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Author Topic: question about audio output and buffering  (Read 1238 times)
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jeanninemtv
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« on: September 19, 2012, 02:00:15 02:00 »

hello,
i'm havin' some fun building a very classic light organ ( you know a 20x gain amp, 3 passband filters and leds ^^) to have lightbeating  from my mp3 player , but, i have a little doubt (using a lm324 and a LM741 it's working now the blink -to-the-music circuit Smiley ).  :

i will put in my pcb two female jacks:  one that will receive a male male cable plugged to my cellphone or mp3 player, the other must go to the audio amplifier, 


ok now the question.   can i just use splitter configuration? (i mean the same point for both jacks?)  or i'm obligated to put a buffer for L and R before send it to the jack that goes to the amp??? (in that case , do i need any impedance stuff for outputting?)


thanks in advance
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solutions
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« Reply #1 on: September 19, 2012, 02:21:24 02:21 »

Make the pickoff for your color organ branch high impedance (buffered) and you should be able to go straight through to get the same signal level and distortion as a straight MP3 to amp connection.

That said, have you tried running the MP3 player into the amp directly to see how it works out?
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jeanninemtv
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« Reply #2 on: September 19, 2012, 02:32:32 02:32 »

That said, have you tried running the MP3 player into the amp directly to see how it works out?

in fact i have no more audio jacks in this moment Sad  just a male that i use to plug and test now, so i can plug the organ to the mp3 or the amp to the mp3 but not both ^^.... Do you think a simple LM324  single supply powered buffer will be enough for output to the audio amp withouth risk (or in worstcase a headphone???) ?  I will buy the rest of stuff to assembly the board... in this moment i'm drawing my final sch prior to do a simple pcb
....

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optikon
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« Reply #3 on: September 19, 2012, 03:27:47 03:27 »

Post your schematic.
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jeanninemtv
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« Reply #4 on: September 19, 2012, 06:08:36 06:08 »

same sch as here but i replaced the single opamp by 741 and the Quad by 2 LM324 (one for each channel)

by now the only amp i got i one TDA2002 based built inside two speakers case that someone give to me few years ago ^^ ...



http://makezineblog.files.wordpress.com/2010/10/ledcolororgan-schem_r2.gif
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solutions
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« Reply #5 on: September 19, 2012, 06:17:01 06:17 »

Your input impedance for the color organ is huge - you should be fine if you tap off the run going from input jack to output jack, IMO. Try to keep the pickoff fairly short before it hits that first op amp.

If it were me, I would not be messing with the signal path to the amp, unless it needs preamplification from your MP3 source. That's why I said you should be testing a connection from the MP3 to the audio amp, sans color organ.
« Last Edit: September 19, 2012, 06:20:03 06:20 by solutions » Logged
jeanninemtv
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« Reply #6 on: September 19, 2012, 06:17:36 06:17 »

Make the pickoff for your color organ branch high impedance (buffered) and you should be able to go straight through to get the same signal level and distortion as a straight MP3 to amp connection.

btw,i was thinking about that:
 assuming the high input impedance of the opamp, can i figure myself the fact of zero input current, then this part is not really "loaded" and i can pass directly the same terminal to the output branch that is tied to the audio amp???

am i severely wrong about that???  (btw maybe i"ll verify it with crocodile clips and wire from the only jack i got available this very afternoon at my office if i have enough place ^^ )
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optikon
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« Reply #7 on: September 19, 2012, 01:54:35 13:54 »

btw,i was thinking about that:
 assuming the high input impedance of the opamp, can i figure myself the fact of zero input current, then this part is not really "loaded" and i can pass directly the same terminal to the output branch that is tied to the audio amp???

am i severely wrong about that???  (btw maybe i"ll verify it with crocodile clips and wire from the only jack i got available this very afternoon at my office if i have enough place ^^ )

The input current is not zero. It is however small enough that you wont care in this case. On the opamp datasheet, the input current will be listed as Ibias, Iinput bias or similar. For bipolar opamp input stages, this is on the order of 10s nA to uA's, for Jfet stages it can be 10s pA to 1nA and for CMOS, fA to pA (specs limited by manufacturer testing capability since it's so small)

In your application, you can tie this node to many places with little fear of loading it down. Be sure you do not exceed the input common mode voltage range and you should be ok.
BTW, for future projects, the 741 sucks --there much better *and even cheaper* devices available today if you get into more serious work. Good luck

« Last Edit: September 19, 2012, 01:57:12 13:57 by optikon » Logged

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jeanninemtv
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« Reply #8 on: September 19, 2012, 02:03:26 14:03 »

Yes of course input current is not zero but quite minimal to not being affraid of kill the rest of signal that goes to the amp ...

yes i know there's much better opamps but is the only stuff available for free at the lab ^^ ...  and for that application, is quite enough a old fashioned opamp because the application is focused for students of 1st year of electronics cours ^^  (you know because the circuit is quite simple :  gain stage , active filters and some glittering leds ^^ ...  and because the circuit is not dealing anything but rough levels, the 741 for buffered virtual gnd and the LM324 for bank filter with raw and even saturated output is quite ok for that ...   

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dotm
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« Reply #9 on: September 22, 2012, 12:38:45 12:38 »

Yes of course input current is not zero but quite minimal to not being affraid of kill the rest of signal

by using an operational amplifier with jfet input stages, like the good old TL072, you will have input currents of some picoamperes.
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sarah90
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« Reply #10 on: November 18, 2012, 07:04:45 19:04 »

The current to the opamp should not be a problem. The output of the mp3 player must be able to drive any existing op amp, how crappy it may be. I would be worried about feeding noise in from your circuit to the folowing audio part. The next stage may be high impedance as well and sensitive to noise. Grounding could be an issue as well.
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Toxible
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« Reply #11 on: November 25, 2012, 06:03:59 18:03 »

This schematic is good for you. An MP3 player can drive a headphone (64 ohm im right?) and this circuit has an input impedance > 47Kohm.... It's fine you can connect the organ and the amp at one time and there will be no problem.
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