Single Ended Tokin SIT THF-51S Common Drain Mu Follower Amplifier , 45W?
I used to be a huge fan of single ended common source SIT amplifiers. I had a 2SJ28 version of Michael Rothacher's Hammond 193V choked loaded L'Amp and I also have a THF-51S version of Mr. Pass's BAF2015 2SK77 amplifier (Zen Mod's version is the Singing Bush). I took the 193V out of the L'Amp and tried it in a higher Watt 2SK180 L'Amp. Unfortunately that did not work out as well as I had hoped in that it had thermal runaway issues. However that turned from bad to good when I tried the 2SK180 in common drain mode with the 193V as load, and I was pleasantly surprised:
https://www.diyaudio.com/forums/pas...93v-choke-loaded-2sk180-lamp-post6601437.html
So now I am a huge fan of SITs in follower mode. I suppose I shouldn't be surprised as Mr. Pass's recent DIY Sony VFET Part 1 is also a follower amplifier, and Mr. Pass has impeccable taste. So I got to thinking that my THF-51S BAF2015 monoblocks are good candidates for conversion to common drain follower (inverted BAF2015 THF-51S?).
For the CCS/mu follower I needed to find a suitable P-Channel enhancement mode Mosfet. Unfortunately they are not in great supply, especially when I was thinking power dissipation of about 100W. So with a safety factor/derating factor of say four, I was looking for a device rated at a minimum of 400W. After some searching the internet I found the IXTQ26P20P. However it was only rated at 300W. I was willing to give that a try though. Its datasheet had a Safe Operating Area chart showing DC parameters at Tc=70C, Tj=150C and it looked very feasible. Then I found an even better device that was used by diyAudio member Lynn Quam in his SIT3X amplifier design: the IXTN40P50P. It is rated at 890W, so plenty of headroom, and it is also the same form factor as the IXFN140N20P in the BAF2015 amp. Also Lynn had the Spice model posted so I didn't need to search for it. Thank you, Lynn.
Since my plan is to repurpose my BAF2015 THF-51S build, I will be using the existing chassis and power supply. The power supply was designed to comfortably output 60VDC at 3.0A. It is currently running at about 2.3A and 62.5VDC. The chassis has a fan cooled heatsink and temperature is not an issue.
So I started out with some LTSpice simulations and decided to go with the mu follower and not just straight CCS, as the mu follower's additional power output was enticing. I found 62VDC and 3.0A was a good operating point. Simulations showed 45W into 8 Ohms was feasible. In reality, to be determined.
EDIT:
New - PCB Available: Start of PCB Design at post #173
I used to be a huge fan of single ended common source SIT amplifiers. I had a 2SJ28 version of Michael Rothacher's Hammond 193V choked loaded L'Amp and I also have a THF-51S version of Mr. Pass's BAF2015 2SK77 amplifier (Zen Mod's version is the Singing Bush). I took the 193V out of the L'Amp and tried it in a higher Watt 2SK180 L'Amp. Unfortunately that did not work out as well as I had hoped in that it had thermal runaway issues. However that turned from bad to good when I tried the 2SK180 in common drain mode with the 193V as load, and I was pleasantly surprised:
https://www.diyaudio.com/forums/pas...93v-choke-loaded-2sk180-lamp-post6601437.html
So now I am a huge fan of SITs in follower mode. I suppose I shouldn't be surprised as Mr. Pass's recent DIY Sony VFET Part 1 is also a follower amplifier, and Mr. Pass has impeccable taste. So I got to thinking that my THF-51S BAF2015 monoblocks are good candidates for conversion to common drain follower (inverted BAF2015 THF-51S?).
For the CCS/mu follower I needed to find a suitable P-Channel enhancement mode Mosfet. Unfortunately they are not in great supply, especially when I was thinking power dissipation of about 100W. So with a safety factor/derating factor of say four, I was looking for a device rated at a minimum of 400W. After some searching the internet I found the IXTQ26P20P. However it was only rated at 300W. I was willing to give that a try though. Its datasheet had a Safe Operating Area chart showing DC parameters at Tc=70C, Tj=150C and it looked very feasible. Then I found an even better device that was used by diyAudio member Lynn Quam in his SIT3X amplifier design: the IXTN40P50P. It is rated at 890W, so plenty of headroom, and it is also the same form factor as the IXFN140N20P in the BAF2015 amp. Also Lynn had the Spice model posted so I didn't need to search for it. Thank you, Lynn.
Since my plan is to repurpose my BAF2015 THF-51S build, I will be using the existing chassis and power supply. The power supply was designed to comfortably output 60VDC at 3.0A. It is currently running at about 2.3A and 62.5VDC. The chassis has a fan cooled heatsink and temperature is not an issue.
So I started out with some LTSpice simulations and decided to go with the mu follower and not just straight CCS, as the mu follower's additional power output was enticing. I found 62VDC and 3.0A was a good operating point. Simulations showed 45W into 8 Ohms was feasible. In reality, to be determined.
EDIT:
New - PCB Available: Start of PCB Design at post #173
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Wow, a lot of followers, I'm thrilled. Thank you all. 
vidi_nenna, that is a good question and I don't have a scientific answer. However I can say that I really like the sound of my Tokin 2SK180 follower amp driven by my Luminaria preamp. The follower has a clarity that my common source SIT amps do not have. I would also like to point out that there are follower SIT amps out there right now, and the most talked about and the most in-demand one is the DIY Audio Sony VFET Part 1.
GnuB, I decided to go with CCS/mu follower because my 2SK180 follower is choke loaded, so I wanted to do something different. Another reason is that I have a set of built Tokin single ended common source monoblocks that I can re-purpose for this build, so most of the work is done.
I do have an amp that I have to finish before I start work on this one. No, it is not a DIY Audio Sony VFET Part 1. I decided not to participate in the draw as I am already having enough fun playing with the Tokins. Thanks to Mr. Pass and Cody, I am working on a F2J that is going into the 2SJ28 L'Amp chassis that donated the 193V chokes to the 2SK180 Fokin follower project.
vidi_nenna, that is a good question and I don't have a scientific answer. However I can say that I really like the sound of my Tokin 2SK180 follower amp driven by my Luminaria preamp. The follower has a clarity that my common source SIT amps do not have. I would also like to point out that there are follower SIT amps out there right now, and the most talked about and the most in-demand one is the DIY Audio Sony VFET Part 1.
GnuB, I decided to go with CCS/mu follower because my 2SK180 follower is choke loaded, so I wanted to do something different. Another reason is that I have a set of built Tokin single ended common source monoblocks that I can re-purpose for this build, so most of the work is done.
I do have an amp that I have to finish before I start work on this one. No, it is not a DIY Audio Sony VFET Part 1. I decided not to participate in the draw as I am already having enough fun playing with the Tokins. Thanks to Mr. Pass and Cody, I am working on a F2J that is going into the 2SJ28 L'Amp chassis that donated the 193V chokes to the 2SK180 Fokin
follower project.
Beh Mah,
I think you will be very pleased with the F2J. I have been extremely happy with it. It works wonderfully with my Fostex speakers.
Thanks again for the help get the F2J up and running, and codyt and NP for the SS JFETs.
I will be following this thread closely, since I have a couple pairs of THF51S sitting on the sidelines wanting for an amp.
I think you will be very pleased with the F2J. I have been extremely happy with it. It works wonderfully with my Fostex speakers.
Thanks again for the help get the F2J up and running, and codyt and NP for the SS JFETs.
I will be following this thread closely, since I have a couple pairs of THF51S sitting on the sidelines wanting for an amp.
After all the praises of the triode-like characteristics this development is indeed a little bit ironic. Once again the ear disagrees with the eye but good literature still makes for great reading.
Next logical and unavoidable step is to replace the SIT with a modern vertical mosfet
Otoh, conventional wisdom has always insisted low distortion / high Df is preferable to the opposite.
Many thanks to Ben Mah for doing the research. I see no alternative than to follow his steps and build both topologies.