hi.
i have a Chinese pre amp of unknown brand/kit and that's an input stage that can be seen on the attached drawing , followed by cathode follower. the preamp sounds reasonably good but i can see there is a feedback between both triodes of 12ax7 tubes. if i wanted to add a switch or reduce the feedback by half than how to calculate the feedback resistors ratio in this case? probably a silly question to most of you but is there any formula.
i would like to reduce the feedback by at least half.
thank you for your help.
i have a Chinese pre amp of unknown brand/kit and that's an input stage that can be seen on the attached drawing , followed by cathode follower. the preamp sounds reasonably good but i can see there is a feedback between both triodes of 12ax7 tubes. if i wanted to add a switch or reduce the feedback by half than how to calculate the feedback resistors ratio in this case? probably a silly question to most of you but is there any formula.
i would like to reduce the feedback by at least half.
thank you for your help.
Could you post the exact circuit that you are using? A cathode follower could be added to the posted circuit in several different ways.
i haven't copied the rest of it yet, i didn't thing it matters, the second cathode follower stage is out of the first feedback loop stage, correct?
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Yes, then you could increase the value of the feedback resistor, which would give higher gain and less feedback. This will also improve the LF response, as the second plate is less heavily loaded by the 120k feedback resistor.
Or, instead split the 820R first cathode resistor into two series resistors, and connect the feedback resistor to their center.
Or, instead split the 820R first cathode resistor into two series resistors, and connect the feedback resistor to their center.
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There is some interaction, because the nfb resistor loads the second plate, reducing the open loop gain. When the nfb resistor is increased in value, the gain of the second stage tends to increase. which tends to increase the amount of nfb.
At the same time, higher closed loop gain due to the increase of the 120k tends to decrease the amount of nfb. You would have to analyze both circuits in detail to see what happens, but it may be that the amount of nfb doesn't change too much if the 120k is, say, doubled or tripled in value.
The other option, splitting the first cathode resistor, avoids that loading interaction. If you start with two equal parts, that will increase gain by 6dB. There's an interaction there also, with the first cathode loading the nfb network, but it should be less.
At the same time, higher closed loop gain due to the increase of the 120k tends to decrease the amount of nfb. You would have to analyze both circuits in detail to see what happens, but it may be that the amount of nfb doesn't change too much if the 120k is, say, doubled or tripled in value.
The other option, splitting the first cathode resistor, avoids that loading interaction. If you start with two equal parts, that will increase gain by 6dB. There's an interaction there also, with the first cathode loading the nfb network, but it should be less.
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it sounds like a catch 22, i will try increasing it until the feedback is less and gain is higher, thank you for your comments.
Yes, bu this is a drawback of your present circuit, and many other two stage tube voltage amplifiers. A cathode follower before the nfb resistor would eliminate this problem.
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The circuit has basic problems. Perhaps a lower gain tube for the second stage, using a smaller value of plate resistor (say 25k or so), would be better.
Then the nfb resistor would not load the second stage so much. The open loop gain would also be smaller, and also have less nfb with the same nfb resistor. The drawback would be more DC power supply current is necessary.
You could instead use just one 12AX7 stage, followed by the cathiode follower, with no nfb resistor. Then the circuit would have only local nfb due to the cathode resistor. The gain might be kind of high, though.
Then the nfb resistor would not load the second stage so much. The open loop gain would also be smaller, and also have less nfb with the same nfb resistor. The drawback would be more DC power supply current is necessary.
You could instead use just one 12AX7 stage, followed by the cathiode follower, with no nfb resistor. Then the circuit would have only local nfb due to the cathode resistor. The gain might be kind of high, though.
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The 20k and 820R in the feedback loop set the gain. Don't change 820r as that sets cathode current. Make 20k twice as big to double gain or make it 10k to decrease the gain by factor of 2. Be careful with increasing gain the 12ax7 can become a bit unstable at high gains. In my designs I nearly always ended up using 12au7's instead as they were easier to tame.
sorry, what 20k resistor?,...did you mean 120k resistor?
amazing, ..in fact i have an apmplifier < and again welcome to China, it has the same 12ax7 input and exactly the same feedback and that drives 6l6 tube that drive 845. i dont have a full schematics drawn yet but i will complete it at some point. and now the question, does it make a difference if the same designe is used to drive 6l6 tube or just being stand alone pre amp?followed by cathode follower?
A generic 12AX7 stage has a high output impedance, and will have trouble driving anything less than say 300k, without the cathode follower.
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