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Old 10th February 2012, 10:28 PM   #1
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Default Adding a Headphone output to my amp.

I have a small and very nice SET value amplifier that with only about 10 watts gives me all the SQ I need in my DEN.

However ther are times that for reasons of family harmony I would like to be able to listen via heahones instead. My study ajoins the master bedroom and my wife can clear be kept awake.

The out put impedence is sufficently low for my phones which are 32 ohm. So I do not think I need any additonal impedence matching transformers but simply an attenuator, if that.

if I simply had a speaker a/b switch would that suffice? The low output Z of the amp, will provide sufiicent dampening for the phones, at 32 ohms I guess the phone provide anought load for the amp.

Am I missing something. I dont want to break my amp nor my custom Grado phones.

Cheers, Tony

P.s. maybe volume 4 of Morgan Jones book holds the answer ;-)
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Old 10th February 2012, 10:34 PM   #2
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parallel a suitable resistor with the headphone output to reach the design Z load of the output transformer.

Either use a separate a/b switch or, if you are a bit lazy, a headphone jack that automatically does the switching.

Done.
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Old 10th February 2012, 10:53 PM   #3
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Thanks for that advice, so use the resistor in parallel to make the 32 ohm phones 8 ohms.

So by my calcluations a 10 ohm resister get me close enough at just under 8 ohms.

Cheers,
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Old 10th February 2012, 11:09 PM   #4
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yeah, I'd aim a little higher - a 15 ohm parallel will give you 10 plus a bit ohm total load. Thats really just a matter of personal preference - Id rather the Z was set up slightly above the nominal since it has a nasty habit of going low at some frequencies. If you can get a frequency plot of your Grados that will make it a little clearer and your decision more fact-based!
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Old 10th February 2012, 11:10 PM   #5
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a fuse in the headphone circuit would be a good idea too - you are going to be able to SERIOUSLY overdrive them, and I'd rather blow a fuse than my Grados
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Old 10th February 2012, 11:26 PM   #6
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Great points. I intend to build his into a small control box on the side rather than make any direct changes to the amp. I intend that the box will contain speaker output posts, amp inputs, a switch for phone to speaker, so I can keep teh phone plugged in, a fuse for the headphone circuit and the parrellel resistor.

Thank you all.
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Old 11th February 2012, 04:41 AM   #7
db! is offline db!  Canada
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Take a look here:
Headphone Adaptor for Power Amplifiers

If you decide to use a fuse, remember to connect the fuse(to your headphones) after the parallel resistor. Otherwise, if the fuse blows, your OPT will become unloaded and will arc itself to death.
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Old 11th February 2012, 05:01 AM   #8
jcx is offline jcx  United States
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fuses only prevent fires - seldom "protect" equipment

serious amps have relays or output crowbar circuits to protect speakers - and by extension headphones that cost as much as speakers

a output transformer prevents DC from ever reaching the load

sensitive headphones may be better matched to a audio power amp with a power rated resistive divider
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Old 11th February 2012, 05:10 AM   #9
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Thanks all.

DB, that circuit seems to be spot on.
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Old 11th February 2012, 07:22 AM   #10
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Quote:
Originally Posted by aardvarkash10 View Post
If you can get a frequency plot of your Grados that will make it a little clearer and your decision more fact-based!
Click the image to open in full size.
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