- Norman Tracy
LSAF 2016 Show Report
An 1/8 inch thick aluminum alloy chassis rigidly supports all of the components and heavy transformers while acting as a heat sink for the tubes. The chassis pulls away heat and dissipates it effectively from the tubes, helping them run cooler increasing their lifespan over cheaper steel chassis. Aluminum transformer cover and back panel, as well as stainless steel hardware, ensure good shielding as well as minimal hum injection into the ground path. Oiled walnut end caps with curly maple inlays furnish the side of the chassis for an attractive finish.
Point to Point hand-wired...
The circuit is all hand-wired using point-to-point method for the amplifying stages. This keeps signal wires extremely short reducing noise and increasing stability. Tube sockets are mounted directly to the chassis to remove the risk of cracking a PCB on cheaper amplifiers caused by changing tubes and heat stress.
The power supply is kept to the back of the chassis with the large capacitors mounted on 1/8 inch thick electrical grade fiberglass eyelet board for easy service and rugged construction. Independent voltage rails run from the supply board to each amplifying stage, helping to keep the coupling of the stages to a minimum. Special attention is made to the grounding and power supply return paths to reduce crosstalk between stages and mantain signal-to-noise integrity. A multiple star ground configuration is used to control ground currents.
"The 22A3 sounded simply marvelous. I have a great affection for and long experience with the sound of triodes driving high efficiency speakers so with some confidence I can state this is the real deal."
For this design, emphasis was placed on low distortion and noise while still maintaining an open loop, or zero feedback design. To help achieve this goal, a total parallel design was implemented. The front-end amplification is handled by a paralleled 6SL7 twin triode reducing preamp noise and improving signal linearity significantly. A high current parallel 6SN7 driver stage is directly coupled to the input 6SL7. Direct coupling of the preamp removes the need for a coupling capacitor in the signal path to diminish the chance of signal degradation. The high output current of the 6SN7 driver achieves maximum signal swing into the 2A3 output stage with high slew rate and bandwidth, extending well beyond human hearing.
Fixed bias for maximum headroom...
The driver stage is capacitor coupled to the final parallel 2A3 output stage and is the only capacitor in the signal path. This isolation allows for a fixed biasing of the 2A3 tubes for maximum output power and low distortion even at very low frequencies. The parallel power stage offers many benefits including a reduction of the amplifier output impedance for better loudspeaker control and increased dynamic range. Hum balance control potentiometers are provided to perfectly null noise caused by the AC voltage used to heat the filaments in a hum-bucking configuration. Extensive design consideration was also placed on a very high quality output transformer. This custom designed 9 lbs transformer enables excellent distortion performance and superb bandwidth unheard of in a parallel single ended design. The large core size also eliminates the risk or core saturation at full power. 4, 8, and 16-ohm speaker impedance taps are provided to accommodate a verity of loudspeakers.
With a GZ43/5AR4 tube rectifier used for generation of the high voltage supply, voltage sag is minimized and a controlled startup of the supply is achieved. A CLC filter topology utilizing a large 10 Henry choke reduces the voltage ripple greatly for a very low noise high voltage supply. A separate voltage supply is used to provide bias voltage for the 2A3 output stage. Bias adjustments can be made via a potentiometer on the fly with the idle current of each 2A3 monitored by analog ammeters for ease of use.
Premium components are used throughout for the upmost performance, sound quality and longevity.
Featuring a total parallel design, including parallel preamplifier, driver stage, and output stage, the 22A3 gets most of the one of the best audio tubes ever produced, the 2A3. Although this amplifier is rated at 5 Watts, the parallel output stage offers robust power and LF control all the way up to its 5W limit.
Why the 2A3...
While the 2A3 is often overshadowed by venerable 300B tube, which can deliver more power, the 300B does not offer improvement in bass control and weight; the 300B internal resistance nearly the same to the 2A3. To worsen the problem, speakers requiring more power also required improved control over low frequencies. Our parallel 2A3 design offers lower source impedance and better LF control than many higher power SE amplifiers giving this 5W amp the detailed and lifelike midrange which SET amplifiers are known for with more accurate and extended bass response.
This amplifier provides stunning detail and nuance throughout the entire audible band while maintaining low distortion and an impressive -95dB SNR all without the use of negative feedback. With high efficiency speakers, this 5-Watt amp is more than ample for high sound levels even in large rooms. If less efficient speakers are used or more volume is demanded, feedback can be applied in Closed Loop mode with the flick of a switch to reduce distortion and output impedance while linearizing the amplifier at higher speaker excursions for even better blow frequency control.
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The 22A3 Monoblock Power Amplifier is a Class-A 5-Watt Parallel Single-Ended Triode (SET) tube amplifier designed for those who prefer higher efficiency speakers for audio listening.
With high efficiency speakers, the amplifier requirements are unique...
Average listing power on high efficiency speakers can be just a few hundred milliwatts. Small details in the back of the mix can approach just tens of milliwatts at very audible levels. However at these power levels, a typical push pull amp is sitting at idle in the crossover region.
Speaker distortion and excursion are very low for high transient accuracy giving the ability to hear the amplifier's sound signature much more readily.
The 22A3 is Bandwidth Audio’s answer to this problem.
Do to the praise of the features designed into the 288, we carried many of these over to the 22A3 as well.
A 24 position stepped attenuator is used for fine tuning the volume. Compared to a standard rotational potentiometer, the stepped attenuator clicks into one of 24 positions setting the volume precisely and accurately through a fixed resistor network while eliminating noise associated with conventional pots. Not only is noise removed, perfect matching between channels is achieved by setting each channel into the same position of the volume control for a perfect stereo image.
A VU or Volume Unit meter on the front illuminates when the amplifier is powered on and indicates the relative power output to the speaker. It offers compression and ballistic control of the needle movement. It is calibrated to show 0dB at 5 Watts of output power. No longer do you have to worry about overpowering and damaging sensitive loudspeakers.
Never visit a technician, adjust bias at home...
External bias adjustment of the 22A3 output stage has been added so the best performance can always be achieved. Analog ammeters monitor the current in each of the output tubes so no additional metering required. To bias, simply set the meters to the indicated level in the User's Manual via the adjustment point on the back panel.
These current meters also serve as an in-circuit tube tester. If there is a discrepancy in the measurement between meters, the output tubes are no longer matched.
Redefining High Fidelity Audio Reproduction!
Parallel Single Ended Triode Tube Power Amplifier
Handcrafted in Austin TX USA
Maximum Output Power
THD+N at 5-Watts 1KHZ
2.75% Open Loop
2.5% Closed Loop
Full Power Bandwidth (5-Watts)
<13Hz - 23KHz at -3dB Open Loop
<13Hz - 32KHz at -3dB Closed Loop
THD+N at 1-Watt 1KHz
0.5% Open Loop
0.35% Closed Loop
<13Hz - 22KHz at -3dB Open Loop
<13Hz - 32KHz at -3dB Closed Loop
Input Voltage for Rated Output
1.3V RMS Open Loop
1.7V RMS Closed Loop
SNR 1-Watt into 8ohms
-90dB A-Weighted Open Loop
-91dB A-Weighted Closed Loop
SNR 5-Watts into 8ohms
-95dB A-Weighted Open Loop
-96dB A-Weighted Closed Loop
14.5dB Open Loop
12.5dB Closed Loop
Negative Feedback (Closed loop only)
4, 8 & 16 ohms
1x 6SL7, 1x 6SN7
100 Watts at idle
20" x 13" x 9.5"
Due to the open loop design of this amplifier, actual performance will vary depending on vacuum tube quality. Specifications subject to change.