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Old 4th August 2020, 05:51 AM   #1681
N101N is offline N101N
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BJosephs,
it is a push-pull topology exactly as it should be. The resistor provides protection by reducing the very high, nonlinear gate-source impedance and the amplitude.

Susan,
under practical load, the frequency response of the input transformer will almost certainly drop below 20 kHz.
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Old 4th August 2020, 07:49 PM   #1682
Susan_Parker is offline Susan_Parker  United Kingdom
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Quote:
Originally Posted by N101N View Post
BJosephs,
it is a push-pull topology exactly as it should be. The resistor provides protection by reducing the very high, nonlinear gate-source impedance and the amplitude.
One could put in a 47K resistor on each MOSFET, but that won't be enough to protect the part for the BOOM!!!

I am curious as to where do you get the "... the very high, nonlinear gate-source impedance and amplitude." from?

Quote:
Susan,
under practical load, the frequency response of the input transformer will almost certainly drop below 20 kHz.
... and???
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Old 4th August 2020, 07:59 PM   #1683
Susan_Parker is offline Susan_Parker  United Kingdom
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Default Current schematic...

Single channel Audio Path only.

Zero Feedback Impedance Amplifiers-susan-parker-zeus-system-1920s-style-audio-path-schematic-1a-950-gif
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Old 4th August 2020, 08:38 PM   #1684
GUNFU is offline GUNFU  Russian Federation
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beautiful schematic

I did a similar one in 1996 on 6C33C

Last edited by GUNFU; 4th August 2020 at 08:54 PM.
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Old 5th August 2020, 12:30 AM   #1685
Ejam is offline Ejam  Australia
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Default Current Schematic

Susan

I believe you are using the IXTH20N50D as the output mosfet in your Zeus amplifier. I have eight of these MOSFETs but the fear that obtaining two match pairs is going to be an issue, given previous experience with matching vertical mosfets. The prospect of buying more to match, at $56.68AUD each (Digikey), puts that as a remote option. May I ask what parameter is most important matching and what tolerance is acceptable, eg. is it Vgs or transconductance? Like to know your thoughts and any suggestions.

Regards

Anthony
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Old 5th August 2020, 01:29 AM   #1686
TonyTecson is offline TonyTecson  Philippines
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for me it is Crss, for mosfet followers..
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if people react to you a certain way, it is not because of them but more because of you....
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Old 6th August 2020, 09:30 AM   #1687
N101N is offline N101N
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Dear Susan,
I have been referring to the same elegant circuit.

The insulated gate can hardly be characterized as anything other than a considerably doped, high impedance, nonlinear electrical interface that is in need of stabilization by resistive elements. The very thin dielectric layer is indeed vulnerable, but such hostile voltages can barely at all arise. For more common MOSFETs, a low value gate-source resistor is a natural part of the biasing arrangement.

Bandwidth is dependent on biasing and loading. Measurements data made under no load conditions are of little relevance.

Confusing information? I am not urging you to make changes to your working amplifier.
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Old 6th August 2020, 10:26 AM   #1688
jan.didden is offline jan.didden  Europe
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Off your meds again?

Jan
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Old 6th August 2020, 11:00 AM   #1689
N101N is offline N101N
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Jan,
you are welcome to make your first ever factual contribution in the discussions instead of the usual systematic mocking around.
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Old 6th August 2020, 11:03 AM   #1690
Susan_Parker is offline Susan_Parker  United Kingdom
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Quote:
Originally Posted by Ejam View Post
Susan

I believe you are using the IXTH20N50D as the output mosfet in your Zeus amplifier. I have eight of these MOSFETs but the fear that obtaining two match pairs is going to be an issue, given previous experience with matching vertical mosfets. The prospect of buying more to match, at $56.68AUD each (Digikey), puts that as a remote option. May I ask what parameter is most important matching and what tolerance is acceptable, eg. is it Vgs or transconductance? Like to know your thoughts and any suggestions.
Hi Anthony,

I sort them by the current flow through a 1 ohm resistor, to get the best match to minimize the DC in the transformer. They need to be all on the same heat-sink as the values will change with temperature.

A few tens of mA difference isn't going to hurt with an EI transformer, might be a problem with a toroid.

You could gap EI transformers with a thin c. 0.1mm shim of copper without killing the bass-response too much as a "lets see how this works" measure.

I have hundreds of the parts, as they were going to be made obsolete in 2011 so I bought a lifetime supply.

For my daily use pair of PP amps and the SE line driver I got close enough matches from a tube of 30 but the variation between individual devices is large; I got a min/max spread of 512mA to 876mA.

I am NOT a seller however I have enough to do some matched sets if people are interested since it is now very unlikely that I am ever going to be able to sell my amps commercially.

Doing a quick search I see them from this place for under $10 each, which is a lot more palatable:

IXTH20N50D | IXYS Corporation IXTH20N50D Stock available from Infinity-Electronic.hk | IXTH20N50D with Best Price at Infinity-Electronic.hk

Best,
Susan.
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