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Old 30th September 2004, 04:51 PM   #1
ble0t is offline ble0t  United States
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Default Headphone Switch Circuit

I'm currently working on a pre-amp with built in headphone amplifier and I would like to mute the output of the pre-amp if headphones are plugged in. The headphone jack has a single switch in it (link here) that I would like to use to send a signal to my PIC to mute the output. Attached is an image of my intended circuit.

Would this work? The PIC has an ADC I would be using, so that is not an issue.

Thanks, and don't be too harsh...I'm only a newbie

EDIT: The little unconnected wire there is supposed to represent a switch...crude I know
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Old 30th September 2004, 08:22 PM   #2
ble0t is offline ble0t  United States
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I don't mean to be pushy, but here's a little thread bump

Anyone have any thoughts on this at all?
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Old 30th September 2004, 09:21 PM   #3
sam9 is offline sam9  United States
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I don't know much (anything) about PIC but I assume that the mute works by applying a voltage within a specified range to a pin. If that's right the key would be to create voltage reference on that range. I don't know how fussy a PIC device is but assuming you already have a clean regulated PS, wouldn't a simple means be to just take a DC feed off the power supply and drop it to the control voltage by means of a zener diode. Then just wire your swith in series with the control voltage.

Alternative: Use the switch to control a DPDT reed relay that switches the preamp output signal either to the headphone amp input or to the preamp output jack. A good reed relay is very quite and should do no harm to the signal. It has the advantage that you can locate where it keeps the signal path short but the contol switch can be where it is convenient. The wire carring the DC control volage can be on any reasonable length and run most anywhere.
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Old 30th September 2004, 09:27 PM   #4
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You are using an NPN-transistor as a "high side" switch here which I would avoid. Use a PNP transistor instead such as BC327. Also, you need a "pull-down" resistor from the emitter (OUT_PIC) to ground to make this circuit work. Something in the 1-100kohm range would work fine. Most microcontrollers have internal pull-ups/downs that can be activated. It was many years since I used the PIC's but IIRC they have internal pull-ups on some ports.

If you want to make this circuit even simpler you could emit the transistor and just use a pull-up resistor from the headphone switch to VCC although you have to watch out for ESD. It's up to you.

/Magnus
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Old 30th September 2004, 09:55 PM   #5
johnnyx is offline johnnyx  United Kingdom
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Default Re: Headphone Switch Circuit

Quote:
Originally posted by ble0t
I'm currently working on a pre-amp with built in headphone amplifier and I would like to mute the output of the pre-amp if headphones are plugged in.
Is there a relay on the main output to do the switching? How would the output be muted? Is there a "power-on delay" to prevent thumps at switch-on?

You don't have to go via the PIC. Whatever mutes and unmutes the output can be arranged so that the output is on when power is ok AND the headphones are unplugged, otherwise it's off.

Hope this helps.
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Old 1st October 2004, 05:20 AM   #6
ble0t is offline ble0t  United States
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Yes, there is going to be a relay on the main output. As for controlling the possible thump from this, I'm using a 'soft mute' circuit design that should prevent it.

Thanks for all the advice...I'm hoping to pick up a few parts to test with next week. If all goes well, I'll report back here...perhaps someone else can use this in the future as well.
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Old 11th December 2004, 10:07 PM   #7
eibwen is offline eibwen  United States
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Default how'd this work?

Hello, just wondering if this worked well or not?

I would like to make something like it.

Actually i'd like to make a 5.1 version, so that when i plug headphones in it turns off all the surround sound speakers, and feeds the left and right to the headphones. Altho its not really needed to do that, but I would like it so that if i forget to go into my speaker configuation thingy for my audio card, the center, sub, rear speakers would be silent no matter what.

Well: so for 5.1 i would need a special chip to control it, right?
That would be hard to find i would think, so i might just make it like yours, and have to remember to go to my speaker config all the time (which i would have to do anyways, otherwise i would hear what was processed to go to the left and right, so much less vocal usually)

Ok well, the big question, how much did it cost in the end?

Thanks
-Greg
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Old 24th March 2008, 06:33 AM   #8
dfdye is offline dfdye  United States
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Wow, no love for this in a while! And no resolution to boot!

I'm running into the same question as to how to mute a circuit (specifically an LM1876) by pulling up the mute pin from ground to 2.5V. via opening the switch on the headphone jack. I can easily get the 2.5V to a transistor that can be controlled to drive the mute pin, but I can't easily figure out how to control the transistor with what would normally be an audio signal.

Any new thoughts?

. . . . . Of course I can always add a manual mute switch, but where is the fun in that?!?!?
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