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Stupid question? Driving headphones with preamp output

Dogen

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Is it really inadvisable to drive headphones with what are essentially preamp outputs? Here’s why I ask.

Until I got my headphone amp, I was driving my headphones with the analog out of an Airport Express, using iTunes to control the volume. It sounded OK. A frequent use case is to hook up an iPhone or Airport Express or such to a stereo system with a 3.5 to RCA adapter. Maybe not perfect but not obviously terrible at all.

I’ll grant you headphones are not designed to be driven such. But in practical terms, what’s the problem with driving headphones with a volume-controlled preamp out? Will there be a change in the frequency response or sound in a significant way? Could the headphones or preamp be damaged? Is the answer dependent on the characteristics of the headphones?

Thanks for your patience (if any) with this basic question.
Bob
 

amirm

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The output impedance is usually pretty high and there is not enough voltage, nor current to drive some headphones. The result is that you will hear music but the frequency response may change and it may sound anemic (e.g. lack of bass impact). Some of this will also depend on the actual preamp out. Some have much higher impedance than others.
 

watchnerd

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Is it really inadvisable to drive headphones with what are essentially preamp outputs? Here’s why I ask.

Until I got my headphone amp, I was driving my headphones with the analog out of an Airport Express, using iTunes to control the volume. It sounded OK. A frequent use case is to hook up an iPhone or Airport Express or such to a stereo system with a 3.5 to RCA adapter. Maybe not perfect but not obviously terrible at all.

I’ll grant you headphones are not designed to be driven such. But in practical terms, what’s the problem with driving headphones with a volume-controlled preamp out? Will there be a change in the frequency response or sound in a significant way? Could the headphones or preamp be damaged? Is the answer dependent on the characteristics of the headphones?

Thanks for your patience (if any) with this basic question.
Bob

I drive headphones from the pre-out of my Devialet 400.

Some notes:

1. I can software configure the output from a max of .1 mv to 4 V

2. Output impedance is 0.1 ohm

3. I use a 10 foot length of RCA to TRS cable to hook it all up

So, yes, it's entirely possible, but "it depends"....
 

solderdude

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As per previous answers... it depends on the headphone and pre-amp used.

Output resistance and max. voltage but above all output current can be the limiting factor.
A design can have a 1 Ohm output resistance yet not be able to provide much current.
Pre-amps are usually connected to power amps with input resistances between 1kOhm and 10kOhm so don't have to supply much current.

About damage.. the headphones are not likely to be damaged that quickly but it cannot be guaranteed that the pre-amp devellops a fault in the output buffer which may be loaded a bit too much and give DC on its output. There will be no DC protection unless the pre-amp has output caps or transformer.
So under the wrong circumstances something may happen but on the bright side... this can also happen to headphone amps.
 

alfredinho

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So, I understand one can connect headphones to rca preout on normal preamp? With this adapter in photo
-90368990-496349492.jpg
 
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