| Author |
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Ricky Romaya
Guest
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 Posted:
Sat Jan 01, 2005 11:04 pm Post subject:
NEWBIE: Designing a Simple Audio Amp |
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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.
TIA
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John Popelish
Guest
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| Posted:
Sat Jan 01, 2005 11:58 pm Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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Ricky Romaya wrote:
| Quote: |
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)?
|
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.
| Quote: | 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.
|
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.
Some examples:
http://www.national.com/ds/LM/LM380.pdf (one of the most common)
http://www.national.com/ds/LM/LM386.pdf (low voltage version of the
LM380)
http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/TDA1905.pdf
http://rocky.digikey.com/WebLib/Panasonic/Web%20data/AN7513.pdf
http://rocky.digikey.com/WebLib/Panasonic/Web%20data/AN7511.pdf
--
John Popelish |
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John Popelish
Guest
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Graham Knott
Guest
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Anthony Fremont
Guest
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| Posted:
Sun Jan 02, 2005 1:21 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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"Graham Knott" wrote:
I have a question about your circuit. R2 and C3; what is their
function? |
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John Popelish
Guest
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| Posted:
Sun Jan 02, 2005 7:23 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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Anthony Fremont wrote:
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.
See Zobel network on this page:
http://www.rane.com/par-z.html
--
John Popelish |
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Bob Masta
Guest
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| Posted:
Sun Jan 02, 2005 9:03 pm Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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On 01 Jan 2005 17:04:46 GMT, Ricky Romaya <something@somewhere.com>
wrote:
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.
| Quote: | 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.
|
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
www.daqarta.com |
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Anthony Fremont
Guest
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| Posted:
Sun Jan 02, 2005 9:22 pm Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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"John Popelish" wrote
| Quote: | Anthony Fremont wrote:
"Graham Knott" wrote:
John Popelish wrote:
Forgot this one (I just bought 10 of these so I should have
remembered
it)
http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/TBA820M.pdf
Have a look at
http://homepage.ntlworld.com/g.knott/elect24.htm
I have a question about your circuit. R2 and C3; what is their
function?
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.
See Zobel network on this page:
http://www.rane.com/par-z.html
|
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. |
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Ricky Romaya
Guest
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| Posted:
Sun Jan 02, 2005 10:32 pm Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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John Popelish <jpopelish@rica.net> wrote in
news:41D6E63A.3E7A85DE@rica.net:
What is the use/significance, and how to use these pins:
1. BYPASS pin on LM380/386
2. FREQ COMPENSATION, RIPPLE REJECTION, and BOOTSTRAP pins on TBA20M
3. 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.
TIA |
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John Popelish
Guest
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| Posted:
Mon Jan 03, 2005 12:14 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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Ricky Romaya wrote:
| Quote: | What is the use/significance, and how to use these pins:
1. BYPASS pin on LM380/386
|
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.
| Quote: | 2. FREQ COMPENSATION, RIPPLE REJECTION, and BOOTSTRAP pins on TBA20M
|
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.
| Quote: | 3. BOOTSTRAP, TRESHOLD, MUTING, and SVR pins on TDA1905
|
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.
| Quote: | 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?
|
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.
| Quote: | 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.
|
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.
| Quote: | 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.
|
Instantaneous power to the speaker is amperes times volts. The
speaker impedance tells you the ratio of volts per ampere.
| Quote: | 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.
|
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.
--
John Popelish |
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Rich Grise
Guest
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| Posted:
Mon Jan 03, 2005 8:14 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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|
On Sun, 02 Jan 2005 13:14:54 -0500, John Popelish wrote:
| Quote: | Ricky Romaya wrote:
....
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?
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.
|
From my intuitive analysis of the two figures and the schematic,
The chip would rather drive the speaker that's returned to the positive
supply, because it just feels better that way. ;-)
....
| Quote: | 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.
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.
|
Any reasonable speaker, i.e. not an audiophool model, can fill a room
uncomfortably loud with one watt. Well, maybe you need a really good
enclosure. :-)
I vote for the TBA820M 'cause it's the coolest. :-)
Cheers!
Rich |
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John Popelish
Guest
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| Posted:
Mon Jan 03, 2005 11:42 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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Rich Grise wrote:
| Quote: | I vote for the TBA820M 'cause it's the coolest. :-)
|
Especially if you glue a piece of aluminum on top of it. ;-)
--
John Popelish |
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Ricky Romaya
Guest
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| Posted:
Thu Jan 06, 2005 12:59 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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NoSpam@daqarta.com (Bob Masta) wrote in
news:41d80947.1263957@news.itd.umich.edu:
| Quote: | On 01 Jan 2005 17:04:46 GMT, Ricky Romaya <something@somewhere.com
wrote:
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.
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).
TIA |
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Ricky Romaya
Guest
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| Posted:
Thu Jan 06, 2005 12:59 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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John Popelish <jpopelish@rica.net> wrote in
news:41D83A1E.CDC853EC@rica.net:
| Quote: |
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.
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.
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).
| Quote: |
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.
Instantaneous power to the speaker is amperes times volts. The
speaker impedance tells you the ratio of volts per ampere.
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?
TIA |
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John Popelish
Guest
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| Posted:
Thu Jan 06, 2005 7:08 am Post subject:
Re: NEWBIE: Designing a Simple Audio Amp |
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|
Ricky Romaya wrote:
| Quote: |
John Popelish <jpopelish@rica.net> wrote in
news:41D83A1E.CDC853EC@rica.net:
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.
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.
|
Those details are amplifier specific.
| Quote: | 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).
|
Each amplifier design may have different requirements.
| Quote: |
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.
Instantaneous power to the speaker is amperes times volts. The
speaker impedance tells you the ratio of volts per ampere.
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?
|
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.
--
John Popelish |
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