This is a review and detailed measurements of the KOSS PRO 4AA headphone. It is on kind loan from a member. He bought it used for $10 but then sent it to KOSS and they sent him a new one. In researching it, I found a review (of a different revision) dating back to 1963 yet it is still on sale on Amazon for US $80!
The 4AA looks like an avionics headphone to me for some reason:
I found the cups very stiff and they became very uncomfortable after 15 minutes of listening.
Note: The measurements you are about to see are made using a standardized Gras 45C. Headphone measurements by definition are approximate and variable so don't be surprised if other measurements even if performed with the same fixtures as mine, differ in end results. Protocols vary such as headband pressure and averaging (which I don't do). As you will see, I confirm the approximate accuracy of the measurements using Equalization and listening tests. Ultimately headphone measurements are less exact than speakers mostly in bass and above a few kilohertz so keep that in mind as you read these tests. If you think you have an exact idea of a headphone performance, you are likely wrong!
Fitment on the fixture was VERY difficult. It would not fully surround the artificial ear, and I could not get it to fully seal no matter what I did. Slight movements and changes would vary the levels but in practice, I noticed frequency response did not substantially change. Still, be on notice that my confidence in measurements are not super high (although EQ settings verified that they were good enough for that purpose).
KOSS PRO 4AA Measurements
Let's start with our usual frequency response:
Ouch. That bass deficiency may be partly due to seal but still, we basically have no bass. There is some kind of resonance around 2.5 kHz and response then drops way off. In other words, this is as close to one-note headphone as we can get.
Relative response puts this in context relative to our target:
Distortion however was extremely low:
Then again we have a ton of boosting to do.
Group delay is clean and not revealing of much:
Sensitivity matches the average of all headphones tested so far:
Impedance is high and somewhat variable:
KOSS PRO/4AA Listening Tests and Equalization
Boy, does this thing sound flat and lacking in just about everything that makes music hi-fidelity. There is no bass or treble. I almost stopped there thinking it could not be salvaged with EQ. But I was wrong:
The transformation was dramatic. It was the classic toad turning into a prince! OK maybe that not far but you get the picture. There was even decent spatial effects now. Deep bass was pronounced but had a weird distortion about it. I took the levels down and it helped with that but then I thought bass was lacking with other tracks that were not bass heavy. After some back and forth, I left it as you see.
Conclusions
In researching this headphone, I ran into this Stereophile review for a slightly different version of it:
I don't think I have ever read such blunt and negative review in stereophile! Was this the norm back in 1991? If so, I wish that spirit was kept until today.
Anyway, I concur with it. Can't recommend the KOSS PRO 4AA.
----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
The 4AA looks like an avionics headphone to me for some reason:
I found the cups very stiff and they became very uncomfortable after 15 minutes of listening.
Note: The measurements you are about to see are made using a standardized Gras 45C. Headphone measurements by definition are approximate and variable so don't be surprised if other measurements even if performed with the same fixtures as mine, differ in end results. Protocols vary such as headband pressure and averaging (which I don't do). As you will see, I confirm the approximate accuracy of the measurements using Equalization and listening tests. Ultimately headphone measurements are less exact than speakers mostly in bass and above a few kilohertz so keep that in mind as you read these tests. If you think you have an exact idea of a headphone performance, you are likely wrong!
Fitment on the fixture was VERY difficult. It would not fully surround the artificial ear, and I could not get it to fully seal no matter what I did. Slight movements and changes would vary the levels but in practice, I noticed frequency response did not substantially change. Still, be on notice that my confidence in measurements are not super high (although EQ settings verified that they were good enough for that purpose).
KOSS PRO 4AA Measurements
Let's start with our usual frequency response:
Ouch. That bass deficiency may be partly due to seal but still, we basically have no bass. There is some kind of resonance around 2.5 kHz and response then drops way off. In other words, this is as close to one-note headphone as we can get.
Relative response puts this in context relative to our target:
Distortion however was extremely low:
Then again we have a ton of boosting to do.
Group delay is clean and not revealing of much:
Sensitivity matches the average of all headphones tested so far:
Impedance is high and somewhat variable:
KOSS PRO/4AA Listening Tests and Equalization
Boy, does this thing sound flat and lacking in just about everything that makes music hi-fidelity. There is no bass or treble. I almost stopped there thinking it could not be salvaged with EQ. But I was wrong:
The transformation was dramatic. It was the classic toad turning into a prince! OK maybe that not far but you get the picture. There was even decent spatial effects now. Deep bass was pronounced but had a weird distortion about it. I took the levels down and it helped with that but then I thought bass was lacking with other tracks that were not bass heavy. After some back and forth, I left it as you see.
Conclusions
In researching this headphone, I ran into this Stereophile review for a slightly different version of it:
I don't think I have ever read such blunt and negative review in stereophile! Was this the norm back in 1991? If so, I wish that spirit was kept until today.
Anyway, I concur with it. Can't recommend the KOSS PRO 4AA.
----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
