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Old Yesterday, 03:31 PM   #1551
lhquam is offline lhquam  United States
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Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
Originally Posted by Ian Macmillan View Post
I misunderstood. So, your goal is to not have the FE degrade the OS performance as measured with a perfect FE.

I'm also guessing that you don't want to remove even harmonics, or at least k2 given that the intentionally mismatched OS FETs are chosen so as to provide more k2?
I would not say the the OS FETs are intentionally mismatched. Yes, the OS produces a lot of k2 due to the transconductance mismatch of those FETs, but that is not why they were chosen.

If you were to build a balanced (bridged) amplifier using the same FE and OS, you should be able to cancel the even harmonics, if that is what you really want.

Based on Nelson's comment in post #1545:
Quote:
They still do, but this is only significant when the open loop distortion is quite high, like more than 1%, and usually noticed only when there weren't significant high order harmonics already.
I realize that I chose an inappropriate example for studying the FE/OS feedback issues. The OS using IXYS FETs and an FE of Sony VFET-2 flavor have distortion too low to really exhibit the kind of harmonic mixing that cause problems with high levels of feedback.

At this point I do not know how the FE "tuning" will affect the sound of this amplifier. I need to build a complete second amplifier so that I can do A/B testing.
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Old Yesterday, 04:26 PM   #1552
Ian Macmillan is offline Ian Macmillan
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Join Date: May 2002
Location: Cambridge, UK
Quote:
Originally Posted by lhquam View Post
I would not say the the OS FETs are intentionally mismatched. Yes, the OS produces a lot of k2 due to the transconductance mismatch of those FETs, but that is not why they were chosen.

If you were to build a balanced (bridged) amplifier using the same FE and OS, you should be able to cancel the even harmonics, if that is what you really want.

At this point I do not know how the FE "tuning" will affect the sound of this amplifier. I need to build a complete second amplifier so that I can do A/B testing.
OK, now I am intrigued: why did you chose this particular pair of OS FETs?

Not saying that I personally want to cancel the even harmonics - I don't, or at least I am not pursuing this as a goal.

Agree with you regarding A/B testing and I look forward to reading about your results.
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Old Yesterday, 05:15 PM   #1553
lhquam is offline lhquam  United States
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Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
Originally Posted by Ian Macmillan View Post
OK, now I am intrigued: why did you chose this particular pair of OS FETs?
...
When using IXYS SOT-227B FETs, there are very few choices for the P-channel FET. The N-channel FET was chosen primarily for its low Crss values. Apparently PassLabs is using the IXFN140N30P in the XA25; look at the photos in the 6Moons review.
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Old Today, 01:18 AM   #1554
lhquam is offline lhquam  United States
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Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Here is a rather nice set of bench measurements where the closed-loop gain is 20dB, the global feedback is only 14dB. The open-loop gain with no load on the FE and no cascode feedback is about 57dB. The FE load resistor and cascode feedback reduce the open-loop gain to about 34dB. This is a good result, but the damping factor is significantly reduced, as would be expected from the reduced global feedback.
In the plots below show the harmonic spectra for 1kHz into 8R for 1W, 5W, and 20W, and the circuit.
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