About OTL Stereo Amp Build
OTL Amplifier Working!
A brave undertaking with suggestion, encouragement, help and guidance from a very dear friend in Pacific Northwest DIY audio club.... Took 2 years from inception of idea to final sound. Lots of planning, discussions, and construction of custom hand-crafted aluminum case. All point-to-point wiring with thoughtfully pl
OTL Amplifier Working!
A brave undertaking with suggestion, encouragement, help and guidance from a very dear friend in Pacific Northwest DIY audio club.... Took 2 years from inception of idea to final sound. Lots of planning, discussions, and construction of custom hand-crafted aluminum case. All point-to-point wiring with thoughtfully planned layout.
Did I mention this is the first amplifier that I built? Learned a lot, and still learning...
Amazing sound -- wide and deep sound stage. Very open and fast response for live, engaging listening experience. Incredible bass, midrange and treble response. Effortlessly driving large Soundlab electrostatic speakers.
- Output power: 80W/channel pure tube OTL (Output TransformerLess) power into 4/8/16 ohm load
- Full power bandwidth: 2Hz (0dB) to 100KHz (-3dB)
- Input sensitivity: 2V p-p
- Gain: 28dB @8ohm load
- Zero negative feedback
- THD+N: 5% at full power
- BIAS: -52V, adjustable -40V to -80V, with digital LED monitor
- DC-offset adjustment with 4.5 precision digital LED monitor: Stable at +/-0.002VDC
- Built-in 4.5 precision digital LED tube tester to monitor each output triode mA current draw (40 triodes using front panel selector switches)
- Tube complement: A lot of tubes... 10 x Svetlana NOS 6H13C per channel (total 20 output tubes for stereo), 4 x Sylvania NOS 6SN7GTA for input stage, 2 x RCA NOS 6SN7GTB for driver stage
Power Supply
- 103 lbs, all aluminum hand-crafted case
- 7 transformers, including 2 x 6.3VAC/28A filament transformers for output and driver tubes
- 2 x 10H/100mA chokes for input and driver stage supply filtering
- High current and voltage custom-built Hexfred bridge rectifiers
- Modular multi-layered construction with multiple shelves, custom layout
Power Supply
- 103 lbs, all aluminum hand-crafted case
- 7 transformers, including 2 x 6.3VAC/28A filament transformers for output and driver tubes
- 2 x 10H/100mA chokes for input and driver stage supply filtering
- High current and voltage custom-built Hexfred bridge rectifiers
- Modular multi-layered construction with multiple shelves, custom layout for optimum placement and wiring
- 72,000uF output HT filtering capacitors, 400 joules energy storage per channel, total 800 joules
- 1,000uF + 2 x 10uF Clarity Cap ESA input/driver stage HT filtering capacitors
- Fully automated relay start and warm up sequencing with a single push button
- 3 slow-blow fuses: 4A output filaments, 8A output bridge supplies, 1A input and driver stage
- 7 Neutrik umbilical connections for remote power to the amp unit on top
- Neutrik PowerCon AC line connector
- Dimensions: 12"H (+2" feet) x 18.25"W x 18.25"D (+2" power connectors)
Amplifier Unit
- 47 lbs of 3-dimensional tube circuitry art
- 3/8"x1/4" solid copper bus bars for output and driver filaments
- Elevated filament supply voltage (+44VDC)
- Balanced filament supplies (3.15VAC + 3.15VAC) for filament to cathode interference cancellation
- Point-to-point solid core silver wiring, teflon sleeve as applicable
- Wire gauges: 20g
Amplifier Unit
- 47 lbs of 3-dimensional tube circuitry art
- 3/8"x1/4" solid copper bus bars for output and driver filaments
- Elevated filament supply voltage (+44VDC)
- Balanced filament supplies (3.15VAC + 3.15VAC) for filament to cathode interference cancellation
- Point-to-point solid core silver wiring, teflon sleeve as applicable
- Wire gauges: 20ga for signal and grids, 19ga for tube plates and cathodes
- Fully balanced differential design
- Bus bar and star ground
- 4 x Clarity Cap MR 0.1uF DC coupling caps from cascode tube plates to driver tube grids
- 4 x Clarity Cap ESA 33uF additional local bypass film capacitance for output stage power supply filtering
- Custom designed calibrated digital LED meters for bias, DC-offset and tube testers
- Russian military NOS switches
- Neutrik XLR input connectors
- Krell speaker terminals
- Neutrik NL4X power connectors (x 7)
- Dimensions: 6"H (+5" tubes + 1" feet) x 18.5"W x 18.5"D (+2" speaker terminals and power connectors)
Thanks to... Ralph Karsten of Atma-Sphere Music Systems...
This amplifier is based on the amazing Atma-Sphere OTL amplifiers
Thanks to Ralph Karsten of Atma-Sphere Music Systems, for inspiration, very generous guidance and support in the DIY Audio forum.
I came across a long thread in the DIY Audio forum about "What tubes for a OTL tube amp?" where Ralph provided great information including the schematic for Atma-Sphere M-60 model. I started the design of this stereo amplifier based on that schematic for my own use, but to scale it larger to the MA-2 model. First I decided to build a stereo version just to prove to myself that I can build it and I can make sound... While researching and planning, I built the cases for the power supply and amplifier units from scratch with aluminum panels using a router, equivalent to 2 half monoblock MA-2s combined in a stereo design.