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 26th December 2009, 12:44 PM #1 jane   diyAudio Member     Join Date: Jul 2004 Location: N-6600 Norway What's wrong with SE Cad? If I make the primary load equal to the plate resistance (Rload=rp), DF should be 1. At the secondary side DF should be 1 or less than 1 if we add up Rdc in the transformer. Why is SE Cad telling me that DF is greater than 1 and Zout is less than 8 ohms? As far as I can see, the transformer has an 8 ohms secondary (10.9 ratio = 950/8). Could someone please explain why SE Cad calculates such generous output impedance? Jan E Veiset __________________ Life is hard - Then you die.
 26th December 2009, 04:02 PM #2 smoking-amp   Sin Bin   Join Date: Dec 2001 Location: Hickory, NC I haven't used SE Cad, but I see a Plate Zo figure of 475 Ohms listed. Maybe the operating point is at a different current than the tube's spec point for the 949 Ohm? Don
 26th December 2009, 04:39 PM #3 Svein_B   diyAudio Member   Join Date: Oct 2005 I have not used John Broskie's SE Amp CAD either. Id seems that it calculates the Z-out from the tube as Rp in parallel with the load Rl. This is as as I understand as per textbook learning - if were to measure at this point. When we measure at the secondary load, the level with no load is twice the level with 8 ohm load. This should indicate that the internal output impedance is around 8 ohm. I have simulated this in LtSpice to confirm my estimate. When we apply a signal to the output load from another generator (with no input signal to the amp), 1 V with an internal impdance of 1k, we measure 4mv across the load, idicating that the output impedance is in fact about 4 ohm as predicted by John Broskies program. SveinB. Last edited by Svein_B; 26th December 2009 at 05:00 PM.
 26th December 2009, 05:32 PM #4 jane   diyAudio Member     Join Date: Jul 2004 Location: N-6600 Norway You are right. The value Plate Zo is rp in parallel with Rl (dcR included) and these value (Plate Zo) is used to calculate the output impedance and DF. If I reduce dcR in the transformer to a minimum (still with rp=Rl), output impedance is calculated to 4 ohm / DF=2. Nice result, but I wouldn't call it output impedance [1]. With the load included in the calculations, DF will never be less than 1. Jan E Veiset [1] Output impedance - Wikipedia, the free encyclopedia __________________ Life is hard - Then you die.
 26th December 2009, 05:34 PM #5 smoking-amp   Sin Bin   Join Date: Dec 2001 Location: Hickory, NC "indicating that the output impedance is in fact about 4 ohm" I assume this was measured with the load connected. In that case it would look like an 8 Ohm output Z in parallel with the 8 Ohm load giving the 4 Ohm result. But the definition of damping factor is Zload/Zsource (or Zload /Zout) so the damping factor should be around 1 (8/8). ?

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