Almost forgot to describe one of the most obvious changes in the DiaLogue Premium design … the use of 6 x 12au7 tubes …. Lots to tell you about this.
One + One = Two, but Two + One =THREE … 3 x 12AU7 per channel
So far we have used one 12AU7 tube per channel as phase shifter and driver for the power tubes. In this situation each leg of the push-pull output stage is driven by only one triode.
That configuration is powerful enough to reasonably drive 1 – 2 power tubes. But … It is a known fact that an output stage benefits greatly from a “powerful” drive. The stronger the better, although there are limits to this of course.
For example in military transmitters, the output stage is often driven by a driver stage which already can supply half the power of the entire amplifier. So a 100Watt transmitter could have a 50Watt driver stage. Just to drive the power tubes …
The extra 12AU7 tube in this new Dialogue Premium design allows for a far more powerful driver stage. Suddenly we can use now for each half of the power amp one full 12AU7, and the two triodes in this tube can be put in parallel. This reduces the impedance of the driver stage by half.
This creates the situation that the power tubes are driven from a 50% lower impedance, which results in a double current supply capacity of the signal drive for the power tubes, which has positive effects on a few key parameters:
1. The bandwidth of the driver stage becomes twice as wide, increasing the entire bandwidth of the amplifier significantly,
2. The linearity of the driver stage becomes twice as linear, reducing the dynamic distortion of the amplifier significantly,
3. The linearity of the power stage is further increased, which results again in a reduction of the dynamic distortion of the amplifier.
This results in:
Less phase-shift in the extreme high frequency range, reason why the amp is more stable and the harmonics of the instruments are reproduced in a more accurate way.
The amplifier has less phase-shift in the extreme low frequency range, so the amp can supply more energy and deliver a tighter bass.
The feedback has less phase-shift to compensate, which makes again the amplifier more stable.
The power tubes are driven with more control, which results in a more dynamic stable output signal.
The static distortion, as measured with a tone on a distortion meter, will not decline significantly. Just a little. But the dynamic distortion, which the human ear perceives as unpleasant distortion, is significantly reduced. It makes the amp sound cleaner.