The AKG BX is a spring reverberator series based on a proprietary design based on a technology called Torsional Transmission Line principle (TTL) which gave the BX range a unique texture and sound.

In 1972, around the time the first digital reverb (EMT 144) hit the market, Austrian based acoustics experts AKG launched the BX range of spring reverberation hardware. The series kicked off with the BX20, a 100 pound refrigerator sized, wooden paneled stereo spring reverb unit that offered a variable 2 second to 4.5 second decay time via the R20E wired remote control. In contrast to previous spring designs on the market, AKG used its own design dubbed the Torsional Transmission Line (TTL) principle. The TTL design itself was all about jamming as long a spring as possible into as small a space to yield the best possible sound.

Over the course of the next decade the BX range was expanded and improved. The BX10, a more compact and portable stereo spring unit was released in 1975. It included some really well thought out design aspects. Despite lacking any rack mounting functionality, the unit itself was built just narrow enough to fit within a 19 inch equipment rack. It also had a side-mounted carry handle which by design would cause the suspension mounted (for acoustic isolation) spring reverb tank to pivot into a resting position that was safe for transport. The 1970’s were closed out with the release of the BX5, a 3u rack mountable unit – the smallest and most cost effective of the BX series retailing at $1195US ($3800US in 2015 dollars).

In 1982 the BX25E followed by the BX15E were released. As the name suggests, the BX25E took the reigns from the BX20 as the flagship model. It came in a smaller form factor not too dissimilar from the BX10 in styling. There were a number of additional features and updates to its design – despite being a third of the size of the BX20, it managed increase spring length by a whopping 20%. It also included a high pass filter on the input and an optional M-250 digital delay module that could provide control over pre-delay and early reflections. The BX15E improved upon the portable BX10’s feature set, offering stepped EQ controls and more decay settings in a unit of roughly the same size.

Despite being discontinued decades ago, the BX series is still incredibly popular among audio engineers. There is an attraction to the rather simplistic appearance and reverb control compared to digital alternatives. Not to mention very few, if any digital plugin or digital hardware reverbs that can match the dynamic nature of these high end professional spring reverbs. Their operation relies on a bunch of elaborately arranged springs that are suspended neatly into a container that isolates them from external mechanical noise. At one end is a transducer that vibrates the network of springs with the audio signal. At the other end is a figure-8 shaped, wire wounded, air-core (there is no metal laminates) coil suspended inside a horse-shoe shaped magnet. Together these two form the magnetic pickup that converts the mechanical vibrations in the spring back to alternating current.

If you are considering an AKG spring reverb for your arsenal, ensure that you get the opportunity to thoroughly test and inspect it. The electronics driving them are rather common and quite easily calibrated. There are a number of tuning trim pots inside each unit that will set the appropriate levels for each side of the stereo signal, and alter parts of the sound like low frequency response which great help in totally dialing out the typical BOIIINNGG associated with spring reverbs. These units are capable of sounding smooth and deep. The area of particular concern is the spring tank itself. It is a proprietary design and AKG ran out of spares quite a few years ago. Snapped springs could potentially be repaired but the air core coil that forms half of the transducer is delicate, unique and at the very center of what makes these units tick. Don’t let that scare you though. Just take it as a precaution rather than as a point of panic. These units are incredibly well engineered and sound absolutely terrific!




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