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Old 15th January 2013, 06:10 PM   #3471
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Quote:
Originally Posted by Antoinel View Post
Tea-Bag. The following explanation of F6 operating in two additioal modes maybe such a diagnostic circuit.
  • Keep Pass NFB
  • Disable the AC drive to the upper FET; but maintain its Vgs bias. Now this FET operates as a constant rather than a modulated current load to the bottom FET. Do your tests and determine whether you like its music performance.
  • Repeat step 2 for the bottom FET instead
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Tea-Bag: Here is potential diagnostic test with the F6 circuit:
  • Disable Pass NFB
  • Reverse the phase of one of the secondary windings. In an ideal world, the power output of this amp will be zero. But; measure it anyway and listen to its performance [signal and music] with headphones.
  • Enable Pass NFB, test and listen again as in bullet #2
Here is another maybe diagnostic test: In F6, the FETs are operating in their best behavior of Class A. Maybe Class AB operation will unravel something else which you could attribute to either FET

Use headphones where possible to disable room effects and standing waves so as to better control the variables under test.
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Old 15th January 2013, 06:25 PM   #3472
lhquam is offline lhquam  United States
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I suspect that transconductance and internal source resistance are the most important parameters of the SS-R100s for use in the F6. Out of 6 R100's in my possession, I have been able to only find one well matched pair.

My experiments with Fairchild FQA28N15 MOSFETs in the F6 indicate that their gate capacitance is too high -- bad hi frequency roll-off.

Quote:
Originally Posted by buzzforb View Post
My guess would be the test suggested in the mosfet testing article by Nelson. I would imagine the differences are in the Ciss and possibly leakage current. ZM eluded to capacitance issues due to interactionwith transformers windings. Based on what Generg and Ilquam have shown, there is some parameter that we are missing that is not seen with basic matching. The SS I have match very well in the general sense, but I have not gone beyond that. Nelson replied to Ilquam on this topic somewhere earlier in the thread, but it might be difficult to find.
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Old 15th January 2013, 06:33 PM   #3473
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Quote:
Originally Posted by lhquam View Post
I suspect that transconductance and internal source resistance are the most important parameters of the SS-R100s for use in the F6. Out of 6 R100's in my possession, I have been able to only find one well matched pair.

My experiments with Fairchild FQA28N15 MOSFETs in the F6 indicate that their gate capacitance is too high -- bad hi frequency roll-off.
You have as much experience with it as anyone, so I cant argue with your thinking.
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Old 15th January 2013, 07:34 PM   #3474
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Originally Posted by buzzforb View Post
You have as much experience with it as anyone, so I cant argue with your thinking.
buzzforb and lhquam. Maybe matching of FETs can also be done dynamically in addition to the standard static one. Suppose that the hardware preceeding the FETs are matched; especially the output voltage, its frequency response, phase etc.. of the secondary windings of the transformer. Reverse the phase of one secondary winding relative to the other. Slowly increase the input signal for the safety of the FETs because both will turn on and turn off simultaneously. The simultaneous turn on is the problematic one in this dynamic test; because they have 46 VDC supply across them. If the FETS are matched, there will be no output voltage. A lack of exact matching will generate an output voltage which may be proportional to the mismatch; small or large.
If the simultaneous turn on of the FETs turns you off then maybe use a variac to lower the power rails and limit the inadvertent over dissipation of the FETs.
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Old 15th January 2013, 08:36 PM   #3475
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phase orientation of lower SS must be held intact

or one can expect some squeeeeeeky sound ......
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Old 15th January 2013, 08:44 PM   #3476
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Originally Posted by Zen Mod View Post
phase orientation of lower SS must be held intact

or one can expect some squeeeeeeky sound ......
Interesting and unexpected observation. Thanks.
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Old 15th January 2013, 08:47 PM   #3477
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positive feedback
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Old 15th January 2013, 08:53 PM   #3478
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O.K......

it took me some hours to find out of my 8 R125 the two pairs that gave me around 0,012% THD and a distortion spectra where k2 and k3 are at the same level.
I wanted this position to go later more easily for negative and positive phase.

If it is possible to find the pairs only by measuring the transconductance of the parts outside of the circuit then I am an idiot.....not to build the circuit Nelson suggested 1993...!

So buzzforb and tea-bag can measure the transconductance, so it would be very interesting to see, if they get a predictable distortion picture.

Tea-bag please insert in your F6 two pairs of SS with same transconductance and let us see the distortion picture. k2 should be reduced, shouldn't it?

Buzzforb can you see distortion?

and of course Ihquam did you see a correspondence between equal transconduction and a reduced k2 compared to k3?

when my 10 Ohm pots are coming I will continue....

because changing the source resistors changes as well the bias, as the offset values...... ! Too much work! Am I correct to suppose that using the pots the bias and the offset is not changed?

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Old 15th January 2013, 09:00 PM   #3479
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Yes, I can measure distortion, although new to visual analyzer and working with buggy EMU.


Perhaps all amps are this way, but I still say, the f6 is tweakers dream.
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Old 15th January 2013, 09:00 PM   #3480
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Quote:
Originally Posted by generg View Post
....... Am I correct to suppose that using the pots the bias and the offset is not changed?

Gute Nacht!
if you meant on ZAP , then that's correct ; they're working strictly in AC domain
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