D3a Line stage

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sbench
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Joined: Fri Jan 28, 2005 3:45 pm

D3a Line stage

Post by sbench »

Here's an interesting comparison of two potential D3a/7721 line stages. Both use the same transformer we're talking about for the EL34/D3a totem pole power amp. For the line stage, I compared two alternatives...
1. Conventional stage with cathode biased with a very low value resistance and to voltage divider to DC source. (This provides cathode bias with low equivalent resistance and eliminates the need for a bypass capacitor.
2. Using the tertiary winding at 58 ohms (25mA bias current).

Attached is schematics, frequency response plots and distortion plot (at 10VRMS output acoss 1.8k). The green is (1), the blue is (2).

Note that (1) has slightly more gain, so the input was padded down (as shown on the schematic) to equalize the outputs.

Note the LOW DISTORTION (which is actually fairly accurate based on real tube tests here) considering the output level and load!

Steve

Steve
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D3aline2.gif
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D3aline1.gif
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