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Old 21st April 2011, 05:51 AM   #1
ddietz is offline ddietz  United States
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Default Guitar headphone amp with aux input questions.

On to my next project. I'd like to build a simple electric guitar practice headphone "amp". Something to use on my wife's guitar later at night. I've seen this recommended several places:
http://www.generalguitargadgets.com/...eamp/lm386.gif

Seems easy and small. The crutch is I'd like to be able to also mix in a cd/mp3 input for playing to. Can I use a simple passive mix at the lm386 parts output, using resistor (ive seen 680ohm to 4k7recommended for this) andd a 10k pot to mix the cd/mp3 source? If not, what is the simplest method. I don't need the cd/mp3 amped or with hifi sound.

Part 2 of all this is I have a couple opa2132/4 and extra CMOY parts laying around. I tried the guitar on a 2228 based CMOY tonight and it worked so-so. I know the impedance and such was wrong and the gain was insufficient, but it sounded good actually with little noise. Could i modify a typical CMOY to work for this? Perhaps changing the input resistor of the CMOY to 1M ohm and boosting the gain to say 20x or even higher? I know 21x works "fine" in a CMOY as I tried I for fun.

Same question applies redarding mixing in an aux source if theodified CMOY will work.

Thanks in advance!
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Old 21st April 2011, 06:21 AM   #2
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I've used one of those LM386 chips to make an amp for a friend... realy easy, but I suggest you look at the datasheet and add the optional gain boost components and a switch....Description"The LM386 is a power amplifier designed for use in low voltage consumer applications. The gain is internally set to 20 to keep external part count low, but the addition of an external resistor and capacitor between pins 1 and 8 will increase the gain to any value from 20 to 200."LM386 - Low Voltage Audio Power Amplifier
Cmoy can put out like 40mA combined into a short circuit... not alot of power to drive a speaker...

Last edited by digits; 21st April 2011 at 06:24 AM.
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Old 21st April 2011, 06:24 AM   #3
ddietz is offline ddietz  United States
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I had planned to do so. Thanks!

Any thoughts on mixing the two sources simply?
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Old 21st April 2011, 06:35 AM   #4
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I'd split them with a buffer, I don;t think whatever the other source is is going to enjoy the output from a pickup on its business end.
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Old 21st April 2011, 01:49 PM   #5
ddietz is offline ddietz  United States
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Thanks. So, what makes a good buffer. Ive found this on on this forumhttp://www.italentshare.com/storage/Jam%20Along%20Headphone%20Amp/Jam%20Along%20Amp.jpg.

I've also seem a simpled buffer for the cd/line in that did not use an opamp. Most designs I've seen show using 10k resisters in series with the aux lines and paralleled with the guitar into the lm386. I gather 10k is sufficient to prevent signal mixing problems while adjusting for differences in input strength between the line in and pre-amplified guitar signal.

What do you mean by buffer?

Thanks again!
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Old 21st April 2011, 05:50 PM   #6
ddietz is offline ddietz  United States
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Would a good solution be to combine the top right, simple virtual ground active mixer with buffer at this link:
http://www.all-electric.com/schematic/simp_mix.gif

and feed the output of that in parallel with the guitar to the lm386 amp? I would connect tip and ring of aux in together for mono.

If I went that route, can I use a 2132/4 for the buffer opamp, using one side of the 2134 to buffer the other?

The problem is, won't that cause lots of distortion to the aux input If I use high gain on the lm386? I'd like to avoid that if possible. Using the 2134 buffered to mix after the lm386 would also work right, and solve the cd distortion issue?

In reality, since the low end, undistorted gain of 20 on the lm386 is easily achievable gain on the 2132/4, wouldn't the simplest sulution be to use a normal CMOY circuit but change the input impedance of it, R2 on tangent style layouts? Maybe 1M resistance tongroumdnat the guitar input to the + pin on the opamp. I lile that solution since I have all the parts and boards I need to do it. If that is a solution, should I only use one channel of the 2134, buffer with the other side, or feed the guitar into both halves of the 2134?

Sorry for all the questions

Would the output of the lm386 and and aux source like a CD player have similar levels/impedance, allowing for passive mixing after the lm386?

Thanks!

Last edited by ddietz; 21st April 2011 at 06:05 PM.
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Old 21st April 2011, 06:15 PM   #7
johnr66 is offline johnr66  United States
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A guitar is a low level high impedance source, while a CD/MP3 player would be more like a line level low impedance source. Buffers would be ideal, but you may be able to work without them.

The LM386 has a 50K input impedance, so using summing resistor of any more than 50k is not ideal as the input voltage would be cut in half when using 50k resistor (neglecting guitar's impedance). Using the 386 in high gain mode, try around 20k summing resistors. The line input for the player would need an additional voltage divider or pot to reduce its level prior to summing.
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Old 21st April 2011, 06:24 PM   #8
ddietz is offline ddietz  United States
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Thanks John for the reply.

Why can't I use summing resistors after the lm386, to connect the 386 output and the cd player output? I don't really need to amp the cd/mp3 player and if I did, I could use a CMOY. Since no one seems to donor this way, there must be a reason, I'm just not experienced to know why.
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Old 21st April 2011, 09:36 PM   #9
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You can do this, but the isolation between the driving amps will be poor, each will drive the other, particularly as you will have to keep the resistances in the passive mixer small in order to avoid wasted power, excess heat generated and poor damping factor in the speaker drive. Best is to make an active mixer (summing opamp) as in the diagrams you have shown, before the 386, be aware that they require a split-rail power supply, although you can use a single rail with virtual ground, but again this is not ideal.

w
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Old 21st April 2011, 09:55 PM   #10
ddietz is offline ddietz  United States
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Wakibaki (great name by the way), thanks for your reply.

A couple of questions, regardless of the mixer type ( I can accept that active buffered opamp is best), why place it before the lm386. That could add lots of distortion to the cd signal.

Second, will a 2134 work for the mixer using it buffered and obviously in mono?

Thanks again.
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