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| AB-class amplifier schematic analysis & optimization |
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| ratatax:
Hello For my sound generator project I want to add headphones amplifier to it. I planned initially to use a digital amplifier IC but since we want the ability to drive high impedance headphones (600 ohms) with studio-quality someone told me it would be better to use a classic approach with a ~1W class-AB amplifier, powered with +15V/-15V rails. EDIT : See more info about my needs & project on my reply post : https://www.eevblog.com/forum/beginners/ab-class-amplifier-schematic-analysis-optimization/msg2425455/#msg2425455 We found this schematic, from http://sound.whsites.net/project113.htm : The original design is very old-school, using through-hole components and big 100uF caps. We'd like to modernize it to integrate into the product we're making. No problems, resistors diodes and small caps are easy, we found SMD transistors equivalents to BD193/BD140, but big caps are expensive and take a lot of board space. I understand the role of some of them in the circuit but I have no idea what C2L, C3L & C4L do. Also I feel this design is old and may use some overly-big capacitors to compensate for a slow linear power supply, but now it's full SMPS, much faster. C2L seems in reverse polarity to me. I'd like to avoid any cap bigger than 10uF to stay with small ceramic ones, and maybe modernize other things if you find flaws... Can you help me ? :) Edit: after looking at different class-AB amplifiers on google, most of them don't have the caps equivalent to C2L, C3L & C4L. Maybe they are meant to optimize something ? |
| Audioguru:
C2 has a very small input offset voltage across it. It is used so that the amplifier does not amplify this DC offset voltage which would cause some DC voltage at the amplifier output. Headphones move the cone to one side if there is DC on them. The capacitors parallel with the diodes reduce distortion a little. |
| dietert1:
The circuit you show is something like a school example. For a real product there need to be: a) Bandwidth limiting If there is a SMPS nearby, for example 1 nF caps to Gnd on both inputs of the OpAmp. Then a 39pF parallel to R4 to get a compensated divider. Pulse response needs to be checked for stability. Also there should be a 20 or 30 Hz high pass somewhere. Modern media include subsonic audio down to 5 or 10 Hz and sound quality will suffer if you pass that to the headphones. C1 could be 470 nF instead and C2 may be reduced to 10 uF as well to get something like a second order filter. That's a matter of taste. b) Short circuit protection For example sensing the voltage on the two 10 Ohm output resistors R7 and R8. c) Turn-on and turn-off transient suppression (load/listener protection) This can be done with a pair of low Rdson mosfets per channel and a supply voltage supervisor including a small turn on delay. Two years ago i tested 150 uF multilayer smd caps. You can use them for C3 to C4, which only see small voltages. Regards, Dieter |
| David Hess:
C2 lowers the gain of the operational amplifier at low frequencies to 1 so it does not amplify its own offset. Because of AC coupling from C1, it never sees significant negative voltages. C3 and C4 provide a low AC impedance path between the operational amplifier output and the bases of the transistors so it can better control them. |
| ratatax:
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. @dietert1 how do you calculate the value of 39pF for the capacitor to be used with R4 ? a) Bandwidth limiting the DAC output already has a lowpass output filter and the datasheet mentions a DC blocking cap isn't needed, so I guess there is already a high-pass integrated but I'd need to check its cutoff frequency. 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 ? c) Turn-on and turn-off transient suppression (load/listener protection) => we need to think about that. I think the easiest solution would be to have mosfets controlled by a pin of our microcontroller, so there isn't any need for a hardware-based delay |
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