Help with stereo audio mixer

[jrfhoutx]

I've been looking around and am having a bit of trouble finding an appropriate circuit for my purpose. I'm a bit new to this so any help would be greatly appreciated.

I'll start by explaining the purpose of this as best I can. I want to combine two stereo audio signals (both trs style connections) into one stereo audio signal (again a trs style connection). The two audio sources are an iPod and a PSP. I would like to keep the circuit as simple and small as possible (not due to lack of skill but because of extremely limited space as all this will be combined into the compact space of the PSP).

I have seen this thread here:
Simple Stereo Audio Mixer (Two RCA in, One RCA out) - Page 2 - All About Circuits Forum

but I am not quite sure that any of these circuits meet my needs. If one of the circuits does and I have overlooked it please let me know.

One other question:

I would like to be able to control the level of each source (ie. if I want to hear more of the iPod and less PSP or vice versa), so if both sources have volume controls wouldn't adding gain/volume controls into the circuit be redundant?

 

[audioguru]

The mixer with transistors has too much voltage gain and too much distortion. it also has volume controls that you don't want.

The mixer with opamps is too complicated.

You need just two resistors per channel in series with each output and joined at the amplifier's input. Depending on the output impedance of the sound sources and the input impedance of the amplifier maybe 1k ohms will be fine.

 

[jrfhoutx]

Thanks, audioguru. would this same method work if connected to a set of headphones instead of an amp?

 

[MikeMl]

Thanks, audioguru. would this same method work if connected to a set of headphones instead of an amp?

Depends on the headphones. If you are using 4,8 or 16Ω headphones like you would use with a home stereo, then the isolation resistors should be about 10 to 20Ω. If using 35Ω-100Ω headphones like the earbuds that came with a portable stereo, then raise the isolation resistors to about 22 to 33Ω.
If using hi-z 500Ω headphones, then the isolation resistor should be ~100Ω

The isolation resistors provide a modicum of isolation between the two source amplifiers. They also define the minimum load impedance seen by either source amplifier. For example, an IPOD is designed to drive an earbud with an impedance of ~35Ω; if you connect it to a 4Ω headphone, you will likely get severe distortion because the IPOD amp cannot supply enough current to drive such a low impedance.

The best way to drive headphones from two sources is to use a "real" mixer which has a "real" audio amplifier in it... This also keeps the listening audio level from changing if one of the sources is removed...

 

[jrfhoutx]

thank you for the clarification on the impedance and resistance. now, if a 'real' mixer with a 'real' amplifier in it would be the best way, what would be the most appropriate way of doing this (is there a schematic available that I could follow and/or duplicate)?

 

[MikeMl]

thank you for the clarification on the impedance and resistance. now, if a 'real' mixer with a 'real' amplifier in it would be the best way, what would be the most appropriate way of doing this (is there a schematic available that I could follow and/or duplicate)?

First, you have to define what you want to drive, at what level? How much "fidelity"?

 

[jrfhoutx]

ok, ~16ohm, max 100mW, 20-20kHz freq range are the specs on the headphones I normally use..

 

[MikeMl]

Ok, regardless of what I suggest, AudioGuru is going to trash my suggestion. So I will just let him suggest something...

 

[audioguru]

If your ipod and PSP can drive 16 ohms and have enough output to allow for a 6dB loss then a 16 ohm resistor can be in series with each output.