Adding filters to class AB audio amp

[d4n13l]

Hi,

I picked up the attached amp from internet and made small modifications, I think it works nicely for audio but is still missing some filtering which I'm still not clear how it works. I tested the circuit as is, except by R4 and C7 which is how I've seen some filters implemented by I'm not sure how it works since the filters I've found on literature take the output from between R4 and C7, they don't put it in parallel with the output as you see here.  Also is C2 making a filter R1 or with the rest of the circuit?

Basically I'd appreciate notes on how to add high and low pass filtering here.

[ledtester]

I am not an audio guru, but I would place any filters before the final amplification stage which is what the TL084 is doing.

Are the 3 other op amps in the TL084 being used?  If not you could use some of them to create active low/high pass filters. This page has pretty good info on the main types of filters used in electronics:

https://www.electronics-tutorials.ws/category/filter

See the page on "Active Band Pass Filter" for an example of how filter and amplifier stages are chained together.

[wraper]

First of all this is not AB but B class amplifier. Secondly, it not a filter on the output but snubber to improve stability of amplifier.

[Zero999]

That circuit is class B, not class AB, and will have some crossover distortion, so is not optimal for audio.
https://www.google.com/search?client=firefox-b-d&q=class+b+crossover+distortion

R4 and C7 form a Zobel network.
https://www.google.com/search?client=firefox-b-d&q=Zobel+network

C2 is an AC coupling capacitor, which enables the audio signal to pass through to the input of the amplifier, whilst blocking DC. Removing it will upset the biasing of the circuit, so its output will saturate close to the positive rail, instead of sitting between both supply rails.
https://www.google.com/search?client=firefox-b-d&q=AC+coupling+capacitor

[wraper]

Also is C2 making a filter R1 or with the rest of the circuit?

Filter consists from impedance of signal source, VR1, C2, R1. It's not good at all because it's parameters depend on signal source and VR1 position,

[wraper]

R4 and C7 form a Zobel network.
https://www.google.com/search?client=firefox-b-d&q=Zobel+network

Yeah, I tried to remember but failed to recall how exactly this snubber network was called.

[d4n13l]

doesn't the op amp make this work as a class AB? as I understand it a class B amplifies only after the 0.6V that takes to activate the transistors thus causing distortions, however due to the negative feedback the op amp put enough voltage to overcome that

[Zero999]

doesn't the op amp make this work as a class AB? as I understand it a class B amplifies only after the 0.6V that takes to activate the transistors thus causing distortions, however due to the negative feedback the op amp put enough voltage to overcome that

No the op-amp doesn't cause the output stage to work in class AB. Each transistor conducts for less than half of the waveform, so it's definitely class B.

The op-amp will reduce the crossover distortion somewhat, but it will still be there, especially at higher frequencies/slew rates. The only way to reduce it further is to bias the output transistors so both conduct slightly all the time, making it class AB.

[Zero999]

I've done an LTSpice simulation. Note that I didn't include the Zobel network or decoupling because it's not necessary for a simulator, which uses ideal components. Of course they should be included in real life.

Look at the horrible crossover distortion.

[soldar]

VR2 provides negative feedback. You can put any filtering network in its place to adjust frequency response.

[mikerj]

VR2 provides negative feedback. You can put any filtering network in its place to adjust frequency response.

Adding additional,variable phase shifts into the feedback is likely to turn this into a oscillator, and even if it could be made stable it will undoubtedly make the crossover distortion even worse.  Filters should go before the amp.

[Circlotron]

R4 and C7 are there to keep a load on the amplifier output at high frequencies. The speaker impedance goes up with frequency meaning less and less load and the chance of high frequency oscillations and instability. The impedance of C7 comes down with frequency, and if R4 and C7 are sized right the amplifier will see a constant load regardless of frequency. I have seen typical values of 100nF and 10 ohms.

[John B]

Not only will the output transistors have crossover distortion, but with only a 12V supply to the op amp, the output of the op amp itself is likely to have crossover distortion above a certain voltage output swing. It will likely need loading down to the negative rail, ground in this case. Something like 2k-10k ohms depending on how much output swing you want.

[d4n13l]

ok I'll work on the other stuff next, now I get the Zobel network (made by R4 and C7) and why is there but in order to calculate the cap and resistor values you need the inductance value of the speaker, at least according to https://diyaudioprojects.com/Technical/Speaker-Zobel/, however I can't find an easy way of calculating it, does anyone now a practical way for doing it or people just take approximate values?

[GerryR]

Speakers, I believe are rated for impedance at 1 KHz (8 @ 1 KHz), so you can measure the resistance with an ohm meter and calculate the inductance at 1 KHz.

[Audioguru again]

Not only will the output transistors have crossover distortion, but with only a 12V supply to the op amp, the output of the op amp itself is likely to have crossover distortion above a certain voltage output swing. It will likely need loading down to the negative rail, ground in this case. Something like 2k-10k ohms depending on how much output swing you want.

The only opamps I know that have crossover distortion at ANY supply voltage (within spec's) are the LM324 quad and LM358 dual (they use the same opamps) because they are designed for very low idle current so they are missing the biasing of their output transistors that eliminates crossover distortion. A load on the output causes one of the output transistors to operate in class-A eliminating the crossover distortion.

[Audioguru again]

This thread title says "adding filters".
Why? To cut low frequencies to make an adult male voice sound like a chipmunk and to muffle high frequencies in speech and music like an old telephone?

[GerryR]

I've done an LTSpice simulation. Note that I didn't include the Zobel network or decoupling because it's not necessary for a simulator, which uses ideal components. Of course they should be included in real life.

Look at the horrible crossover distortion....

The OP's schematic shows a 100 nf (.1 uf) cap, not a 10 uf like in your model.  It is a ~160 Hz hi-pass into the op-amp.  If the OP wants a lower response, then changing to 1 uf will drop it to ~16 Hz.  I believe the 10 uf will cause other problems at the low end.

[d4n13l]

This thread title says "adding filters".
Why? To cut low frequencies to make an adult male voice sound like a chipmunk and to muffle high frequencies in speech and music like an old telephone?

Hi audioguru,

Well, since the intended purpose of the amp is to work with audio I'd say the obvious answer is to restrict the amp output to the appropiate 20hz to 20kHz bandwith. But I'm actually doing this to learn how they work so is more of and excersise, and since I happend to have a bunch of sort of crappy small speakers I'm thinking it would be a good idea to implement some kind of bass cut.

[d4n13l]

Going with GerryR's note on speakers having 8 ohms at 1kHz I came out with a value of 10 ohms for R5 and ~10uF for C5 for the Zobel network.

As for the filters, I take it C6 and the speaker are making a high pass filter so a better value for C6 would be 1000uF as to allow freq over 20hz?

But I'm having trouble as to where to put the low pass filter, maybe at pin 2 of the op amp as seen in the diagram? which would be 795nF (closest I have is 1uF) for C7 to cut frequencies below 20hz, or 63nF if I wanted to cut the bass at 250hz?

[GerryR]

.... But I'm having trouble as to where to put the low pass filter, maybe at pin 2 of the op amp as seen in the diagram? which would be 795nF (closest I have is 1uF) for C7 to cut frequencies below 20hz, or 63nF if I wanted to cut the bass at 250hz?

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.

[John B]

Not only will the output transistors have crossover distortion, but with only a 12V supply to the op amp, the output of the op amp itself is likely to have crossover distortion above a certain voltage output swing. It will likely need loading down to the negative rail, ground in this case. Something like 2k-10k ohms depending on how much output swing you want.

The only opamps I know that have crossover distortion at ANY supply voltage (within spec's) are the LM324 quad and LM358 dual (they use the same opamps) because they are designed for very low idle current so they are missing the biasing of their output transistors that eliminates crossover distortion. A load on the output causes one of the output transistors to operate in class-A eliminating the crossover distortion.

Mea culpa. I probably got bitten by the fake bug again. As not to derail this thread, I documented it here: https://www.eevblog.com/forum/beginners/tl07x-tl08x-ebay-fakery/

[d4n13l]

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.

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?

[Audioguru again]

Throw away that horrible class-B amplifier with its awful crossover distortion. Use an opamp made for audio to drive the output transistors.
If C5 has the extremely high value of 10uF then the 10 ohms resistor in series with it will overload the amplifier above only 1600Hz. An LM386 IC amplifier uses a 0.05uF capacitor in series with 10 ohms for its Zobel network. Then it begins loading the amplifier output at very high ultrasonic frequencies where the speaker is a very high impedance and the amplifier will try to oscillate without the load.

EDIT: I forgot about the capacitor to ground you added at pin 2. It does nothing at pin 2 because this inverting input has the input signal cancelled by the negative feedback. Make a lowpass filter parallel with the feedback resistor.   

[fourfathom]

The only opamps I know that have crossover distortion at ANY supply voltage (within spec's) are the LM324 quad and LM358 dual (they use the same opamps) because they are designed for very low idle current so they are missing the biasing of their output transistors that eliminates crossover distortion. A load on the output causes one of the output transistors to operate in class-A eliminating the crossover distortion.

Doesn't adding this resistor at the output merely move the crossover distorting to a different place?  Of course with small signals the modified crossover point won't be reached, so the distortion as a % of the signal will be improved.