VFET Absolute Audio Labs SIT-A15 power amp

[MattyW]

Ok, so I'm finally going to sate my curiosity regarding SIT or VFET amps in my system. I've a Absolute Audio Labs SIT-A15 SE being built by amp guru Watchara Salakaew in Thailand. It's described as a Single Ended Static Induction Transistor amplifier with MOSFET Constant Current Source. Watchara does not have a website though rather operates entirely off his Facebook page. I've had a brief discussion with him how it compares with the First Watt SIT-3 and have gleaned the following:

 

PASS SIT has lower transconductance because it has a small DIE surface on TO-247 package
My SIT can drive into 300B spec without ant degrade on Lower and Higher frequency
If you see my Spectrum and Harmonics you can notice a smooth NEGATIVE 2nd Harmonics
Only TOKIN SIT can do this spectrum, Japanese style

SIT-A15 has 18db gain & 15WPC @ 8ohms (enough to work with my passive pre) vs FW SIT-3 has 11.5db gain & 18WPC @ 8ohma

Unlike FW SIT-3 the SIT-A15 does not like lower impedances.

 

With my particular amp I'm paying a little extra for Cardas wiring & binding posts, his top grade of matched output devices and lowest noise transformer. Also getting a smaller front badge (10cm rather than 15cm) and red power LED rather than blue. I think this amp has to potential to be a little special 

 

Video of an amp he did in 2020 for the standard SIT-A15 (video downloaded from Facebook): https://drive.google.com/file/d/10FCYrTefDyHImzYH85WXfmQEcnxcQUgw/view?usp=sharing

 

Anyway, here's the eye candy

 

 

 

 

 

 

 

 

 

Edited April 23, 2022 by MattyW

[muon*]

You may e digging out another gem with this one, Matt

 

 

[MattyW]

13 hours ago, muon* said:

You may e digging out another gem with this one, Matt

 

 

I certainly hope so Ian. I struck gold with my current DAC and speakers. OCXO clock is also excellent and I’m convinced the passive pre can’t be beat without going to the balanced version. Too much money for balanced though 

Edited April 13, 2022 by MattyW

[MattyW]

Incidentally, the rear panel is done. Given that the build began immediately after payment I daresay the build will take less time than I expected touch wood.

 

 

 

 

 

Edited April 13, 2022 by MattyW

[ICUToo]

Photos look fabulous- can't wait to hear about the sound!

[MattyW]

Not sure how long the amp will take to build, and I believe Watchara has components on order though based on the fact that he started assembly the same day payment was sent I'd say this will be completed fairly quickly 

[rocky500]

Looks great!

I had those Cardis binding posts put on my speakers when built.

Nice connectors but not so good if using spade connectors I found. A bit hard to tighten them up easy.

Perfect for banana plugs though.

 

Got some mundorf ones on my amp and they are a dream to tighten down on spade connectors. But can not take bare wire as no hole in the shaft but the positive is they should be stronger.

 

 

 

 

Edited April 14, 2022 by rocky500

[Jehuty]

7 minutes ago, rocky500 said:

Looks great!

I had those Cardis binding posts put on my speakers when built.

Nice connectors but not so good if using spade connectors I found. Perfect for banana plus though.

A bit hard to tighten them up easy.

 

Got some mundorf ones on my amp and they are a dream to tighten down on spade connectors. But can not take bare wire as no hole in the shaft but the positive is they should be stronger.

 

 

 

 

Mundorf looks great!

 

I use Cardas CCBP in some of my amps, took ages to burn in. I checked the Mundorf on PCX and man they are expensive  I think I'd get KLEI Naked Harmony instead next time.

Edited April 14, 2022 by Jehuty

[rocky500]

4 minutes ago, Jehuty said:

Mundorf looks great!

 

I use Cardas CCBP in some of my amps, took ages to burn in. I checked the Mundorf on PCX and man they are expensive  I think I'd get KLEI Naked Harmony instead next time.

Just looked now and the Mundorf's are expensive!

I am pretty sure they where quite a bit cheaper when I used them 3 to 4 years ago.

[muon*]

I like the look of the EIZZ EZ-301 Binding Posts, look like nice low mass design but only suitable for bananas I think, I have used the old ETI Pods.

 

OT sorry, Matt.

[MattyW]

No biggie, my speaker cables use Furutech locking banana plugs so the Cardas binding posts will be fine hey 

[MattyW]

58 minutes ago, rocky500 said:

Just looked now and the Mundorf's are expensive!

I am pretty sure they where quite a bit cheaper when I used them 3 to 4 years ago.

Those look fantastic. I may or may not change to those in future. They look fairly low mass which bodes well. 

[MattyW]

Eh, quoted the wrong post. It was the KLEI Naked Harmony posts I meant. Still, I think I'm just going to leave the amp stock and let it burn in for a month or two so I know what it does before I even consider changing anything in it.

Edited April 14, 2022 by MattyW

[rocky500]

I posted about binding posts back when I was looking and Aperalim posted this pic which helped me decide at the time.

 

https://www.stereonet.com/forums/uploads/monthly_2018_11/1234463645_ECHOLELimitedEditionspeakercables11_08_18b.jpg.e3e5e9fd636785445674ffddae552cb1.jpg

 

[MattyW]

I had a bit of a Google about what Watchara meant by negative 2nd harmonics and found this on DIYAudio:

 

”Mr Pass mentions some people prefer higher 2nd harmonic than 3rd harmonic and some prefer higher 3rd than 2nd.In addition 2nd harmonic can be either positive or negative phase as referred to the fundamental.A perfect negative phase 2nd harmonic reads as +90 degrees on the 2nd harmonic line in the spice error log.Conversley perfect positive phase 2nd harmonic reads as -90 degrees.Audibly I have found neg. phase 2nd gives a wider soundstage and mellower tone,positive 2nd is more focused and in your face.Higher 2nd harmonic is a mellower sound whereas higher 3rd harmonic is a more direct in your face sound.These are my observations and others have commented similar findings.”

 

So a wider soundstage….. I’ve also confirmed greater treble and bass extension than the First Watt SIT-3. I like the sound of this. So basically it will certainly be different to a First Watt SIT-3. Maybe even subjectively better. 

Edited April 18, 2022 by MattyW

[MattyW]

Power consumption is 200W and temperature over 60 degrees Celsius. Good thing we have a big solar system and aircon. Hungry and toasty.
 

That said apparently an ambient room temp of 25C is necessary. If I can’t train my wife (very likely) then I guess I’ll be switching amps between summer and winter.

 

Final assembly is today and testing (someone else’s amp done right before mine):

 

 

 

Can’t touch it long it seems:

 

https://drive.google.com/file/d/1eV83fOb_QLPPcM7wTbPtQxc9vKnfrO85/view?usp=drivesdk

 

Sounding nice though:

 

https://drive.google.com/file/d/146qnEzrJVGLWywRcisnLdJAv9LxQGjgC/view?usp=drivesdk

 

Hope my wife doesn’t make me sell it due to heat. Good thing winter is coming absent the whitewalkers. 

Edited April 20, 2022 by MattyW

[MattyW]

 

Resistive load of output bias section, 4.7 OHM 1% 100W
 

 

Matched pairs CCS of output stage. L/R
 

 

 

Suitable Hfe for current limitter was 178 from Zetex : ZTX450
 

 

Original brand stocked since 2012
 

 

All soldering points were coated with KONTAKT GOLD 2000

Edited April 17, 2022 by MattyW

[rocky500]

Its great to get of sort, a hands on approach of the making of your amp.

 

[MattyW]

Yes, the last time I had in progress pictures  was when I had my previous Abbas DAC and tube pre built.

[Rottalpha]

looking good Matt. Can't wait to hear your thoughts on sound.

Do you also own a PASS SIT-3?

Edited April 16, 2022 by Rottalpha

[MattyW]

5 hours ago, Rottalpha said:

looking good Matt. Can't wait to hear your thoughts on sound.

Do you also own a PASS SIT-3?

No, and no more being made either. When I found out about Watchara’s amps and that there was a SIT offering I got in touch immediately. I’ve had SIT envy since reading about the SIT3 in 2019. I’m rather hoping this will be the last amp I will ever want to buy.  
 

Ready for testing:

 

 

 

 

 

 

I just messaged to confirm red LED’s used. I don’t want bright blue LED’s lighting up the room.

Edited April 16, 2022 by MattyW

[MattyW]

Ok, rather than a badge I’m getting the make and model CNC’d into the centre of the front panel, the model below the brand and red power LED below. I’m undecided on whether to use silver or black screws at this point. I guess it’d the CNC’d text shows silver then silver screws, and black screws if it’s black.

 

 Either way it’ll look good. All black will blend into any system better yet a little silver adds some class to things. Guess I’ll see. I suppose I could always paint the lettering silver and use paint thinner to remove the excess around the lettering.

Edited April 17, 2022 by MattyW

[MattyW]

Very interesting. Came across this on the Tokin VFET based SissySIT amps. Bodes well for my amp:

 

"There is a kit version. Sort of.

the kit version is very nice as they can use the tokin transistors from japan. 

Those transistors are actual V-fets, from the last batches of high current RF use design criteria. 

Tokin used the original process for making V-fets, IIRC. Japanese made and in the last run of Japanese V-Fets.

I’d have to check, but I suspect the Tokin transistors have a better spec than the First Watt SIT transistors. As in higher frequency capacity, greater current capacity and possibly more linear. Or at least individually tested and quality spec’ed, sorted and QT’d for release to the public (like all high end expensive transistors). 

To clarify, the SIT transistor that Nelson had made up, came from an offer to do so, from Semi-south, a smaller transistor manufacturer. Nelson had the proto batch made up, and then Semi-south was bought up by another manufacturer. The end...( of that ride). The Tokin, is another matter. It came from a decades long run of making the best in V-fets for extremely demanding applications of extreme voltage swing and extreme current loading (think radar power amplifier). That run had a chance to be perfected over decades of iteration.

Thus, a Tokin version, might be capable of having a higher voltage and higher current rail, for a notably higher wattage and impedance handling range than the SIT-3. 

The operative word being ’might’. 

Importantly, the (very) few who’ve had the chance to make the comparison say the Tokin version outperforms (according to them) the SIT-3. "
 

This fits with what Watchara has said about better treble and bass extension vs the SIT-3, also bigger soundstage due to negative 2nd harmonics (apparently not achievable with Semi-South SIT’s).

Edited April 19, 2022 by MattyW

[Niktech]

13 hours ago, MattyW said:

Very interesting. Came across this on the Tokin VFET based SissySIT amps. Bodes well for my amp:

 

There is a kit version. Sort of.

the kit version is very nice as they can use the tokin transistors from japan. 

Those transistors are actual V-fets, from the last batches of high current RF use design criteria. 

Tokin used the original process for making V-fets, IIRC. Japanese made and in the last run of Japanese V-Fets.

I’d have to check, but I suspect the Tokin transistors have a better spec than the First Watt SIT transistors. As in higher frequency capacity, greater current capacity and possibly more linear. Or at least individually tested and quality spec’ed, sorted and QT’d for release to the public (like all high end expensive transistors). 

To clarify, the SIT transistor that Nelson had made up, came from an offer to do so, from Semi-south, a smaller transistor manufacturer. Nelson had the proto batch made up, and then Semi-south was bought up by another manufacturer. The end...( of that ride). The Tokin, is another matter. It came from a decades long run of making the best in V-fets for extremely demanding applications of extreme voltage swing and extreme current loading (think radar power amplifier). That run had a chance to be perfected over decades of iteration.

Thus, a Tokin version, might be capable of having a higher voltage and higher current rail, for a notably higher wattage and impedance handling range than the SIT-3. 

The operative word being ’might’. 

Importantly, the (very) few who’ve had the chance to make the comparison say the Tokin version outperforms (according to them) the SIT-3. 

Interesting stuff, Matty. Thanks for sharing

[MattyW]

I’ll likely hang onto my Pass Mini Aleph to run during the summer months as Watchara has advised ambient room temp needs to be 25 degrees or less. If it were just me this would be no problem and I’d close all windows and turn on the aircon. My wife will not do that though.