I built a 2SJ28 L'Amp back in 2017 and I have been enjoying it very much in my bi-amped system since then. Thank you, Michael Rothacher, and also for the Luminaria, which I also built. The L'Amp is choke loaded with Hammond 193Vs and also has Hammond chokes in the dual CLC power supply. Add all this up, it is heavy, weighing in at about 60 lbs (27 kg). Alas, I'm finding that I can barely lift it safely; something had to be done. So, I figured the Hammond 193V chokes had to go. They weigh about 9 lbs or 4.1 kgs each and removing them would make a big difference.
My initial plan was a simple one - replace the chokes with current sources. I would use the existing 2 x 18V power transformer windings in series to produce a single 45VDC power supply instead of the existing 2 x 23VDC supplies. This would slim down the L'Amp to a manageable weight, and I would be able to move it occaisionaly to dust under it.
I then fired up LTSpice to explore how this all could come about. One thing lead to another, and I came up with this design which I built. It is a 2SJ28 source follower with a CCS load. The 2SJ28 is driven by a choke loaded 2SK82. So, trade a huge inductor for a small one.
Being the frugal type, I bought two Hammond 156C inductors for the 2SK82 load. They are cheap and some people have used them in tube amps with good results. They are only good for about 7mA of current though, so there is a downside to them. But they are cheap!
I managed to re-use the power transformer and the CLC filters. The transformer windings are connected in series to a single bridge rectifier and then to separate CLC filter stages for each channel. The 2SK82 has its own transformer and separate CRC filters for each channel.
The voltage amp stages are fed 175V and the self biased 2SK82s are operating at 122Vds and -25.5Vgs with 8mA current. You can see the FFT for 2,5Vpeak and 20Vpeak out.
The CCS for the 2SJ28 is supplied with 46V. Its output is connected to the 2SJ28 source. The 2SJ28's gate is biased by a voltage divider connected to the 46V supply. No need for a separate bias power supply.
I ran FFT measurements to confirm the performance of the choke loaded 2SK82 voltage amp. They are shown below. The FFT measurements for the complete amp are also shown below.
Overall, I am pleased with the results. Aside from LTSpice simulations, I did not know how well this would work. I was worried about hum pickup by the Hammond 156C inductors in the voltage amp. No noticeable hum, so I am happy. And also, the amp now weighs 46 lbs(21 kg). I can handle that.
I have just put the amp in my main system and I will listen to it for awhile before commenting on its sound.
My initial plan was a simple one - replace the chokes with current sources. I would use the existing 2 x 18V power transformer windings in series to produce a single 45VDC power supply instead of the existing 2 x 23VDC supplies. This would slim down the L'Amp to a manageable weight, and I would be able to move it occaisionaly to dust under it.
I then fired up LTSpice to explore how this all could come about. One thing lead to another, and I came up with this design which I built. It is a 2SJ28 source follower with a CCS load. The 2SJ28 is driven by a choke loaded 2SK82. So, trade a huge inductor for a small one.
Being the frugal type, I bought two Hammond 156C inductors for the 2SK82 load. They are cheap and some people have used them in tube amps with good results. They are only good for about 7mA of current though, so there is a downside to them. But they are cheap!
I managed to re-use the power transformer and the CLC filters. The transformer windings are connected in series to a single bridge rectifier and then to separate CLC filter stages for each channel. The 2SK82 has its own transformer and separate CRC filters for each channel.
The voltage amp stages are fed 175V and the self biased 2SK82s are operating at 122Vds and -25.5Vgs with 8mA current. You can see the FFT for 2,5Vpeak and 20Vpeak out.
The CCS for the 2SJ28 is supplied with 46V. Its output is connected to the 2SJ28 source. The 2SJ28's gate is biased by a voltage divider connected to the 46V supply. No need for a separate bias power supply.
I ran FFT measurements to confirm the performance of the choke loaded 2SK82 voltage amp. They are shown below. The FFT measurements for the complete amp are also shown below.
Overall, I am pleased with the results. Aside from LTSpice simulations, I did not know how well this would work. I was worried about hum pickup by the Hammond 156C inductors in the voltage amp. No noticeable hum, so I am happy. And also, the amp now weighs 46 lbs(21 kg). I can handle that.
I have just put the amp in my main system and I will listen to it for awhile before commenting on its sound.
Attachments
Thank you, Zen Mod and Ben Mah.
Forgot to mention that the 2SJ28 is running at Vds = 18.5V, 1.5A.
Attached are FFTs comparing the choke loaded 2SK82 voltage amp to a resistor loaded 2SK82. The resistor load is 4k ohms which is approximately the 4k ohm DC resistance of the choke.
Finally, a picture showing the VFET power amp family in action - the rebuilt amp on top and the BAF THF-51S twins below.
Forgot to mention that the 2SJ28 is running at Vds = 18.5V, 1.5A.
Attached are FFTs comparing the choke loaded 2SK82 voltage amp to a resistor loaded 2SK82. The resistor load is 4k ohms which is approximately the 4k ohm DC resistance of the choke.
Finally, a picture showing the VFET power amp family in action - the rebuilt amp on top and the BAF THF-51S twins below.
Attachments
The output stage of post #1 is quite similar to the soon-to-be-released DIY Sony
VFET amp (P version), which runs less voltage but more bias. As we will also be
releasing boards and such for those who already have the P channel parts, you
may find that a convenient vehicle. Separate front ends - 1 from me, and 4 from
MarkJ, will also be available, and they fit on UMS heat sink patterns.
VFET amp (P version), which runs less voltage but more bias. As we will also be
releasing boards and such for those who already have the P channel parts, you
may find that a convenient vehicle. Separate front ends - 1 from me, and 4 from
MarkJ, will also be available, and they fit on UMS heat sink patterns.
My VFET impressions
My system's electronics, with the exception of the phono preamp (tube) and modified LX-Mini active crossover, includes VFETs in all amplification stages. I have the LuminAria (2SK82) preamp driving the active crossover which in turn feeds the THF-51S amps for the bass and this new 2SK82-2SJ28 amp for the highs.
My speakers are based on old JBL prosound drivers (2225, 2445/2397, 2405). The THF-51S amps drives the bass while the 2SK82-2SJ28 drives the 2445 compression drivers and also the capacitor coupled (high pass filter) 2405 tweeters.
I used to be exclusive committed to single-ended directly heated tube amplifiers until I discovered VFETs through diyAudio. So, to replace all of them with VFETs is a good indication of what I think about the VFETs. Basically, comparable sound without the hassle (limited life, noise and hum, expense).
Interestingly, before I built my VFET amps, I bought a used Sony TAN 5550 power amp (with VFETs) which I refurbished and used to power my bass speakers. At that time, I had started using an old Cyrus One integrated amp that I had. I wanted to try more power with the bass, and I thought that it sounded pretty good. When I replaced it with the Sony TAN 5550, I couldn't believe how much better it sounded. The lower frequencies just sounded more alive and tuneful, not flat. This is with a crossover frequency of about 900 Hz.
Then, when I built the THF-51S amps, they replaced the Sony TAN 5550. Again another obvious improvement. The actual bass frequencies and drums got better. Harder hitting. Rock and roll really rocked and rolled. Felt like being in a bar listening to live music.
With respect to the upper half of the musical frequencies, I've always liked the 2A3 and 45 tube single ended amps. When I put the 2SJ28 L'Amp with 193V into my system, it sounded very similar to the tubes. But, there was a big improvement in the background noise and hum. With the tubes, there was always a bit of noise between songs. Not so with the VFETs. Dead quiet. And they don't go bad over time like tubes.
I've been listening this new 2SK82 - 2SJ28 amp for a day or so now, and I can say that it is as good as the L'Amp. It may be better in some respects, but I cannot tell for sure, There was a period of four or five days when I was taking the L'Amp apart and rebuilding it to its present form, so I can't trust my memory. I know that nothing bad stands out. I find that sonic faults generally surface quickly and become obvious. Nothing here.
Seeing as my recollection may be faulty, I think that I am now listening to songs (rock and roll, blues) at a higher sound level. Luckily, my home environment allows me to do this without worry about disturbing anybody. Maybe, the lower distortion of the new amplifier is allowing me to crank up the volume. However, it has only been a couple of days with this new amp, and I am probably biased, so who knows?
With respect to my preamp, before the LuminAria, I had a 01A tube preamp. It had all the good stuff. I find the LuminAria comparable, but once again, without the hassle. Also, good 01A tubes are becoming harder to find.
Interestingly, I think the inductor loaded 2SK82 may also work as a low parts count preamp.
My system's electronics, with the exception of the phono preamp (tube) and modified LX-Mini active crossover, includes VFETs in all amplification stages. I have the LuminAria (2SK82) preamp driving the active crossover which in turn feeds the THF-51S amps for the bass and this new 2SK82-2SJ28 amp for the highs.
My speakers are based on old JBL prosound drivers (2225, 2445/2397, 2405). The THF-51S amps drives the bass while the 2SK82-2SJ28 drives the 2445 compression drivers and also the capacitor coupled (high pass filter) 2405 tweeters.
I used to be exclusive committed to single-ended directly heated tube amplifiers until I discovered VFETs through diyAudio. So, to replace all of them with VFETs is a good indication of what I think about the VFETs. Basically, comparable sound without the hassle (limited life, noise and hum, expense).
Interestingly, before I built my VFET amps, I bought a used Sony TAN 5550 power amp (with VFETs) which I refurbished and used to power my bass speakers. At that time, I had started using an old Cyrus One integrated amp that I had. I wanted to try more power with the bass, and I thought that it sounded pretty good. When I replaced it with the Sony TAN 5550, I couldn't believe how much better it sounded. The lower frequencies just sounded more alive and tuneful, not flat. This is with a crossover frequency of about 900 Hz.
Then, when I built the THF-51S amps, they replaced the Sony TAN 5550. Again another obvious improvement. The actual bass frequencies and drums got better. Harder hitting. Rock and roll really rocked and rolled. Felt like being in a bar listening to live music.
With respect to the upper half of the musical frequencies, I've always liked the 2A3 and 45 tube single ended amps. When I put the 2SJ28 L'Amp with 193V into my system, it sounded very similar to the tubes. But, there was a big improvement in the background noise and hum. With the tubes, there was always a bit of noise between songs. Not so with the VFETs. Dead quiet. And they don't go bad over time like tubes.
I've been listening this new 2SK82 - 2SJ28 amp for a day or so now, and I can say that it is as good as the L'Amp. It may be better in some respects, but I cannot tell for sure, There was a period of four or five days when I was taking the L'Amp apart and rebuilding it to its present form, so I can't trust my memory. I know that nothing bad stands out. I find that sonic faults generally surface quickly and become obvious. Nothing here.
Seeing as my recollection may be faulty, I think that I am now listening to songs (rock and roll, blues) at a higher sound level. Luckily, my home environment allows me to do this without worry about disturbing anybody. Maybe, the lower distortion of the new amplifier is allowing me to crank up the volume. However, it has only been a couple of days with this new amp, and I am probably biased, so who knows?
With respect to my preamp, before the LuminAria, I had a 01A tube preamp. It had all the good stuff. I find the LuminAria comparable, but once again, without the hassle. Also, good 01A tubes are becoming harder to find.
Interestingly, I think the inductor loaded 2SK82 may also work as a low parts count preamp.
Thanks a lot for this write up. Very inspiring. Shame though that the 2SK82 is now harder to find than good DHT preamp tubes. Hope some of the more easily available SITs turn out to be useable in an adapted Luminaria circuit...
I see that 2SK180, 2SK182ES, and THF-51S SITs are still available on eBay at less than US $100 per device. This is still cheaper than DHT power tubes. Also, expensive no output transformer required.
These have higher input capacitance than the smaller SITs, but that can be overcome with a buffer at the input. I say, grab a pair and start experimenting. That's the beauty of DIY!
These have higher input capacitance than the smaller SITs, but that can be overcome with a buffer at the input. I say, grab a pair and start experimenting. That's the beauty of DIY!
I have listened to this new amp extensively for a few days now so I can provide an update on my experience. First, I'll say that all the VFET amps that I've listened to sound very good. One may sound better than another in some respect, but the differences are not big. However, there are differences that I can hear.
Another thing to consider is my preamp, the LuminAria, which has a single ended 2SK82. So, it has second order harmonic distortion that is thrown into the mix. As a result, the sound from the amplifier is also influenced by the preamp.
This new 2SK82-2SJ28 amp, being a source follower, has lower distortion and lower output impedance than the L'Amp that it replaced. After the initial listening period when I had the volume cranked to 11, I toned things down and started listening at a more reasonable volume and a wider variety music. It was then that I noticed a subtle difference from my previous setup. There seemed to be more clarity in the midrange and highs. Not a huge difference, but the notes seemed more pure. After thinking about it, I think that previously, the combined harmonic distortion of the Luminaria and the L'Amp was enough to make an audible difference. The lower distortion of the new amp brought the total second harmonic to below a certain detectable threshold for me. I think that if my preamp were not the LuminAria, but one which does not produce this harmonic profile, the difference would not be noticeable.
I have a biamp setup so I thought, why not switch the amps around and use the new 2SK82-2SJ28 amp to power the lower frequencies and the BAF THF-51S to drive the higher frequencies. After all, I had not tested the bass performance of the new amp. So, I did and I am happy to report that this combination works well. I wasn't sure about this at first, because I had found previously, that in my setup, the 193V L'Amp was not great in its bass performance. Its power output is about the same, but its output impedance is higher. The lower output impedance of 2SJ28 as a follower is likely the main factor in the improved bass performance. I think that the THF-51S is still better in bass performance, but then it is also several times more powerful. As well, my JBL 2225 bass cabinets are quite efficient so there is no lack of power for the home environment.
So, the bottom line is, I'm happy with this amp. I was worried about how it would turn out, as I was trying out new things (for me). It's a big weight off my shoulders.
Another thing to consider is my preamp, the LuminAria, which has a single ended 2SK82. So, it has second order harmonic distortion that is thrown into the mix. As a result, the sound from the amplifier is also influenced by the preamp.
This new 2SK82-2SJ28 amp, being a source follower, has lower distortion and lower output impedance than the L'Amp that it replaced. After the initial listening period when I had the volume cranked to 11, I toned things down and started listening at a more reasonable volume and a wider variety music. It was then that I noticed a subtle difference from my previous setup. There seemed to be more clarity in the midrange and highs. Not a huge difference, but the notes seemed more pure. After thinking about it, I think that previously, the combined harmonic distortion of the Luminaria and the L'Amp was enough to make an audible difference. The lower distortion of the new amp brought the total second harmonic to below a certain detectable threshold for me. I think that if my preamp were not the LuminAria, but one which does not produce this harmonic profile, the difference would not be noticeable.
I have a biamp setup so I thought, why not switch the amps around and use the new 2SK82-2SJ28 amp to power the lower frequencies and the BAF THF-51S to drive the higher frequencies. After all, I had not tested the bass performance of the new amp. So, I did and I am happy to report that this combination works well. I wasn't sure about this at first, because I had found previously, that in my setup, the 193V L'Amp was not great in its bass performance. Its power output is about the same, but its output impedance is higher. The lower output impedance of 2SJ28 as a follower is likely the main factor in the improved bass performance. I think that the THF-51S is still better in bass performance, but then it is also several times more powerful. As well, my JBL 2225 bass cabinets are quite efficient so there is no lack of power for the home environment.
So, the bottom line is, I'm happy with this amp. I was worried about how it would turn out, as I was trying out new things (for me). It's a big weight off my shoulders.
Thanks, everyone, for the kind words.
By luck, using my preset bias points in my LuminAria, BAF THF-51S amps, and this new amp, the combination is producing a sound that I'm happy with. Just the right amount of second order harmonics.
The good thing is, all the bias points are adjustable, so I may experiment with them in the future. The beauty of DIY!
A final picture, showing case with more vent holes added for cooling.
By luck, using my preset bias points in my LuminAria, BAF THF-51S amps, and this new amp, the combination is producing a sound that I'm happy with. Just the right amount of second order harmonics.
The good thing is, all the bias points are adjustable, so I may experiment with them in the future. The beauty of DIY!
A final picture, showing case with more vent holes added for cooling.
Attachments
Thank you, Rodeodave.
I was pleasantly surprised by how well the Hammond 156C worked with the VFET. Also, the 2SK82 at high voltage and low current works well, too. Low distortion, good voltage swing and frequency response. I was expecting to have to fight with hum pickup by the Hammond, but it was not an issue.
If I did not already have the LuminAria, I would build a choke loaded VFET preamp. Maybe, I'll do it anyways.
I was pleasantly surprised by how well the Hammond 156C worked with the VFET. Also, the 2SK82 at high voltage and low current works well, too. Low distortion, good voltage swing and frequency response. I was expecting to have to fight with hum pickup by the Hammond, but it was not an issue.
If I did not already have the LuminAria, I would build a choke loaded VFET preamp. Maybe, I'll do it anyways.
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