Very simple passive mixer driving me nuts. Please help.

[Bobhof]

Hi all,

I am trying to make a simple passive mixer to sum the outputs from my computer (minijack) and my DVD player (RCA out) and send them into my sub (RCA inputs). I also need the output to be cut off when I plug my headphones into to a switched jack. Diagram is attached.

Here's the problem: it works perfectly when going out to the sub (whether I plug the sub into the RCA or the jack out) but when I plug in my headphones the sound is hardly audible. I have tried various phones with the same effect. It's driving me nuts! Surely if there is sound coming out it should work for either headphones or the amp?

Any help would be greatly appreciated.

Rob

 

[tschrama]

You are trying to feed your headphone through 10K resistors, correct? This might attenuate the signal to much for 32 Ohm or even 150 Ohm headphones...

 

[Bobhof]

Yeah, but the level is fine when going to the amp. Should I just remove the resistors or do I need them? Sorry, really don't know much about this stuff.

 

[lndm]

The amp will have a relatively high input impedance. The 10k resistors will not be significant here, they will be significant to the headphones.

 

[Bobhof]

I see. Is there any reason not to take the resistors out of the circuit? I just put them there because I read that that was how to sum inputs.

 

[tschrama]

Yes, you need those 10K resistors for the mixing. But nor the RCA output of a DVD nor the line out of a PC is meant to be used for a headphone. You need a headphone amp for that.

 

[Bobhof]

But the line out from my pc is a headphone out. Works fine if I plug the phones into my pc. What do you think I should do?

 

[tschrama]

replace the 10K with 470 Ohm and see if that helps...but it might be that the dvd RCA output will not be suiteble for driving a headphone...

gl,
Thijs

 

[Bobhof]

Thanks for all your help.

 

[Nigel Goodwin]

You really need an amplifier to feed the headphones, what you're trying to do is WRONG - which is why it doesn't work!.

 

[Bobhof]

Thanks Nigel. I understand now why it is WRONG. However, my headphones do work just fine straight out of the computer without an amp so I have just modified my design to put the switched jack before the mixer. Now, when I plug in the phones it cuts the computer output to the amp. The DVD player bypasses the headphone jack so DVD output is not affected when headphones are plugged in (ie I can listen either to computer output on headphones or a mix of computer and DVD through amp). Seems to work fine and no need to add another component (also works without power).

Thanks again to all who helped.

 

[lndm]

You want both inputs at once and that's the thing. You want a mixing stage but it needs to be a stage that can supply power output. It should be active. You might look at buffers. For something simple, just connect the headphones through an existing stereo amp.