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Old 26th November 2012, 12:10 AM   #11
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Amp is up and running on bench supplies, and sounding just as nice as all the other single ended projects I've done lately. Once I fixed an open solder joint, both channels came up without any protest. I need to change the bottom resistor in the bias/Schade feedback network for the output to get the bias current up a bit (it's 130mA vs. the planned 150 mA), but that's just the difference between the simulated device and actual. Stability does not appear to be an issue.

Next will be to gin up a power supply and enclosure for extended listening, as the heat sink I'm using is not at all challenged by the power dissipation.
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Old 26th November 2012, 06:00 PM   #12
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Just about any mosfet rated 600V and above should work in this amp. I suspect that even 500V would be ok in most places, but I want to be safe in a case where the amp is slammed into saturation with a light output load. There are lots of candidates, including a whole mess of 600V IGBTs. No one so far has proposed using IGBTs in this kind of amp, but I see no reason against it.
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Old 26th November 2012, 07:28 PM   #13
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It turns out Michael Koster has tried the Schade dodge with an IGBT, using a somewhat different circuit...

Most linear triode-strapped pentode
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Old 27th November 2012, 03:27 AM   #14
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Koster's implementation is similar to Pete Millett's "E-Linear" amp - another way to skin the cat. Pros and cons - no coupling caps, but a cap (the "Ultrapath" cap) that is tied up with whatever is happening with the power supply. Also, lots of dissipation in the source resistors for the output stage. Still, it's another approach. I had a look at the 1kV IXYS depletion mode FETs used for the input - I have a tube of these I haven't broken into just yet. Compared to the DN2540 Supertex parts, the input capacitance is a lot lower - 100pF vs 300 for the DN2540N5. Having said that, the Supertex part sounded nice in the "SiC Puppy".
At any rate, it's nice to have more circuit options to try. The IXYS part could be used to cascode other parts that have even lower input capacitance, such as my favorite of the moment, the 2SK117, or the J309/310, as used in the current implementation of this amp....
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Old 27th November 2012, 04:20 AM   #15
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I may add - I always add an input coupling cap to my amps. as I can't vouch for the purity (the lack of DC offset) for any source they might encounter. I have been castigated for this and other additions from individuals who will remain unnamed (they wouldn't know anyways, since I'm supposedly on their "ignore" list). I prefer stuff that can play in an imperfect world. A minimum-components implementation is just that - not necessarily the best sounding.
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Old 23rd September 2013, 08:50 PM   #16
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I may drag this setup in to work to play on the HP4194A gain-phase analyzer, as I'm curious as to how the UBT-1 outpit transformers will look. On my "SiC Puppy" Amp, I have two UBT-1s of different vintage, and they appear to have very different frequency response characteristics. I may swap transformers between channels to see if the problem migrates, but not right away...
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Old 23rd September 2013, 09:36 PM   #17
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hi there,
If you don't mind me asking, in the photo you posted what are the dimensions of that heat sink?
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Old 24th September 2013, 06:46 AM   #18
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It's about 10" X 11".
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Old 28th October 2013, 08:37 PM   #19
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In the schematic attached to post 1, there is no value for R13. I need the value to run the circuit in LTspice.
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Old 28th October 2013, 09:17 PM   #20
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To answer my own question, I took an educated/wild guess and a value of 68k for R13 seems to work the best. Actually, I just started off with a value of 10k and ran a few sims to see which way the wind was blowing.

I substitued an FQA1190C_F109 (what a mouthful) for the STW7NB80 because I could not find a SPICE model. For the transformer I used a model for a Trace-Elliot 15W that I had on hand.

The simulation shows about 7W into 8 ohms at 1kHz with 3.7% THD. Am I in the ballpark with these results?
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