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Old 31st May 2014, 11:19 PM   #1
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Default Driving headphone from speaker output - filtering layout

(Posted here since it involves filtering *after* the amplification stage.)

A collegue and me were discussing driving a headphone directly from an amplifier, and I pointed out that you cannot just merge the 2 grounds from the speaker outputs. (I haven't looked up the details on that yet. I think it has to do with the internal amplifier layout.)
But I came up with something that I think can circumvent possible problems and I wanted to do a short reality check to see if it could theoretically work.

- First put an appropriate capacitor on both the left and right grounds/minus
- THEN join the both capacitor ends together to make the headphone common ground.
- Put an appropriate resistor in the merged ground line to reduce the voltage range to safe headphone levels.

By putting the 2 capacitors there, it is not possible to get DC between the grounds, therefore preventing a short cirquit right?


We're not planning on trying the above, I just posted it here to settle a dispute, but any comments or ideas are appreciated. The Elliot Sound website has a cirquit for one channel, which is not quite what we were wondering, but other than that there is little information to find on the internet.
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Old 31st May 2014, 11:45 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

You can merge common ground amplifiers and you
can't merge non-common ground, i.e. bridged.

However for bridged its easy to to derive the
headphone output from the real ground and
two halves of the bridge, more complicated to
interrupt the speakers using socket switching.
(Haven't thought about this much.)

Generally you are wrong, the ground is common
on many amplifiers, except when it generally isn't,
like bridged car amplifiers and low V class D stuff.

FWIW your schema for non-common earth
simply won't work, at AC frequencies it
will short the channels together, very bad.

rgds, sreten.
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Last edited by sreten; 31st May 2014 at 11:59 PM.
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Old 1st June 2014, 01:13 AM   #3
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Quote:
Originally Posted by sreten View Post
Generally you are wrong, the ground is common
on many amplifiers, except when it generally isn't,

like bridged car amplifiers and low V class D stuff.
LOL. Sounds like doubletalk to me.
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Old 1st June 2014, 09:15 AM   #4
AndrewT is offline AndrewT  Scotland
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Do a check first.
Measure the resistance (inverse of continuity) from Spkr1 return to Spkr2 return.

If that measures zero ohms, then it is safe to common these two returns to feed a 3pole headphone socket
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Old 1st June 2014, 09:32 AM   #5
Zero D is offline Zero D  United Kingdom
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If it's a stereo NON bridged Amp, then you don't to merge the 2 grounds from the speaker outputs.

Just use either, one as they will be connected together internally !

You will need to attenuate the Left & Right outputs to the HP's with suitable resistors.
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Old 3rd June 2014, 03:12 PM   #6
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- First I tought that AC wasn't the issue since it is rectified, but then I realized you are talking about the difference between the L/R channels, and since electrified soundwaves are AC ... indeed a very bad idea! Maybe if one would put 2 resistors before the capacitors, then any AC between the terminals would have to travel over both of them, while only trough one of them to go to the headphones. It wouldn't be a short cirquit anymore, but that's just semantics.
- KISS indeed: if it has common ground there would be no difference between them and only use one! Smart thinking! Easly checked by opening up the case or whip out ye olde multimeter.

Funny car stereos are mentioned, since the whole thought experiment was actually started with that. Guess it wouldn't work.

Thanks for the input anyway,

cordraconis
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Old 3rd June 2014, 03:24 PM   #7
Fsatsil is offline Fsatsil  United States
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This article may be of some interest if you would like drive your headphones with a power amp.
Headphone Adaptor for Power Amplifiers
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