Axiom DC300B Data and Specifications
The development of the Axiom is complete or almost. We have made significant modifications since the last update. The Axiom is now sporting more diodes. The most significant change was the addition of a diode in the anode circuit of the 300B. In addition the cathode resistor in the 6Y6 circuit has also been replaced by a diode. There are no resistors or capacitors in the audio circuit only wire and tubes.
Along with the on going "ear" testing we have just completed weeks of analysis using a HP FFT Dynamic Signal Analyzer. The test results confirm what the listening tests were telling us.
Total Harmonic Distortion
Before reporting on the Axiom let me give you the numbers from a well-regarded traditional design 300B amp. This amp was constructed using highly regarded caps, resistors, and transformers and has received rave reviews from Central Florida audio community. The Total Harmonic Distortion (THD) for the amp was measured at 1 kHz.
1 Watt = THD 1%
2 Watts = THD 2%
5 Watts = THD 5%
6 Watts = THD 200%
The Axiom measured using the same measurement techniques
1 Watt = THD 0.042%
2 Watts = THD 0.110%
5 Watts = THD 0.512%
6 Watts = THD 1%
7 Watts = THD 5%
The low distortion measurements are reflected in the many listening test evaluations, the Axiom Amp has great dynamics and sounds exceptionally "clean". The THD distortion numbers are those that would be expected of an amp with significant global feedback. The Axiom, of course, is single ended without global feedback.
Another important difference between the two amps is the shape of the distortion curves. The Axiom had virtually no harmonics beyond the third. The higher order harmonics were buried in the noise floor. Whereas, the traditional amp had harmonics that were distinct and measurable out to the ninth harmonic and the odd order harmonics did not roll off evenly.
Another important comparison is how the amps respond at higher outputs. Since the harmonic distortion of the traditional amp changes significantly as the power increases the apparent timbre of the instruments change as the level is increased. The Axiom, on the other hand, maintains low distortion numbers throughout the power spectrum; thus the instruments maintain the same quality throughout the dynamic range.
The large jump in distortion that occurred in the traditional amp was the result of clipping. The traditional design begins clipping at approximately 5.25 watts. The Axiom clips at approximately 6.25 watts. When the Axiom clips however the clipping is more benign. During listening test the Axiom amp was pushed to approximately 8 watts without any noticeable distortion at the peaks. The headroom of this design is noticeable when listening to uncompressed orchestral CDs. The amp effortlessly reproduces the sound of single violin through the sound of the full orchestra at a triple fortissimo.
Following are some additional test results that will help put the Axiom's design into perspective.
All of the diodes were removed and equivalent resistors substituted.
1 Watt - THD 0.557%
Compared to the traditional design the Axiom did well even without the diodes. The following tests were done sequentially with resistors being removed and diodes inserted.
The resistor in the cathode circuit of the 300B was replaced with a diode.
1 Watt - THD 0.123%
The resistor in the cathode of the 6J5 was replaced with a diode.
1 Watt - THD 0.079%
The diode was added to the 300B's anode circuit (with the previous diodes still in the circuit)
1 Watt - THD 0.051%
The resistor in the cathode circuit of the 6Y6 was replaced with a diode.
1 Watt - THD 0.042%
Blind listening tests for the various iterations were as follows:
Diode in the Cathode of the 300B
Significant improvement in presence and low-level detail
Diode in the Cathode of the 6J5
Better dynamics and a slight improvement in low level detail
Diode in the Anode of the 300B
Improved open-ness and slight improvement in low-level detail
Diode in the Cathode of the 6Y6
Slight improvement in low-level detail
The THD test was also performed at low frequencies (60 Hz and 400 Hz) and at high frequency (5 kHz and 10 kHz) Although there was a slight increase in THD it was always less than 1% at all frequencies. This also is very unusual for single ended amps.
Generally single ended amps have significant increases in THD at both the low and high frequencies. It is not unusual to find distortion on the order of 2% or more at 1 Watt. The fact that distortion and the distortion ratios remains relatively constant throughout the frequency spectrum of the Axiom ensures that the timbre of instruments do not change throughout the range of the instrument. In listening test, this improvement was particularly noted on solo piano. The piano didn't take on a harshness or edginess that is often noted in the upper octaves.
The bandwidth of the traditional amp was found to be 12Hz | 57 kHz +/-3dB
The bandwidth of the Axiom Amp is 20 Hz to 90 kHz +/- 1dB the bandwidth performance is not reduced as the power output goes up. In fact it gets wider!
The Axiom produces virtually distortion free signals down to 20Hz whereas a traditional amp has significant distortion in this range. The result is that the bass produced by the Axiom is cleaner and tighterh and therefore sounds gbetterh than the traditional amp. Much the same can be said for the high end.
In part, the reason for the extremely low distortion numbers across the entire frequency spectrum is the use of plate load chokes. It was noted during listening tests that a change in THD of less than 0.03% was perceived by the listeners. Since distortion is so critical to how we perceive music, maintaining low distortion across the bandwidth is critical.
We feel distortion ratio curves are as important as distortion levels. The Axiom has a steep downward quadratic curve. Third Order is about 1/3 of Second Order and Fourth Order is about 1/3 of Third Order, Fifth and Higher Orders are below the noise floor of the HP Spectrum Analyzer. The Axiom ampfs distortion ratios do not change as power output goes up. In fact the amp was quite listenable at power peaks of 13 Watts.
We must caution you that how effective using a tube diode is in a circuit depends on matching the correct diode to a specific amplifier tube. A diode tube has both static and dynamic resistance, see diode tube curves. It turned out that the same diode turned out to be the best for the 6Y6 and the 300B tube. The 6Y6 has a mu of 6-9 and a plate resistance around 900 ohms. However that tube diode did not perform as well as a different diode on the 6J5.
We have lost count on the number of revisions made and, like a tube diode on the anodes of the 6Y6 and 300B, why the revisions made a difference. Is the Tube Diode acting as a Cascode circuit with the top tube having a gain of zero? In that case is the reduction of Miller Effect responsible for reduced distortion and improved sound performance. Or is the additional tube diodefs dynamic curve combining with the amplifying tubefs curve to glinearizeh the tube and reduce distortion for improved sound performance.
Once we did the math and established the original circuit, we only used listening evaluations, for parts selection and to optimize and refine the circuit. Once we had reached the circuit on the published schematic, a very powerful HP spectrum Analyzer was used to verify what our ears told us to do. I firmly believe if we used the test equipment after every revision, we would have never reached the performance level the amp has.
Bear in mind there are a lot of design elements that are responsible for the Axiomfs performance; no resistors or capacitors in the signal path, plate load chokes for loading the tubes and establish bias voltage for the following tubes, separate stacked power supplies with high Henry chokes. The use of hum bucking interaction in the separate power supplies to reduce hum.