Sonic Frontiers SFS-80 Vintage Tube Amplifier - Serviced

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General:
The SFS-80 is an exceptional tube amplifier in every respect. Not only is it visually stunning, but its sound quality is equally impressive. Delivering 80 watts into 8 ohms, it offers more than enough power for the vast majority of speakers, making it a remarkably powerful all-tube design. The amplifier is fitted with 6550 output tubes and EH 6922 driver tubes.

The build quality and overall fit and finish are also standout features. Nearly every surface is finished in beautifully brushed aluminum, giving the amplifier an elegant appearance that catches the light unlike anything else in our shop. If you decide to pair it with the Sonic Frontiers SLF-2 preamp we also just listed, it would make a pretty awesome system.

During testing, we found the SFS-80 to deliver an exceptionally clear, detailed, and engaging sound that made it a genuine pleasure to listen to.
Included with the amplifier are its original box for safe transport and a printed copy of the owner’s manual.

SkyFi Cosmetic Notes:
Overall condition is excellent all around with very minor signs of wear on the casing.The included manual is a printed copy.

SkyFi Technical Notes:
This amplifier came in with some minor issues. We replaced the bias sense resistors, a few other stressed parts, and re-tubed the unit. The output tubes are a strong used set of Svetlana 6550. The driver tubes are new EH 6922. After completing the service, we tested the amplifier and verified proper testing at the bench and in our listening area on long term tests.

Ownership:
Single Owner

Connections:
RCA inputs, Speaker Binding Posts

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
7/10 = Good. One or two minor scratches. Well Maintained. See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Unit, Box, Manual

Packing:
Original Manufacturers Packing

Original MSRP:
$2,750.00

Specs:
Power output: 80 watts per channel (stereo)
Frequency response: 6Hz to 50kHz
Input sensitivity: 0.95V
Valve complement: 4 x KT99, 4 x 6DJ8

Dimensions:
18 x 14.5 x 9

Weight:
70 lbs.

Approximate Age:
1992

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Kimber Summit Series BiFocal XL Bi-Wire Speaker Cables (PAIR) - Best If Applicable

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - Best

Testing Process:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use a modern Amplitrex AT-1000 Professional Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy.

We document the results of each tube and replace any weak or suspect tubes before proceeding.

When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable.

If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged.

Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1 KHz test signal, slowly increasing its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance.

We finish off the bench evaluation with a 1 KHz square wave check and a 20 Hz to 20 KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours minimum.

For tube amps we like to run this test at least twice.

This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.