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20th July 2012, 08:53 AM  #181 
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my Papa is smarter than your Nelson ! clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM 
20th July 2012, 09:09 AM  #182 
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20th July 2012, 09:48 AM  #183 
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run the amp direct on the higt voltage mains 10.000V+

20th July 2012, 11:48 AM  #185 
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A group or a mosaic photo [if any] for the members of DIYAudio to Emeet the principals.

20th July 2012, 10:37 PM  #186 
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Does anybody know how that was calculated? The only thing I know (or think I know) is that without feedback (infinite value for R12) it would be a current source (tapped at the drain of Q1) which would be close to an infinite output resistance. But the gain would be too high and distortion would be much higher without a resistor on the source of Q1 which is also a kind of feedback. The gain is R12/R11.

20th July 2012, 11:29 PM  #187 
The one and only

Yes, I do. I looked at my thumbnail and in it I saw a reflection of the resistor
bins, noting that I had 100K and 150K available. 150K seemed a bit high, although you might want to try it. Infinite R12 was measured in the article, and the gain was 23 dB. Last edited by Nelson Pass; 20th July 2012 at 11:32 PM. 
21st July 2012, 12:22 AM  #188  
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Quote:


21st July 2012, 01:56 AM  #189 
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Join Date: Feb 2002

Oh I never thought of it that way (power devices as resistors). I guess it makes sense. I assume this implies output resistance and there by the damping factor also depends on the rail voltage and bias current. We could also assume without adding a resistor on the output we cannot make output Z higher than 4.8 ohm.
Would I be correct in assuming the reduction in output resistance can be calculated? If I look at the open loop gain of 23dB(14.1x) and the closed loop gain 14dB(5x) and if we take your 4.8 Ohm output Z. Then we could assume 14.1 * 4.8 / 23 = 2.9 Ohm output Z, which is fairly near to the stated output Z of Mr. Pass of 2.9 ohms calculated from the stated damping factor of 3 = 8ohm speaker / 2.6ohm DF. So I guess my gain formula is wrong for values higher than the amps open loop gain. I forgot to take that into account. Woops. I blame high gain opamps. 
21st July 2012, 03:27 AM  #190 
The one and only

Perhaps it's worth working through this a bit. The IRFP240 Mosfet has
a transconductance of about 2 when biased at 1 amp. This means that 2 amps of current flows for every volt of input at the Gate. So for our purposes at the moment we will treat it as a Mosfet of infinitely high transconductance with a Source resistor of 0.5 ohms. Since this circuit does not supply an external resistance between the Source pin and ground, that is all we need for a quick calculation. If the output resistance (Plate resistance if this were a tube) was infinite, then the gain into an 8 ohm load would be 8/0.5 = 16X, which is 24 dB. But it's more complicated than that. The Mu follower current source on top of the circuit also contributes to gain, adding maybe 5 dB. So now we estimate that with infinite Drain resistance the gain is 29 dB, which is 28X. OK so far? But we actually experience 23 dB gain due to the finite Drain resistance(s) and the contribution of the DC feedback loop. This 5 dB drop is a factor of 0.56X, and is equivalent to our high Drain resistance example driving 8 X 0.56, which is 4.5 ohms. So now we have an apparent Drain resistance which is equivalent to that value which paralleled with 8 ohms gives us 4.5 ohms. That resistor is close to 10 ohms. Without the R12/R11 loop, the output impedance of the amplifier is 10 ohms. To get 6.5 ohms, apply a little over 3 dB of feedback to take the gain down to about 20 dB. Probably my estimate of 100K was a little low for R12. 
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