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 12th November 2008, 03:06 AM #11 zigzagflux   diyAudio Member     Join Date: Oct 2006 Location: Charlotte, NC So did some testing tonite, came up with somewhat questionable results. All measurements taken with calibrated Fluke 87 and uncalibrated Fluke 8060A. Source is an unimpressive sine/tri/square frequency generator; you can see the glitch on the sine wave, indicating they generate the sine wave from the triangular. 47nF cap with 99.5K resistor on secondary of 1.95 ratio xfmr = 108Hz cutoff. Being the capacitance measurement is probably the least accurate, the numbers would indicate an actual capacitance of 56nF. Quite a difference, with a 10% tolerance cap and a more accurate multimeter. Next test was same cap, 9.857K resistor, no other load, no xfmr. Just a simple RC HPF. Measured -3dB is 309Hz. Still results in a calculated capacitance of 52nF, outside normal tolerances. Tried moving to a 33nF 10% tolerance capacitor, measured by Fluke 87 as 32.7nF. With transformer in circuit, 129.3K resistor on secondary, -3dB = 118Hz, resulting in calculated capacitance of 39.7nF, still outside tolerance. Simple RC network with no xfmr, same capacitor, 9.86K resistor, -3dB = 410Hz, results in calculated capacitance of 39.4nF. Pretty close to previous test, but well outside direct measured capacitance and nameplate tolerance. Is it possible the quality of the sine wave is messing up the measurements that bad? I know the measured resistances are very accurate, as is the frequency and voltage measurements. Confirmed with a scope, the frequency generator magnitude is constant over the swept frequency range. Maybe a valid question would be does anyone know of a Mac app that produces fixed frequency clean sine waves ?
 12th November 2008, 07:07 AM #12 revintage diyAudio Member   Join Date: May 2007 You must include transformer inductance and RDC in your calculations. Do it the empirical way as calculations won´t be reliable anyway as inductance is an unknown factor.
 13th November 2008, 01:52 AM #13 zigzagflux   diyAudio Member     Join Date: Oct 2006 Location: Charlotte, NC Well, after some further testing, there is a combination of two issues. One, the purity of the sine wave drastically affects the results, even with a simple RC HPF with no transformer involved. Downloaded a function generator, using the sound card of the Mac. Scope verified a very pure sine wave, much cleaner than the previous function generator used. Numbers turned out right on the money. Then, inserted the transformer and repeated the tests. Still off the numbers, so the transformer is definitely affecting the ideal response. Essentially, the transformer makes the overall response appear as though the input capacitor is about 15% higher than actual. Empirical testing is really the only reliable way to get the proper behavior. End result is theory with ideal transformer required 47nF cap with 110K. Actual test results show 33nF cap with 130K for same response. Fortunately, there are no peaks or odd behavior going on with these values, all the way down to 20Hz. Looks like from a test equipment standpoint this methodology should work; we'll see how it sounds. At least I have obtained a good clean sine wave generator out of this fiasco...
 13th November 2008, 05:53 AM #14 revintage diyAudio Member   Join Date: May 2007 Do you intend to drive the amp balanced or unbalanced?
 13th November 2008, 11:31 AM #15 zigzagflux   diyAudio Member     Join Date: Oct 2006 Location: Charlotte, NC Entire system is balanced from DAC forward.

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