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- Jan 15, 2018
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Just showing off a build I finished today, idea for this amplifier started over a year ago, it has been a long development...
At a high level, it is a 6F5 input with CCS load, cap coupled to a 6BX7 cathode follower, which is direct-coupled to the grid of the 801A output. The 801A is biased at near 0V on the grid, so the grid swings both positive and negative, A2 operation with the grid drawing current, provided by the cathode follower (traditional SET designs are A1 operation only, typically, without grid current). There is a modest amount of negative feedback from the plate of the 801A to the cathode of the 6F5, it is buffered by a PMOS. NFB is necessary to get an acceptable output impedance from the 801A, which has a relatively high plate resistance.
The mains and output transformers were custom-made for this design by Sowter UK. I was going for a "vintage" look, something like an old Hickok TV-7D/U tester. Sockets are vintage Amphenol, knob is vintage Kurz-Kasch. The chassis was designed by me in CAD software, then machined and powder coated by Dave at Landfall Systems, he does all of my DIY chassis, great guy and highly recommended.
This is a 7W into 8ohm amplifier, around 0.3% THD at 1W output, output impedance 1.7ohm.
Here is a schematic of the amplifier circuit. The power supply is pretty complex, so no simple schematic to post, but features one Maida regulator per channel in the B+ supply, one TL431 shunt regulator per channel in the B- supply, and Rod Coleman filament regulators.
Here is the circuit.
And the amplifier itself.
This powers my restored Snell J/II. Of the amplifiers I have built, this one has by far the best sound! Incredible bass dynamics, clarity and staging. Really makes the speakers disappear.
Thanks for checking it out!
At a high level, it is a 6F5 input with CCS load, cap coupled to a 6BX7 cathode follower, which is direct-coupled to the grid of the 801A output. The 801A is biased at near 0V on the grid, so the grid swings both positive and negative, A2 operation with the grid drawing current, provided by the cathode follower (traditional SET designs are A1 operation only, typically, without grid current). There is a modest amount of negative feedback from the plate of the 801A to the cathode of the 6F5, it is buffered by a PMOS. NFB is necessary to get an acceptable output impedance from the 801A, which has a relatively high plate resistance.
The mains and output transformers were custom-made for this design by Sowter UK. I was going for a "vintage" look, something like an old Hickok TV-7D/U tester. Sockets are vintage Amphenol, knob is vintage Kurz-Kasch. The chassis was designed by me in CAD software, then machined and powder coated by Dave at Landfall Systems, he does all of my DIY chassis, great guy and highly recommended.
This is a 7W into 8ohm amplifier, around 0.3% THD at 1W output, output impedance 1.7ohm.
Here is a schematic of the amplifier circuit. The power supply is pretty complex, so no simple schematic to post, but features one Maida regulator per channel in the B+ supply, one TL431 shunt regulator per channel in the B- supply, and Rod Coleman filament regulators.
Here is the circuit.
And the amplifier itself.
This powers my restored Snell J/II. Of the amplifiers I have built, this one has by far the best sound! Incredible bass dynamics, clarity and staging. Really makes the speakers disappear.
Thanks for checking it out!