It's alive. I'm barely, though. I was quite nervous getting this thing going. This morning, I checked and adjusted the 2SK180 bias with the SIT still not connected and then made the final connections. Had lunch and then rechecked my work again. Oh! Found a wiring error. Fixed that and checked everything a few more times.
Decided to take the super safe approach to firing up the 2SK180. I wired in a lab power supply and set the current limiter to 1A and 0V to the 2SK180 and and turned the rest of the amp on. Gradually adjusted the bias to about 0.2A and 15V at the drain. Voltages looked good, so I removed the lab power supply and connected the 2SK180 to the rest of the amp.
It was now showtime. I used my dim-bulb tester and turned the amp on. Everything looked good for about 30 seconds, and then the 2SK180 current started oscillating between about 0.1A and 2.2A. I shut her down and said, "What the heck?". Well, maybe not quite those words.
I though about it and then I guessed that the 60 watt bulb that I was using in the dim bulb tester was the cause of this. It was too resistive so when the SIT started conducting, the AC to the power supply dropped along with the unregulated bias supply so the current increased until the drain voltage dropped enough and the AC voltage increased. This in turn lead to the current dropping and the bias voltage rising and then a new cycle started.
I switched to a larger wattage bulb in my dim bulb tester and things stabilized. Got rid of the dim bulb tester and increased the bias current to 1.5A. I let things warm up a bit and everything was good. Hooked up a speaker and a radio for source and was rewarded with the sound of music. I was happy; stress gone.
Fired up my 1 kHz oscillator and REW and did a quick distortion test. I could only attenuate my oscillator to produce an 11.5Vpp output, so that's what I measured. Into 8 ohm load, this is just over 2 watts (I think my math is correct). The 2SK180 operating point was 40 Vds and 1.5A. Distortion spectrum looked nice and THD was 0.11%.
Enough for today. More tests tomorrow.
Decided to take the super safe approach to firing up the 2SK180. I wired in a lab power supply and set the current limiter to 1A and 0V to the 2SK180 and and turned the rest of the amp on. Gradually adjusted the bias to about 0.2A and 15V at the drain. Voltages looked good, so I removed the lab power supply and connected the 2SK180 to the rest of the amp.
It was now showtime. I used my dim-bulb tester and turned the amp on. Everything looked good for about 30 seconds, and then the 2SK180 current started oscillating between about 0.1A and 2.2A. I shut her down and said, "What the heck?". Well, maybe not quite those words.
I though about it and then I guessed that the 60 watt bulb that I was using in the dim bulb tester was the cause of this. It was too resistive so when the SIT started conducting, the AC to the power supply dropped along with the unregulated bias supply so the current increased until the drain voltage dropped enough and the AC voltage increased. This in turn lead to the current dropping and the bias voltage rising and then a new cycle started.
I switched to a larger wattage bulb in my dim bulb tester and things stabilized. Got rid of the dim bulb tester and increased the bias current to 1.5A. I let things warm up a bit and everything was good. Hooked up a speaker and a radio for source and was rewarded with the sound of music. I was happy; stress gone.
Fired up my 1 kHz oscillator and REW and did a quick distortion test. I could only attenuate my oscillator to produce an 11.5Vpp output, so that's what I measured. Into 8 ohm load, this is just over 2 watts (I think my math is correct). The 2SK180 operating point was 40 Vds and 1.5A. Distortion spectrum looked nice and THD was 0.11%.
Enough for today. More tests tomorrow.
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Congratulations
Thank you very much, Soundhappy. This design is an extension of my 2SK82-2SJ28 follower amp design, but I did try a couple of new things. I have to give credit to Mr. Pass and the people of this forum for inspiration. I probably would not have heard about VFETs and SITs otherwise. I've already got some new ideas for my next SIT amp.
The electronics are now complete for the left channel monoblock. All that is left to do is to sand and polish the 3/8" face plate, and to drill ventilation holes in the top cover plate.
I ran some tests today and these results will follow.
I ran some tests today and these results will follow.
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I changed the 2SK180 operating point a bit today. Everything was stable at 1.5A so I just arbitrarily bumped it up to 1.8A. Vds dropped a bit to 39.5V. I did not try to optimize for power or distortion; I'll wait until I finish the second monoblock before playing with that. My 2SK180s were not matched, so no point checking operating points until I can test the other one.
So, I warmed the amp for about one-half hour and checked the operating parameters. Voltage and current were stable, so I ran distortion tests at a few output levels into an 8 ohm resistor load. The distortion measurements are shown for these approximate power outputs:
8.0Vpp = 1W
12.4Vpp = 2.4W
18.0Vpp = 5W
25.6Vpp = 10W
31.2Vpp = 15W
I can increase the bias quite a bit more, so I don't know where this will end up.
Finally, I did some listening in mono-mode using a slapped together speaker system fed by a boom-box CD player. Nothing bad was noticed. Mids and highs are nice, something about source followers which I first noticed when I converted my 2SJ28 common source amp to a source follower.
So, I warmed the amp for about one-half hour and checked the operating parameters. Voltage and current were stable, so I ran distortion tests at a few output levels into an 8 ohm resistor load. The distortion measurements are shown for these approximate power outputs:
8.0Vpp = 1W
12.4Vpp = 2.4W
18.0Vpp = 5W
25.6Vpp = 10W
31.2Vpp = 15W
I can increase the bias quite a bit more, so I don't know where this will end up.
Finally, I did some listening in mono-mode using a slapped together speaker system fed by a boom-box CD player. Nothing bad was noticed. Mids and highs are nice, something about source followers which I first noticed when I converted my 2SJ28 common source amp to a source follower.
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I finished the electronics for the second monoblock today. I took my time powering it up stage by stage, with and without the dim-bulb tester until I finally ran out of precautions and took just turned it on. I did have the bias turned down though, and I slowly brought it up to about 1.6A with a Vds of about 39V.
All the voltages seemed about right, so I connected a speaker and music source and I was a happy camper. No obvious problems with the sound; no hum or unusual noises. I listened to it for a while to let the amp warm up a bit while monitoring the bias. Current flow was stable.
Next up is to play with the operating points and take test measurements. Also, see how the two monoblocks compare. The 2SK180s were not matched, so it will be interesting to see how they measure.
All the voltages seemed about right, so I connected a speaker and music source and I was a happy camper. No obvious problems with the sound; no hum or unusual noises. I listened to it for a while to let the amp warm up a bit while monitoring the bias. Current flow was stable.
Next up is to play with the operating points and take test measurements. Also, see how the two monoblocks compare. The 2SK180s were not matched, so it will be interesting to see how they measure.
Attachments
I set out to test my latest monoblock, the right channel, today. I had the 2SK180 biased to 1.6A, 39Vds, but I wanted to get right to some higher currents, so I increased the bias slowly to 2.2A. Vds dropped a bit to about 38.5V and everything got warmer.
After playing some music for about half an hour, I set up everything for testing. I thought this wouldn't take too long, but first thing I noticed was at 1W out, THD was about 0.5%. This couldn't be right. I fiddled with the wires, disconnecting, and rerouting them until I finally managed to get things working. I really need to build some proper cables for testing.
So, what did I find? The maximum output for the 2SK180 in my setup is about 17 watts into 8 ohms at 0.7% THD. Increasing output after this point produces clipping.
At 2 watts output, THD is about 0.07%. I didn't test at 1 watt ouput because the gain of the 2SK28 stage is too high for the test oscillator output to produce a 1 watt 2SK180 output. Given my cabling problems, I didn't want to bother with a voltage divider for the oscillator.
I also played around with the 2SK180 operating current and found that lower THD can be produced at lower currents but the 2nd harmonic drops and eventually the 3rd harmonic is greater. I prefer to have lower 3rd harmonic, so I'll leave it as it is.
Next, I'll test the left channel monoblock.
After playing some music for about half an hour, I set up everything for testing. I thought this wouldn't take too long, but first thing I noticed was at 1W out, THD was about 0.5%. This couldn't be right. I fiddled with the wires, disconnecting, and rerouting them until I finally managed to get things working. I really need to build some proper cables for testing.
So, what did I find? The maximum output for the 2SK180 in my setup is about 17 watts into 8 ohms at 0.7% THD. Increasing output after this point produces clipping.
At 2 watts output, THD is about 0.07%. I didn't test at 1 watt ouput because the gain of the 2SK28 stage is too high for the test oscillator output to produce a 1 watt 2SK180 output. Given my cabling problems, I didn't want to bother with a voltage divider for the oscillator.
I also played around with the 2SK180 operating current and found that lower THD can be produced at lower currents but the 2nd harmonic drops and eventually the 3rd harmonic is greater. I prefer to have lower 3rd harmonic, so I'll leave it as it is.
Next, I'll test the left channel monoblock.
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Here are some test results for the left channel monoblock. Harmonic distortion spectra are a little different from the right monoblock, but output power before clipping is about the same.
This 2SK180 has noticeably less 2nd harmonic. I just left it as is since I didn't want to increase the operating current above 2.2A. Lowering the current decreases the 2nd harmonic more. Realistically, I don't think I'll hear the differences between the two monoblocks when I listen to music through them.
Time to finish the cases and do some real listening.
This 2SK180 has noticeably less 2nd harmonic. I just left it as is since I didn't want to increase the operating current above 2.2A. Lowering the current decreases the 2nd harmonic more. Realistically, I don't think I'll hear the differences between the two monoblocks when I listen to music through them.
Time to finish the cases and do some real listening.
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You are only using half your input DC power. The rest is waste heat.
The V/I of the active device should be similar to the V/I of the load. Or here, 8 Ohms.
If you can run 2.2 Amps, then you need 8r*2.2A or 17.6V supply. (Plus losses in choke and FET, small but non-zero.)
If you want to throw 40V at it, then you want to bias near 5 Amps (and 200 Watts heat!!). If the supply will sag near 32V, only 4 Amps.
Alternatively find a 16 Ohm load.
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I am running at a higher voltage to try to operate in a more linear region. Lower voltage and higher current would work too, if the choke can take the current; but it my case, it's not rated for huge current. Anyways, power dissipated would not be much different. But you do have a good point.