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SIT measurements, Mu Follower, and amplifier build
SIT measurements, Mu Follower, and amplifier build
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Old 7th February 2020, 06:26 AM   #11
Zen Mod is online now Zen Mod  Serbia
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SIT measurements, Mu Follower, and amplifier build
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Old 7th February 2020, 06:56 AM   #12
jjasniew is offline jjasniew  United States
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Wow. I look at this and think "Wonder if a similar analysis would work with tube amplifiers?" Assign the control grid DC bias value with a function generator and move the tubes plate voltage (B+ voltage) value with another one. Try to find operating points with minimized harmonics along the line of constant quiescent power dissipation?
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Old 7th February 2020, 11:06 AM   #13
lhquam is offline lhquam  United States
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Replace the SIT with the triode in the schematic in post #2 and collect data triples: (Vp, Ip, Rload), where Vp it the plate voltage and Ip is the place current. I have no idea about the shape of the surface. Similarly, I have no data to suggest that all SITs behave as shown in post #10.
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Old 9th February 2020, 09:19 PM   #14
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SIT measurements, Mu Follower, and amplifier build
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Old 9th February 2020, 09:22 PM   #15
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SIT measurements, Mu Follower, and amplifier build
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.......


The significance of this result is that the parameters of the mu follower can chosen for a sweet spot at the desired idle state Vd and Id. More about this in later posts.
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Old 10th February 2020, 12:36 AM   #16
lhquam is offline lhquam  United States
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Default SIT Mu Follower Amplifier

Here is the Common Source SIT Mu Follower amplifier that I have built, similar in topology to the FirstWatt SIT-2. The 2SK182ES SITs are capable of disipating considerable power. According to the datasheet, the package is rated at 500W dissipation, but the SOA is not specified. My inclination was to go for as much class-A power output as "reasonable", consistant with my favorite packaging approach which has the following characteristics:
  • Simple chassis fabrication. Only three metal parts (plus heatsinks).
  • Heatsinks mounted fin-to-fin:
    • 100W/chan passive
    • >200W/chan with fans
  • Easy board removal.
  • Access for adjustments and scope probes.
This amplifier is power a power hog, requiring 2.9A * 120VAC = 348 Watts from the mains. The power supply output is about 94V. There is no problem keeping the heatsink temperature at 50C with the fans at low speed.
Attached Images
File Type: jpg amp-1.jpg (451.4 KB, 206 views)
File Type: jpg amp-2.jpg (284.4 KB, 198 views)
File Type: jpg amp-3.jpg (219.5 KB, 210 views)
File Type: jpg amp-4.jpg (118.9 KB, 209 views)

Last edited by lhquam; 10th February 2020 at 12:47 AM.
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Old 10th February 2020, 12:37 AM   #17
lhquam is offline lhquam  United States
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The amplifier topology is similar to that of the FirstWatt SIT-2, but uses an optocoupler instead of a simple transistor for controlling the bias current, and because of the high input capacitance of the 2SK182ES, a single-ended JFET buffer is added to drive the SIT gate. In the schematic shown below, the currents and node voltages are shown with a question mark (?), since many circuit parameters have not been yet specified.

Mu follower parameter adjustment:

The primary challenge was to determine the values for Rhi and Rmu. The sum Rhi+Rmu determines I0, the idle bias current of the two FETs. Rmu controls the mu-follower gain of the amplifier. Rmu=0 creates a constant current source. To simplify the experimentation with different values of Rhi and Rmu, I temporarily used a 20ga nichrome wire with 3 Molex connectors attached to nodes X, Y, and Z, as shown below. 20GA nichrome wire has a resistance of about 0.635 Ohms/ft. The connector at node Z can be adjusted to change the bias current without changing Rmu. Similarly, the connector mode Y can be used to change Rmu without changing the bias current. After changing the tap locations on the nichrome wire, the SIT gate bias is adjusted using the 500R pot to obtain the "sweet spot" using a 6R output load.
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File Type: jpg SIT-2-schematic-1.jpg (117.5 KB, 224 views)
File Type: jpg nichrome.jpg (387.0 KB, 151 views)
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Old 10th February 2020, 12:39 AM   #18
lhquam is offline lhquam  United States
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The 3rd figure shows a collection of sweeps of THD vs Watts sweeps of the for the left and right channels, respectively.

I experimented with many combinations of Rhi and Rmu. For each combination, the SIT bias voltage was adjusted to obtain the minimum 2nd harmonic H2 using a software spectrum analyzer. The voltage across the precision resistor Rsen in series with mu follower drain was used to deterimine the bias current Id0. The 500R potentiometer controls the gate voltage of the SIT which in turn controls the SIT drain voltage X0 (Vd0).

For each parameter combination THD was measured at Watt levels from less than .1 Watts to 30 Watts, as shown for the left and right channels in the images below. The parameters from those THD sweeps which give highest power output and lowest distortion are:

  • SITno2_wl25 left channel Id=1.88A Vd=55.7V Rmu=0R443 Rhi=0R251
  • SITno1_wl24 right channel Id=1.82A Vd=59.6V Rmu=0R327 Rhi=0R369

For my "final" build I removed the nichrome wires and installed fixed 3 Watt resistor combinations for Rmu and Rhi.

  • SITno2 left channel Id=1.81A Vd=53.7V Rmu=0R447 Rhi=0R266
  • SITno1 right channel Id=1.59A Vd=58.5V Rmu=0R402 Rhi=0R409

The 2nd figure shows the THD vs. Watts sweeps for the left and right channels with the fixed resistors.
Attached Images
File Type: jpg SK182no1-SK182no2-sweet-spot-sweep-collage.jpg (218.8 KB, 110 views)
File Type: jpg SIT2-build-20191102-sweeps.jpg (80.5 KB, 87 views)
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Old 10th February 2020, 12:41 AM   #19
lhquam is offline lhquam  United States
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Finally, here are LTSpice simulations of the left and right channels at 1kHz, 1 Watt into an 8 Ohm load,
with the parameter values used in my build.
Attached Images
File Type: jpg SIT-2-as-built-20191104-LCH-sim.asc.jpg (508.8 KB, 124 views)
File Type: jpg SIT-2-as-built-20191104-RCH-sim.asc.jpg (514.1 KB, 110 views)
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Old 10th February 2020, 01:05 AM   #20
flocchini is offline flocchini  United States
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brilliant
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