DC Filament:
The Stereomour II DC filament supply upgrade is a DC supply conversion for the AC heated 2A3 filaments of the stock Stereomour II amplifier. There is a separate DC supply for each 2A3, composed of low voltage drop Schottky rectifiers, a CLC filter using a common mode choke. This supply will reduce the already exceptionally low noise floor of the AC heated Stereomour II amplifier to levels acceptable for use with the most sensitive horn speakers.
Its installation is recommended in particular for use with speakers above 96dB@1W@1M sensitivity. It may also be useful in systems employing speakers with a lower sensitivity, enhancing the sense of “black background between the notes” in quiet passages of music.
Shunt Regulator:
Directly heated triodes like the 2A3 output tubes used in the Stereomour II amplifier are exceptionally linear, which contributes to a DHT amp’s great sense of realism. One of the challenges of designing a fully capable DHT amp is to make the preceding driver stage as close as possible to the output stage in terms of linearity without sacrificing gain. We use a couple of different means to optimize the performance of the driver stage.
The stock Stereomour II amplifier uses our well know Camille Cascode Constant Current Source (C4S) to regulate current through the 12AT7 driver tubes. This gives the first amplification stage exceptional isolation from power supply ripple for low noise, and creates a very high impedance load on the tube to elicit the maximum gain from the driver stage while keeping the tube in a linear operating condition to higher input signal levels than a typical resistor loaded driver stage.
The Stereomour shunt regulator upgrade kit adds shunt voltage regulation to the C4S loaded driver stage with the application of the same hybrid shunt regulator circuit used in our premium Kaiju 300B amplifier, BeePre 300B preamplifier, and Mainline headphone amplifier kits. This hybrid tube/solid state regulator tightly controls the supply voltage to the C4S current source loading the driver tube, creating a sense of greater bandwidth and better dynamic performance.
Attenuator:
When we were developing the BeePre 300B preamp kit we built the prototype preamp circuit itself on a pair of chassis, and we built several outboard chassis platforms that allowed us to try various attenuator schemes. We tried all sorts of schemes – inexpensive carbon potentiometers, our old standby Sweetest whispers attenuator, high end stepped attenuators from other manufacturers, transformer volume controls, ladders, shunts, series steps, etc., etc. The end result was the popular BeeQuiet attenuator upgrade for the BeePre.
This design beat out the others by quite a sonic margin, and now the same 36 step attenuator design is available as an upgrade for the Stereomour II amplifier (this kit will not fit in earlier versions of the Stereomour amp).
We felt that an “ultimate” attenuator for the BeePre needed to be done to a higher standard and we felt that it would be reasonable to make the kit similar in price to the other high end attenuators on the market. So we got very high quality rotary switches and PB put his math degree to work coming up with a very useful thirty-six 1.5dB steps, yielding a total -52.5db of attenuation. Yes, you read that right, 36 steps. We accomplish this with two six position switches, one a stereo coarse control with 9db steps and the other a stereo fine control with 1.5 db steps. Simply set the coarse control in the general range you want to listen and then use the fine control to get the level perfect. Now that high quality attenuator is available for the Stereomour II.
The switches are populated with Vishay Dale metal film resistors, a resistor that has remained a favorite of ours over the years for attentuator use. The TwoQuiet coarse and fine stereo switches replace the stock carbon pot volume and balance controls in the Stereomour II. They mount on a custom aluminum sub-chassis, and we provide a neat overlay for the top of the chassis, with attenuation settings, input selector setting and a cool vintage look.
As always the kit comes with an extensive, detailed manual. Assembly and installation is probably a one or two evening affair.
And now to address the question in everyone’s mind – why install this upgrade?
When we put the protoype in our production prototype BeePre the bass punch and dynamics became like Wow! Can’t say for sure why, as both the stock pots and the BeeQuiet are about the same impedance at around 50K ohms. So this is another case where we won’t try to come up with some far fetched theory. We will just suggest strongly that you build the Crackatwoa with the stock volume and balance pots first, then install the TwoQuiet upgrade and share your impressions.
