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In Reply to: Re: Feedback Good or Bad.? posted by mark on January 19, 2001 at 06:37:42:
There is one more thing to keep in mind. The 2M feedback resistors voltage divides into the 4.75K input resistors. Yet the 4.75K input resistor is in series with the output impedance of the preamp connected to the amplifier.
So it if the preamp has a 600 ohm output, the 2M is divided into 5.35K (4.75 + 600) ohms.
Now lets say the preamp has an output impedance of 20K. The 2M feedback resistor is divided into 24.75K (4.75K + 20K) ohms, and the result is more neg feed back.
My point is the amount of neg feedback of these great amps is dependant on both the speaker impedance AND the source impedance driving the the amps.
I recommend running a constant test tone through your preamp AND your M-60 / MA-1 and measure the voltage at the speaker with and without feedback. Then do the math on those two numbers. You may also want to try different test tone frequencys because of frequency varying speaker impedances. Or simply install a nominal (8 ohm) resistive load in place of the speaker.
The Math:
V1 = voltage at speaker with NFB
V2 = voltage at speaker without NFB
Feedback in dB = 20 times the LOG of V1/V2
Enjoy, Paul Speltz
p.s. I am happy runnig no feedback in my system.
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Follow Ups
- Re: Feedback measurement - Paul Speltz 12:06:25 01/24/01 (0)
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