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Old 6th November 2006, 04:48 PM   #1
CarlosT is offline CarlosT  United States
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Default Xtreme Noob - Summing Channels For Mono 3875

I just ordered a 3875 kit and I want to build a small single powered speaker for a kid to plug in his iPod. The problem is that the iPod as all MP3 players has stereo output. What is the simplest way to combine the channels to feed a single 3875?

I also need help picking out a pot volume control. I tried to follow a thread on the PEC pot but got lost by page 12

How much output does an MP3 player put out? Would the gain be set higher in the 3875 for this application?

The speaker can only handle about 30W-50W or so.

Thanks for all the help...I need LOTS of it. I'm an artist...not a scientist
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Old 6th November 2006, 06:13 PM   #2
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Use the inverting schematic in the datasheet, and connect each input via its own 10K resistor. Headphone out is likely to be a bit more than line level, so I'd start with a gain of about 15. Start with a normal 20K or so log pot, you can always go more exotic later, but to be honest, I wouldn't worry that much for your application. You could always forget about a pot at all, and just use the volume on the ipod as normal.
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Old 6th November 2006, 07:31 PM   #3
CarlosT is offline CarlosT  United States
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I'm sorry...I'm using only one LM3875...what do you mean by "its own 10k resistor"? Both inputs go into a common pin on the LM3875? Do you have a drawing or schematic?
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Old 7th November 2006, 03:01 AM   #4
Bob0513 is offline Bob0513  United States
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He means that you the left signal and the right signal from the ipod and attach them both to the input of your amp through 10K resistors.
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Old 7th November 2006, 03:19 AM   #5
CarlosT is offline CarlosT  United States
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Would both channels going to let's say a single 25K pot work just as well? Why the need to attenuate the signal from the Ipod?
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Old 7th November 2006, 08:13 AM   #6
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You are not attenuating the signal, you are setting up a "virtual ground" mixer circuit, and the 10Ks give a high impedance to the output of the iPod. I still think you should forget the pot and go with the volume control on the iPod.
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Old 7th November 2006, 04:08 PM   #7
CarlosT is offline CarlosT  United States
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I was going for a vintage Fender look with a "chickenhead" knob...wanted to give the kids some kind cool volume control...

http://www.angela.com/catalog/guitar...KnobNo.One.gif

Tell me more...thanks!
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Old 7th November 2006, 04:34 PM   #8
Bob0513 is offline Bob0513  United States
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Here is a scematic of what you can use to sum the inputs and have a volume control potentiometer.
Attached Images
File Type: jpeg volumecntrl.jpeg (16.1 KB, 338 views)
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Old 7th November 2006, 04:37 PM   #9
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Yup, and then just use the schematic for the inverting amp in the datasheet. You have downloaded it already haven't you?
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Old 7th November 2006, 05:31 PM   #10
CarlosT is offline CarlosT  United States
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My kit is the Audio Sector LM3875 PCB...I believe that it's a non-inverting layout. What's the big deal with inverting vs non-inverting? Dam_ I ask stoopid questions
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