Electronics > Beginners
AB-class amplifier schematic analysis & optimization
David Hess:
--- Quote from: Alex Nikitin on May 24, 2019, 10:04:00 pm ---The main problem is not that loudspeaker is a reactive load (at least that is a reasonably measurable and easy to deal with effect) , but that it is a very non-linear load, and not only load but a source as it acts as a microphone picking up vibrations and sounds. All this rubbish gets back to the amplifier and enters the feedback loop, as well as other drivers in a multi-way speaker.
--- End quote ---
I have difficulty imaging that being a problem with the low dampening factor at the feedback point of a solid state amplifier combined with the cable and speaker coil resistance.
Speakers are specifically designed to be linear because otherwise excessive intermodulation distortion is produced by the speaker itself. This takes the form of using a narrow voice coil within a large field of magnetic flux or a wide voice coil with a narrow field of magnetic flux. Woofers present the most difficulty because they must support a large throw to produce good loudness at low frequencies and base-reflex enclosures help considerably in this respect by lowering the impedance seen by the driver.
Interaction between drivers should be prevented with active or passive crossovers.
But I would agree that a lot of speakers intended for the consumer market are trash. But hey, the customer knows what sound they want.
dietert1:
Yes, i agree there is a huge difference between somebody ignorant using some sound system and a technical person using a sound system designed or combined by himself. Sound causes feelings! I know lots of well educated technical people who go crazy about horn speakers and the selection of the right foil capacitor.
But human hearing is extremely flexible.
Think about the differential analog transmission still used in professional audio. The input stage can be made such as to yield -6 dB when one of the two contacts in the connection fails. Or it can be made to yield -60 dB in that case. Now you may wonder how many of those connections in the real world are jumping between 0 dB and -6 dB without anybody taking notice.
Regards, Dieter
Zero999:
--- Quote from: David Hess on May 24, 2019, 11:55:01 pm ---
--- Quote from: Alex Nikitin on May 24, 2019, 10:04:00 pm ---The main problem is not that loudspeaker is a reactive load (at least that is a reasonably measurable and easy to deal with effect) , but that it is a very non-linear load, and not only load but a source as it acts as a microphone picking up vibrations and sounds. All this rubbish gets back to the amplifier and enters the feedback loop, as well as other drivers in a multi-way speaker.
--- End quote ---
I have difficulty imaging that being a problem with the low dampening factor at the feedback point of a solid state amplifier combined with the cable and speaker coil resistance.
Speakers are specifically designed to be linear because otherwise excessive intermodulation distortion is produced by the speaker itself. This takes the form of using a narrow voice coil within a large field of magnetic flux or a wide voice coil with a narrow field of magnetic flux. Woofers present the most difficulty because they must support a large throw to produce good loudness at low frequencies and base-reflex enclosures help considerably in this respect by lowering the impedance seen by the driver.
Interaction between drivers should be prevented with active or passive crossovers.
But I would agree that a lot of speakers intended for the consumer market are trash. But hey, the customer knows what sound they want.
--- End quote ---
Yes, speakers and room acoustics are the weakest links in the chain as far as distortion is concerned and dominate over even mediocre amplifiers. The audio amplifier is a problem which has been solved a long time ago. The only improvements made recently involve reductions in sized and increased power efficiency.
Headphones are better than speakers distortion-wise, because they're so small and the path between the ear drum and transducer is short (especially for in ear buds), transmission line effects are far less apparent, so it can be treated as a diaphragm changing the pressure levels inside the ear canal, especially at lower frequencies.
Try this. Wearing a decent pair of headphones, put some music on with a decent bassline and set the volume to a safe, comfortable listening level. Notice the deep thumping bass. Now take the headphones off, hold them around 30cm away from your ears and crank up the volume until you can probably hear the music. Notice how tinny they sound: the bass has magically gone! To get bass, there needs to be a decent seal between the headphones and the ear drums, otherwise the changes in pressure which produce the bass escape and are lost.
--- Quote from: dietert1 on May 25, 2019, 07:14:06 am ---Yes, i agree there is a huge difference between somebody ignorant using some sound system and a technical person using a sound system designed or combined by himself. Sound causes feelings! I know lots of well educated technical people who go crazy about horn speakers and the selection of the right foil capacitor.
But human hearing is extremely flexible.
Think about the differential analog transmission still used in professional audio. The input stage can be made such as to yield -6 dB when one of the two contacts in the connection fails. Or it can be made to yield -60 dB in that case. Now you may wonder how many of those connections in the real world are jumping between 0 dB and -6 dB without anybody taking notice.
Regards, Dieter
--- End quote ---
Sound causes feelings, so many of the perceived differences in amplifiers are often down to the placebo effect, rather than anything real. There is nothing wrong with this, until people start to make extraordinary claims, which cannot be proved.
magic:
--- Quote from: Alex Nikitin on May 24, 2019, 09:12:23 am ---It is getting completely off-topic, however I will only ask a simple question: can you provide a link to an example of a statistically valid controlled DB listening test with a positive result?
--- End quote ---
I can't because I don't care, but of course such tests must have been done if there are people talking about "scientifically established" thresholds of audibility of various defects.
But that's not the point, I don't really insist that they were valid or anything. I only remarked about the suspicious tendency of audiophiles to show up everywhere and say a lot about what they hear while making zero effort to convince that what they hear is determined by the actual sound rather than by their beliefs about what produced the sound.
Say that I swapped a few paralleled NE5532 into one of those LM7171 amplifiers of yours, would you tell it apart?
After all those years, somebody could perhaps show something like that. Maybe a vendor looking for publicity, maybe hobbyists looking to stick the finger to nwavguy and his religion, anyone :-//
And the problem of people hearing their beliefs rather than sound is obviously real. Gold plated USB plugs, compact disk demagnetizers, NE5532 relabeled as LM4562, all that junk sells and fetches positive feedback from satisfied customers.
Zero999:
Let's not continue the off-topic stuff.
I'm surprise no one has done a simulation.
The quiescent current was around 25mA, which seems a little high, although may not reflect reality.
Overcurrent protection could easily be added by connecting some back-to-back diodes between the output (the emitters of Q1 and Q2) and the op-amp's output. This will limit the voltage drop across R7 and D8 to the diode's foraged voltage. Ordinary diodes will clamp at 0.6V, giving a current limit of 60mA, although there may be some distortion before then. The output current will be a little higher, than that because the op-amp's output current will be added to the output, which will make around 100mA, for the NE5532. A couple of red LEDs connected back-to-back or use a bipolar LED, will limit the current to a couple of hundred mA.
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