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Old 11th December 2014, 05:55 PM   #1
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Default DC protection for headphones

Hi,
I found this The ε12 Muting / Protect Circuit on amb & this Mini UPC1237 Headphone Protector Protection FOR Headphone Amplifier AMP Project | eBay. UPC1237 is good for spk. protection as its dc detection threshold starts from approx. 2V DC but useless in HPA application as offsets involved are under 20mV.
My question is can the dc detection & relay driving ckt (outlined in the E12 schematic) be replaced by upc1237? Also omitting TL082 as a rail splitter in the process as I'm planning to use dual supply for TL082(for 20X to 100X DC offset amplification) & a separate power supply (30V DC) for upc1237(for DC detection & relay driver).Any suggestions on better but cheap jellybeans replacement opamps than the stock TL082? like TL072...
Pls see attch. for original & moded E12.
Thanks All,
availlyrics
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Old 15th December 2014, 12:33 PM   #2
oshifis is offline oshifis  Hungary
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DC is catastrophal for loudspeakers, but not so much of concern for headphones. The major difference is the higher resistance and lower driving voltage (generally speaking). One should use reliable amplifier that does not give out DC.
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Old 15th December 2014, 01:17 PM   #3
AndrewT is offline AndrewT  Scotland
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I don't agree with Osh.
A very small DC offset is potentially more damaging to headphones than to the "big" voice coils in room driving speakers.

Offset can push the "at rest" position of a headphone voice coil so far off centre that the remaining Xmax is too small to give clean reproduction of the audio signal.

One way to avoid offset is to use DC blocking capacitor/s that are so big, they don't roll off any of the audio.
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Last edited by AndrewT; 15th December 2014 at 01:20 PM.
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Old 15th December 2014, 04:57 PM   #4
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@oshifis Some sensitive h/p may not like DC in excess of 100mV. DC in the range of 20mV or above cause audible noise when connectin/disconnecting h/p.

@AndrewT I want to add DC protection for headphones as I'm using direct coupled designs like O2/CMOY. The offsets are quite low (< 3mV) but I faced -ve voltage regulator failure & this made me worried about component failures and the resulting damaging DC.
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Last edited by availlyrics; 15th December 2014 at 05:26 PM.
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Old 16th December 2014, 09:38 AM   #5
AndrewT is offline AndrewT  Scotland
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With a normal speaker we see a general recommendation for less than 100mV offset for 85dB to 90dB/2.83Vac @ 1m, or 77dB to 82dB @ 2.5m (let me call that an average of 80dB listening volume @ 1W)
That 100mV is equivalent to ~-29dB ref 2.83Vac

For a 300ohm headphone with a sensitivity of 100dB/mW, the voltage required for the same listening volume is 20dB below 1mW, i.e. 54mVac
A comparable -29dB offset would be ~2mV
i.e a 100mV offset for a 8ohms 88dB/2.83Vac @ 1m is very roughly equivalent to 2mV offset for a 300ohms 100dB/mW headphone.

Many power amplifiers are specified @ < 20mV and I regularly build amps that are <5mV offset.
Headphone amps are in a completely different league due to sensitivity of the Headphone voice coil.
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