
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Chip Amps Amplifiers based on integrated circuits 

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 

Thread Tools  Search this Thread 
26th May 2015, 02:45 PM  #1 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

Attenuator input and output impedance
I know this subject has been on many previous threads but I am trying to find out how to calculate the input and output impedance of an attenuator based on a simple type potentiometer, pi type and a pi type with a series resistor at the inlet to the pi network. Its the theory I am after.
These resistor layouts appear on various switched type attenuators. I think I understand how to work out the input impedance but not completely sure. I am looking at using a switched type attenuator using two 2 pole switches 6 way with pcb mount terminations to give course and fine volume control. 
26th May 2015, 03:30 PM  #2 
diyAudio Member
Join Date: Feb 2011

okay, show us what you got sofar, and then you can get help working out the rest.
__________________
V*A=VA V*A*PowerFactor = WATT 
26th May 2015, 04:03 PM  #3 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

for pi attenuator I add the series resistor with the second resistor to ground and then take parallel sum, ie for r1 (series), r2 first resistor to gnd, r3 2nd resistor to gnd,
input imp = r2  (r1+r3). for potentiometer I take input impedance as r1 section of pot where total r = r1 + r2. 
26th May 2015, 04:49 PM  #4 
diyAudio Member
Join Date: Dec 2001
Location: Bremerton, WA.

I think you should post a diagram so everybody is clear exactly what configuration you're talking about.
A simple voltagedivider scheme is very straightforward. Input resistance is = r1 + r2. Output resistance is = r1  r2. Dave. 
27th May 2015, 10:19 AM  #5 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

ok will do some diagrams hopefully today

27th May 2015, 03:34 PM  #6 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

Have attached attenuator layouts with my best guess of input and output impedance

27th May 2015, 03:55 PM  #7 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Hi,
Your neglecting the load impedance for Rin. Source impedance can usually be ignored. For 2 Rout = R3 (R2+R1) For 3 Rout = R3 R2R1 For 4 Rout = R4(R2+R1R3) For 5 Rout = R1 rgds, sreten.
__________________
There is nothing more practical than a really good theory  Ludwig Boltzmann When your only tool is a hammer, every problem looks like a nail  Abraham Maslow 
27th May 2015, 04:23 PM  #8 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

Just to clarify what you are saying; is it that the load impedance will be much greater than Rin meaning Rin can be neglected.
I can understand the theory for Rout from your formula except for 5, why does R2 have no effect. 
27th May 2015, 04:55 PM  #9  
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Quote:
Rload can be neglected assuming its very high, it usually isn't. Assuming Rsource is very low, an ideal voltage source is safer. For 5 Rout = R1 + R2Rsource and if Rsource is zero .... rgds, sreten.
__________________
There is nothing more practical than a really good theory  Ludwig Boltzmann When your only tool is a hammer, every problem looks like a nail  Abraham Maslow Last edited by sreten; 27th May 2015 at 05:14 PM. 

28th May 2015, 08:25 AM  #10 
diyAudio Member
Join Date: Aug 2011
Location: West Midlands, UK

Thanks for the help, very good information

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
determining input/output impedance  ksporry  Tubes / Valves  11  15th February 2015 06:15 PM 
Balanced stepped attenuator with constant input impedance  tubeing  Analog Line Level  6  17th October 2014 07:43 AM 
Low amp Impedance input and high dac output  tulip  Tubes / Valves  1  30th November 2007 08:24 PM 
Input / Output impedance, Resistor network Vs. Dynamics  _nando_  Everything Else  3  24th March 2007 11:19 PM 
Balanced line stage input or output attenuator  nobody special  Pass Labs  4  7th April 2006 01:18 PM 
New To Site?  Need Help? 