Negative Feedback

Jack Thomas · 3rd May 2003, 02:26 PM

When changing a negative feedback connection from 16 ohms to 8 ohms, how does one calculate the change in feedback resistor value?

SY · 3rd May 2003, 03:27 PM

0.707 times the 16 ohm value.

EC8010 · 4th May 2003, 11:43 PM

If it has a capacitor across it, then you need to scale that by 1.414 (sqrt of 16/8) to maintain the correct time constant.

fdegrove · 5th May 2003, 12:03 AM

Hi,

Usually I just leave it as is....

EC8010 · 5th May 2003, 12:21 AM

If you scrap an old MW/LW valve radio for its (large) variable air-spaced capacitor, you are likely to have a 2 x 365pF or 2 x 500pF capacitor that is ideal for quickly determining optimum feedback capacitor value.

fdegrove · 5th May 2003, 12:23 AM

Hi,

Yup...good lateral thinking.

Cheers,

EC8010 · 5th May 2003, 12:38 AM

Rotary, rotary...

Bung a square wave up it, and two seconds find the answer. I was quite surprised. You can put a variable resistor in series with the capacitor, if you like, and waggle both.

3rd May 2003, 02:26 PM	#1
Jack Thomas diyAudio Member Join Date: Jul 2002 Location: Baltimore, Maryland, USA	Negative Feedback When changing a negative feedback connection from 16 ohms to 8 ohms, how does one calculate the change in feedback resistor value?

3rd May 2003, 03:27 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	0.707 times the 16 ohm value. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

4th May 2003, 11:43 PM	#3
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	If it has a capacitor across it, then you need to scale that by 1.414 (sqrt of 16/8) to maintain the correct time constant. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

5th May 2003, 12:03 AM	#4
fdegrove diyAudio Senior Member Join Date: Aug 2002 Location: Belgium	Hi, Usually I just leave it as is.... __________________ Frank

5th May 2003, 12:21 AM	#5
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	The radio boys will hate me for this, but... If you scrap an old MW/LW valve radio for its (large) variable air-spaced capacitor, you are likely to have a 2 x 365pF or 2 x 500pF capacitor that is ideal for quickly determining optimum feedback capacitor value. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

5th May 2003, 12:23 AM	#6
fdegrove diyAudio Senior Member Join Date: Aug 2002 Location: Belgium	THE VARICAP? Hi, Yup...good lateral thinking. Cheers, __________________ Frank

5th May 2003, 12:38 AM	#7
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	Empericism rules Rotary, rotary... Bung a square wave up it, and two seconds find the answer. I was quite surprised. You can put a variable resistor in series with the capacitor, if you like, and waggle both. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

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