The 2A3 pictured here is a custom job for Jim Denton. This amplifier uses a conventional 2A3 output circuit driven by the Aikido as developed by John Broskie. You can read all about the Aikido line driver configuration in the following links:
Aikido Amplifier
Aikido Variations
More Aikido
The Aikido's claims to fame are extreme neutrality and very low noise. The noise floor is limited mostly by the choice of tubes, resistors and wiring techniques. Power supply noise is virtually eliminated by injecting anti-phase noise into the triode loaded cathode follower final driver stage. The input stage is conventional SRPP. By incorporating the Aikido circuit, the driver circuit can take its voltage earlier in the power supply allowing higher pre-stage voltages while maintaining the customery output stage supply voltage. This aids in optimization of overall input stage linearity and more headroom for large, low distortion signal swings without sacrificing the PSRR figure. In addition, similar currents in the input and driver stages swing in anti-phase, minimizing noise tranfered back into the power supply.
The result is that a dead quiet driver stage of relatively low output impedance to drive the 2A3. The only limiting factors are the 2A3 AC hum and output PSRR which can be mostly cancelled via a conventional hum balance control and by virtue of making the noise rejection circuit adjustable, as is done in this amp. What we end up with is the true sound of the 2A3 without coloration by the driver stage(s) and an output noise figure reading less than 3mV peak to peak as viewed on the scope (which includes quite a bit of local noise not related to the amp).
Here's the offer!
For the first time ever, I will build a series of this amplifier for the discerning audio enthusiast for only $750 per unit. This will be a limited run of units. Unlike the amplifier pictured, the output and other trannys will NOT be painted. You'll get a basic black and silver theme. See side panel note below. Hammond transformers in the power chain. Excellent quality standard parts used throughout.
Tubes:
The tube complement will be the Valve Art (or Electro-Harmonix - your choice) 2A3 output tubes; new stock.
Other 2A3's may be used at additional acquisition costs (you specify).
A 5U4 rectifier from NOS or used tested as NOS. The 5U4G upon request additional $25.00.
After this you have a choice of configurations as follows:
(These tubes are NOS or used tested as NOS with brands and tubes taken from inventory as available.)
1. Octal set includes 6SL7 for SRPP stage and the 6SN7 for the driver stage.
2. 9-pin set includes 12AX7 for SRPP stage and the 12AU7 for the driver stage.
Optional: Using the 5965 in bypassed mode for the SRPP stage.
Other tube combinations can be used according to your tastes, but tubes such as the 6DJ8 will have low gain for this amp.
Other useful input tubes for this amp include: 5751* and the 5755.
If you prefer, tubes such as the 6CG7 and 12BH7 can be used in the driver stage.
The input stage will be capacitively bypassed for all version EXCEPT the 12AX7.
NO bypassing is optional for the 6SL7, 5751* and 5755 but at slightly lower gain according to your tastes.
* I do not have inventory on the 5751
Construction:
Options:
1. Centered power supply as in the picture.
2. Power supply to one side, both channels together (preferred for improved power, ground and signal routing).
Chassis and parts:
Heavy duty .080 clear-anodized lightly brushed aluminum w/internal shield and support
"Natural" black and silver color scheme, black painted side panel (default).
Painted or natural pine wooden side panels (see side panel note below)
Hammond power and choke transformers
Output transformers 3K by Transcendar Transformers of California USA made.
High quality metal film, carbon comp and wire-wound resistors throughout
Silver bearing solder used throughout
Silver coated copper wire in signal path
Single-strand silver input wire option (no charge)
Input bias-wire option (no charge - adds input coupling cap)
My default input and output connectors as shown above
ALPS 100k dual audio taper volume control
Paper foil coupling cap to 2A3 grid
Specifications:
Power at 1% THD 3.2W/ch.
Hum & Noise: 80db or better below rated power
Freq. Resp.: 10Hz to 30KHz at rated power +0db -3db points
Output Impedance: Dual connectors for 4 or 8 ohms (sorry, 16 ohms not available)
Approx. size: 16"W x 13.5"D x 3"H Height from top of chassis to shelf. Depth includes connectors and knobs.
Options:
ALPS 100K (or 250K) audio taper precision step volume control add $35
5751 input tube (Works in all 9-pin versions. Increases gain in 5965 version, lowers gain in 12AX7 version.) $$$Current market value or you supply. The 12AT7 also works in the 5965 version with slightly higher gain.
Tube Note:
2A3's are new production either Valve Art or Electro-Harmonix (your choice). All other tubes may be NOS, new current stock or used tubes thoroughly tested as new according to stock and availability. Other 2A3 brands can be supplied at appropiate increase in cost. All tubes tested for microphony.
If the input stage uses the 5965 the cathode resistor is capacitively bypassed. If you do not desire input bypassing, you may substitute the 12AX7. 12AX7's will be supplied from existing USA made inventory or purchased as required and may or may not be NOS. RCA is likely the default, but other USA brands may be used according to availability. Tubes such as the 6N1P-EV, 6922 and 6DJ8 cannot be used in this circuit due to too low gain.
Side Panel Note:
Side panels are made from craft finish-quality pine. The default is painted black. You can opt to have these painted in color or urathane for a natural look. Urathane tends to darken the pine a little. I can also stain the pine followed by a clear coat. Exotic woods can be used. You supply/purchase the wood and I will install it for free. I am also offering your choice of using oak (sanded and clear-coated) or cherry (oiled or clear-coated) at no additional cost.
There will be a limited run of units over the summer months on a first come first served basis.
You may also specify optional parts to be placed, such as the final coupling cap to the 2A3. For any such specification, simply add in the cost of the part(s) you desire.
Estimated turn around according to work load on a first come first served basis.
Estimated time 60-80 days.
NOTE: Actual construction details may vary from amp to amp. Pictures are of a completed unit and reflect the same general appearance as the one you order.
Terms:
A 50% deposit is required to reserve your amp.
Payment by money order or Paypal (add 3% for Paypal).
Deposit 100% refundable until I start your chassis. After that deposit is 50% refundable.
Delivery times are estimates and do not form a contract.
Balance is due within 30 days of completion.
These are "hobby devices" and may or may not meet any specific consumer or local codes for safety etc. All amps are fused against catastrophic failure. The chassis design is open-bottom. Sale assumes user is familiar with tube amp usage and high voltage risks. Complete instructions and documentation are provided with each amp.
Final note:
I have worked up an all direct coupled version of this amp thus eliminating the coupling cap to the 2A3 grid.
This version uses a 12AX7 as the SRPP stage and is unbypassed also, eliminating all capacitors in the direct audio chain.
Email me for a discussion about this amp if you are interested. Estimated price: $850.00.
EMAIL:
abraxasaudio (at) att (dot) net
