 |
Project Author |
 |
Joined: 04 Jun 2008, 20:59 Posts: 4308 Location: Arizona, USA
|
Hi, The easiest way to calculate the NFB is to not calculate it but determine it on a scope. I have not found calculating it to be very accurate as there are many unpredictable variables. Far easier IMO to feed in a low power square wave at various frequencies above 5K HZ (I use 10K) and see what it looks like. Use a resistive load. I like to start with the resistor. BTW the ratio of the cathode resistors in the driver stage can be changed as well. But the range I like there is between about 1:5 and 1:10. Adjust the NFB resistor to have about 3 db effect on the signal output. You can use sine waves for this part. Then look at the square wave and see if it shows overshoot and/or ringing. Starting with small value capacitors (33pf, 50pf, 100pf 220pf and so on) place them across the resistor until the wave form is optimal. It may not be possible to get it perfect. A lot depends on the actual circuit components and especially the output transformer. Usually there will be a value that works well. If you find you are going much over about 500 pf then you may have to decrease the resistor in size and thus drop the gain a little more and start over with the capacitors. Usually 3 db is enough. What you are doing is forming a high pass filter with a first order slope. The corner frequency is somewhere below the range of the resonance point and when placed inside a NFB loop will reduce the gain at that frequency sufficiently to insure stability.
Good listening Bruce
_________________ Some of my DIY Tube Amplifier Projects:
|
|