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mu feedback
Alex wrote:
Restoring an old 1953 radio I came across a funny way of applying negative feedback in an audio 2 stage amplifier (EBF80 + EL80). First stage was using a EBF80 pentode. Input signal was applied to the control grid, while the feedback from the speaker went to the screen grid via a 0.1uF capacitor of course. Thus the gain of the amplifier was determined by the internal mu (mu g1-g2) of the EBF80, which is in this case is about 18. I am wondering what order of THD one can expect from such mu based feedback "divider"? Also the only way to reduce gain is to find a pentode with a lower internal mu. I am wondering if a pantode exists with the internal mu of say 10? I tried to expand on the mu feedback concept. What about arranging a cascode (series connection) of two triodes and apply input signal to grid of the bottom one, and the feedback -- to the grid of the top one? Then the mu of the bottom one will solely determine the gain of the amp. Advantage of this topology -- very high impedance on the feedback input (grid of the top triode). It is convenient to put a high impedance tone control circuit in the feedback path. I tried to find any low mu low power dual triodes. The lowest mu i found was from 12AU7 (17), closely followed by 6CG7 (20). Again what order of THD can be expected from such a trick? Consider putting a triode in the feedback path. That would tend to cancel out the loose input triode (or g1,g2, k combo of the pentode). This idea probably has a name, and a treatise to match, which may in turn answer your questions. Ian |
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