Yes, speakers are usually designed to be driven by voltage sources. A voltage source is one that regulates the voltage applied to a load (the speaker, in this case) while supplying whatever current is required to apply that voltage across the load. If the speaker was a resistive 8 ohms, that would mean that for every 8 volts the amplifier applied across the load, it would have to deliver 1 ampere through it. So just because it is a voltage source, does not mean that you can neglect the current side of the equation, it just means that voltage is what the amplifier regulates most precisely.
One of the big differences between a typical opamp and a power amp is that the opamp has a maximum current capability somewhere between 10 and 30 milliamps while a power amplifier that operates from the same supply voltage may have large enough output transistors to pass more than an ampere.
There are integrated power amplifiers (essentially high output current capability opamps with some internal bias and gain setting resistors) that are available for very little cash. I think you might start with one of these for your experiments. The data sheets show the typical speaker amplifier circuit diagram and additional parts needed.
I want to design and build a very simple audio amplifier to drive my
8ohms speakers from (onboard) pc soundcard. I figure I'm using op-amps. I'm a newbie and all I know about op-amps are basic, mostly from text book.
Basically, I'm a little confused. Usually I see that the type of amplifiers to drive speakers are called 'power amplifiers'. From the basic building blocks of op-amps, I could only find circuits for voltage and/or current amplifiers. Do those 'power amps' are really 'voltage amps', cos I figure the main drive factor for speakers are voltage variations (voltage are converted into air preassure by the speakers, and hence creating audible sound)?
Further more, I figure that the input stage of the amps I want to build will simulate that of LINE-IN/AUX/CD-IN lines found on commercial sound systems. Anybody knows whether there are standards to the input voltage and input impendance of those lines (LINE-IN/AUX/CD-IN), and if there are, what are the values? And how about output impendance standards for driving 8ohms speakers?
Also, anybody knows how the input parameters (voltage, current, power) to the speaker relate to the SPL (Sound Preassure Level) generated by it? This indirectly means I'm asking the relationship of the amplifier gain to the loudness of the sound generated by the speaker.
Last, but not least, what are the recommended op-amp series for audio processing (I only know the multi-purpose u741 series)?
Well, that's a long list of questions, and there are more. Any help is appreciated.
At the high frequency end of the amplifiers capability, the speaker tends to look inductive (rising impedance with frequency), which raises the open loop gain, just as the phase shift of the amplifier is increasing and tending to decrease the stability of the closed loop. This network absorbs energy at this part of the spectrum, loading down the gain, so that the amplifier will not oscillate at a few hundred kilohertz, especially if you disconnect the speaker. At ordinary audio frequencies, it consumes very little power.
THe internal bias network tries to set the output voltage to 1/2 of the supply, so that it can swing equally far in either direction. But if the supply has ripple voltage, this would also couple half of that to the speaker, producing hum. Once you decide which rail you are going to use as speaker return, you bypass the middle of the bias divider to that rail, to low pass filter the ripple to that reference point, so that the output is biased not to half the supply, but to half of the average of the supply over a time longer than the ripple period.
Frequency compensation pin is a place to tie a bit of high frequency feedback from output to near the input to stabilize the amplifier as its phase shift increases as frequency rises. It also rolls off the high frequency response of the amplifier.
Ripple rejection is similar to the bypass pin on the LM380.
Bootstrap is positive feedback from output back to the output driver, to increase the output swing possible by adding to the positive supply during positive output swings. This allows a bigger swing from a given supply with lower distortion.
Bootstrap same as for TBA820M
Treshold (threshold) sets the logic switching threshold voltage for the muting function to match it to whatever logic is driving the muting function.
Muting is an input that silences the amplifier (useful to get rid of the blast of noise an FM detector produces if no valid signal is detected.
SVR (supply {ripple} voltage rejection) similar to bypass on LM380.
I am not sure I have figured them all out, but the speaker to positive rail saves one capacitor (the bootstrap) since the speaker coupling cap can do both duties. But in many applications, you have a speaker that is already tied to the negative rail by some other requirement, so they show that configuration, also.
Wire (twinned pair or twisted pair) rated for the current and voltage is all I usually worry about. What else do you have in mind? If the run is long, I use oversized wire, to keep total voltage drop a small fraction of the total.
Instantaneous power to the speaker is amperes times volts. The speaker impedance tells you the ratio of volts per ampere.
That depends on the efficiency the speaker and enclosure have at converting electrical energy into sound energy. If you are sitting right in front of a typically efficient speaker (say, a 6" by 9" automotive type speaker with big magnets in a cubic foot or so box), 1 watt will shake your teeth a bit. If you use tiny computer speakers or want to fill a large room with sound, 1 watt will probably be weak.
You almost have to start with the speakers and space and work backwards ot the power needed, then make sure those speakers can handle that power without distorting or burning up.
To add to what others have said, the circuits for discrete power amps are basically the same as ordinary op amps, with the difference that instead of integrated low-power devices the power amp uses big discretes on heat sinks.
Assuming the speaker impedance is constant with drive level (a reasonable assumption over normal ranges), then the power is proportional to the square of voltage just as if the speaker were a simple resistor: P = V^2 / R If you double the voltage, you get 4 times the power, which is
+6 dB. Lower-impedance speakers (4 ohm nominal instead of 8 ohm, for example) will give you more power.
SPL is an absolute measurement of sound power, so the efficiency of the speakers enter into it, as well as the geometry and the room. The sound measured at a particular point in space will be louder if the speaker is beaming most of its energy there, versus sending it off into other unmeasured directions. Speaker efficiencies are usually rated as the SPL output you get for a 1 watt input, measured at 1 meter from the cone. Or instead of 1 watt, they may use a particular voltage that gives a similar output. Typical values are around 90 dB SPL.
Power amp gains are typically set so a "line level" of about 1 VRMS will drive them to their maximum output, which depends mostly upon the power supply rails... you size your devices to handle the rails.
Hope this helps!
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
Thanks for the link, I've never heard that term before. I kinda figured it was being used to "dump" higher frequencies to ground, but didn't know if it was an oscillation preventer or just some kind of tilt compensation (though I would have expected that the amp would have lost gain as frequency increased anyway, so I didn't understand bypassing the higher frequencies to ground). I wasn't thinking about the changing (rising) impedance of the load causing an increase in gain and leading to potential oscillations.
John Popelish wrote in news: snipped-for-privacy@rica.net:
Thanks for the pointer. I have more questions though.
What is the use/significance, and how to use these pins:
BYPASS pin on LM380/386
FREQ COMPENSATION, RIPPLE REJECTION, and BOOTSTRAP pins on TBA20M
BOOTSTRAP, TRESHOLD, MUTING, and SVR pins on TDA1905
On TBA20M, there's 2 application circuits which shows 2 alternatives on connecting the load (i.e. speakers): to the supply or to the ground. Why two alternatives? What are the strengths and trade-offs of each alternatives? And does anybody care to point me to some web resources on how to drive a speaker (apart from the amplifier stage), what are the required interconnections and percautions, and how it works.
This may be basic, but how to calculate the output power of an op-amp network? I mean, with those extra precautions in place (like the Boucherot's filter and DC gain compensation capacitors) things get a little hairy for me.
Last question (for the moment), I really need to know the relationsip between input power (or voltage/current) to SPL to estimate how much power need to be fed from the amps to the speaker. I don't really know whether 1W, 2W, or 6W is enough for me. My only guide is by vague memory that the old SB16 cards uses 4W amps, and I'm not really sure.
snipped-for-privacy@daqarta.com (Bob Masta) wrote in news: snipped-for-privacy@news.itd.umich.edu:
Care to elaborate on this? I've noticed that the power amp op-amps have their gain preset. Do you mean that to drive them to max output, you need
1 VRMS input? Take TBA820M for example, it's gain is preset to 20 (if not mistaken), then a 1 VRMS input will give 20 VRMS output, and the power on the 8 ohm speaker is then (20^2)/8 = 50W? But it says that TBA820M is a
2W power amp (on 8 ohm load and 12V rails).
John Popelish wrote in news: snipped-for-privacy@rica.net:
I mean what is the guidelines of connecting the amplifier stage to the speakers. What are the precautions. Like in text books (ones I read at least) when explaining op-amps, it only explains the op-amp in both open and close loop configuration. They seldom explains about the capacitors networks for unity gain only for DC range, or baucherot's filter for example. They only explains op-amps in terms of cute triangles with 2 input pins and output pins, and resistor feedback networks. This made me confuse when comparing real schematics with what I get from the textbook.
Yes, in theory the only thing needed to couple the amp stage to the speakers are a single wire. But from what I saw in the app circuits of the datasheets, there are capacitor networks involved as well. This is what I want to know, or if there are some resources out there explaining all the parts (this device/network do what) step by step of a simple power amp (like the app circuit of TBA820M).
What I meant is does those precaution networks, like baucherot's, will be involved in the equation, cause the speaker is connected after it. By your reply above, it's still unclear to me, what voltage and amperes to use. Is it the input voltage times the op-amp network gain (for voltage), and the rated output current from the datasheet?
Each amplifier design may have different requirements.
Only for signals that don't saturate the amplifier, For a single supply amplifier, like the TBA820M, the largest voltage the amplifier can impress across the speaker is half the supply, minus the output device saturation voltage. A general estimate for the saturation voltage is about 1.5 volts. So if you power the amplifier with 12 volts, the largest voltage it can impress across the speaker is about
12/2-1.5=4.5 volts. If the speaker has an 8 ohm impedance, the implies a peak power of 4.5^2/8=2.5 watts. If that peak is part of a sine wave signal, the average power is half that. If the amplifier has a gain of 20, this implies that the output will clip if the peak input voltage is greater than 4.5/20=.225 volts.
Sorry for the confusion. I was speaking of discrete power amps, like for home stereo. But the numbers are about what you mention: a gain of 20 or so. Most home stereo discrete amps are in the 50-100 watt range.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
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