general question about feedback, input impedance, and noise - diyAudio
 general question about feedback, input impedance, and noise
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 26th August 2005, 05:31 PM #1 diyAudio Member     Join Date: Oct 2002 Location: USA general question about feedback, input impedance, and noise In a feedback amplifier where there is an input resistor, say 2k for example, a 47k resistor from gate to ground, and a feedback resistor of 100K on the same input (inverting amp), does the 2k in series affect noise performance? Also, how do you calculate the input impedance of such a stage (JFET input)? __________________ Steve
 26th August 2005, 08:14 PM #2 diyAudio Member   Join Date: Jul 2004 Location: Ottawa Input impedance will be 2K, gain 50. The value of resistor to the ground (47k) does not affect input impedance and gain.
 26th August 2005, 08:39 PM #3 diyAudio Member     Join Date: Nov 2003 Location: Indiana What if the input impedance of the LTP is 50k... in that case I'd think the input impedance is in the order of 25k and not 2k correct?
 27th August 2005, 11:13 AM #5 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Thanks Prr. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
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Join Date: Nov 2003
Location: Indiana
Quote:
 > What if the input impedance of the LTP is 50k If it is JFET: then something is mighty wrong. Even for high-current BJTs, the naked input resistance is increased by feedback. For any well-defined high-NFB design, it will be "infinite enough". If low-NFB, you need to do some elaborate calculations, throw it at SPICE, or whomp-up a breadboard (the best way to get the real answer). [/B]
That is what I thought when the statement was made that the input impedance with a 2k in series and 25k to gnd will have an input impedance of 2k... I thought it would be the lower of the 25k or LTP input impedance not 2k which is in series...

 28th August 2005, 12:36 AM #7 diyAudio Member   Join Date: Jul 2004 Location: Ottawa From the post #1 it is clear that this is inverting amplifier. In this case it does not matter if transistor is JFET or BJT and to be more precise the gain will be 100k divided by 2k plus source output impedance. For the source input impedance of amplifier is 2k.
 29th August 2005, 04:24 PM #8 diyAudio Member     Join Date: Oct 2002 Location: USA Thank you for the replies. Would I be correct to assume that I never see any phono stages with feedback and an inverting amplifier because the series input resistor degrades noise performance, and also makes for too low of an input impedance? I'm speaking of MM here, not moving coil. __________________ Steve
 29th August 2005, 09:23 PM #9 diyAudio Member     Join Date: Jun 2003 Location: Maine USA > I never see any phono stages with feedback and an inverting amplifier because the series input resistor degrades noise performance, and also makes for too low of an input impedance? I've seen the idea used; you have the disadvantages figured out. The noise may not be a killer fault once you drop the needle on vinyl. Surface noise is generally far higher than the noise of a modern device. In fact the disk levels and cartridge sensitivity were all balanced to work well with the noise of a vacuum tube, say 2 microvolts good and 5 uV typical. A hot-chisel cut on acetate carefully pressed to pure vinyl will have surface noise a little higher than tube noise. When we go to <1uV transistors, we get better test-bench specs but not better listening. A perfect 47K resistor will have 3uV noise. More than the best tube, less than many popular tube phono amps. Going to lower input impedances gets confusing because the impedance of the pickup is not well specified. That is, until you get to zero input impedance. Then the electrical response is flat (per your feedback impedance). However the custom has been to allow the pickup inductance and cable capacitance to resonate at the top of the audio band, limited by the Standardized 47K resistance. Some cartridges have a few dB electrical bump to compensate a falling mechanical response; others with excess mechanical response may be balanced for a slump; some (old Grado) just don't resonate much with normal capacitance and 47K. Many fancy preamps have switchable C and R loading so you can fiddle the top octave to spec or to taste. With a zero-Z input you lose this fine-trim on the top, and may need a EQ trim in the electrical system.

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