How to find the transfer function of this volume control?
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davidel94
diyAudio Member

Join Date: Jul 2012
Location: Verona
How to find the transfer function of this volume control?

what is the transfer function of this circuit? R3 and R4 are a potentiometer divided into two resistors.
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 1st December 2012, 04:46 PM #2 richie00boy   Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK R4 is providing in effect negative feedback, because in the loop is an inverting amplifier. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
 1st December 2012, 04:47 PM #3 willysan   diyAudio Member   Join Date: Dec 2005 Location: Buenos Aires Let's say Vi is the input voltage and Vo the output voltage. Let's pray my knowledge of circuit theory is OK. In such a case: Vo/Vi=-R4/{[R1/(1/R2+jwC2)](1/R5+1/R4+1/R3)-1/R3}
 1st December 2012, 05:10 PM #4 davidel94   diyAudio Member   Join Date: Jul 2012 Location: Verona i've simulated your transfer function and it is not right because it should give with the values in the photo -2.63dB which is a gain of 0.738 V/V, with your formula i have a gain of about 13 V/V
 1st December 2012, 07:10 PM #5 DF96   diyAudio Member   Join Date: May 2007 The title asks how to find the function. The text ask what it is. I will answer the first question. Circuit theory plus some algebra.
 1st December 2012, 07:27 PM #6 davidel94   diyAudio Member   Join Date: Jul 2012 Location: Verona i now what a transfer function is but i don't know how to calculate the transfer function of this circuit
 1st December 2012, 07:59 PM #7 DF96   diyAudio Member   Join Date: May 2007 Where did you find the circuit? What is your application? Please assure us that this is not a student assignment.
 1st December 2012, 08:52 PM #8 djoffe   diyAudio Member   Join Date: Oct 2008 this is a variant of the Douglas Self/Baxandall high level gain stage circuit. It does a fairly good approximation of log gain with a linear pot. He uses essentially this in his latest preamp. Akitika GT-101 Update My Dynaco
 1st December 2012, 11:06 PM #9 willysan   diyAudio Member   Join Date: Dec 2005 Location: Buenos Aires The first time I derived the transfer function in a hurry. I checked it later and found a mistake. Now it's my turn to ask. How did you know the mid-frequency gain was -2.63 dB?
 2nd December 2012, 10:33 AM #10 davidel94   diyAudio Member   Join Date: Jul 2012 Location: Verona i simulated it with LTspice but yesterday i calculated by myself the transfer function so now i have it and i don't need your help. thanks anyway

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