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 19th December 2007, 02:53 PM #1 cbutterworth   diyAudio Member   Join Date: Oct 2006 plotting load-line for OTL amp? Following on from my previous loadline thread, I have another question: How do I plot a loadline for an OTL amp, such as the one shown in http://headwize.com/projects/showfil...aarde1_prj.htm ???? Would I use the value of R5, which is 4.7K in this example? Or would I use the impedence of the headphones being driven (32R in my case)? If I were to pick my choice from above, then I'd go for the 4.7K, but I could be totally wrong. Thanks, Charlie
 19th December 2007, 08:41 PM #3 cbutterworth   diyAudio Member   Join Date: Oct 2006 Jeb-D, OK does this sound right? 1. Load line for E88CC would be drawn between 150V at Ia = 0 and 0V at Ia=17mA. 2. The operating point for cathode resistor of 820R would be at -3.2V @ 4mA. I got this figure by plotting Ia for each value of Vg/820. Where this line (shallow curve) intersects with the loadlne is the operating point. Vp = 115V, Ia = 4mA 3. OP tube loadline is plotted with Ia = 0 at 150V and Ia = 45mA (150/3K3) at 0V. 4. OK, so it seems that an operating point of -50V with a Vp of 100V looks good, but how do I calculate the voltage on the grid? Is this value the same as the Vp for the input tube? If so then it is around 115V, so the cathode voltage would actually be 165V?? 5. AC reference loadline 3K3//10K//32 = 31.5R or 32R So, for 2V p-p on the input, I would get a voltage swing from 90V to 140 on the plate of tube 1, which is 50V p-p. This would give something like voltage 70V p-p on the output? In other words a total amplification factor of 35??? Charlie

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