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Author Topic: op amp audio amplifier circuit help/suggestion  (Read 3070 times)
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Im trying to make a simple amplifier
im using an lm324(which may change I think I might have blown it up) on a 0-12 supply
and with a 460ohm resistor from the input to the - in, and a 10k pot to the the output, + in is gnded
I had it working(very crudely) so I figured id add a voltage offset and just have a capacitor after my transistor to get my nice ac to.a 8ohm speaker
however I put a 10k pot and had the leg go to the input before the 460 ohm resistor and it didn't do anything other than kill the signal at both ends of the pot not biasing as I thought
I added that last part and know Im not getting even the crude amplification( like I said probably blown being its one am and I think I shorted something somewhere by accident), anyway im giving up on the crude design
I know there are tons of op amp circuits on the web but I was wondering what circuits you guys have used and find work well, some reason I've always had bad luck with audo circuits so simplest is better
thanks for listening
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Im trying to make a simple amplifier
im using an lm324(which may change I think I might have blown it up) on a 0-12 supply
and with a 460ohm resistor from the input to the - in, and a 10k pot to the the output, + in is gnded
I had it working(very crudely) so I figured id add a voltage offset and just have a capacitor after my transistor to get my nice ac to.a 8ohm speaker
however I put a 10k pot and had the leg go to the input before the 460 ohm resistor and it didn't do anything other than kill the signal at both ends of the pot not biasing as I thought
I added that last part and know Im not getting even the crude amplification( like I said probably blown being its one am and I think I shorted something somewhere by accident), anyway im giving up on the crude design
I know there are tons of op amp circuits on the web but I was wondering what circuits you guys have used and find work well, some reason I've always had bad luck with audo circuits so simplest is better
thanks for listening
For audio, the LM324 is not the best one... So can you tell us more about what you need it for, and if possible an explanation of your idea ?!
If its to power a small speaker maybe the lm386 will do ( you have several options as LM386N-1: 0.25W Audio Amplifier; LM386N-3 0.5W Audio Amplifier and LM386N-4 1W Audio Amplifier for example).
Normally, datasheets will have example circuits, and this Amp is a good example of a well known IC with many a circuits out there.
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If its to power a small speaker maybe the lm386 will do ( you have several options as LM386N-1: 0.25W Audio Amplifier; LM386N-3 0.5W Audio Amplifier and LM386N-4 1W Audio Amplifier for example).
Normally, datasheets will have example circuits, and this Amp is a good example of a well known IC with many a circuits out there.

For example: http://www.runoffgroove.com/ruby.html

Look around on the runoffgroove.com site for more examples / info.
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Hi,
   I can second the lm386, Its a great little amp on a chip. I had one driving a PC Speaker from an iPod nano using the very minimal circuit suggested in the data sheet, it's hard not to be impressed with the sound given it takes about five minutes to build. I also have one on the output of my Audino, again it's driving a PC speaker.

If you still have the lm324, you can make a nice vu meter for your 386 amp.

Duane B

rcarduino.blogspot.com


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The speaker im driving is an 8ohm 75w, 6" but the main purpose is more a learning experience of a general purpose audio amp
how can I use the lm324 for a vu meter?
And for audio is it better to get a dual supply somehow and use an according opamp or to use a dc offset until it gets to the output?
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In that case, you need something much more powerful , and a nice well known would be the LM3886 ( as it deliveries 68W, close to the 75W, you mentioned, ).
But mind you, we are talking about a much more complicated project, with a double rail PSU. If it is a first( or one of the first) projects, id advise you to get a kit with it, including of the PSU, as at least you will have a tested circuit, wont have much space for mistakes as it will probably be quite well documented,and have others experience to learn and avoid their previous mistakes. Cause in this case we are talking about messing with AC and High voltage here... So unless you have a good basis on electronics, id deffo recommend the KIT, or start with something small( like the LM386 as i mentioned).
It is not enough to know how to do it but why it is done as it is...
After the 386 maybe a TDA2030 ( which is already a double rail one), and if your intention is to learn and experiment a good bench PSU would be desirable.
Hope it helps.
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Yeah im gonna need one of those soon, my current homeade supplies aren't keeping up with me,
is there a similiar device that would work with 12vdc as the sole supply? Im thinking most likely my project will be powered off a car battery and hopefully be useful (as an amp I can stow in my friends pickuptruck bed for some mobile music outdoors)
im not terribly worried about perfect quality so long as it doesn't horribly distort, I was using the 324 as a preamp basically to drive an npn transistor rated for 8a that I found that actually said it was meant for audio
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Yeah im gonna need one of those soon, my current homeade supplies aren't keeping up with me,
is there a similiar device that would work with 12vdc as the sole supply? Im thinking most likely my project will be powered off a car battery and hopefully be useful (as an amp I can stow in my friends pickuptruck bed for some mobile music outdoors)
im not terribly worried about perfect quality so long as it doesn't horribly distort, I was using the 324 as a preamp basically to drive an npn transistor rated for 8a that I found that actually said it was meant for audio
As i said the LM3886(do not confuse with lm386) is a audio power amp that is quite well spread and deliver 68W onto 4 ohms, 34 to 8ohms.. It can be used with a single supply voltage (just double it-ex:24V+/0V/24V- = 48V). Datasheet will have an explanation of a basic circuit, but theres loads functional others online, im sure. Normally id not recommend breadboarding this, mind, as most probably wont even get to work in a breadboard(though u might get away with a stripboard if u follow closely the datasheet. That is what they are there for.
But, wouldnt hurt you get some experience behind you with other simpler formats, mind, or just buy a kit as i also mentioned and learn from it by researching !.
Pro audio electronics is not as easy as breadboarding a circuit for arduino. Far from it !! Specially remember of keeping safe as we are talking about possible instances where these type of voltages might cause permanent damage.
Let us know how it goes.
There are these nice documenting videos and instructional at same time by this CAT that might guide you as he will give you a lot of insight into the theory while building things himself, or experimenting ( His owner is just his helper in building the stuff lol).
Subwoofer amps, amd amps included http://www.youtube.com/user/vegmatic1966 .
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Forgot to say- you can always use an inverter in the car. Or just have a lower Voltage that will also lower the power, of course.
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Hi,
  For the vu meter a google search for lm324 vu meter circuit returns many results. It's a very simple circuit but if you have a few lm324's you can add filters to give you vu meters for different frequency bands and some very basic music visualization. There is also a dedicated vu meter ic the LM3916, I have both, but have only used the 324 so far.

Duane B

rcarduino.blogspot.com


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I've used the lm3916, there pretty cool
That link is nice, thanks for the insight
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and with a 460ohm resistor from the input to the - in, and a 10k pot to the the output, + in is gndedI had it working(very crudely)
An op-amp needs negative feedback to control/set the gain.   A theoretical op-amp has infinite gain.   Real op-amps have gain somewhere in the thousands.  So, a pair of feedback resistors are used with the gain  set by the ratio.   With too much gain (or no feedback), you get a full-power square wave (severe distortion).   Positive feedback can cause cause oscillation (like "feedback" in a PA system), and that's usually a bad thing,.. unless you are trying to build an oscillator.


With a 10K pot on the output, between the LM324 and the speaker, the voltage is going to be divided between the pot (depending on its setting) and the speaker.     With a 10k Ohm pot and an 8-Ohm speaker, most of the voltage (signal) is going to be dropped across the pot and you won't hear anything.    With a 10k pot cranked to full-volume (minimum resistance), it still might measure more than 8-Ohms, since pots are not perfect and relative to 10,000 ohms, 8-ohms is practically zero....

The LM386 has built-in feedback resistors, so you don't need to add them.  You may still want a pot on the input.

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...to a 8ohm speaker
The LM324 is not designed to drive a speaker.  If you use Ohm's Law to calculate the current through the speaker, you will find that the LM324 is "trying" to put-out about 500mA.   It's only rated for 10 or 20 mA.  You could fry it!

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so I figured id add a voltage offset and just have a capacitor after my transistor to get my nice ac...

...And for audio is it better to get a dual supply somehow and use an according opamp or to use a dc offset until it gets to the output?
You can do it either way.  Usually, you are find dual supplies used, except where you are running of a battery.    But, I have a 50W power amp that's got a single supply.    With a single supply, you need a capacitor on the input & output.   On a power amplifier, you need a physically large high-uF, capacitor to get low frequencies (bass) into the speaker.   With dual supplies, you can get "bass" down to zero-Hz (DC).   Not that you'd want  DC actually going to the speaker...  And of course, with a dual power supply, you need an extra capacitor in the power supply, so basically with a stereo amp, you can save one large capacitor by using dual supplies.

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how can I use the lm324 for a vu meter?
As I mentioned...  Op-amps have "infinite gain".

If you run it open loop (maximum gain) the output will be either on or off.   If the + input is greater (more positive) than the negative input, the output will go full-positive.

If the +input is less than (or more negative) than the -input, the output will go full-negative.

So, if you put a "reference voltage" on the -input, and put an LED on the output, the LED will come-on whenever the + input goes above the reference.  (Or, it will go off if you wire the LED the other way).

   
This circuit is called a "comparator".  With several different reference voltages (or a voltage-divider ladder) and several op-amps wired as comparators, you can make a VU meter!


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I had the 324 driving the base of a power transistor thru a pot with hfe 100, ce max 8amps 100v
the 324 was mainly just to amplify the voltage so the transistor would work
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Hi,
   For the VU Meter as above but in different words, you use a resistor ladder to set the trigger voltage for each LED, the LEDs are each connected to the output of one the op-amps, the signal is connected to the + input of all of the op-amps. Each op-amp then has a individual reference voltage above which it will turn on and light the connected LED, the reference voltage is determined by the resistor ladder which is simply a chain of resistors connected in series between the supply voltage and ground.

As your supply is probably much higher than you signal voltage, you use a single high value potentiometer as the first resistor in the chain, this is used to adjust the sensitivity of the rest of the resistor ladder by dropping more of less of the input voltage across the pot or the resistor ladder. After this first variable resistor, add fixed value resistors to set your trigger ratios (I had mine all equal using a 10K pot for sensitivity, then 680 Ohms for each of the reference voltage resistors, logarithmic might be better). The resistor ladder is giving us our reference voltages so we connect the -  input of each op-amp to a step in the ladder. As the input signal rises and falls so each opamp will continually be comparing it to its reference input, if it goes above the reference, the op-amp goes high and lights your LED.

Duane B

rcarduino.blogspot.com
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The VU meter is a cool idea. I did this with a stack of comparators and a resistor ladder which set reference voltages corresponding to increasing dB intervals. A good resistor kit will get you close to one resistor per value in the stack. Very cool.

As far as the Op Amp goes, if you suspect it is blown, check to see if it is passing one of the supply rails to its output. With no DCv in, it should have no DCv out. If the output is showing DCv nearing supply voltage of the a power rail, it is fried. Replace it and go again.
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