Electronics > Beginners

Adding filters to class AB audio amp

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GerryR:

--- Quote from: d4n13l on August 19, 2019, 02:21:54 am ---
--- Quote from: GerryR on August 18, 2019, 11:17:06 pm ---The 100 nf and 10 K is a high pass with a break frequency of ~160 Hz; nothing below that (at 6 db per octave) is going to "get in."  Change the 100 nf or the 10 K to change what you "let in" to the amp at the low end.

--- End quote ---

Ok thanks, but that takes me to my original post, I'm having trouble analyzing the filter in series like that, all literature that I've seen would take the output of the filter from between C1 and R2.. so are VR1 and C1 making a variable low pass filter as well?

--- End quote ---

VR1 is simply a volume / level control.  C1 and R2 make up a simple 1st order high-pass filter with its break frequency at
f = 1/ (2pi RC).  Changing R and / or C will change the break frequency.

Also, I might mention that good audio amps have a frequency response from below 20 Hz to 100 to 200 KHz, the reasons being the "feel" of the bass in the low end (below 30 Hz) and the better transient response in the high end.  (One amp of a receiver I have goes to 100 KHz, and a power amp that I have (150 Watts RMS out per channel) has a response out to 200 KHz, which I have tested to spec.)   Most people can't hear below 40 Hz and above 15 KHZ, and at my age, I'm sure the band is much narrower, but hey, the specs are great.  :)




soldar:
Just for reference and comparison I am attaching a schematic of a preamp with treble and bass filtering. I seem to remember this type of RC network was pretty standard at the time and most preamps had similar controls. This schematic is half of a stereo unit so it had a similra circuit in parallel.


GerryR:

--- Quote from: Zero999 on August 19, 2019, 08:18:10 am ---
What problems do you think having a 10µF capacitor will introduce? It's generally good practise to make DC coupling capacitors much larger than the bare minimum because it reduces phase shift and attenuation at the lowest frequency of interest. The only downsides are a bigger audible click at turn on it taking longer for the steady state condition to be reached. 10µF and 10k is an RC time constant of 100ms, so it will be fairly close to steady state after being turned on of 0.5s.

--- End quote ---

With the 10 uf and 10 K resistor, your cut-off is down to 1.5 HZ.  Any "drift" on the input to that filter is going to affect the output of the amp.  (I believe most amps stop at 15 Hz, or above, on the low end to avoid this issue.  If you have a super power supply and the circuit feeding the amp is really stable, you might get with it.

Zero999:

--- Quote from: GerryR on August 19, 2019, 12:41:23 pm ---
--- Quote from: Zero999 on August 19, 2019, 08:18:10 am ---
What problems do you think having a 10µF capacitor will introduce? It's generally good practise to make DC coupling capacitors much larger than the bare minimum because it reduces phase shift and attenuation at the lowest frequency of interest. The only downsides are a bigger audible click at turn on it taking longer for the steady state condition to be reached. 10µF and 10k is an RC time constant of 100ms, so it will be fairly close to steady state after being turned on of 0.5s.

--- End quote ---

With the 10 uf and 10 K resistor, your cut-off is down to 1.5 HZ.  Any "drift" on the input to that filter is going to affect the output of the amp.  (I believe most amps stop at 15 Hz, or above, on the low end to avoid this issue.  If you have a super power supply and the circuit feeding the amp is really stable, you might get with it.

--- End quote ---
What do you mean by drift? Are you talking about power supply rejection or thermal effects?

I don't see how having an oversized input capacitor supply affects the power supply rejection.

C1 filters the bias voltage, keeping the power supply rejection high. Any "drift" as you say (due to thermal effects?) will be much below 1.5Hz and will be rejected.


I admit, there's less of a reason for oversized coupling on the power stage because it will be the final one in the chain, but when you've got many amplifiers, it's a good idea. Try chaining several stages together, each with a cut--off of 16Hz and you'll see there's quite a lot of phase shift and attenuation at 20Hz.

Audioguru again:
When the audio engineering company I worked for began selling business telephone systems when the government allowed competition with Bell, the cheapest telephone system was made by Goldstar from South Korea. The hand soldering was the worst I have ever seen and customers complained about the speakerphones producing severe crossover distortion.
Later, Goldstar Electronics grew up and changed their name to LG (Lucky Goldstar) Electronics.

Since the speaker in each telephone was about 32 ohms then I added a 33 ohms resistor from the output of the opamp to the emitters of the class-B output transistors to allow the opamp to fill the crossover gap.
 

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