The Kingsway preamp was designed initially around the prototype tone control project built previously. The idea was to incorporate that basic circuit and round it out by adding a volume control, buffering the output to give it good long-cable, low-impedance drive capabilities, and by adding a gain stage set to about 4dB gain with local negative feedback, overcome the losses in the rest of the circuit to provide unity gain.
I also had planned to incorporate a 3-triode phono stage.
At the time the design was being considered, a customer got in touch looking for a line-level preamp, they didn’t need a phono stage but they did want XLR (Balanced) inputs and outputs, in addition to the usual RCA single-ended ones.
Since I’d already designed boards for the preamp, I re-worked the 3-triode phono stage board to be an XLR input/output board instead.
The aesthetics were designed in conjunction with the customer who chose the font and colour scheme and the front panel hardware (switches and knobs)
The XLR output is provided by an active stage consisting of a concertina phase splitter (just like in a push pull amp) followed by buffering of the outputs.
The XLR input is processed to SE by a pair of Sowter 3575 transformers seen here
The preamp has 3 linelevel RCA and 2 XLR inputs leading to a very busy back panel
The total draw of all the stages in the amp amounts to around 60mA at 270V.
To keep things quiet the power supply uses a modified version of the well-known Maida regulator which has been modified to take a MOSFET as the current pass device.
This regulator circuit then uses an LM317 regulator which while not traditionally used at high voltages, can be used floating if suitable provision is taken with blocking diodes and zeners to limit the voltage across it.
The conceptual schematic for the power supply is below
The power supply circuit provides a perfectly stable and ripple-free DC high voltage power.
Performance is excellent: Freq response is flat 20Hz to 20khz as expected, distortion around 0.05% and noise at -85dB (referenced to output level of 1v p-p)
But what does it sound like?
The sound is everything you could hope for. Bright, spacious, laid-back yet intricately detailed, all the things you could hope for in a tube preamp.
About the Name
The Kingsway preamp is named after Kingsway Hall in London, which was regarded as having among the finest acoustics of any recording venue, and which was the principal recording venue for Decca Records until 1984. Sadly now demolished, it lives on in the excellent acoustical quality it gave to the many performances recorded there. Our preamp honours the venue and by extension those musicians and engineers involved with that rich legacy of recordings.