Sumo "The Nine" Vintage Amplifier, Pure Class A


The Nine is loved by audiophiles because of its Class A operation and compact footprint.

You see most Class A amps are huge and heavy which makes them a bit of a drag. Try moving around an equivalent Krell or Classe Class A and your back will remind you why you should have gotten a Sumo.

Cosmetically this unit is in very nice condition with no significant signs of wear. The faceplate is clean and all bits are intact.

In the lab it tested well and needed little attention.

Lab Results:
• Function testing. Both channels equal. Tested clean up to 60W into 8 Ohms
• Fairly flat frequency response.

More from Sumo:
Sumo gives you the real thing. The nine is a pure class A power amplifier that remains class A even when driving 4 Ohm loudspeakers.

The Nine's approach to class A is so unique that it actually earned a US patent. An amazing achievement for an audio component.

Sumo's exclusive class A engineering provides all the advantages and none of the disadvantages of typical class A designs.

It features an all aluminum anodized chassis with a massive extruded heat sink and built-in whisper fan for efficient heat dissipation. Plus military grade G-10 glass epoxy printed circuit boards throughout.

The power supply incorporates a 1.4KW shielded toroidal transformer and 42,000 microfarad computer grade filter capacitors. The combination results in a supply voltage regulation better than 3%.

Please see our last photo or the manual linked below for full specifications:

Sumo "The Nine" Vintage Amplifier, Pure Class A - Owner's Manual

The SkyFi Testing Process for Solid State Amplifiers:
We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and transistors are checked for signs of stress or damage.

We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally, we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If everything reads clean, we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier then remains powered on to "burn in" on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will be also be tested.

Final testing involves pushing actual music thorough the amplifier. We've learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage KEF speakers that we're very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice.



Original Box





Not Applicable


Yes - Power

Physical Condition


Working Condition


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