Xtreme Noob - Summing Channels For Mono 3875

[CarlosT]

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]

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]

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]

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]

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]

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]

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-amp-parts/FenderAmpKnobNo.One.gif

Tell me more...thanks!

 

[Bob0513]

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

 

[pinkmouse]

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

 

[CarlosT]

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

 

[CarlosT]

...Headphone out is likely to be a bit more than line level, so I'd start with a gain of about 15...

Actually I recently drove an Adcom GFA5200 with a CD player directly from the headphone jack and then with an Adcom GTP350 pre-amp and the pre-amp drove the amp much harder. Or am I thinking about this the wrong way?

 

[Bob0513]

He means that the headphone jack will provide more power than the line-level jack. The line-level jack powered by a pre-amp may provide more power than the headphone jack.

 

[CarlosT]

I like how you guys translate for each other

 

[pinkmouse]

OK, you could have mentioned you had a kit earlier! You will need to put a opamp in first, with the virtual earth circuit at it's input. Then you have the volume control, then the chip amp board. I'll post a schematic later when I find it, or you can google.

 

[CarlosT]

Thanks, Al. So you're saying that using the LM3875 kit pcb negates your simple 10K Ohm series and summing schema as a solution? I was starting to dig it...the simplicity of it

 

[CarlosT]

How does inverting vs non-inverting have anything to do with summing stereo channels?

Thanks.

 

[pinkmouse]

Have you googled yet? There are plenty of good sites out there that will help, and I don't have time to type out a reasonable explaination.

 

[CarlosT]

I usually Google the sh_t outta stuff. What are my keywords at least, Pragmatic Al? You're asking me to Google how to slice off my own silicon and mica wafers for homemade transistors?

I actually searched the topic here extensively but most cool threads here die quickly yet the ones expounding one's prowess seem to go on forever...strange forum this DIYaudio

 

[Nordic]

Texas is that away...

 

[pinkmouse]

http://www.diyaudio.com/forums/showthread.php?postid=1050285#post1050285