BSM Technology News

After valves (tubes) had already asserted themselves in electronics for a considerable timespan, engineers started from about 1940 to revise certain valve stages in a most clever way. The objective was to specifically optimize the characteristics of particular circuits. In order to give individual amplifier stages a considerable boost in their gain, new concepts – some of them quite daredevil-ish – were developed and patented. In most of these two triodes (we shall limit ourselves to triodes here) were connected "on top of each other" ... which is why the configuration is sometimes designated a "totem pole" amplifier. From a large number of such solutions, only single one (together with its electrical relatives) is still remembered: this is the SRPP circuit (US-Pat.-No: 2.310.342 by M. Artzt, filed 1940). Especially in the field of High-End-HiFi, the further development of the circuit has become and remains well-known as the "Aikido"-amplifier. As field-effect-transistors (short: FETs) were developed at the beginning of the 1960's, the nifty traditional triode-circuits were quickly adapted for this new technology. Now, while a JFET (junction FET) has a sound similar to that of a triode, it unfortunately sports a rather meagre gain. This limits its application. In particular the seemingly simple high-gain "mu-amplifier" (US-Pat.-No: 3.286.189 by E.E. Mitchell, filed 1964) with its highly tricky arrangement of two serially connected JFET's failed to attract much attention at first – it vanished in the depths of the patent offices, escaping almost any notice. It was not before a short introduction of the circuit in the data book of the manufacturer National Semiconductor in 1973 that the public was casually shown the corresponding simple approach to realizing a small FET-circuit with high amplification factor. Subsequently, it still took some time until the circuit found its way into musical instrument electronics. It is, after all, the area of applications for electric guitars where we look – beyond the triode – for great sounding, overdriven stages. Using regular transistors, this was not successful: while those have a high gain, they sound harsh and not musical. Here, the little "mu-amp" put together from JFET's yielded the missing link. At first used only in the DIY-field, it soon found acceptance with the stomp-box manufacturers. By now, whole tube-preamp concepts derived from historically well-proven guitar amps are equipped such that mu-amps replace triodes at the latter's position, with the overall contraption fitted into medium-sized stomp boxes.

BSM finds the old "mu-amp" and SRPP concepts both interesting and great sounding to the extent than we have designed a sweetly crunching Treble Booster using these concepts, beyond our Booster line that we have carried for decades. The first "crunchy booster" based on the "new old" concept is the "Heritage Crunch".