Headphone Amplifiers

The Glensound Origin Series:  The AIM Headphone Amp System.  The Origin Of The World’s Most Influential Headphone Amp

Glensound, Maidstone, UK.   January, 2012

Glensound was asked by the BBC to produce a unique headphone amp and mixing system to be used at the multiple arenas for their coverage of the London, 2012 games.  The usage scenario was quite unique.  There was a broadcast ‘cell’ in each arena.  This cell consisted of 2 x presenters, 1 x cameraman, 1 x engineer, and 1 x producer.  The BBC needed a portable headphone amp system to provide outputs for each of the users, and to produce 2 separate mixes, basically the talent mix and a technical mix.

Six outputs was deemed to be the magic number of headphone connections.  Now consider this:
-          The unit needed to be battery powered
-          The cameraman could be wearing 600 ohm stereo headphones
-          The talent could be wearing a 32 ohm mono earpiece
-          Of the 6 headphone outputs, any type of headphone could be plugged into any connection at any time.

Those of you that understand the technical implications will see that this is a very challenging environment for a headphone amp.

The system that made it all possible was a brand new design of headphone amplifier, developed by Glensound’s Chief Engineer Paul Grant.  The impedance of the connected headphone is constantly being monitored.  When a new headphone or earpiece is connected, the impedance is measured and this level directly affects the gain range of the volume pot for that individual output.  It gives the pot a useable range for the individual headphone/earpiece, meaning that the lower end of the pot is low level, and the upper end is high level.

The elegance of the system is that the pot range is constant regardless of the impedance of the connected headphones or earpiece, making the system one of the most versatile and practical systems in real world broadcast applications.  Low impedance earpieces could still be turned down to a sensible level, still with a usable range on the pot.  High impedance headphones could still be turned up to high levels, also still with a useful range on the pot.  It was this practicality for use with a wide range of headphones and earpieces that made the system a totally unique solution and a resounding success. 

The design was come to be known as the AIM (automatic impedance matching) headphone system, and it was so popular that it is still the standard used on most Glensound headphone outputs today.

The first product to feature the AIM system was the TopHAT M3, and was used at all the portable broadcast locations by the BBC at all the arenas around the London 2012 site.  The AIM system is also used in the world’s most popular network audio headphone amplifier, the VIRGIL.