Expanding understanding of audio amplifiers

Discussion in 'General Electronics Chat' started by BrainFog, Sep 9, 2012.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
    Hello everyone

    Recently I have decided to to expand my understanding of electronics into an area that I am sure means a great deal to many of you, making noise!

    At first I was going to just buy myself some small pre made speakers with a built in amp for the TV in my room until I randomly came across this kit
    I have never actually bought a kit like this before as I have been a bit dubious of them. I tend to do some research into what I am making then buy the parts myself and make it on a prototype board. I like having full control of what I am making and understanding how and why it works but this one was cheap and could be good for experimenting and using as a starting point. Still the biggest selling point was letting me see the schematic before I bought it.

    I have also bought some TDA2030A Class AB op amps, like the above link uses, they were so cheap on ebay and are great to play with.

    From what I have read about op amps they require a +- power supply (I am still developing my understanding of them) but the kit I bought (which works perfectly) uses a standard single power supply. I suspect that it manages to work without a +- power supply by either creating a voltage divider (but can't see how with the components it uses) or it adds a voltage gain so the output is all positive which is then removed by the output capacitor (not sure of technical term) to produce the +- output. Can anyone explain this to me please?

    I have seen numerous references to LC(inductance capacitance) high/low pass filters and the first thing I picture in my head when I think of inductors is the voltage spike they produce when the magnetic field collapses. My first real electronics project took advantage of the voltage spike which I know can be damaging. I have found no mention of the voltage spikes in filter designs online. Is it simply not a problem or do they safely discharged into the capacitor?

    With the op amp kit when I first tested it I used an old variable plug in transformer with selectable voltages just to test it out. I was surprised that the voltage had no affect at all on what was coming out of the speakers. I began at 12 volts and slowly reduced the voltage down to 3v and it worked perfectly at a comfortable volume. I did get clipping at 3v if I turned the volume up. It was interesting as 3v is well below minimum voltage on the datasheet. As long as there is no clipping does the input voltage matter at all?

    Does sound quality benefit from voltage regulation, such as the 8712 voltage regulator? Or is a big capacitor all that is needed to regulate voltage from a mains transformer (not smps)?

    How do I measure power consumption? I know audio amps don't follow the normal rules because the consumption is far from linear.

    I realise I asked quite a bit, I have more questions but will leave them for now, I don't want to scare people off with a 1000 questions.

    A couple of interesting/geeky facts my research found while searching online about audio amplifiers that may interest novices who are interested in this sort of thing.
    If you have 2 speakers with an identical audio output and you unplug one of them what you hear is only 3dB quieter. So increase of 10dB doubles subjective loudness is but takes 10 times the power. Therefore if you want to upgrade to something that is twice as loud get something with 10 times the wattage.
    The decibel is one tenth of a bel and named for Alexander Graham Bell
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Hi there yourself.

    Not anymore...op amps can be designed for a single supply... and even the good old ones like a 741 can run off a single supply if you give it a mid point voltage reference.

    I suspect the designers of this amp are targeting portable stereo units (boom boxes) and the like, so a single supply means less batteries to run the thing.

    Well if you break the connection to an inductor yes they spike, as in a relay coil. But if you run an AC wave thru them they act like a low pass element: they have lesser resistance (actually reactance) to low frequencies and greater reactance to higher ones.

    But better to start another thread for that and start with the schematic of the filter you saw.

    A cap is OK as long as you also have a bridge rectifier to make it pulsing DC.

    Op amps have a built in immunity to supply noise. As long as you have the spec sheet there look up the "power supply rejection ratio." It should be a large number of dB's.

    With an ammeter and a signal generator. ammeter goes in series with the supply (like duh!) and measure first with no signal, then give it a pure tone. You'll need a load of a speaker (if you can stand the squeal) or a resistor (if you have a big 8 ohm one) as a dummy (and quiet) load.

    You can use music too but the meter will be jumpy.

    Nah, you were most polite and we don't scare that easily. :D
  3. Audioguru

    New Member

    Dec 20, 2007
    You should look at the datasheet of the TDA2030A Power Amplifier (it is not an opamp).
    It shows 1 ohm, not 22 ohms for the resistor at the output side of the amplifier IC (not the output side of the output coupling capacitor) plus two important protection diodes are missing in the kit (a speaker might produce an inductive voltage spike).

    The kit people lied about its output power. With an 18VAC transformer which makes 23.5VDC the power into 8 ohms is only 6W per channel and into 4 ohms is 10W per channel, not 15W, but you might not notice.

    The text in the Chinese kit has horrible Engrish so it is difficult to understand how it simply biases the inputs of the amplifiers.

    A 12V voltage regulator is much too low and a voltage regulator is not needed anyway.

    The datasheet for similar power amplifiers shows the (full output) power consumption is about double the output power. So with an output of 6W into 8 ohms the power from the 24VDC power supply is 12W and the current is 12W/24V= 0.5A for each channel when the load is 8 ohms.
  4. BrainFog

    Thread Starter Member

    Jan 24, 2011
    Thank you for the replies. It is quite important to be polite as I am asking for something. I have seen topics that contain something like “tell me how to do blah blah blah” and that is it. Not exactly something I would feel any motivation to respond to. :p

    It is good to know that I can use almost any op-amp with a single power supply as next I plan to have a go at an active low and high pass filter and I may be able to use a LM324N I have sitting around.

    I did not realise there was a difference between a power amplifier and an op-amp. I simply thought that power amplifiers for audio purposes only differed from op-amps in they abilities to manage large amounts of current. Are there major internal differences or is it more to do with how you set up the external components?

    I did notice the missing protection diodes shortly before posting this topic. I will add them to the board before I next use it. I will also swap out that 22ohm resistor for a 1ohm. They are not the only oddities I have found with the kit. If you look at the schematic there are 6 4.7uF capacitors but if you look at the circuit board position codes there are 2 4.7uF and 4 47uF capacitors (which were supplied correctly). But comparing the components on the photos on ebay I would say they only used 47uF capacitors. Despite the drill holes also not being done in quite the right places, the heatsinks having a slightly lower surface area than the ones in the photos I am happy with the kit as I have learned a lot from it. Still I don't see myself buying another kit in the foreseeable future.

    I actually enjoy slightly redesigning it. I may also lower the values of c1 and c5 to use it to power some tweeters. Or try my active low pass filter and use it to power a sub-woofer.

    I have been thinking a bit more about input voltage and I am wondering if the TDA2030A is a good power amplifier to use in some usb powered speakers for my laptop? Obviously it would be limited to only about 1w output but that may be enough if I am next to it.

    Would it be an effective method to use a bridge rectifier across a speaker leading to a capacitor in order to measure the amplifiers output peak voltage?

    I have read much conflicting information that I would appreciate being cleared up:
    Does speaker impedance affect sound quality in any way?
    Will putting speakers in series/parallel really degrade sound quality?
    Does it matter which polarity you connect the speakers as long as they all match polarity?

    Have a good day
  5. Audioguru

    New Member

    Dec 20, 2007
    A lousy old LM324 quad opamp or its sister an LM358 dual are horrible for audio. They have crossover distortion (because they are low power), are noisy (hiss) and perform poorly above only 2kHz. You should use an audio opamp like a TL071 single, TL072 dual or TL074 quad that have extremely low distortion and noise and work well up to 100kHz.

    An opamp has a max output current of 20mA. An 8 ohm speaker with only 10W of power needs a peak current of 1600mA.
    Higher power uses more current. A power amp has an output of as many Amps as is needed.

    Each opamp or power amp is different. The datasheet tells you exactly how to use it and has example circuits and pcb layouts.

    Capacitors with higher values filter power supply ripple better and pass low frequencies better.

    Reducing the value of C1 forms a simple passive poor performance highpass filter.
    C5 stops high frequency oscillation. Do not change its value.
    Reducing the value of C4 forms another simple passive poor performance highpass filter.
    Then you have two poor filters in series which is bad. You should make an active highpass filter.

    Then the normal woofer amplifier needs a complimentary highpass filter.

    USB is only 5V. The minimum supply voltage for a TDA2030A is 12V so it will not work from only 5V.
    With a supply of only 5V, an ordinary audio amplifier (LM386) will produce only 0.14W into an 8 ohm speaker with high distortion or only 0.11W at clipping because the output voltage swing is small.
    A TDA2822M has two amplifiers in a bridge with the speaker between them so its output at low distortion will be 0.8W. The TDA2822M will get pretty hot but not too hot if it has some free air around it (when it is not enclosed).

    My stereo computer speakers have a big heavy 12V/1A wall-wart and produce 3W each. They are 3" speakers with huge magnets in pretty big sealed enclosures.

    A bridge rectifier produces a voltage drop of 1.4V to 2V.
    Use an active fullwave peak detector circuit instead (has no voltage drop).

    High impedance = low power. Low impedance = smoke or high power.

    A speaker has a few strong resonances that are damped by the extremely low output impedance of a modern amplifier. Some speakers in series sound "boomy" and resonate like crazy.
    Speakers in parallel do not affect each other but the power load on the amplifier is increased and might cause the amplifier to be destroyed.

    You do not know which is the correct phase. I think an explosion sound should be a pressure wave not a vacuum wave. Try it both ways to see if they sound different.
    Last edited: Sep 12, 2012
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Just a minor suggestion. I can go to my local University surplus property store, and buy a set of used computer speakers, including a wall wart, for $5. If I want to spend more, I can get a wall-powered Bose system with a third speaker (subwoofer) for $10. They would do everything you propose building...
  7. Audioguru

    New Member

    Dec 20, 2007
    A $5 set of used computer speakers will probably have no power (not very loud) and produce no bass and hardly any treble.

    A sub-woofer produces frequencies from 20Hz to 80Hz. It is pretty big.
    A Bose system uses a little 4.5" speaker as a "sub-woofer" and it does not produce sounds below 80Hz so it is not a sub-woofer and is barely an ordinary woofer.
  8. BrainFog

    Thread Starter Member

    Jan 24, 2011
    Thank you for your replies once again.

    There seems to have been a bit of confusion. Sorry. My ultimate goal at this time is improving my knowledge rather than constructing a full on sound system. Any mentions of projects are more just projects to see how it turns out or seeing how it affects performance. I find it is the best way to learn.

    Thank for for that information, I got the impression they were good for audio quality after seeing them being used as a mini ear bud amplifier in an introduction to op-amps video on youtube.

    When I mentioned this I simply put it down as a statement supporting how difficult it was to understand and the clear lack of understand the seller had for the kit itself. And as a bit of a warning to anyone who is contemplating getting it after reading this thread.

    Sorry I mixed up the capacitor numbers here, I was referring to the input capacitors with the intention of creating a low pass filter for both the left and right speaker. Can I assume that passive filters are not suitable for audio purposes? I will most likely test it out just to see I can hear the difference due to curiosity.

    At this time I am not concerned with power output for my experiments. I know that the TDA2030A works at as little as 3v DC with low output power because I have tested this out as part of my experimentation. I do sort of understand what you are saying.

    I have seen schematics of 2 amplifiers on either side of a speaker. Could someone please explain what its purpose is and why it is done this way from an engineering point of view?

    I am aware of the direct relationship between power and impedance however I am interested to know if it affects the sound quality itself. As I have said I have read conflicting claims. Ultimately I value sound quality over how loud it is.

    Before now I did not think much about resonant frequencies of speakers, I will have to look into it.

    This was another "out of curiosity" questions. I know that if the polarity is different it can create all sorts of distortions (I will try it out to see what it is like) but I was wondering if humans can perceive the difference between pressure and vacuum of sound waves.

    There is not a place like that near me. In my whole life I have never seen a shop that even sells electronic components. I can only obtain them online. There are however plenty of off the shelf options if I were to buy one.

    An off the shelf option would not satisfy my interest in electronics, my desire understand how things work and experiment. The purpose of this really isn't about constructing speakers and an amplifier to improve the quality of my TV's audio any more but a step into understanding amplifiers.
  9. Audioguru

    New Member

    Dec 20, 2007
    There are lots of old garbage circuits with poor performance on the internet.

    No because ordinary bass and treble tone controls have the same performance as simple RC passive filters.
    my point is that a tweeter is very fragile so it can move air at high frequencies. Low frequencies will quickly destroy a tweeter. A simple filter gradually reduces low frequencies Then a tweeter might be damaged. Most tweeters need a filter with a flat response down to the cutoff frequency then a sharp dropoff. A Butterworth or Linkwitz-Riley filter is usually used.

    It is called a "bridged amplifier". The amplifiers are connected with opposing phase so one has its output voltage going up and the other has its output voltage going down. Then the signal voltage across the speaker is almost doubled. Ohm's Law says when the voltage is doubled then the current is also doubled. Double the voltage and double the current gives 4 times as much output power as a single amplifier. Ordinary car radios operate from only 12V so the output signal voltage from one amplifier is low and the power output is low (3.1W into a 4 ohm speaker). But with a bridged amplifier the output power is much higher (almost 12W into a 4 ohm speaker).

    Most home and professional speakers are 8 ohms. Most car radio speakers are 4 ohms and 2 ohms. The good ones all sound fine.

    Cheap speakers and amplifiers make a boomy sound like a bongo drum due to the low frequency resonance of a speaker. High frequency resonances sound like a shriek.
    You want the speaker to do exactly what the amplifier wants it to do. So the amplifier should have an extremely low output impedance (a good damping factor).

    I have never tried switching the phase of all frequencies.