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Old 6th May 2013, 10:13 PM   #1
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Join Date: Mar 2013
Default Audio controlled voltage mod.

Hey guys. You've been an incredible help so I thought I would give back by sharing a small circuit I came up with last night to solve my quiescent current draw problems. Essentially what this circuit does is connect to the feedback pin on the LT3579 DC converter IC I'm using and while low or no voltage is present at the headphones it parallels the resistance of R3 with the existing feedback resistor (also 113k in this case) halving the feedback resistance and halving the voltage output.

When the voltage on the headphone output exceeds approx. 3v peak the positive portion of the wave passing through D1 is enough to switch Q2 on grounding out the gate of Q1 and shutting it off removing R3 from the circuit and doubling the output voltage from the DC converter.

With the values used this circuit it changes the output voltage to my audio section from +/- 6v to +/- 12v when there is sufficient output voltage swing to warrant the extra headroom. The result is more than double the avg battery run time. Not too bad for 40 cents in components

Pretty much any small N channel mosfets may be used for this circuit. I used

DMG1012UW-7 Diodes Inc | DMG1012UW-7DICT-ND | DigiKey

Adjust C1 to change the time to fall back to the lower voltage output. 0.1uf in my case gave me about 15seconds after the output voltage threshold is below the trigger level before the DC converter voltage drops back to +/- 6v. This value will change depending on the leakage on Q2, D1 and PCB so some tweaking will likely be needed. Also if you insert a 100k potentiometer after R1 you can fine tune the output voltage the circuit will trigger at or insert more series diodes for a larger voltage drop to the gate of Q2 and C1. You just have to be careful you don't drive the headphone output to clipping before the circuit triggers and increases the voltage headroom to the audio components.

Click the image to open in full size.

There are far more robust ways to go about this but this is extremely cheap and seems to work very well.

Last edited by Amperage; 6th May 2013 at 11:03 PM.
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Old 10th May 2013, 11:20 AM   #2
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Updated the circuit a bit and bolted on a visual indicator to show if the amp is operating in battery saver or not. Added another series diode to make circuit less sensitive and the zener diode to limit the voltage on Q2's gate to 5.1v just in case.

When operating in low voltage mode the status LED pulses on and off slowly then goes steady when the circuit kicks the voltage up on the converter / regulator.

I should note that I'm using logic level mosfets. They work like normal mosfets but have much lower "on" voltage requirements. Often 1-2v is enough for full on vs. 5v+ for normal mosfets.
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Click the image to open in full size.

Last edited by Amperage; 10th May 2013 at 11:28 AM.
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