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Sonic Frontiers SFS-80 Review (Vintage Tube Amplifier)

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GXAlan

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Then the next question is 'would it be worth testing SFS-80 shod with KT88s'?
ThanX, @GXAlan.
I have the KT-77 that came with the amp when I bought it but I would probably say there are a few other amplifier tests I want to do first. I had a MC2102 and switched the KT88’s for 6550A’s and thought it was cleaner. But that was a long time ago.
 

Burning Sounds

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Thanks so much @GXAlan for testing this amp. As a long-time owner of a Curcio ST-70 it's great to see a related amp being tested. In my experience this is an extremely reliable design with a very clean sound. Some may decry its un valve-like sound, but I always loved it. Mine drove my Maggie MG1s for years and later the ribbon portion of my MG2.5Rs with my Beard P100 driving the bottom section.

There is a link in this post to a test of a Curcio ST-70 done with an AP1.
 

kemmler3D

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This is really interesting, the results have changed my mind about tubes somewhat, especially the point about performing better at lower power. If you have a certain type of system, that might make sense for you. The "analog dithering" theory is pretty interesting too.

Of course I think you can probably get the same results via software and a budget Purifi implementation these days, but it's very cool tech to see measurements of.
 

SIY

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A couple comments, if I may:

1. Low feedback? I'm guessing not. Joe Curcio's go-to topology was a differential cascode input/driver and pentode outputs, which yields a rather high open loop gain. If memory serves, the input stage gain was about 400 for the ones I had built, but that's been some years and I could be mistaken.

2. Not your fault, this is a common thing, but we need to not use "bias" as a synonym for "idle current." Increasing bias decreases idle current and vice versa. The ambiguity of saying "increasing the bias" can lead to misunderstanding.

3. Output impedance is a critical measurement. That makes more difference in the sound than nearly any other amp measurement since it directly affects frequency response.

Thanks for your hard work and the write-up!
 

Sbrtoy

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Had a Line 3 SE two-box preamp back in the day, one of my favorite preamps I've owned. Curious how it would perform under testing now, no fuzzy warm distortions, sounded very clean to me with a massive soundstage I never got from anything else.
 

Marc Bungener

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Sonic Frontiers SFS-80 Tube Amplifier: A "new" benchmark to beat from 1992

Up next on testing gear that would be impossible to ship safely... the Sonic Frontiers SFS-80 tube amplifier. 70 lbs, all concentrated in the transformers.

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Believe it or not, Sonic Frontiers is a company we all know very well... today, they're known as Anthem AV. In the late 80's or early 90's, a Canadian part distributor began to ship DIY kits for tube amplifiers. They began to sell their finished products under the Sonic Frontiers brand and rapidly grew. As home theater and the DVD era was introduced, Sonic Frontiers established a "budget line" known as Anthem. Generally, Sonic Frontiers focused on tube-based products while Anthem focused on solid-state. Fast forward to 1998 and Sonic Frontiers/Anthem were purchased by Jerry VanderMarel and Scott Bagby, the founders of Paradigm speakers. The 2 channel tube line was retired and Anthem focused exclusively on solid-state products and has become the company we all know and love today.

One of the questions asked is if tube amps are just intended to be distortion factories? Some of the worst measuring devices here are the pricey Carver tube amplifiers. We've seen questionable modifications to the Dynaco ST-70 which measure poorly, though potentially attributable to "questionable" tubes. The SFS-80 is built around the tried-and-true Dynaco ST-70 platform but with over-engineered transformers and power supplies to handle up to 80 watts per channel. The circuitry is a Joe Curcio design. What's great about the SFS-80 for tinkerers and hobbyists is that the front meter is NOT a VU meter which is pretty but not all that useful. Instead, this is a bias meter which allows you to fine tune the bias of your power tubes by turning a potentiometer. Modern designs have gone with self- or fixed-bias designs or "fool-proof" turn the knob until the LED light turns on or off. With an actual bias meter, you can really tinker with the setup.

Today we'll be looking at a specimen that has never been restored, oxidized bindings posts left as is, but does have the following tubes which are different from factory
1) Joint-Army-Navy GE 6550A tubes, biased at 40mA and 50mA
2) Joint-Army-Navy Sylvania 6DJ8 tubes

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Unlike McIntosh tube amps, the SFS-80 does not have multiple binding posts for different output impedances and you're going to have to de-solder/re-solder the output transformers to change the setting. You can go with 3.5 ohms or 8 ohms, and my testing load is 4.48 ohms.

These tubes were bought off eBay and were tested on a B&K 707 Mutual Transconductance tube tester to be "according to spec" (according to the booklet). Joint-Army-Navy genuinely means that these tubes were designed for the US military and met the specs mutually agreed upon by the Army and Navy. This was the terminology developed during World War II and provided an un-classified designator for electronic components. Sometimes JAN tubes were superior to the mainstream consumer products. Other times, they came from the same production lines and since every tube met the military specifications, the tubes sold to the US government were printed with the JAN nomenclature. These tubes don't carry any special audiophile fairy dust. They weren't matched specifically nor are they the "super musical" Mullards or Gold Lion. What they do represent are tubes when tubes were used for scientific instrumentation, radars, and other military gear. You know, when you were trying to bring down distortion as much as possible.

The test setup is:
Panasonic UB9000 -- balanced-2V --> Marantz PM-11s2 as pre-amp --single ended--> Sonic Frontiers SFS-80 --Vishay Dale 1% NH-250 4.48 ohm resistor--XLR-in-> E1DA Cosmos ADC
I have decided to ground the E1DA to the pre-amp.

Based upon my Marantz amplifier testing, my test setup is as much as 9 dB worse than @amirm 's APx555.

Here is the 2W and 5W output at 40 mA bias, where I normally keep the amplifier and think it sounds better. This is typically what an EL34 tube amp would be biased at.

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And here are the results at 50mA which would be typical for a KT-88/6550 tube and will net more peak power before clipping.
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Whew! I'm glad this tube amp measures well.

Before measuring this amp, I'd my tell audiophile friends and family that this SFS-80 really does sound great even against expensive solid state gear as long as I keep the volumes low. I really don't think it's a distortion factory like everyone says. Thankfully the measurements correlate with my opinions.

Obviously the appeal of tube amplification besides the nostalgia/anachronism is the presence of non-linear distortions which anecodatally sound great. Looking at these measurements, I wonder if tubes produce the effect of analog dither with all of the noise spikes at the -100 dB range that continue well into the critical mid-range. However, when tube coloration can also measure this well, you certainly don't have to worry so much about feeling guilty of sighted bias. We also see evidence that UNLIKE solid state where a bit more power can improve SINAD as you get out of the noise-limited region, this tube amp performs better at lower power and in fact better at the lower bias. One missing detail from these measurements is that when playing the 1 kHz test tone, the output transformers started to hum at 1 kHz. I don't think these transformers, which were made in Canada at the time, were encapsulated in epoxy resin like McIntosh transformers/autoformers are. The noise would have been masked by actual musical content, but I wonder if there are microphonic effects that may be non-linear as well. Last, it's not obvious to me that there are differences to the even or odd harmonics. It's been explained that topology has a bigger difference than the actual use of transistors or tubes, and this data seems to confirm that. Last this tube amp has minimal negative feedback -- you potentially could get better results if a stronger feedback loop was applied.

Remember that my measurements are as much as 9 dB worse than what Amir can get with his carefully designed setup. Still, my 5W SINAD of 74 dB is pretty impressive. It means I'm beating the Lyngdorf TDAI-3400, Anthem MRX520, Pioneer Elite VSX-LX504, and every tube amp tested to date at ASR. I don't know if we'll really get the full 9 dB when measured on a proper setup, but I'm very confident that this unit would break past the mean amplifier SINAD of 78. If this amp can break into the 80 dB range, that really would be saying something about the types of audio gear being passed off today as "audiophile premium products" wouldn't?

Subjective Impressions
I actually think the PM-90 and SFS-80 sound very close to my ears, at low volumes. I remember thinking to myself that the PM-90 measured pretty well at ASR, but this tube amp sounds like it's in the same general performance category. At least with my measurements it does in fact measure very similarly. The SFS-80 is just a plain old amplifier whereas the PM-90 is a full integrated amp with preamp, phono amp, etc.

Conclusion
Today, Anthem is known as a company with premium room correction technology and a premium audiophile manufacturer that still produces some of their products in Canada. Thirty years ago, they were hobbyists working with tubes and it seems as if this tube amp really showcases how the ethos of science and engineering can be applied to tubes. You can have all the nostalgia of tubes and North American manufacturing along with whatever positive colorations exist with tubes while simultaneously getting measurements that remain "on par" with today's solid state electronics. Since tubes cannot match the performance of solid state electronics, offering tinkers the ability to fine tune the bias is a worthwhile feature in my opinion. It's much easier to dial in the bias with an actual meter like this as opposed to having to bring out a multimeter or rely on an LED that turns on/off.

What isn't clear to me is if the performance showcased today, which sets a new benchmark for tube amps, is a reflection of special tubes or just an amplifier design that had the goal of "making tubes as accurate as possible." All I can say is that this combination of Canadian electronics and US-manufactured tubes is another 1992 Dream Team that sets the new benchmark to beat when it comes tube amplification.

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thank you for this review which touch me as until last year and for about 20 years I had the monobloc version of these, the SFM-160, which essentially is 2 units with double the power output, that I used with my Wilson Audio Sophia Serie 1 from 2001. Now I use Bryston 7b sst from 2006.
The main difference I noted is that to get details and deep bass with the Sonic Frontiers I needed to up the decibels while with the Bryston even at mid volume you get more of both. I always wondered if this was a function of design or if some component in the Sonic Frontier started to be too old.
Since it’s the first time I post a reply after registering time to also thank you for all your good work and insights. If you were closer to home I would lend you all my gears for testing but sadly would be too expensive as I live overseas in Geneva, Switzerland.
 

SIY

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The main difference I noted is that to get details and deep bass with the Sonic Frontiers I needed to up the decibels while with the Bryston even at mid volume you get more of both. I always wondered if this was a function of design or if some component in the Sonic Frontier started to be too old.
It might be an issue of just plain gain.
 
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GXAlan

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I'm not sure that's entirely true.
Voltmeter is good from the standpoint that you get much cleaner signals than you do with an SPL meter.

You still are dependent on testing the output voltage at a fixed frequency like 1 kHz.

The only way I can match well is to record, analyze, retest because that will get me both RMS and peak volume and account for each amplifier’s frequency response variation. You can image integrated amplifiers with boosted bass and treble. At 1 kHz, if you matched volume the amplifier with the boosted bass and treble would be louder in real content.
 

Labjr

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I used to check 9 volt batteries by licking them. I wonder if I could level match the same way?
 

restorer-john

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I used to check 9 volt batteries by licking them. I wonder if I could level match the same way?

Absolutely! Use this album, and turn the volume right up...

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PS Don't do this at home. A big amplifier could easily kill you if you put the speaker wires on your tongue...
 

James Romeyn

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Great review! Thank you! Can the SF amp be tested to get the maximum output @ 1% THD?

Did anyone else see the crazy specs John Atkinson got for the new Western Electric 91E integrated 300B? WE rates the amp for 20W @ about 10% THD. Instead Atkinson tested about 1.75W @ some insanely high THD I can't recall. Me don't think WE is shipping one soon for testing here.

And only $20k! SUCH A DEAL! I got a couple that fell out of a guy's van in the box, perfect condition for only $8k/ea! /sarc off
 

fpitas

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Did anyone else see the crazy specs John Atkinson got for the new Western Electric 91E integrated 300B? WE rates the amp for 20W @ about 10% THD. Instead Atkinson tested about 1.75W @ some insanely high THD I can't recall. Me don't think WE is shipping one soon for testing here.

And only $20k! SUCH A DEAL! I got a couple that fell out of a guy's van in the box, perfect condition for only $8k/ea! /sarc off
And yet, someone thinks it's wonderful sounding. And to them, it is. C'est la vie...
 

Labjr

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My Wavelength 300B amps have never failed to impress. Don't knock it before you try it. But obviously not for every system.
 
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GXAlan

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Great review! Thank you! Can the SF amp be tested to get the maximum output @ 1% THD?

Did anyone else see the crazy specs John Atkinson got for the new Western Electric 91E integrated 300B? WE rates the amp for 20W @ about 10% THD. Instead Atkinson tested about 1.75W @ some insanely high THD I can't recall. Me don't think WE is shipping one soon for testing here.

And only $20k! SUCH A DEAL! I got a couple that fell out of a guy's van in the box, perfect condition for only $8k/ea! /sarc off

I can try to do that in two weeks or so. 1% is 40 db sinad. I run my tubes at 40 mA for longevity so it won’t get to peak.

I think the really high distortion single ended tube amplifiers provide unique sound signature which may be pleasant to listen to.
 
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