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Old 6th October 2009, 05:36 AM   #1
zigo3 is offline zigo3  Italy
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Default Headphone attenuator

Hi
when my source is computer I plug directly the headphone in the analog output of an RME digi 96/8 PAD ( ~9,5V, 75 Ohm) and I use the software fader to regulate the volume.
But I read that attenuate the volume using software degrades the signal reducing the bits.

My can is 40 Ohm.

I was thinking to a passive potentiometer (attenuator) to put between the output and the headphone and use the maximum output volume of RME, because in this way the signal doesn't suffer degradation.
But I don't think that usual stereo potentiometer can work correctly, because they are kOhms impedance (20...100 or more).
Am I right?

My purpose is to have a very good signal, then I'm looking for a very good solution, not objects who downgrade the sound. And I don't want to use preamplifiers or amplifiers, because I want that the signal remains original.

What can I use?

TIA

Rob
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Old 6th October 2009, 05:46 AM   #2
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I suppose what you really want is a higher impedance version of the old 8 ohm L-pad. That would let the card see a constant load as you varied the level to the cans. You could make one up using rotary switches, but it seems like overkill. I'd probably just use either a series rheostat (level won't go to zero), a conventionally wired rheostat picking the signal off the wiper, or a series resistor of 40 or so ohms and the rheostat to ground the way people do shunt controls.
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Old 6th October 2009, 03:58 PM   #3
zigo3 is offline zigo3  Italy
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Hello Conrad
I didn't know the L-pad.
Interesting...
I think you are right, but do you think that a rheostat could take an HIFI signal?
I looked for it and I found several 50Ohms objects, not 40, and they are industrial objects. Do you think that they are "corrects" for my purpose?
Interesting your idea of resistors on the positive signal and a rehostat on ground.
But how can I do it?
Sorry for my beginner questions, I have a "humanist" background, not electronics; but I prefer electronics, more objective

Quote:
Originally Posted by Conrad Hoffman View Post
I suppose what you really want is a higher impedance version of the old 8 ohm L-pad. That would let the card see a constant load as you varied the level to the cans. You could make one up using rotary switches, but it seems like overkill. I'd probably just use either a series rheostat (level won't go to zero), a conventionally wired rheostat picking the signal off the wiper, or a series resistor of 40 or so ohms and the rheostat to ground the way people do shunt controls.
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Old 6th October 2009, 04:25 PM   #4
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Is an external hi-fi headphone amplifier/buffer too complicated of a solution?

You could then use any standard audio potentiometer.

There are ready-made headphone attenuators, but I don't know if these could ever be considered "hi-fi"......

http://www.radioshack.com/product/in...ductId=2102975
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Old 6th October 2009, 04:50 PM   #5
zigo3 is offline zigo3  Italy
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Quote:
Originally Posted by theAnonymous1 View Post
Is an external hi-fi headphone amplifier/buffer too complicated of a solution?
Think Anonymous1: why I "MUST" use an amplifier if the signal is sufficienlty amplified? Why I "MUST" use something who is absolutely unnecessary, exaggerate and something who modify (then dirty) the original signal if I don't need it???

Last edited by zigo3; 6th October 2009 at 04:52 PM.
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Old 6th October 2009, 05:21 PM   #6
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Quote:
Originally Posted by zigo3 View Post
Think Anonymous1: why I "MUST" use an amplifier if the signal is sufficienlty amplified? Why I "MUST" use something who is absolutely unnecessary, exaggerate and something who modify (then dirty) the original signal if I don't need it???


I didn't say you "MUST" do anything.

I was asking you if you thought using an external amplifier would be too complicated of a solution. Using an external amplifier would allow the use of a standard analog volume control.

Who says a quality amplifier would mess with the sound any more than a l-pad or rheostat?

Do what you want, it was just a suggestion.
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Old 6th October 2009, 05:30 PM   #7
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An "industrial rheostat" will be made with Evenohm or some similar decent high performance wire, just like a wirewound resistor. There are few turns so the inductance will be minimal and I can't imagine one degrading the sound. The unknown to me is how the headphones will react to various driving impedances. I used to think they should be driven from a zero ohm source, but more reading suggests they're probably designed for something like a 40 ohm source. Many devices have been designed with a separate little headphone amp rather than using the main outputs, but IMO the main outputs are fine. I drive my phones from a fixed wirewound divider off a 60W/ch amp. That brings up another solution. Why not set up a fixed divider out of good quality wire or film resistors, such that the listening level is just a bit higher than you like. Then use software controls to bring it down slightly. The slight loss of bits should be minor, plus if you decide to bring it down a lot for some reason, any quality degradation won't be audible anyway. IMO, the object is to keep the signal to noise ratio as high as possible, so you want the amp running at a good percentage of it's maximum output. That way the signal gets attenuated by the divider but so does the noise. BTW, 40, 50 or something else makes little difference here, as long as it's within the capability of the amp to drive it- don't use a 4 ohm divider!
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Last edited by Conrad Hoffman; 6th October 2009 at 05:33 PM.
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Old 6th October 2009, 06:04 PM   #8
AndrewT is offline AndrewT  Scotland
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a 50ohm wirewound pot would do the job.
maybe 2W would be about right.

at -6dB the source impedance seen by the headphones would be 25r//[25r + Rs]
~=15r.
As you increase the attenuation (less loud) the impedance lowers.

You could consider an attenuator followed by a buffer to feed the headphones.
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Old 7th October 2009, 05:50 AM   #9
zigo3 is offline zigo3  Italy
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Thanks to both Conrad and Andrew.

A rheostat/potentiometer could be the right solution because often the output level depends by the source level, who is not fixed.
Then an adjustable pot is very useful.

If impedance falls down with attentuation (in linear pots is it a direct proportion?) I could consider a higher impedance pot since it's difficult use it with 0 attenuation.

The challenge now is find a stereo rheostat: using google I didn't find them. "Double rheostats" was produced a lot of time ago, but I can't find them now.

@ Conrad
your idea to use a fixed attentuation, a little higher, and use the software attenuator for fine andjusting is not bad. Maybe the simpler way to do it is a T attenuator, something like this T-link attenuator with constant impedance

Maybe the best solution is this Constant impedance relay-resistor logarithmic attenuator , very good but by sure a lot less easy.
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Old 7th October 2009, 04:22 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by zigo3 View Post
If impedance falls down with attentuation .....I could consider a higher impedance pot ........
no you can't.

The maximum current through a 2W 50r pot is 200mA. 0.2*0.2*50 = 2W
The maximum I would recommend is ~140mA.

A 100r 1W pot has a maximum current of 100mA.
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