Sony VFET 2SJ28 Single Ended Choke Loaded Follower Amplifier

[Ben Mah]

ra7, Congratulations on your build.

For your bias supply did you build it stand-alone as my original schematic or did you do as I did in post #17? In the post #17 schematic I used the V+ supply to boost (add) to the 12V bias supply, so the higher voltage stand-alone bias supply was not necessary.

I have been listening to my THF-51S common drain mu follower until recently when I put in my choke loaded 2SK180 follower. However after reading your post, I became curious so I put the choke loaded 2SJ28 follower into the system.

On first power up, there was a very mild thump. My speakers are 103dB sensitive and the thump wasn't very loud. Subsequent powers up were just a bit louder but still not objectionable to me. I put a meter on the output for the subsequent power ups, after checking that all of the capacitors were fully discharged. At the speaker output, I saw 4 to 5VDC but that fell immediately to 0VDC. It definitely didn't go to 18VDC and slowly drop.

The main difference I can see is that I bypassed my speaker output caps with a 10uF film (motor run) capacitor. I don't know whether that makes a difference or not.

My choke loaded 2SK180 follower has no power up thump at all. Apart from the 2SJ28/2SK180 difference, that amplifier has the power supply on the drain and the source/choke is grounded. The reason I did it that way was because that was left over from the original common source configuration that I changed to follower configuration. However the drain to ground with the power supply to the source/choke configuration that I use for this 2SJ28 follower is supposed to be superior due to the lower noise, as explained by Mr. Pass in his DIYAudio VFET Part 1 amplifier writeup:

You may wonder why the Drain of the VFET is attached to ground. The reason is that the Drain impedance of the VFET is quite low compared to regular transistors, and any noise that appears on the Drain is going to make it to the output signal. Since Ground is the quietest spot in the circuit, it is logical to attach the Drain there. In the N channel version this is still true, although the supply voltage for the current source becomes negative instead of positive.

As I recall people have written that the DIYAudio VFET amp also has a power up thump.

Regarding the VFET temperature, if the VFET body is only at 40C, I wouldn't worry about it. I haven't used Keratherm but I have used similar pads that were designed to be installed without goop with no problem.

My amp is very quiet, even with my 103dB speakers. I am a firm believer of minimizing loop areas (twisting wires) and keeping power AC far away from signal AC and DC. It is possible to get your amp to be hum free. The 2SK180 follower with the power applied to the drain, which is supposed to be noisier, is also hum free. Wire management does make a difference.

 

[ra7]

ra7, Congratulations on your build.

For your bias supply did you build it stand-alone as my original schematic or did you do as I did in post #17? In the post #17 schematic I used the V+ supply to boost (add) to the 12V bias supply, so the higher voltage stand-alone bias supply was not necessary.

No, I built a separate 30 VAC supply, with a voltage divider and a pot to go from about 40 V to 20 V. Works nicely.

I have been listening to my THF-51S common drain mu follower until recently when I put in my choke loaded 2SK180 follower. However after reading your post, I became curious so I put the choke loaded 2SJ28 follower into the system.

On first power up, there was a very mild thump. My speakers are 103dB sensitive and the thump wasn't very loud. Subsequent powers up were just a bit louder but still not objectionable to me. I put a meter on the output for the subsequent power ups, after checking that all of the capacitors were fully discharged. At the speaker output, I saw 4 to 5VDC but that fell immediately to 0VDC. It definitely didn't go to 18VDC and slowly drop.

I will try with an 8 ohm dummy load and see what I get.

The main difference I can see is that I bypassed my speaker output caps with a 10uF film (motor run) capacitor. I don't know whether that makes a difference or not.

My choke loaded 2SK180 follower has no power up thump at all. Apart from the 2SJ28/2SK180 difference, that amplifier has the power supply on the drain and the source/choke is grounded. The reason I did it that way was because that was left over from the original common source configuration that I changed to follower configuration. However the drain to ground with the power supply to the source/choke configuration that I use for this 2SJ28 follower is supposed to be superior due to the lower noise, as explained by Mr. Pass in his DIYAudio VFET Part 1 amplifier writeup:

You may wonder why the Drain of the VFET is attached to ground. The reason is that the Drain impedance of the VFET is quite low compared to regular transistors, and any noise that appears on the Drain is going to make it to the output signal. Since Ground is the quietest spot in the circuit, it is logical to attach the Drain there. In the N channel version this is still true, although the supply voltage for the current source becomes negative instead of positive.

As I recall people have written that the DIYAudio VFET amp also has a power up thump.

Yeah, I see that the P-channel VFET amp doesn't have a relay, at least on the schematic (I didn't enter the lottery because I had VFETs... waiting for Round 3 to come about). I figured Nelson didn't put it there because the thump wasn't a problem.

Regarding the VFET temperature, if the VFET body is only at 40C, I wouldn't worry about it. I haven't used Keratherm but I have used similar pads that were designed to be installed without goop with no problem.

My amp is very quiet, even with my 103dB speakers. I am a firm believer of minimizing loop areas (twisting wires) and keeping power AC far away from signal AC and DC. It is possible to get your amp to be hum free. The 2SK180 follower with the power applied to the drain, which is supposed to be noisier, is also hum free. Wire management does make a difference.

Yeah, mine is quiet too, hum is audible only when you put your ears near the speakers. But I'm sure with wire twisting and final layout, I can get rid of it. I have joined the three PS grounds using alligator clips right now

My speakers are 95 db/2.83V (15" JBL 2226H woofer coupled to the 2380A horn). Curious to hear what kind of speakers you have. The bass response with the 193T inductor seems quite good. Deep organ notes on the Saint Saens Organ Symphony come through quite good, though the speakers only go down to about 40 Hz.

Yet to put it on the test bench... enjoying the sounds

 

[Ben Mah]

I was out for a while so the amplifier had a chance to cool down. From cold, the turn-on DC at the speaker was about 1.5V which went down to 0V immediately. So it's a quiet thump.

My speakers are Avantgarde Uno Series 2 from about year 2000. No noise at all with my ear at the drivers.

I am very happy with the sound too. I like all of my VFET/SIT follower amplifiers. I think I may have a slight preference for choke loading though - maybe a bit more relaxed and 3D compared to the common drain mu follower, but the difference is very small.

 

[ra7]

Ha… same here. Tested with resistor first. There is a very low level thump… not objectionable nor a problem for the speakers. No relay needed. Back to the music.

Thanks for the feedback.

Avantgarde Unos… nice!

 

[Ben Mah]

After a cool and wet spring, summer is finally here. The Sony VFET choke loaded follower amp is back making music and only dissipating 33W of heat per channel. It's a great sounding summer class A amp.

The sun was out, temperatures over the weekend in the mid to high twenties, and lots of outdoor activities, including live music!

 

[Vunce]

Great pics Ben!

 

[nicoch58]

After a cool and wet spring, summer is finally here. The Sony VFET choke loaded follower amp is back making music and only dissipating 33W of heat per channel. It's a great sounding summer class A amp.

The sun was out, temperatures over the weekend in the mid to high twenties, and lots of outdoor activities, including live music!

interesting 33w is half of my SE Sony kit

 

[Ben Mah]

Thanks, Vunce. It was great to finally enjoy the sun again.

nicoch58, the difference is my amp is choke loaded whereas the diyAudio kit is CCS loaded. The choke is low DC resistance and dissipates very little energy whereas the CCS dissipates nearly as much energy as the VFET. So my choke loaded amp runs on a 22V power supply at 1.5A per channel whereas the diyAudio amp runs on a 36V supply at 1.6A per channel.

 

[nicoch58]

HI Ben thanks, as I have 34° I really like your amps 😉

 

[Ben Mah]

Canada Day today, and my VFETs took the day off. I went downtown to Canada Place Convention Centre and Cruise Ship Terminal and listened to some great music. There's nothing like loud music with bass that can be felt. It was rocking!!