Vintage Koss Pro4AA Headphone Restoration

Vintage Koss Pro4AA Restoration


Renovated Koss Pro4AA Recently I gained possession of a vintage pair of Koss Pro4AA Headphones, age unknown, but circa 1970’s. My initial inspection of the headphones proved favourable, but with any electrical equipment which is 40 years old, some work was required to restore these to what hopefully approaches their former glory.

The Koss Pro4AA is a headphone that has a good reputation.  A closed headset design (i.e. it isolates the user from outside noise) it was first produced in 1970 and remained in production until the late 1980’s.  By popular demand Koss resurrected the design in 1988 and a modern equivalent of the 1970 design remains in production today.  The modern version shares the looks of the original but internally is a different design.

This project was a quickie really, taking only around 8-10 hours of my time (across 3 evenings) to complete.  To be honest when I took the project on, I kind of hoped that it would be more involved.  I suppose it is a tribute to Koss’s robust design and industrial strength build quality that there was not really that much work required to get these headphones back up to scratch.

Of course those original owners of the Koss Pro4AA Headphone will recall with misty eyes how the original design is the better.  I hope that I will eventually have the opportunity to compare, I tend to think that they may be right, but then I am just as guilty as any to misty eyed nostalgia.  Certainly the look and feel of those big heavy “cans” takes me back to my childhood.

Initial Inspection

Koss Pro4AA Renovation The Koss Pro4AA cans that I acquired looked pretty good, at first glance.  I was quick to give them an electrical test and get them plugged into an amplifier for a quick listen.  It was clear from the build up of dust that these headphones had been in storage for a while.  I was unable to find out for just how long, but a while.

The Koss Pro4AA is a ruggedly constructed head phone.  From the top, the headband is made from a rubberised plastic with a foam insert on the underside to offer the listener some comfort.  The legs of the Koss Headphones are made from stainless steel and are adjustable.  They retract inside the rubberised headband.  The headband on my vintage pair of headphones is still supple although had many years of stains and dirt to be cleaned.

Koss Pro4AA Renovation This was good news, though a closer look revealed that in fact the headband had sustained some damage over the years and had been hastily repaired.  On the ends of the rubberised headband are plastic end caps.  on both left and right hand sides the plastic had been split.  Someone had used some strong hard glue to repair the damage (a few years ago judging by the yellowing) but had inadvertently glued one of the adjustable legs in place.  Flexing the headband released the stuck leg, but reopened old wounds!

Koss Pro4AA Metalwork Corrosion The shape of the legs on either side has changed slightly, no doubt as only one of the legs was adjusting before.  There was also a little rust staining on the adjusting legs around the hinging points, though they moved freely as no doubt intended by Koss

Trying out the headphones things didn’t sound too bad. Moving hinges and jiggling chords did reveal a little crackle in the right hand earpiece telling me that the cabling had a break/loose connection somewhere in the signal path.  Checking the cabling revealed the chords to be in good connection otherwise and no stretching or brittleness in the chords was evident.

Koss Pro4AA Renovation Moving to the cans, again the plastics showed staining and yellowing.  This concerned me a bit.  i am unaccustomed to renovating yellowed plastics and was concerned that the yellowing was more than dirt.  Plastics of a certain age can yellow, not because of dirt or grease, but due to a chemical reaction in the plastic itself.  I have seen it many times in old computer equipment and feared that the yellowing on my Koss Pro4AAs.  By all accounts there are ways to bring these plastics back to new, but this involves making chemical cleaners up yourself such as retrobright from some caustic household chemicals.

Other obvious damage to the cans was that the original badges were missing from the Koss Headphones.  The Koss Pro4AA headphones normally sport a very 1970’s chrome badge either side, on the left hand side with blue lettering and logos, the badge on the right detailed in red.  I was not sure how easy these would be to replace.

Koss Pro4AA Renovation On the Koss Pro4AA headsets from the 1970’s the ear pads are of an interesting design.  The original design included oil filled rubber pads.  Looking closely at my set, it looks as though the original oil filled pads have never been replaced.  Unfortunately after 30-40 years, the oil has long since dried up in the pads leaving them saggy and only half filled with air.

Replacement Oil filled pads are no longer available to buy, and it was very clear from my tentative listening to the headphones that sticking with the originals was not going to be practical.  The Koss Pro4AA headphone design uses the rubber pads to enclose the ear of the listener.  With the pads only half inflated (at best!) instead of my ears being enclosed, the floppy rubber was more sort of pressed against my ears.  Within a short space of time my earlobes were sweating, not the most pleasant of sensations.  If that was not bad enough, under those pads resides some screws which now were being pressed against my ears in a painful manner.  At least the headband still had most of its original gripping power!

Koss Pro4AA Renovation The other issue with the pads took me a little time to work out.  In the design, behind a soft mesh inside the pads is a flat foam insert which had entirely disintegrated.  The only clue that it was ever there was the dust that remained.  It was everywhere!  When i began disassembling the headphones, the foam remnants seemed to get everywhere and now no doubt will remain in my flat for years.  The foam dust seems to be impervious to dusters and vacuum cleaners.

So to sum it up.  Despite being in existence in excess of 30 years during times of being loved and cared for and no doubt through less happy times stored goodness knows where, these Koss Pro4AA Headphones were still in pretty good shape.  Question was could they be renovated?

I didn’t want to spend too much money on these.  The modern versions can be purchased for around £70 in the UK.  So any work to be done on them had to be reasonably priced.  Little was I to know how much this project would cost!

Sourcing Parts

This was always going to be the scary part of the restoration, Finding parts.. After all this is a 30-40 year old piece of equipment.  What I identified was that I was going to definitely require the following:

  • 1 Pair of pads
  • 1 Pair of foam inserts.  i.e. the foam that sits over the speaker itself.
  • 1 Pair of badges

From looking around forums, I worked out that the replacement pads for the new Koss Pro4AA design, whilst of a different construction were available for purchase and would fit.  Koss moved away from using oil filled pads and now use foam filled alternatives.  These are lighter and are said to be more comfortable to wear.

Koss Pro4AA Replacement Pad PartsI could not find anyone advertising the other parts, so i sent off an e-mail to Koss themselves.  Koss it seems are very intent on keeping their products alive.  On a lot of their products they offer lifetime guarantees and other Koss Pro4AA owners have reported that they have sent their vintage phones to Koss and have had them restored under guarantee!

Koss Europe responded quickly to my e-mail and passed me on to Hama.  A UK based distributer and service supplier of their product.  Hama’s engineers managed to source me the pads, foam inserts and also a pair of badges.  These were dispatched for free to me.  Now that is service!   I think it is very refreshing to find a company so committed to its customers and even more so it’s product (I am after all a second owner of the headphones).  I would like to take the opportunity to thank Simona Vanin of Koss, Jerry Lunn and Les Morris of Hamas for their kind assistance.

I was confident that any other parts (i.e. Cable) could be sourced easily should I have found they were needed.

Stripping Down

Koss Pro4AA Renovation Disassembly of the Koss Pro4AA Headphones turned out to be fairly easy.

The first step was to remove the existing pads from the cans.  They initially seemed to be pretty well stuck in place and i feared that they were bonded there.  It turns out the trick to removing them is to get a finger under the pad from the inside and pry them loose that way.  Once the pad started to lift it was easy to remove the pads.

Koss Pro4AA RenovationThis was the point that the horrible dust that used to be the foam inserts became obvious.  It was simply everywhere, All over the phones, inside the drivers and pretty quickly all over me and my living room.  I am still finding it places.  Fortunately my partner has not equated the arrival of this foul stuff with the restoring of the headphones!

Once removed, a set of screws which hold together the speaker assembly are revealed.  Undo these and the cans just fall apart.  The cabling to the drivers is held in place with little terminals.  These were in good condition on both the left an right hand drivers.

Koss Pro4AA Renovation In less good condition was the connection between the main coiled chord and the cable that connects the right hand driver.  This connection is contained within the left hand can and in my Koss Pro4AA was soldered together.

I am not sure if this was how Koss had originally joined these two cables, but they had been soldered and then wrapped in insulating tape which had perished.  Looking at the solder joint a fair bit of corrosion had occurred.  This was no doubt the source of my occasional crackle in the right hand speaker.

The corrosion was either caused by moisture over the years possibly aided by the original flux from the soldering process.  Flux is mildly corrosive.  Good news was that when I cut and stripped back the cable I found shiny copper pretty much instantly.

The silvered ring around the cans it turns out is made from plastic which is painted with chrome paint.  I remember seeing this kind of brittle plastic in my toy cars when I was young. I removed all grommets from the cable entry holes as well as the nut and washers that is provided for a boom mike on the left can.

Disassembling the headband turned out to be a very easy affair.  The foam insert is glued in place and the glue had long since dried making it easy to remove without damaging the foam.  This was good.

The metal band is held in with a couple of sharp spikes that grip the inside of a channel in the headband.  It was relatively easy to take part.


As I said before, the ease of this restoration was almost a disappointment.  I had booked more of my time to do this.

Once the headphones were in pieces the bulk of the work involved was in cleaning the plastics.

Koss Pro4AA Cleaned plastics Comparison The rubber from the headband was easy.  Gentle scrubbing with washing up liquid and hot water removed the years of stains and dirt pretty quickly.  Using a toothbrush cleared the dirt from the embossed surface without too much effort.  On the inside I did have to scrape out the old glue, but it was so brittle that a flat headed screwdriver was all that was needed to do this job.

The harder plastic of the cans proved to be a bit more resistant to cleaning.  This took several hours of scrubbing… Per Can!. The picture to the right shows the cleaning half way through with just one of the cans clean.

The best solution that I eventually found. (And I tried most of the cleaners that I found under the sink) was to scrub with Mr Muscle Orange Kitchen Cleaner using a toothbrush.  Then I scrubbed with ordinary washing up liquid.  I alternated between the kitchen cleaner and the washing up liquid over and over until the dirt and yellowing was removed.  There is a little difference in the restored colour of the headband and the plastic cans but it is more obvious in the photographs than real life.

DSCN1892 Once I had the plastics were clean, I turned my attention to the stainless steel parts of the headband assembly.

These were treated to a polish using first of all my Dremel with a mini polishing pad and a wax metal polishing bar.  This removed the slight rust stains on the metal parts.

Next up was a polish using a metal polish and paper towels.

Finally I used a soft cloth to give the metal work a final sheen.  In places there is a little pitting where corrosion has occurred, but you have to look pretty closely to notice this.  Otherwise the metalwork came up as good as new.

With all clean, it was time to reassemble the Koss Pro4AA Headphones.

  • They went back together pretty much in the reverse of the tear down.

1.  The cables were fitted first,

2.  Koss Pro4AA Sticking Headband Foam Closeup Then the metal headband parts,

3.  Then the foam was fitted.  I chose to use double sided craft tape to glue the foam insert into the headband.  The double sided tape is flexible and can be used with no mess.  I figured it would not dry out like a lot of glues do.

Koss Pro 4AA Reworked Connections 4.  Next the cans were assembled.  The cables, grommets and microphone fittings first.  I then soldered the right hand drivers cables to the main (curly) chord.  I used proper heat shrink to protect the soldered cables from short circuits or moisture.  This should be more durable than the insulating tape that was there before.

5.  Once the speaker assembly was all screwed together properly is was time to fit the new pads and foam inserts.  These were tight to fit on but just hook over the rim of the speakers mounting plate.  I found that turning the pads inside out made a difference here.  They were then “folded” on to the rim. It took a couple of goes to add the replacement foam pads.

Renovated Koss Pro4AA Replacement Pads In Place All done.  A quick test confirmed that the Koss Pro4AA headphones were back in working order.


Restored Koss Pro4AA Left Cup

Hopefully the photographs do the restored Koss Pro4AA Headphones justice.  I think that you have to look pretty hard to spot that these headphones are anything else but brand new from the manufacturer.

If you want to be picky, where I re-repaired the end caps of the headband, the edge of the cracks are still visible.  The cables as well don't have the new sheen that chord of that type seems to have these days.  Other than that they look like new!

The difference in comfort is very noticeable, well lets face it with the sweaty knackered pads and solid grip of these headphones, the “before” experience was nothing short of painful.  The new pads offer a big leap in comfort for the wearer.  However after a few hours of use your ears do get a bit warm.

The level of isolation that they provide is excellent in either direction.  My partner reports that when I am wearing the headphones she cannot hear a sound from the Koss Pro4AA’s.  We are developing our own sign language now as even at low volume levels I cannot hear much outside noise.

So how do they sound?

I will update this in time.  Let me explain why…

Renovated Koss Pro4AAI currently am driving them from my EeePC901’s onboard soundcard which is far from ideal.  In the pipeline are a couple of headphone amplifiers.  One a tube (or valve as we say in the UK) equipped hybrid amplifier and the other the Little Dot MKV transistor headphone amplifier.  I am looking forward to hearing what they can do with a decent source and decent amplification.  I will update this section when I do.

However even using the EeePC901’s output already i can tell these headphones are good.  I can only compare them to my Sennheiser CX400 ear bud headphones.  I would say that they compare more than favourably.  The other thing I am noticing is that the sound seems to be improving with continued play.  I would think that the drivers have sat idle for a long time and need to regain some of their suppleness.

The bass is weighty with these speakers although it could do with a little more control.  I am pretty sure that this is getting better as they are played more.

Restored Koss The mid range currently is sounding pretty good, vocals in particular are very nice providing that the recording is up to scratch.  I have a mixture of digital media on my network and some of the older stuff is in pretty crumby compressed formats.  The elderly Koss headphones seem to have a bit of get up and go about them as well.

The high end is ok though I would prefer to hear a bit more detail.  Overall though these headphones sound pretty good.

The sound is enclosed.  I normally listen to music from a set of large speakers.  The headphone sound is taking a bit of getting used to.

I think though they need to be driven with a bit more power and control than a lowly EeePC901 can supply.  I look forward to hearing them driven with a dedicated amplifier and a quality source.

I would also very much like to hear the more modern design which is currently being sold by Koss.  If only to find out if the original is indeed better, or whether nostalgia has sweetened the memory of the original owners of this decent (even by today's standards) set of cans.



Looking at the cost that headphones of this headphone should you try and find one on an auction site, they seem to sell between £20 and £30.  Even without the generosity of Koss, replacement pads for these are available for around 5USD.  Re-cabling with even half decent microphone cable should set you back no more than a few pounds.  The rest comes down to scrubbing (although I was tempted to try to clean the parts in a dishwasher!)


For that you get a modern classic.  That is not to say that you can take just any 40 year old headphone and grab a bargain like this.  Because the Koss Pro4AA is still in production, replacement pads are readily available and affordable, even if they are not true to the oil filled design.  There are companies out there that will make custom pads for older headphones, but this may cost too much to be justified for all but the most valuable headphones.

Is this the best headphone ever?  Well probably not, but it’s very very good and worth finding a pair even if it costs a few pounds to restore them.  Value for money you won’t find better

Closed designs have advantages as well if you share a living space with someone else.  I have on order a set of Sennheiser HD650s, but that is not to say I don’t anticipate that my newly restored Koss Pro4AA won’t be getting a lot of love in the future.  They are practical and so far sound very good.

And Finally…

If you are looking to purchase new headphones and have been considering a Koss set, their care for their product, even after 30 odd years should give confidence to you.  They really do mean lifetime when they say it on a guarantee.  I have been very impressed with their service!   It’s fair to say that I could not have done so much so quickly without their help.  Thanks again Koss.