Das ist aber Schade... - Page 2

wrenchone · 26th November 2012, 12:10 AM

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.

wrenchone · 26th November 2012, 06:00 PM

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.

wrenchone · 26th November 2012, 07:28 PM

It turns out Michael Koster has tried the Schade dodge with an IGBT, using a somewhat different circuit...

Most linear triode-strapped pentode

wrenchone · 27th November 2012, 03:27 AM

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....

wrenchone · 27th November 2012, 04:20 AM

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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26th November 2012, 06:00 PM	#12
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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.

26th November 2012, 07:28 PM	#13
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	It turns out Michael Koster has tried the Schade dodge with an IGBT, using a somewhat different circuit... Most linear triode-strapped pentode

27th November 2012, 03:27 AM	#14
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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....

27th November 2012, 04:20 AM	#15
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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.