finding feedback line and gain controlling resistor

guitar89 · 2nd August 2011, 11:39 AM

The question i'm going to ask is that where is the feedback line in this amplifier and which is the resistor will determine the gain ?
I want to vary the gain for optimum performance, but couldn't find it.

Thanks in advance

DF96 · 01:32 PM

R18 (2k7) is the feedback resistor. Bear in mind that altering feedback doesn't just vary the closed-loop gain, you will also change bandwidth and potentially affect stability.

Or is it R4 (1k). I'm not sure. Maybe both.

This circuit looks like it has had HF stability problems which have been patched up by sprinkling capacitors everywhere.

guitar89 · 01:46 PM

df96,
just to be sure, will it destroy my amp or lead to component failure ? or quality reduction ? (permanent changes, that can't be change even after i return to its original value.

Wanted to try to vary these values, see what will happen X.x

my friend said maybe R11 (2k2), and R10 (2k7), but just according to his feeling ^^

juma · 2nd August 2011, 02:01 PM

Gain (roughly) equals R18 || R4 / R5

guitar89 · 2nd August 2011, 02:10 PM

Quote:

Originally Posted by juma

Gain (roughly) equals R18 || R4 / R5

what is the ||| you written meant for

hm...... R5 ? seems unrelated ? (well.... i'm a beginner)

DF96 · 2nd August 2011, 02:27 PM

|| means in parallel. I think the answer may be ((R18||R4)/R5)+1, which means calculate the resistance of R18 and R4 in parallel, then divide by R5, then add 1.

Bear in mind that some resistors (possible R4?) have a DC bias effect as well as AC signal. This is quite a complicated circuit so I would not fiddle with it before understanding what each component does.

guitar89 · 2nd August 2011, 02:39 PM

Quote:

Originally Posted by DF96

|| means in parallel. I think the answer may be ((R18||R4)/R5)+1, which means calculate the resistance of R18 and R4 in parallel, then divide by R5, then add 1.

Bear in mind that some resistors (possible R4?) have a DC bias effect as well as AC signal. This is quite a complicated circuit so I would not fiddle with it before understanding what each component does.

hm.... the designer of this circuit had mentioned that R2,R12,VR2,VR12 is used to adjust bias and trim DC offset, so means those are not for bias ?

anyway, you mean parallel means by multiplying them ? or what should i do if i want to tweak the gain ?

DF96 · 2nd August 2011, 02:50 PM

Google "resistors in parallel". Almost any resistor in a circuit may have an effect on bias, which is why I suggest you understand the circuit before changing it.

guitar89 · 2nd August 2011, 02:53 PM

hm..... i'm a beginner, so is there any link you could offer me to read about ? (all details about amplifier circuit ?

juma · 2nd August 2011, 03:55 PM

Quote:

Originally Posted by DF96

...((R18||R4)/R5)+1....

Exactly

That's why I said "roughly"

2nd August 2011, 01:29 PM	#2
DF96 diyAudio Member Join Date: May 2007	R18 (2k7) is the feedback resistor. Bear in mind that altering feedback doesn't just vary the closed-loop gain, you will also change bandwidth and potentially affect stability. Or is it R4 (1k). I'm not sure. Maybe both. This circuit looks like it has had HF stability problems which have been patched up by sprinkling capacitors everywhere. Last edited by DF96; 2nd August 2011 at 01:32 PM. Reason: extra

2nd August 2011, 01:43 PM	#3
guitar89 diyAudio Member Join Date: Dec 2010 Location: Malaysia, Selangor	df96, just to be sure, will it destroy my amp or lead to component failure ? or quality reduction ? (permanent changes, that can't be change even after i return to its original value. Wanted to try to vary these values, see what will happen X.x my friend said maybe R11 (2k2), and R10 (2k7), but just according to his feeling ^^ Last edited by guitar89; 2nd August 2011 at 01:46 PM.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Wanted: Zero feedback 2-10x gain line stage...prefer symmetrical	Ric Schultz	Analog Line Level	14	26th February 2011 07:26 PM
Volume control : - pot' or pot' variable feedback resistor (gain) ?	ash_dac	Solid State	5	1st August 2006 05:07 AM
Heretical Unity Gain line stage PLUS a little gain ?	yagas	Tubes / Valves	7	24th January 2006 01:30 PM
Finding an L-pad or resistor... help?	Feandil	Multi-Way	6	6th April 2005 05:06 AM
need some help finding a resistor	offroadbum	Solid State	3	27th January 2004 01:59 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

2nd August 2011, 02:01 PM	#4
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	Gain (roughly) equals R18 \|\| R4 / R5

2nd August 2011, 02:27 PM	#6
DF96 diyAudio Member Join Date: May 2007	\|\| means in parallel. I think the answer may be ((R18\|\|R4)/R5)+1, which means calculate the resistance of R18 and R4 in parallel, then divide by R5, then add 1. Bear in mind that some resistors (possible R4?) have a DC bias effect as well as AC signal. This is quite a complicated circuit so I would not fiddle with it before understanding what each component does.

2nd August 2011, 02:50 PM	#8
DF96 diyAudio Member Join Date: May 2007	Google "resistors in parallel". Almost any resistor in a circuit may have an effect on bias, which is why I suggest you understand the circuit before changing it.

2nd August 2011, 02:53 PM	#9
guitar89 diyAudio Member Join Date: Dec 2010 Location: Malaysia, Selangor	hm..... i'm a beginner, so is there any link you could offer me to read about ? (all details about amplifier circuit ?

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)