Mission 753

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.

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Hammer Dynamics Super 12 Fullrange Loudspeaker

imageBrendon Lee made a set of these Hammer Dynamics Super 12 loudspeakers for himself.  You may be more used to seeing loudspeakers with multiple drivers per cabinet.  There is a good reason for this.  Most speaker drivers are good at reproducing a certain part of the sound spectrum.  Generally speaking the smaller the speaker cone, the better the speaker driver is at producing high frequencies, The larger the cone, the better at low frequencies.(i.e. bass).

This is not the whole story, and indeed using multiple speaker drivers carries with it its own problems.  One such problem is matching the sound of multiple drivers to get a consistent sound (volume wise and also tone wise) across the frequency range.  Other issues arise in the electronics that filter the signal to each speaker.

Another solution then is to find a single speaker driver that is capable of reproducing the full range of audible frequencies (at least with the assistance of a cabinet).  This is the principle of a full range loudspeaker.  Brendon’s Hammer Dynamics 12 Full Range speaker does use a super tweeter to help at the very top end.  Here is what he thought of the Hammer Kit.

This article has been republished with Brendon Lees kind permission, previously being homed at Brendon’s Geocities hosted site.

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Zigmahornet Speaker Cabinets

Full Range Speakers offer some advantages over the more traditional 2 way or 3 way designs, though have their weaknesses as well.  For the hobbyists, a Full Range design negates the need for a crossover and with a lower component count can appear attractive.  But do they match up to good 3 way designs?  In this article Robert Powell lends his experiance and impressions of the Zigmahornet design.  A slim floorstander with a suprise or two up its sleeve.

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Brendon Lees, Wayne Parham's Theatre 4Pi Loudspeaker Build.

imageIn this article Brendon Lee gives a description of his build and listening impressions of the 4Pi Loudspeaker kit.  This kit is available here for a princely sum of $380 dollars.  Comprising of a 15’ woofer and horn tweeter, this kits background is routed in the PA market both in parts count and appearance.

The original design was by engineer Wayne Parham, the kit components can still be purchased here.

This article is republished with kind permission of Brendon Lee and was originally published on his Geocities website.

Read more: Brendon Lees, Wayne Parham's Theatre 4Pi Loudspeaker Build.

Mission 753 Crossover Capacitor Swap

altI love my pair of Mission 753. I have used these speakers for several years now and been very happy with them. However they are of an age now where guarantees have long since become distant memories. Time to tinker with them. This article deals with the crossover, in particular the benefits of a capacitor swap.

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