Xtreme Noob - Summing Channels For Mono 3875

CarlosT · 6th November 2006, 04:48 PM

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

pinkmouse · 6th November 2006, 06:13 PM

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.

CarlosT · 6th November 2006, 07:31 PM

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?

Bob0513 · 7th November 2006, 03:01 AM

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.

CarlosT · 7th November 2006, 03:19 AM

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?

pinkmouse · 7th November 2006, 08:13 AM

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.

CarlosT · 7th November 2006, 04:08 PM

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!

Bob0513 · 7th November 2006, 04:34 PM

Here is a scematic of what you can use to sum the inputs and have a volume control potentiometer.

pinkmouse · 7th November 2006, 04:37 PM

Yup, and then just use the schematic for the inverting amp in the datasheet. You have downloaded it already haven't you?

CarlosT · 7th November 2006, 05:31 PM

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

6th November 2006, 04:48 PM	#1
CarlosT diyAudio Member Join Date: Oct 2006	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

6th November 2006, 06:13 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

7th November 2006, 08:13 AM	#6
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

7th November 2006, 04:37 PM	#9
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Yup, and then just use the schematic for the inverting amp in the datasheet. You have downloaded it already haven't you? __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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6th November 2006, 07:31 PM	#3
CarlosT diyAudio Member Join Date: Oct 2006	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?

7th November 2006, 03:01 AM	#4
Bob0513 diyAudio Member Join Date: Feb 2006 Location: home	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.

7th November 2006, 03:19 AM	#5
CarlosT diyAudio Member Join Date: Oct 2006	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?

7th November 2006, 04:08 PM	#7
CarlosT diyAudio Member Join Date: Oct 2006	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!

7th November 2006, 05:31 PM	#10
CarlosT diyAudio Member Join Date: Oct 2006	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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