| 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 :rolleyes:
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 :D |
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| 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. |
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| 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? |
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| 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. |
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| 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? |
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| 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. |
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| Bob0513 |
| Here is a scematic of what you can use to sum the inputs and have a volume control potentiometer. |
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| pinkmouse |
| 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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| 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 :D |
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| CarlosT |
| quote: | Originally posted by pinkmouse
...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? :D |
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| 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. |
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| CarlosT |
| I like how you guys translate for each other :D ;) |
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| 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. |
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| 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 :D |
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| CarlosT |
How does inverting vs non-inverting have anything to do with summing stereo channels?
Thanks. |
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| 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. ;) |
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| 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? :D
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 :D |
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| Nordic |
| Texas is that away... |
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| CarlosT |
| Oh...OK...circular paths...you got me going in circles, Al. Thanks, man...touching :D |
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| waltona |
Hi CarlosT,
Are you sure your kit doesn't have 2 LM3875's? As far as I know AudioSector kits have all the boards and components to build a stereo amp (apart from transformer and pot). At least mine did!
Al |
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| CarlosT |
Yes...but in my application, I'm splitting the kit into two separate mono kits. I wanna build a retro cabinet with a single driver. The problem lies in that the iPod puts out a stereo signal thhrough its common ground 1/8" stereo jack and I need to combine them to drive the SINGLE LM3875. This thread started with the suggestion of a simple passive "resistive Y" to combine L-R positive signals but then Pinkmouse recanted because the Audio Sector kit is a non-inverting LM3875 kit. Supposedly this inverting vs non-inverting issue is the showstopper. It went on from there to highlight my ignorance and to set me on my path to discovery :D
The most promising thing so far appears to be this SSM2141 chip but I'm still just marginally more savvy than when I started this thread.
ANY HELP IS APPRECIATED! |
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| pinkmouse |
| That's for balanced lines Carlos, and is no good for your application. All you need is a basic opamp, and three 10K resistors. Two on the left and right input lines connecting to the -input of the chip, and one connecting between the -input and output. +input goes straight to ground. Simple. ;) |
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| Bearman |
| If you read the datasheet, it can be also used for a summing chip and the resistors are in the chip. |
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| CarlosT |
Thanks, Bearman...that seems very similar to the SSM2141 except that the SSM2141 is supposedly cleaner running.
Al...thanks...I'm still confused.
What happens in an OP amp if the feedback loop is switched from the negative to the positive input? |
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| Bob0513 |
| You don't need any special chip. You can sum the signals with two resistors like we stated in the beginning of the thread and pass that through a op amp to strengthen the signal a bit. You are making this way more complicated that it needs (should) be. |
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| pinkmouse |
| quote: | Originally posted by CarlosT
Al...thanks...I'm still confused.
What happens in an OP amp if the feedback loop is switched from the negative to the positive input? |
That is getting quite involved. If you want to know theory, your best bet is to download Walt Jungs's Opamp Applications from the Analog Devices website. He explains it much more lucidly than I could. |
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| CarlosT |
I read the Wikipedia stuff on virtual grounds and it's all fascinating stuff.
I just wondered how compatible Fig. 3 of the attached would be with the Audio Sector non-inverting LM3875.
http://www.analog.com/UploadedFiles...ets/SSM2141.pdf
Fig. 3 shows a very neat summing circuit with zero external parts. The problem is that it's non-inverting.
Can someone please look at it besides just spewing patronizing wisdom? Thanks! |
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| Bob0513 |
Carlos,
I don't want to sound mean, but why ask a question and then ignore the answers? The simplest and most effective way to sum 2 signals, have volume control, and invert that signal is with the basic resistor divider mentioned back on page 1. I attached a schematic of the whole thing. |
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| pinkmouse |
| Sorry Carlos, I don't have the time to go through schematics for you. I gave you a workable option, which Bob illustrated above, and references if you want to take it further. The rest is up to you. Build them and see what you prefer! ;) |
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| CarlosT |
| quote: | Originally posted by Bob0513
Carlos,
I don't want to sound mean, but why ask a question and then ignore the answers?... |
Bob your answer from Page 1...the simple passive resistive Y would not work according to Pinkmouse because of the non-inverting schema of the Audio Sector LM3875 kit. The schematic that you just posted is closer to what appears to be the right answer but it basically took my grovelling for 4 pages worth of forum and countless hours of Googling to answer my own question. That wasn't too efficient.
I appreciate you hanging in there though and continuing to respond. |
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| CarlosT |
So after considering the mounting costs and complexity of using the SSM2141/INA134 buffer in front of the AudioSector non-inverting GC, I'm thinking of maybe using the Techdiy inverting GC kit instead.
Al et al. would you say that it's OK to just use the simple "resistive Y" to combine the channels then?
Where would you place the volume pot? I was eyeing up the spot where Jacknnj has the little trim pot. |
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| Bob0513 |
Whats wrong with the solution I gave 2 posts ago?
Bob |
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| CarlosT |
| Bob...I was trying to save the money and additional complexity of building another board and having to step down my 25+- supply down to 18V or 15V. Supposedly the original problem posed by Pinkmouse was that the non-inverting GC kit from AudioSector was not well geared towards taking combined channels. |
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