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AB-class amplifier schematic analysis & optimization

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drussell:

--- Quote from: ratatax on May 20, 2019, 01:08:46 pm ---Ooh I see ! The polarity of C2 confused me since it will see positive and negative voltages, but it's not the first time I see electrolytics used like that, it doesn't seems to matter when the current/voltage is limited enough with a resistor. I was planning to replace them all with 22uF ceramic caps.
--- End quote ---

You should try to avoid using ceramic capacitors in the audio signal path.  Their capacitance tends to be highly non-linear with changes in the applied voltage.  Best to use film when possible if permitted by space / cost constraints.

For decoupling and stabilization type uses they're great.


--- Quote ---b) Short circuit protection => you're right. Since the circuit is ready to drive already high-impedance headphones (600+ ohms), isn't the simplest solution simply to replace the 10 ohm resistors by a higher value ?
--- End quote ---

If you go too high, you then remove the ability to drive low impedance, low efficiency headphones to high level, although with your original circuit that can theoretically supply multiple watts, that may not be an issue.

On some older typical hi-fi stereo amplifiers / integrated receivers, the headphone jack was just run right off the main speaker power amplifier output with, say, 390-560 \$\Omega\$ resistors.

dietert1:
If you use 1 nF parallel to R3 then for a compensated divider C = 1 nF * 1 KOhm / 22 KOhm = 45 pF should be parallel to R4. By looking at the transient response you can determine whether a 47 pF or a 39 pF works better. The 1 nF close to the OpAmp inputs can make a big difference on sound quality in a RF poisoned environment.
Concerning the 20 or 30 Hz high pass, this is an problem i have seen in many audio solutions. Engineers think the lower Hz they handle the better, but this may just not be true, when the speaker/headphone system starts producing lots of intermodulation. I did not propose to use ceramic capacitors in the audio path, but only for C3 and C4, that are uncritical in this respect (effects eliminated by negative feedback). The 10 uF i proposed for C2 can be film, maybe 2x 4u7 50V.
Concerning short circuit safety: It's fairly easy to test how long your final design survives a short cicuit at reasonable audio levels and how it dies. Certainly you don't want a fire.

Regards, Dieter

drussell:
Just a few quick additional notes...

Regarding the Elektor magazine example I posted,

--- Quote from: Yansi on May 20, 2019, 02:31:02 pm ---No real measurements of that amplifier provided.
--- End quote ---

Indeed, there are no published curves or in depth measurements provided for the unit as-built, only the specifications table according to the original article:
https://archive.org/stream/ElektorMagazine/Elektornonlinear.ir2011-01#page/n47/mode/2up


--- Quote ---Input impedance (without P1)10kΩBandwidth3.4 Hz -2.4 MHzTHD + Noise (1 kHz, 1mW/33Ω)0.005%(B = 22 kHz)THD + Noise (20 Hz - 20 kHz, 1mW/33Ω)     0.01%(B = 80 kHz)Signal to noise ratio (ref. 1mW/33Ω)89 dB(B = 22 kHz)92 dBAMax. voltage (into 33Ω)3.3 V(THD+N = 0.1%)Max. input voltage0.57 V(with P1 set top maximum volume)
--- End quote ---

And, then to the OP, regarding this advice from:

--- Quote from: dietert1 on May 20, 2019, 03:22:54 pm ---The 1 nF close to the OpAmp inputs can make a big difference on sound quality in a RF poisoned environment.
...
I did not propose to use ceramic capacitors in the audio path, but only for C3 and C4, that are uncritical in this respect (effects eliminated by negative feedback). The 10 uF i proposed for C2 can be film, maybe 2x 4u7 50V.
...
Concerning short circuit safety: It's fairly easy to test how long your final design survives a short cicuit at reasonable audio levels and how it dies. Certainly you don't want a fire.
--- End quote ---

These are all very good points.  I agree wholeheartedly!  :)

ratatax:
Thanks for all those informations !

Now I have more for you.

Yeah I'm looking for something 'quite' simple, it's for my product which is a synthesizer I plan to build for my upcoming business, so I can't go crazy on the costs. I'm only in the design phase currently, but the budget for heaphones amplifier is only a small part of the whole product so I think I need to keep it under $5 (this is not including the +15V/-15V power supply and connectors. Price is for buying components in qty ~100 on LCSC).

That's why I'm looking either for an amplifier IC or a quite simple but good enough solution with opamps (TL072 series) and transistors. Amplifier IC are almost always specified for 8ohms loads in their datasheet, I lack the knowledge here to understand if they would work well enough for higher impedance loads. I thought it was only a power thing, but I heared frequency response may also be affected if the amplifier isn't designed for high impedances ? (even if it is powerful enough)

I'm not searching for the Audiophile thing, if it has no audible/obvious distorsion or noise it's fine :)

I routed the schematic to see how it looks on the board :



yup it's a TL074 with two unused modules in this implementation... !

22uF ceramic caps here but I may have to switch back to aluminium caps @drussell :(

current transistors are MMBT100 / MMBT200

I have seen opamps with decoupling caps between V+/V-, some others between V+/GND and V-/GND. Best practice would be the latter ?

Audioguru:
Will the tiny little surface mounted output transistors survive the heating in this amplifier?

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