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Old 24th October 2013, 12:11 AM   #1
jimminy is offline jimminy  United States
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Default Leveling line signals to power amp

This is probably too basic to be interesting for most people around here, but I have a number of devices I'm feeding into a 100W power amp (all from their headphone out jacks) driving a set of speakers for my home audio system and I'm having trouble getting the relative levels of all these devices balanced.

My TV, a couple different MP3 players, and a computer all seem to have different headphone out levels, so one might be turned way up and others down or in the middle for me to hear them at a similar volume.

So the goal is to have the amp's volume static and control the volume on the individual devices using their remotes. Ideally, having the volume somewhere in the middle on all the devices would result in roughly the same volume overall.

The Amp is has auto-sensing and will turn itself on only when there's a signal, so made to be stashed in a closet and forgotten.

Anyway, I think I need something to boost the low signals and attenuate the high ones. I was looking around and found some options and would like to know if anyone here can tell me if one would work or if they have a better solution. Below are the things I found but not sure if they are appropriate to what I'm going for. Thanks for any advice.

DC Input 12V 2 Channel Stereo Power Amplifier Module Kit 1W Based on TDA2822M | eBay

Low Voltage Audio Stereo Amplifier Module Based on NJM2073D | eBay
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Old 24th October 2013, 04:17 AM   #2
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Quote:
Originally Posted by jimminy View Post
So the goal is to have the amp's volume static and control the volume on the individual devices using their remotes.
Isn't that what you have now? Not sure I understand.
Anyhow, the links you provided are for small low-voltage power amps; that won't work*.
The only solutions I'm aware of are 1) an automatic gain control (AGC) circuit, 2) a compressor/expander circuit, or 3) a multi-input preamp with each input individually processed to equalize the output level.

*"Won't work" may be too extreme. They could be used as a variation of #3 above. Less than ideal, but they could be used as preamps for the sources with lowest output. Some, like the TV and computer, should be capable of level matching without additional circuitry.
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Old 24th October 2013, 08:41 AM   #3
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Use a little mixing desk to set the levels. Compression and expansion will make everything sound over processed, much like a CD.
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Old 25th October 2013, 12:46 AM   #4
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Yeah, that was poorly worded.

What I meant to say is that the goal is the set the volume control on the power amp once, stick it somewhere out of sight, and control volume using the volume controls on the individual devices.

Each device seems to have fairly different gain levels coming out of their headphone jacks and I would like to be able to compensate for that to even them out a bit.

What I'm describing is basically mixer, with controls to boost or cut each stereo channel to what would be a good strong headphone out level for probably 4 devices.

I hope that's more clear. Thanks again.
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Old 25th October 2013, 03:50 AM   #5
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I dug around the forums a bit more and found this , which seems like someone with a similar issue using small transformers to get the impedance corrected to get a better signal to the power amp.

So, if I use this transformer solution to correct the impedances of each device, will that level out the different signals going into the power amp or do I still need a way to adjust the level of each stereo channel with a pot in the signal chain after the transformers?

I get that the impedance of the headphone outputs need to better match the amp's input and that will give me a stronger signal, but I don't know if that will correct the difference in volumes between the devices or just give me better sound but still very different volumes when I switch between them.
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Old 25th October 2013, 04:14 AM   #6
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It isn't just the impedances (which to me is questionable and/or confusing, eg. there are plenty of low output impedance preamps driving high input impedance amps) but the voltage across those impedances.
If you connect a pair of those transformers to each source device, you'll still have the large difference in signal strength. You'll only have shifted it all to another level.
DIY solution... an op amp for each device output channel, with the op amp gain adjusted for each device so all outputs are more or less equal, i.e. #3 above. Use op amps that are stable at low gains because you won't need much. Of course the device with the strongest output signal doesn't really need to be tweaked - it can be the target level for the other outputs. Or you could buy preamps to do the same thing.
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Old 25th October 2013, 04:17 AM   #7
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DIY it's very easy to feed each input through a pre-amp, each pre-amp can be gain adjusted to suit and the outputs summed together then fed to the power amp.

Basically that is what a mixer is comprised of.
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Old 25th October 2013, 04:27 AM   #8
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If you look at E-Bay item --- 121196799893 --- This is such a pre-amp. Use one per input.

All you then need to do is feed each output via a 2K2 resistor to the input of the power amp. Ideally you also need a 2K2 then between the commoned resistors coming in and 0V.

This is known as a summing circuit, just connecting all the pre's together wont work satisfactorally.

If you google "summing amplifier" you will see the way this works...Op Amp Summing Amplifier

Last edited by KatieandDad; 25th October 2013 at 04:34 AM.
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Old 25th October 2013, 08:10 AM   #9
jimminy is offline jimminy  United States
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It's a lot less fun, but this might be an easy off-the-shelf solution, and probably cheaper.

I figured I would end up getting 4 kits like katieanddad's ebay suggestion and then do the summing circuit exactly as described but maybe not worth the effort. Or maybe I can still do the impedance thing and be able to get my hands dirty after all. If I do, would it best to add those transistors to each stereo channel before the mixer or just do it to the summed stereo channel after the mixer before the amp?
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Old 25th October 2013, 01:47 PM   #10
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As a lot less fun off-the-shelf solution, 3 of those Rolls mixers will do the job.
A very good DIY solution (#3 again) could be built for half that price, or less.
FWIW, a "mixer" isn't really what you need, unless the plan includes playing more than one source at a time. Though the circuit is virtually the same, the function you need is isolation of the four variable stereo outputs. This could be achieved with a 4-position switch.
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