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Old 4th December 2011, 06:52 PM   #31
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Originally Posted by PSz. View Post
Also, any thoughts on cap bypass for the source resistor?
.
For a 1ohm source resistor, you'll need about 50,000uF....

Roscoe
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Old 4th December 2011, 07:09 PM   #32
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@PSz

The higher the leakage current, the higher the voltage drop (I X R)

I didn't try a source bypass cap. I didn't want to suffer the wrath of the purists, and I knew the source resistor alone would be pushing it.

I'll be interested in your opinion once you've tried it.

Last edited by Michael Rothacher; 4th December 2011 at 07:12 PM.
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Old 4th December 2011, 08:55 PM   #33
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so in case of fig10 with 15/16V on the Vd can i use a 3V batterry on the gate for the bias like in tube amp grid bias?
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Old 5th December 2011, 12:11 AM   #34
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Look at the schematic I pointed you towards previously. L'Amp: A simple SIT Amp

A power supply (B+) of 45-50V will work nicely. Adjust the bias for 2.5-2.75A.

Bias batteries are not a good idea here.
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Old 5th December 2011, 05:49 AM   #35
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ok michael and thanks for your time! a last dumb question , for the bias supply i suppose that P1 is a 10k pot but what is CW and CCW on the shcematic? and is 10000uF output cap the minimum value to get fullrange?
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Old 5th December 2011, 06:25 AM   #36
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CW = clockwise

CCW = counterclockwise turning of the pot....
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Old 5th December 2011, 10:34 AM   #37
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i learn everyday thanks!
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Old 5th December 2011, 11:57 AM   #38
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120V lightbulb is very difficult to find in europe, is it not as good using a 300W OHMITE - C300K10R -Click the image to open in full size.
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Old 5th December 2011, 12:30 PM   #39
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and is 10000uF output cap the minimum value to get fullrange?
The cutoff frequency (-3dB) for the filter formed by the 10,000 uF output cap and an 8 Ohm speaker is:

Fc = 1/(2*PI*R*C(Farads)) = Hz
Fc = 1/(2*PI*8*.01) = 2Hz

Then the frequency response at 20Hz will be:

20*LOG10(1/(SQRT(1+(Fc/F)^2)) = dB
20*LOG(1/(SQRT(1+(2/20)^2)) = -0.04dB

You can do the math for a smaller cap and decide if it's suitable.

Also, the big resistor will work fine. You want 11-13 Ohms total. It will get very hot.

You could potentially string several 2 Ohm resitors in series and tap off to try different load values. But that might best be left for later.
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Old 5th December 2011, 02:34 PM   #40
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or use this variable expensive one .. Click the image to open in full size. VISHAY SFERNICE 280W 5% 22R
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