The BeePre 300B preamp has been Bottlehead’s top line preamp for several years now. The idea was to produce a single gain stage tube preamp with a low output impedance and gobs of drive to work with amps like the Kaijui 300B and MonAmour 2A3 amps – which we achieved. BeePre 2 takes this concept and refines the circuit and layout for better sound, an easier build, lower noise, the capability of working with solid state and push pull tube power amps (with the addition of upcoming upgrades), and more reliable performance worldwide with a universal power transformer.
The quick backstory – it was decided in 2013 to build a prototype 300B preamp. The result, BeePre, was the best preamp we had made.
As happens with any product user feedback over the years helped us to prioritize some improvements we felt useful. The basic concept of a super linear 300B Directly Heated Triode (DHT) as a preamp tube was – pardon the pun – sound. However this type of design is very challenging to dial in and we felt we could improve things a bit more. So we have addressed a few issues that we and our users found to be a little bothersome in the original BeePre.
One was to come up with a way to reduce the slight ringing (microphonics) that all DHTs generate. This we did by moving the volume control to the output of the preamp. It’s an unconventional move that requires some mathematical trickery regarding the attenuator values and gain structure. But it works well to reduce the noise floor when the attenuator level is reduced.
The next thing we addressed was the need for a universal power transformer. The massive, well designed PT-9 universal power transformer used in the Kaiju 300B preamp was employed. The PT-9 also allowed us to improve the BeePre’s handling of voltage sag, which in a few cases caused some noise when a user’s AC mains voltage dropped below nominal and affected the original voltage regulators. The new fliament supply regulator adopted from the Kaiju is much more tolerant of those variations.
Other changes were made with an easier build in mind. We found that some of the improvements we had incorporated allowed us to create a more basic and simple overall circuit that still retained the goodness of the original BeePre while making the build far less complex and much easier to debug. The original 12″ x 12″ chassis has also been rescaled to 10″ x 16″ to better match the size and shape of the Kaiju 300B Stereo amp.
The original BeePre preamp came with a quasi-balanced XLR output as well as single ended RCA output. That added some complexity and some limitations to the circuit, and not that many users ended up using the balanced output anyway. So a buffered balanced/single output option will be part of a shunt regulated high voltage supply upgrade that will be released in the near future. Mounting holes for a balanced input transformer will be carried over from the original BeePre as well, for those wishing to perform their own balanced input mod.
What is the bottom line? The improvements to the circuit make the Basic BeePre2 a match to the sonics of the original Beepre. The upgrades will take it past the original BeePre, with a very noticeable improvement in bass punch, lower noise floor, and more natural vocals. The upgrades will also make the BrePre2 a good match to many solid state and push-pull tube amps. Digital sources and analog sources like the Eros phono preamp and Tube Repro tape head preamp will all work exceptionally well with the BeePre2.
Performance? Input impedance is 70K ohms with balance control centered. Output impedance is 3K ohms at maximum (this can be lowered to about 400 ohms with the upcoming shunt reg/buffer upgrade kit). Gain of our prototype at 1kHz measures 6dB. Frequency response -3dB points at 7Hz and 110kHz. Noise level will depend upon the brand of 300B you choose. Our basic BeePre prototype’s S/N measures -70 dB with 2V signal input. THD measures 0.21% at 1kHz at 2Vrms output. Maximum output before the onset of clipping (which presents itself as a very gradual rounding of the sine wave trough) is 32Vrms.
