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In Reply to: Re: how can I know or calculat my amp feedback ? posted by lovetube on September 11, 2003 at 05:29:24:
Formula for the relationship between amplification factor with feedback (Af), amplification factor without feedback (Ao), feedback resistor (Rf) and cathode resistor on the input tube where Rf is connected:
Ao = Af ( 1 + Ao x Rk /(Rf + Rk))
Negative feedback factor is the ratio between Ao and Af: NF = Ao/Af
Or, expressed in dB, NFdB = 20log(NF).
In another way, you can measure the amplification with and without neg. feedback and their ratio expressed in dB is your amount of neg. feedback in dB. For example, Ao=300, Af=30 - NF=10 times or 20 dB.
Follow Ups:
Hi.
Thanks for the great post . can you tell me how to meansure the amplification ?
I try to meansure the current in the input without preamp connected but I can't get any things there . am i wrong on this ?
thanks for your time.
In a simplified manner - measure the AC voltage on the output (speaker) - you`ll need a sinusoidal input signal (signal generator or test CD). Let`s say that is 15 Vrms. Now measure the input signal, let`s say it`s 0,5 Vrms. Now, your Af = Uout/Uin = 30. You can disconect the feedback loop (can be tricky) and measure again, or just calculate Ao, knowing Rk and Rf. Let`s say Ao=300 - for 15 Vrms out, input signal will be 50 mV. NF=Ao/Af = 10 or 20 dB.
This is simplified, we have not inserted the speaker impedance, (changes with frequency) and the internal impedance of our amp (changed with feedback),etc.; but for our amateur purposes it is "good enough"...
Thanks for your post again .
last question please .how can I meansure the input singal in Vrms ?
can I use the multi meter ?
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