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Old 9th May 2017, 03:01 AM   #11
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This afternoon, I ran some PSpice simulations on the buffered "Fake SIT" that constitutes the bottom half of this amplifier, fed on top by a simple 1.5A current source.

What I was after was the effect of breaking the ground connection to R1 and adding a bias supply to tweak the output voltage centering independent of the overall gain as set by R5. This works, and I was able to adjust the gain from ~4X to ~12X by juggling R5 and the bias setting, still keeping the output centering at ~1/2 Vcc.. A volt or less does the job, something you could set up using a cheapo yellow-green LED as the voltage reference.

Since I was trying to push up the value of R5 for more gain, a positive bias does it, so no additional bias supply is needed. The consequence of pushing up the gain is less feedback around the output mosfet, hence more distortion, entirely dominated by 2nd and 3rd harmonics.

Next up is developing a rationale for for predicting the gain of the buffered fake SIT.
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Old 9th May 2017, 05:35 PM   #12
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Here's what I was messing with, and the distortion profile for a gain of ~12.
Attached Images
File Type: gif LT_SCH.GIF (16.1 KB, 363 views)
File Type: gif LT_Dis.GIF (8.4 KB, 353 views)
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Old 9th May 2017, 05:57 PM   #13
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This is what the distortion looks like with a gain of ~6, R3 = 2.2k, Bias V2 = -0.3V. If you push the gain down far enough, you need a negative voltage for bias V2 to get proper centering. I suspect the results will be different for a real IRFP260 and ZVP3306A as compared to the simulation. When I was building the "Aleph McSIT", I had to considerably tweak the values around the fake SIT portion.

At any rate, food for thought as always...
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File Type: gif LT_Dis_G6.GIF (8.4 KB, 261 views)
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Old 10th May 2017, 04:25 AM   #14
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One thing that has not been apparent to me regarding all the playing around with Tokin, Yamaha, Sony, Papa SITs is their effective voltage gain. Does anyone have some words of wisdom that has messed with these devices?
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Old 10th May 2017, 05:01 PM   #15
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Biased by a CCS and driving 8 ohms, the typical sorts of gain figures
are around 18 dB or so for both the 2SK77B and the SIT-1.

I recall the Sony parts as having somewhat less.
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Old 10th May 2017, 05:08 PM   #16
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Thank you. It looks like similar gain is readily attainable with the fake SIT setup. Big, low-mu triodes are where it's at...
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Old 10th May 2017, 05:36 PM   #17
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Just for grins, here's the simulation tweaked for a gain of a little over 8, or 18 dB. Since the extra bias needed for centering is positive, no extra supply is required and you can generate the bias from the Vcc supply (though it's a bit wasteful..). I would use an old-school GaAsP red or yellow-green GaP LED as the reference source - quiet, cheap, and pretty.
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File Type: gif LT_G8_SCH.GIF (16.3 KB, 208 views)
File Type: gif LT_G8.GIF (8.3 KB, 201 views)
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Old 10th May 2017, 11:22 PM   #18
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Lowering the impedance of the drive network for the bottom fet lowers the distortion, possibly by increasing the bias current in the p-channel fet as well as decreasing the driving impedance to its gargantuan n-channel sibling. You could possibly be very generous with the bias current in the p-channel, as the voltage across it is small. In this case, the bias current in the p-channel fet is 8.5 mA vs. 4 mA in the previous example . The gain is the same for both.
Attached Images
File Type: gif LT_LOIMP.GIF (13.9 KB, 54 views)
File Type: gif LTloimp_DIS.GIF (8.4 KB, 49 views)

Last edited by wrenchone; 10th May 2017 at 11:25 PM.
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Old 11th May 2017, 05:34 AM   #19
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Funny thing -I tried adding a supplemental current source to the source of the p-channel from Vcc, but it did nothing to help the distortion, while lowering the feedback resistors from the n-channel drain to the p-channel source worked well. This time, simpler is better. I'll try lowering the impedance even more tomorrow. Cranking the dissipation on the p-channel up to a staggering 100 mW might work wonders.
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Old 11th May 2017, 05:45 AM   #20
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just to let you know - I'm (as always) following with interest
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