Electronic – Hobbyist: Is this a reasonable design for an audio amplifier? (very sketchy design)

amplifieraudiooperational-amplifier

I have always wanted to design and build my own audio amplifier. Is the following a reasonable design?

  1. The input stage takes in line level signals (say, up to +/-3 V, but typically more like 0.3 – 0.8 V).

  2. TI OPA 134 "audio op amp", in buffer configuration, to impedance match for…

  3. Low pass filter (ideally one I can turn on and off, so I can put the amp in "subwoofer mode")
  4. TI OPA 134 in non-inverting amplifier configuration, aiming for gain of 5-20
  5. A push-pull BJT amplifier stage, with enough gain to push 100 W through 8 ohms of impedance.

All of this powered by a, say, a 150 W power supply with high voltage rails.

I originally wrote +/- 15 here. I had op amps on the brain, since I am actively working on designing the line level circuit and want to figure out what gain factor I'll eventually need.

Obviously, there are a lot of details/features I'm not including or even designing at the moment. The BJT part is especially sketchy at the moment. I'll get to that, but I want to know if it's even "plausible" and reasonable.

Questions:

  1. Is this a reasonable first attempt at sketching out design?(This is actually version 0.0.4 of my design). Any additions/advice for a beginner?

  2. Given that I'll have a low pass filter, is the buffer op amp necessary or does modern audio equipment typically have a high enough output impedance at line level? Does it make sense to use an audio op amp in the buffer configuration?

  3. I'm guessing it makes more sense to do the low pass filtering on a line level signal as opposed to after the BJT amplifier stage, but that's just a guess. Is it right?

  4. How do I calculate the voltage I need across my 8 ohm load? If I do a DC approximation, it comes to about a 28 V difference (i.e., +/- 14 V). (Did I flub this math? P = I^2 R = 100 W / 8o = 12.5 A^2, so I is about 3.5 A, which means the voltage difference must be about 28 V give or take.) But presumably, AC imparts less power at +/- 14 V than DC at +/- 14 V. (But then again — perhaps as a naive guess? — a square wave swinging between +14v and -14v "should" intuitively push out as much power as +/- 14 V DC…) So what output voltage do I really need to get 100 "real" watts out of my speaker? What assumptions should I make about the input signal and it's spectrum? Square waves? White noise?

Or am I overthinking this, and should just aim for some rule of thumb?

Despite some of the comments, I am going to do this. Even if it takes me 50 practice circuits and and auditing a course on EE. I am not trying to kill myself or start a fire. I am trying to plan out a long term project — to figure out how much time to invest in learning the line-level components and to figure out what kind of transistor technology and amplifier layout to learn about next, in order to accomplish my goal. Assuming this sketch of a design (barring the power supply issues mentioned) is even plausible. Which is why I asked in the first place. I am working on a roadmap, not trying to sit down and solder random parts together.

I'm totally green when it comes to building circuits, though I did learn a lot as a kid (Mims III) and vaguely remember some circuit design/analysis from college (passive components, Ohm's law, DC analysis, RCL analysis (though I don't remember the formulas right now and would have to look them up to solve for capacitances, etc.), transistor basics, and op amps, Fourier analysis for signals and in real analysis. I also have a math degree and took a bunch of physics classes.

Best Answer

To get 150Wrms on an 8 ohm load you'll need 35xsqrt(2) volts on your DC rails... Plus and minus 50V. Plus a little bit for losses etc. 150 watts rms is a lot of power for home use. 150 watt amp will melt the voice coils of most home HiFi speakers. Use P=V^2/R, and remember Vpeak is sqrt(2)xVrms

Don't bother with toroids bridge rec etc... Making your own psu is a mugs game unless you're a professional psu manufacturer. Buy a pair of isolated switch mode ones. Don't believe the audiophile nonsense about switch modes being noisy etc... Technology has moved on in leaps and bounds since they all formed their opinions.

Likewise, the leach amp is a relic of a bygone era. Lm3886 is an excellent beginner choice (I've built 2 of them myself, with 2x 24V switch mode PSUs) the sound is subjectively and objectively excellent if done right-but beware of fake chips) and irs2092 is more complicated to do yourself, definitely not beginner friendly, but a nice solution.

You don't normally need to buffer modern outputs from e.g your mobile phone, but older kit may have higher output impedance so suffer voltage drop and distortion without a buffer. The buffer amp is useful if you have a weak signal or your filter is liable to create harmonics on your input line (filters impedance changes with frequency so presents a different load to the input at different frequency).

You should absolutely do the filtering at line level. Filtering at power level is expensive since it requires large components and more likely to not work with the reactive load of the speakers/resonate/...

For building and testing, get yourself some power resistors rather than using speakers at first, an oscilloscope if you can afford it, and a power supply with a current limit and variable voltage (start low and turn up the volts once it is working).

Edit: you said 100W not 150W... For which you'll need 28sqrt(2)V, +/-40V rails. This is still a pretty monsterous amp for home HiFi.

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