Circuit design help?

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Dr. Webster's picture
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Hey all,

I need a little help with a circuit I'd like to design.

Here's a little background:

At work, I spend a good portion of my time on the phone. I use a headset that has one earpiece and a mic boom. The connection from this headset is a TRS 3.5mm minijack (aka a stereo minijack), and this connects to a special phone amplifier/adapter.

What I'd like to do is be able to:

1. Use a normal, stereo headset with boom mic, and
2. Listen to music in stereo while taking phone calls.

The headset that I want to use has separate connectors for the mic and headphones, so I built a small adapter box. The telephone amp connection is like this:

Tip--send
Ring--receive
Sleeve--ground

...so I tied tip and sleeve to a jack to plug the mic connector into, and I tied ring and sleeve to the headphone jack. I then tied another jack into the headphone jack so I could connect another audio source (but not have it feed into the phone amp).

The problem I ran across is this: When I plugged the headset into the phone amp, it worked great. But as soon as I plugged in the 2nd audio source (in this case an iMac), it killed not only the phone amp but the iMac's sound controller.

Not good.

I'm positive that the polarity is correct and that all the solder joints are good. I think the problem is that I need to isolate the phone audio from the computer audio when they reach the headphones.

So this is where I need some help. Anyone know of a simple circuit that I can build that will effectively mix two audio (headphone-level) signals? Source one would be mono audio (tied to go to both headphone drivers) and source two would be stereo (which I would want to keep stereo).

If this description causes confusion, I can draw up a schematic of what I made to clarify.

Thanks!

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jt
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Joined: Dec 20 2003
Posts: 447
It'd be a butt-ugly hack . . .

. . . but why not try two of the stereo->mono mixing adapters to get a "mono" feed into each earphone speaker from the two "stereo" sources by mixing Right/Right(actually mono voice split right?) and Left/Left(same deal?) to get what you want?

jt Evil Block Company

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DanR's picture
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Joined: Jan 28 2004
Posts: 48
Maybe this could be adjusted

Maybe this could be adjusted for headphone output:
Simple two line output combiner

Also: G4 Headphone Jack specs

Dr. Webster's picture
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I think the resistors are wha

I think the resistors are what I needed.

Next time I'm out I'll pick up a couple and try wiring 'em in.

But here's another question: does anyone have any good, easy-to-build headphone amp schematics? I'd get a Boostaroo but I want this whole thing integrated in one neat package. (Dr. Bob, you're the whiz at this stuff, any suggestions?)

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DanR's picture
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Stumbled across this gizmo wh

Stumbled across this gizmo while I was poking around:
http://www.telecomaudio.com/voice_over.html

Dr. Webster's picture
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Update

Well, I e-mail the guy who did this page:

http://www.epanorama.net/circuits/linesum.html

...and he says that while it will indeed mix two headphone-level signals together, it'll reduce their signals to line-level.

The VoiceOver looks promising, but it'll only do mono audio. Plus, it sits upstream of my headset amplifier, so that'll make the amp's batteries run down much faster.

There has to be a way to do this. If I could find a headphone amplifier circuit I could put something together that way.

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DrBunsen's picture
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Joined: Dec 20 2003
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You could probably pick up co

You could probably pick up complete headphone amp chips or modules from ebay for a trifle, in the lots more/business and industrial/electronic components section. Then it's just a case of making up a small mixer circuit and deciding whether you want to mix before the amp (at line level) or after. Before is probably easier.

Or try paia.com

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mixer

Here's the schematic for the Audio mixer that I designed.

http://vader.inow.com/~drbob/temp/schematic.gif

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martakz's picture
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Probally being dense...but wh

Probally being dense...but why the transformers Dr Bob?

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JetStar's picture
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I think their 1:1 isolators,

I think their 1:1 isolators, so that you won't get ground loops, which is very important. Otherwise you can get a nasty hum.

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It must be user error.

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Joined: Dec 20 2003
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yep

Guessed it in one.

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