) and amplify it too (with current gain).
That way it's like you're doubling the voltage supply available, without boosting it up.

@iLEEZi , please consider my posts, as I have been through what you facing. Please provide more details, like if you can use ICs.

 

I seriously doubt that you will ever see a commecrically available amplifier with the circuit show in post #44.
Really! it is not at all similar to any paractical amplifier circuit that I have ever seen!!

AND, what sort of transistor IS the BC327??
There are quite a few audio amplifier circuits published, and some of them are very good. Many of them are variations of "the classic"
With one transistor serving as a phase-splitter and then one stage or two stages of buffering followed by the two output transistors.

 

I seriously doubt that you will ever see a commecrically available amplifier with the circuit show in post #44.
Really! it is not at all similar to any paractical amplifier circuit that I have ever seen!!

I agree, it's a bit odd. Never thought you could use the buffer part that way. I'm going to simulate that to see if it has any drawbacks. I'll be back here with the conclusions, hopefully.

 

It's a circuit that has crossover distortion, but the op amps are fast enough to feedback it and adjust for it. Thus, the last stage of BJT pair is a true class B, with no bias, being more energy efficient.

 

The mcp1632 IC shown in post #44 is a PWM circuit, so that's a Class D amp, not AB.
Crossover distortion is not a factor with such an amp.

 

The mcp1632 IC shown in post #44 is a PWM circuit, so that's a Class D amp, not AB.
Crossover distortion is not a factor with such an amp.

MCP1632 is indeed a PWM modulator. But in post #44 there's only the MCP632T. You've added an 1 in the middle.

 

MCP1632 is indeed a PWM modulator. But in post #44 there's only the MCP632T. You've added an 1 in the middle.

Mea culpa (egg on my face).

 

Mea culpa (egg on my face).

I found that it's romanian, and it's amazing how similar it's to portuguese and spanish. Minha culpa (PT), mi culpa (SP).

 

I found that it's romanian, and it's amazing how similar it's to portuguese and spanish. Minha culpa (PT), mi culpa (SP).

Off topic: "mea culpa" means "my fault" and is the same in Catalan, French, and English, with roots from Latin.

 

Thanks for the language lessons guys (ha ha).

Ok now as to the bridge setup, it's basically just two of the same amplifiers, where one of them is fed an inverted signal. The speaker output is then taken from across the two amplifier outputs. The drawback is there is no common ground between input supply and output, so you cannot use a vehicle chassis as one of the speaker wires. The advantage is of course we can get 2x the output voltage swing which greatly improves the max power output.

 

It looks to me like quite a bit of extra circuit to avoid using an output transformer. BUT since transistors cost much less than good transformers, it is probably still cost effective. The downside is that a single shorted transistor will result in burned out speakers. So speaker fuses become a very good choice again.

 

The downside is that a single shorted transistor will result in burned out speakers. So speaker fuses become a very good choice again.

Since the output DC bias voltages of those two simple amps will likely differ, each output should be capacity coupled to the speaker.
This will also prevent damage if a transistor shorts.

 

It looks to me like quite a bit of extra circuit to avoid using an output transformer. BUT since transistors cost much less than good transformers, it is probably still cost effective. The downside is that a single shorted transistor will result in burned out speakers. So speaker fuses become a very good choice again.

You should know by now that transformers are big and bulky and weigh a ton compared to even 100 amp bipolar transistors. If there is a transistor solution, it is probably going to be much better than a transformer.

In my past experience I have worked with transformers both in design and application.
One application I had a tube guitar amplifier that had two transformers. It was fairly high power so the transformers were big. It was a b**** to carry around from gig to gig. The speaker cabinets were also big and bulky though too
In the power control industry, we design and used transformers from a small as 1 inch in diameter toroids to huge laminated EI core transformers that were too heavy to pick up without a lot of effort. These were slid around into place mostly. Some of them had 3 inch square center legs and were used for very high power AC converters. They were expensive, heavy, hard for the transformer department to wind, and hard to move around. If there was a transistor-only solution everybody would have really loved that but there wasn't

Really though the main question for audio work with somewhat lower DC voltages available is do we want to boost the input voltage or the output voltage. We do not necessarily need a transformer to boost the input voltage but if we want to boost the output voltage we need a transformer.

 

There ARE higher powered transistor amps that do not use an output transformer. MAYBE a big power ttrans. A transformerless power supply may not be adequately isolated from the mains.

 

There ARE higher powered transistor amps that do not use an output transformer. MAYBE a big power ttrans. A transformerless power supply may not be adequately isolated from the mains.

Even if it is indeed isolated, somehow, you can't trust it. A component fail would lead to powerful shocks. I think that technology will eventually lead to it but will be put a great amount of effort to really trust it. For now, there isn't such reliable thing.

 

Hi Guys,
Please keep On Topic, the TS asked for help on this point.
Hi, im currently designing a class AB amplifier to drive two 8 ohm speakers, but I am not able to understand the concept of impedance matching and input/output impedance.

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