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Sonic Frontiers SFS-80 Review (Vintage Tube Amplifier)

GXAlan

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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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MattHooper

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Great post!

Sonic Frontiers certainly was popular back in the day. My friend still uses a Sonic Frontiers Pre-amp (special mods from the SF guys). I've heard it get hissy when the tubes were tired, but when that's not an issue, boy his system sounds clean.

I had an Audible Illusions preamp for a while that was heavily modded at Sonic Frontiers. That was beautiful.
 

VintageFlanker

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Great job again, @GXAlan! You are doing what I didn't dare to try so far: measuring amps with the Cosmos! ;)

As many pointed out in my former reviews, maybe REW should be more accurate for THD+N figures. I personally use :
- REW for 1Khz THD+N and DR.
- Multitone for... Multitone (yup!) and Jitter (the latter not being a thing for amps)
- RMAA PRO for regular tests: FR, Noise floor, SMPTE IMD and Crosstalk.
 
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Gorgonzola

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I'm delighted to see @GXAlan 's review for the SFS-80 amp mainly because I use a Sonic Frontiers preamp, the Line1.

I am please but not surprised to see that the legacy SFS-80 is a superior performing amplifier in its class. The irony is that many audiophiles have criticized the Sonic Frontiers offerings as "too solid state-like", that includes both the amps and preamps. I speculate its because these self-acknowledged audiophiles actually like the higher sort of distortion the more typical tube equipment produces.

My Line 1 has had the "SE+" upgrade/restore from Parts ConneXion, (pcX). pcX's owner is Chris Johnson who was a principal of Sonic Frontiers back in the day; he and his team are able to repair and upgrade virtually all Sonic Frontiers equipment and other equipment as well, (such as McIntosh, etc.). Their upgrade/restoration policy is to maintain the original design but substitute better performing, current components in place of the original; see their policy in that regard HERE.

I would very much like that my Line 1 SE+ be tested by @amirm or GXAlan, however I really don't trust the carriers to get it to them and back safely, especially given the cross-border hassle.

Here's a pic of my Sonic Frontiers Line 1 SE+ ...
gi.mpl


And here's a pic showing most of the extensive upgrades and restores done my pcX ...
gi.mpl
 
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GXAlan

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I would very much like that my Line 1 SE+ be tested by @amirm or GXAlan, however I really don't trust the carriers to get it to them and back safely, especially given the cross-border hassle.

Amplifier testing is very anxiety provoking. At one point I thought I broke my expensive PM-10 today but it was a loose cable. Still a scary thought.

Thankfully, line level testing is super easy.

1) Step one, but an E1DA Cosmos ADC. Amazon has Grade B. I bought my Grade A from Audiophonics in France, shipped to the US. This lets you avoid retailers that you are not familiar with.

2) Have a good dac. My testing is extra hard because I am using the UB9000 which has no USB control. I have to do all sorts of tricks to fiddle with it. If I was testing for real, I would buy a new DAC. Problem is that I am happy with the sound and ergonomics of my UB9000 and sound and ergonomics of my standalone SACD player.

3) software like @pkane’s Multitone test application is FREE and he even will fix any bugs you tell him about.

4) set the input sensitivity on the E1DA and then run your tests with the software.

5) a multimeter to know what your AC voltage output is all you need.

It helps if you have a known DAC that has been measured here to get a sense of your discrepancy.
 

MattHooper

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I speculate its because these self-acknowledged audiophiles actually like the higher sort of distortion the more typical tube equipment produces.

*Raises hand.

I'm in the "if I wanted solid state sound I'd buy a solid state amp" camp. :)

(Actually, I did also buy a solid state pre-amp. I like it and my tube preamp).


gi.mpl


And here's a pic showing most of the expensive upgrades and restores done my pcX ...
gi.mpl

Beautiful preamp!
 

restorer-john

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Amplifier testing is very anxiety provoking. At one point I thought I broke my expensive PM-10 today but it was a loose cable.

It isn't really. You'll get used to it.

But as you are testing amplifiers, we really need to know what they can achieve in terms of power output.

You can still use your ADC, but run it 2 channel (as the improved THD is not needed for power output at clipping tests), use two loads capable of absorbing the power you are going apply and run some elevated power tests. I suggest buying 4x8R loads and setting them up to be 2x8/2x4 even single 2R. If you don't want to run full bandwidth power output tests on old/vintage/valuable gear, just do 1kHz at clipping (1%) first.

And some frequency repsonse sweeps. When you get confident, you can run FR sweeps at full rated power. You can run loaded vs unloaded FR sweeps and determine damping factor/output impedance vs frequency using the data points and a simple calculation.
 
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GXAlan

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I am please but not surprised to see that the legacy SFS-80 is a superior performing amplifier in its class. The irony is that many audiophiles have criticized the Sonic Frontiers offerings as "too solid state-like", that includes both the amps and preamps. I speculate its because these self-acknowledged audiophiles actually like the higher sort of distortion the more typical tube equipment produces.

I had a MC2102 which I really liked and sold at the beginning of the pandemic for good financial reasons. That also was thought to be a tube amp that sounded solid state like by audiophiles in comparison to the MC275.

It does correlate with why some people like Carver amps or the VTA-70 over a plain ST-70, on in the modern day a SMSL D300 over other brands or R2R DACs. I do like the sound of tubes, and a little bit of coloration is a little bit of make-up.

There are different opinions on how much is too much. What I need to figure out is how my most clinical amplifiers measure differently.

I am not an electrical expert but I recall the belief in audiophile harmonic distortions is only accurate for single ended tube designs.

As for the SFS-80, Bascom King liked the budget SFS-40 version. Yes, that Bascom King. The fit and finish on the SFS-80 is “mid fi” so that probably hurt credibility among the elitists. No surprise that Luxman tubes look beautiful when turned off :)
 

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Chrispy

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Curious, why, with good packing, would such be difficult to transport or do they at some point in aging simply become unreliable in that respect?
 

DHT 845

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I used to own Sonic Frontiers stuff, my friend had their top Line 3 preamp and T3, P3 cd transport + DAC combo, 2 of my friends still own SFD-2 mk2 DAC's :)
 

Gord

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Great review. Most of my system is DIY, so glad to see a DIY review. My amp is a 300B based on a TubrLab board. Preamp and phono amp are Vacuum Tube Audio, also DIY. With my 300B I've noticed ouput tubes and output transformers make all the difference in the world. Just discovered higher biasing can also make it sing.
 

Chrispy

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So why is this tube amp particularly "good" otoh?
 
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GXAlan

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Hi, would it be possible to graph THD vs power? Results are fair (and superior to many tube amps) at the relatively low 2-5 W power output.

I have a bit of time each week to try measuring stuff. Since my DAC isn’t so straightforward I can do a series of tests at different voltages and then increase in a stepwise fashion. That said, tubes are a really bad idea when you need lots of power.

I made this chart in a different thread where I show my work for the calculations which sort of shows in both directions why tubes work well for moderate listening levels and why “even though a watt gets you a lot of output, you still want 300-600W juggernaut amplifiers if you are playing music with bombastic peaks.

SpeakerdB at 0.05W (5 vs 15 ft)dB at 0.5W (5 vs 15 ft)dB at 5W (5 vs 15 ft)dB at 50W (5 vs 15 ft)
Revel PerformaBe F328Be80.3 / 70.8 90.3 / 80.8100.3 / 90.8110.3 / 100.8
Focal Aria 90676.3 / 66.886.3 / 76.896.3 / 86.8106.3 / 96.8
Elac Uni-Fi 2.0 UB5272.3 / 62.882.3 / 72.892.3 / 82.8102.3 / 92.8

Peak advertised power handling
SpeakerPeak Advertised Power HandlingIn room Peak SPL (5 vs 15 ft)
Revel PerformaBe F328Be300 watts118.1 / 108.6
Focal Aria 906120 watts110.1 / 100.6
Elac Uni-Fi 2.0 UB52140 watts106.8 / 97.3

I don’t know if this actually measures better than a MC275/MC2102. From what I understand the 12ax7/12at7 combo is even better than the 6DJ8. Then the question is how good something like an Audio Research, current production amp can perform.
To this very day, I simply do not understand how they retired the "Sonic Frontiers" name. What a truly phenomenal branding.
Definitely! Searching USPTO, I cannot find evidence that they ever trademarked the name and Sega has trademarked the name for their latest Sonic the Hedgehog game.

Great review. Most of my system is DIY, so glad to see a DIY review. My amp is a 300B based on a TubrLab board. Preamp and phono amp are Vacuum Tube Audio, also DIY. With my 300B I've noticed ouput tubes and output transformers make all the difference in the world. Just discovered higher biasing can also make it sing.

I’d love to hear a 300B system. It sounds like the power limitations are real, but with a matching high efficiency speaker or appropriate listening level, it’s supposed to be very impressive.

You should put your gear on the test bench too. I have always been told that higher biasing is better too, but I subjectively thought 40 mA was better but I also told myself that it was probably the bias of knowing that these tubes have a finite lifespan and have no reliable replacement.

But at least in this case, the lower bias measures better! The 6550A should easily handle even more power than 50mA. Then you have all the hand waving that EL34’s sound better than 6550a’s if you don’t need power, but EL34’s are typically biased at 40 mA.

I don’t use this tube amp in any of my primary systems but it might be interesting to see if 42 mA, 45 mA, or 47 mA ends up having the “best” results. I just have two datapoints.

No matter how good your 300B setup may be, it would be nice to see if there is a sweet spot for your bias. Certainly, the new current production 300B’s from Western Electric would be interesting to measure.
 

oscar_dziki

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So basically a good amp as long as you don't expect it to amplify speakers. Maybe it would be OK for headphones?
 
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