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In Reply to: Re: Two stage differential amp ?? on NFB posted by Eli Duttman on March 3, 2007 at 08:30:44:
Hi Eli,I'm thinking about a ridiculously simple design with the driver stage being a pair of EL84s transformer coupled to another pair of EL84s for outputs and they all share the same cathode bias resistor which is bypassed with a huge capacitor. They’ll also share the same screen grid supply. The NFB loop goes from the 8 ohm tap via two resistors to the negative leg of the driver stage or even a separate feedback winding sans resistors. My question is weather or not it’ll be balanced that way? I’m hoping that with the huge quantity of NFB in this very simple topology that the circuit will balance itself. The whole shootin match has either two of four resistors and one capacitor but four tubes and two transformers with two power supplies, one for the screens one for B+ and heaters. All held together with a single loop of feedback.
Follow Ups:
Scott,I have reservations.
An interstage trafo inside a NFB loop is asking for TROUBLE. You'll need something with huge bandwidth. Also, top/bottom bandwidth limiting at the amp's I/P is going to be needed. Good bye simplicity.
Eli D.
I've had low levels of loop NFB around an interstage before without trouble but I'm thinking the NFB will be very high considering gain here. On the bench the interstages look great but I'll have to try a real world bread board. If all else fails it's back to a Mullard or Dyna design.
With transformer coupling a single EL84 driver is also an option. They tend to be microphonic used in low-level stages like that though, I've tried.
Thermionically addicted.
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