Hey guys, this is my first post and I have no idea what I'm talking about here.

I'm trying to put together a rig to directly compare various headphone models through consistent A/B testing. The difficulty in doing this is that each model has different sensitivity and impedance (anywhere from 16 to peaks of 1600 ohms). This means that, if two models are receiving the same input, one could be much louder than another. Since studies have proved that a small volume boost is almost universally regarded as an increase in quality (simply the way our brains interpret the sounds), this is a problem I want to fix.

What I planned to do is setup a switch box which would receive a constant input and master volume control. The output would be changeable between the various models of headphones. Somewhere in the path, though, I'll need to be able to accurately attenuate the SPL (volume) for each model- say, the highest impedance headphone receiving the reference output at -0db, and more sensitive models receiving various drops (-2.5db, -8db, etc). The actual measuring I can do- what I really need to know is if such a device actually exists.

I have no idea if this makes sense, so please ask questions if needed.

 

I think it is more complicated than you imagine. Headphone impedance is only one of the factors that affect subjective listening judgement. I think you will find "golden ears" with all sorts of opinions on this subject. Even if such a device existed, I'm doubtful that the results would be believed or trusted.

 

You need switchable attenuators at the input to the headphone amp. which could be achieved with a rotary switch and two (for L+R) multi-turn preset potentiometers per headphone type -1 (since you won't need to attenuate the least efficient headphone).

So, a 12-pole 6-way rotary switches would give you 5 levels of attenuation and 0dB. Use multi-turn potentiometers to set the levels.

For example, one channel would look like this:





But, adding extra stuff in the signal path does raise other questions so it's best to use this kind of thing as a tool for evaluation but not the only method.

And, of course, this is a passive device so is somewhat dependent on the output impedance of the source and the input impedance of the amp. If you know the input impedance of the amp (and it isn't too high) you could get rid of the 10k resistor and use whatever resistance the amp presents as the other half of the voltage divider but you may need different values for the pots to give the range of attenuation required.

 

The usual method is to provide some power at 1KHz and adjust that power to a certain loudness as your reference to do a frequency scan. Thus, all headphones will have equal loudness at 1KHz.

After you realize that they will all have different frequency response, you must also include a number to describe the required power for each one. That takes care of the numbers. As a matter of opinion, I find it completely acceptable to have a low efficiency speaker with a wide bandwidth if that is what it costs to get good quality in a headphone. Everyone else will have a different opinion, but that's why you are doing this, so everyone can decide which ones fit their definition of, "good".

I hope you have a reference point now.

 

Knowing how hard it is to make truly "neutral" or "transparent" audio gear, I'd be inclined to buy something ready made if you can. If you look at sweetwater.com, guitar center, etc. you'll find lots of headphone amplifier options with individual volume controls per output. The volume knobs aren't marked in dB or anything, so depending on how scientific you want to be and how often the rig gets rearranged, it might not be ideal.

I've got the Behringer Powerplay Pro, which sounds fine and gives good control, but it's turned out not to be as roadworthy as I had hoped - one output is no good anymore and another has a sometimes-noisy volume knob, leaving me only two truly reliable channels at this point. If I had it to do over again, I'd probably go for the PreSonus model, having had much better luck with various products of theirs over the years.

If you do go for the passive solutions discussed above, be sure your amplifier can keep up. A lot of headphone outputs on non-professional gear don't handle headphones of different sensitivities well. If the amp can't deliver enough clean power with headroom to spare, you might just end up making them all sound bad!

 

Thanks guys, I appreciate the input. This is for an unusual application - I don't want to go into paragraphs of details, but imagine going into a showroom and seeing a lineup of headphones. The idea is to be able to send a single, consistent signal (a song, in this case) through a switcher which will relay it to one of the various headphones connected to it. The goal, then, is to make sure that I can use a powerful amplifier in order to fully run the most difficult of headphones (leaving ample headroom for easier to drive models), but then use set-and-forget attenuators to cut the volume in order to not blow out the headphones which require less power, and also to create a consistent volume across all of them- (XX)dB at 1kHz. That way, a person could walk up and switch between headphones without having to constantly adjust the volume. If I have to match similar impedance headphones that is certainly doable, but I was hoping that it wouldn't be necessary.

 

Headphones use so little power! You can install a volume control for each one and put the controls where the customers can't touch them. Good enough?

 

As long as the amp you're driving them with is powerful enough (not something that should generally be assumed, but sounds promising from your description) then I'd agree with #12. Here are a couple places to kick off the parts search:

http://www.mouser.com/Passive-Compo...yzvm31&Keyword=dual+trim&FS=True&Ns=Pricing|0

http://www.digikey.com/product-sear...ee=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25

Seems like the dual-pots would be more convenient, since you'd only need one pot per stereo headset instead of two, but there are far fewer to choose from. Also, I filtered both searches for logarithmic pots, which are generally what you want for audio... Although since you don't need full range volume control, but instead just need to make more subtle trim adjustments, that may not be necessary.

 

A couple of things to be aware of; you should not put an attenuator in series with the headphones as this will effectively raise the output impedance of the amplifier and would noticeably affect the sound, and a dual-ganged pot, unless it is quite expensive, may not have particularly good channel-balance and has no way to correct this error.

Impedance is not really an issue for you, providing your amplifier can deliver enough power to drive the most difficult headphones.

 

A couple of things to be aware of; you should not put an attenuator in series with the headphones as this will effectively raise the output impedance of the amplifier and would noticeably affect the sound, and a dual-ganged pot, unless it is quite expensive, may not have particularly good channel-balance and has no way to correct this error.

Impedance is not really an issue for you, providing your amplifier can deliver enough power to drive the most difficult headphones.

Good point about dual ganged pots and potential channel balance issues!

 

The frequency response and phase response of speakers and headphones is critically dependent on the output impedance of the source. Therefore, if your stepped or continuous attenuator does not present the same output impedance at all attenuation settings, you will be introducing more error into your system than the errors you are trying to correct. The "right" way is to have two matched headphone amps connected to the A and B devices, and have the attenuators at the input to the amps, not the outputs. Now it really can be as simple as a bunch of pots and a rotary switch.

ak