Fisher 800 push-pull output stage not symmetrical. Why? - diyAudio
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Old 21st January 2012, 05:28 PM   #1
pixpop is offline pixpop  United States
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Default Fisher 800 push-pull output stage not symmetrical. Why?

I'm messing with a Fisher Ta-800, and according to the schematic the output stage is not entirely symmetrical. Here's the schematic for one channel:

http://dl.dropbox.com/u/8094054/ta-800.jpg

Note that the output transformer primary is split 90/105, and R97 is different from R99.

Can someone splain this to me? I'm expecting the output primary to be split into 2 equal halves...
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Old 21st January 2012, 05:42 PM   #2
TheGimp is offline TheGimp  United States
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DC Resistance of most OPTs is not equal for both windings unless extreme care is made by the manufacturer.

This is because each successive layer in a transformer has a greater circumference, and hence greater resistance.
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Old 21st January 2012, 05:46 PM   #3
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If the primary is wound side by side on a split bobbin, then the the two halves will be identical in number of turns, inductance and resistance. If the two halves are wound one layer on top of another, then as the coil gets bigger, the resistance per turn goes up slightly. So, if you had taps every 25 turns, say, you would be able to see the rising resistance. Its not important. Only the turns and inductance matter. Hope this is Ok.
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Old 21st January 2012, 05:46 PM   #4
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The slight difference in winding resistance is normal. It happens because of the increased wire length due to coil build-up as the winding pack gets physically larger. As for R97 & R99, I suspect it may be a typo. The schematic looks like a Sams and mistakes are not uncommon for them. It may also be that the particular unit they reversed engineered (assuming that's what they did) just had an off-color resistor. Or maybe even an improper value. I'd say they should both be 1K ohm grid stoppers.
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Old 21st January 2012, 05:47 PM   #5
jjman is offline jjman  United States
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I think it's normal to have a difference in DC ohms there. Length of wire can differ (DC ohms) whereas the number of turns (AC performance) would be the same. R97 and R99 are grid stoppers so I wouldn't expect their difference to affect balance although I don't know why they would be different values. Maybe since there is a mystery cap under R97 (on the scheme) and it's removing some high frequencies and the higher R99 is acting as a low pass to equalize. Guessing.
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Old 21st January 2012, 05:48 PM   #6
jjman is offline jjman  United States
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4 simultaneous answers.
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Old 21st January 2012, 05:51 PM   #7
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So we must be right.
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Old 21st January 2012, 05:51 PM   #8
pixpop is offline pixpop  United States
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Thank you, thank you, thank you, thank you.

Did I miss anyone?
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Old 21st January 2012, 06:27 PM   #9
pixpop is offline pixpop  United States
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BTW, I just checked, and the two resistors are indeed different. 1k and 10k.
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Old 21st January 2012, 08:58 PM   #10
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It was very common for Fisher to use two different values for grid #1 stopper resistors, for the sole purpose of helping to control any potential parasitic oscillation tendencies in the output stage.

Dave
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