Tubes Asylum

how can I know or calculat my amp feedback ?

198.142.132.25

[ Follow Ups ] [ Post Followup ] [ Thread: Display Email Next ] [ Tubes Asylum ]

lovetube

(A)

please advice.
thanks

Follow Ups:

check your email (n/t) - steve oda 15:23:14 09/12/03 (0)
m/t
Re: how can I know or calculat my amp feedback ? - steven oda 04:53:22 09/11/03 (6)
LT,
ROUGHLY... take the resistance off the secondary output transformer. Now, divide this by the resistance of the resistor from the voltage divider point to ground.
For example, if the feedback resistor is 4700 k-ohms and the resistor from ground to the divider point is 150 ohms:
(4700)/(150) = 31.33. So if the speakers "sees" 34.0 volts---31.33 volts will be dissipation and lost by the feedback resistor and 2.67 volts will present at the divider point.
In general, the greater the resistance of the NFB resistor the more voltage off the secondary winding is absorbed. And the less amount is available for feedback control, at the divider point. And vice-versa.
- thanks steven. < NT> - lovetube 22:54:39 09/12/03 (0)
  NT
- Re: how can I know or calculat my amp feedback ? - lovetube 05:29:24 09/11/03 (4)
  Hi Steven.
  thanks for your post . I been play around with tube amps quite a lot but never play with NFB .
  but I do understand all you said above except what is the 2.67v means ?
  I ofent seen NFB in DB. like 20db or 26db or so .
  if I can calcula the Volatges on the device point . then how can I work out the amount of feedback in DB ?
  thanks
  Peter
  - Re: how can I know or calculat my amp feedback ? - Damir 04:57:28 09/12/03 (3)
    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.
    - Re: how can I know or calculat my amp feedback ? - lovetube 07:23:10 09/12/03 (2)
      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.
      - Re: how can I know or calculat my amp feedback ? - Damir 11:34:45 09/12/03 (1)
        
        In Reply to: Re: how can I know or calculat my amp feedback ? posted by lovetube on September 12, 2003 at 07:23:10:
        
        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"...
        
        Re: how can I know or calculat my amp feedback ? - lovetube 22:57:00 09/12/03 (0)
        
        In Reply to: Re: how can I know or calculat my amp feedback ? posted by Damir on September 12, 2003 at 11:34:45:
        
        Thanks for your post again .
        last question please .
        how can I meansure the input singal in Vrms ?
        can I use the multi meter ?

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

You can not post to an archived thread.