
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Analog Line Level Preamplifiers , Passive Preamps, Crossovers, etc. 

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 
19th March 2011, 08:10 PM  #21  
diyAudio Member
Join Date: Feb 2007
Location: Δραμα  North Greece

Quote:
__________________
Best Regards FOTIS ANAGNOSTOU 

19th March 2011, 08:59 PM  #22  
diyAudio Member
Join Date: Sep 2003
Location: The Milky Way

Quote:
I just posted an updated Excel chart here with measured data verifying computed data here...http://www.diyaudio.com/forums/analo...ml#post2509261
__________________
Life simplified! Leap into the Warpspeed wormhole... http://www.6moons.com/audioreviews/warpspeed/1.html 

20th March 2011, 10:16 PM  #23 
diyAudio Member
Join Date: Mar 2011

Can some please give me the formulae for calculating input impedance and output impedance for a single voltage divider, with the Rx being the resistor at the input Ry the resistor between the output and gnd?
I'd like to calculate Zin and Zout for this attenuator before I buy any parts, but I can't find the relevant formulae anywhere. I was actually under the impression that a series stepped attenuator always maintained a constant Z, regardless of switch position. Now I know it varies, I'd like to do a few calcs. Thanks, Leon 
20th March 2011, 11:58 PM  #24 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

I know its stepped, but think of it as a pot for the moment. Whatever is below the wiper is in parallel with the amplifier impedance, so calculate that. Add that to whatever is above the wiper and that's your divider input impedance (or at least resistance). For output impedance, parallel whatever is below the wiper, with whatever is above. Include the amplifier in parallel with the bottom, and the output impedance of the preamp in series with the top. Hopefully that very nonrigorous verbal description helps!
Conrad
__________________
I may be barking up the wrong tree, but at least I'm barking! 
21st March 2011, 01:11 AM  #25 
Banned
Join Date: Jan 2008

OK, here we see the source impedance RS, the attenuator (RX + RY) and the load impedance RL. Taking a minimalistic view, we might say that the input impedance of the attenuator is RX + RY, and the output impedance is RY.
io_impedances.jpg Taken in isolation the source sees the attenuator, RX + RY. If taking into account the load, the source sees RX + (RY  RL).  means 'in parallel with'. The current in the circuit I = V / (RS + RX + (RY  RL)). This enables you to calculate the voltage drop across, and current through all the resistances, whether discrete components, or characteristics of the source and load. It's normal to calculate a source resistance by measuring the open circuit voltage and then the voltage across a known load. Knowing that the voltage is being split between the source resistance and the load, it becomes possible to calculate the source resistance. The meter is taken as having infinite resistance (although this may not be an acceptable approximation). This means that there is no current (!) through the source resistance while measuring, and consequently no voltage drop across it. The load sees RY, but this doesn't make much sense without the source connected, in which case the load sees RY  (RX + RS). Voltage sources have a very low impedance, looking at the junction of RX, RY then there are 2 parallel paths to ground, the one through RY and the one through RX, RS and the voltage source. w 
22nd March 2011, 10:53 AM  #26 
diyAudio Member
Join Date: Mar 2011

OK thanks fellas. I've added to my spreadsheet 2 more columns, Zin and Zout.
For Zin I have "Rx + ((RL x Ry)/(RL + Ry))" For Zout I have "(Ry x (Rx + Rs)) / (Ry + Rx + Rs)" Where Rx is everything above the wiper, Ry is everything below, Rs is the output from my soundcard (32 Ohms) and RL is the input to my power amp (15 kOhm). With Rx + Ry = 1.8 kOhm, my Zin and Zout have now been calculated, but the numbers surprise me slightly. Zin is 1800 Ohm at mute and remains at this maximum value from 60 dB up to 37 dB. Here it begins to fall, slowly at first but with a rapidly increasing negative gradient, finishing at 1607 Ohm at 0 dB. I had no idea what to expect but this seems feasible. Zout, however, is slightly curious. Hopefully someone can explain. It starts at 0 Ohm at mute, then 1.8 Ohm at 60 dB, 3.6 Ohm at 54 dB. It seems to follow a pretty tidy positive exponential from 0 Ohm at mute, up to a maximum of 458 Ohm at 6 dB. It then extremely rapidly falls away: 388 Ohm at 3 dB, finally down to 31.4 Ohm at 0 dB. Why the sharp peak at 6 dB? Also, is my maths correct? Thanks guys, Leon 
22nd March 2011, 12:14 PM  #27 
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders

Hi,
I don't know about you or other Members, but I find your naming convention odd. Zin = Power Amp input impedance. Rs = Source output resistance. Rup = everything above the wiper (except Rs) Rlo = everything below the wiper Your first formula is correct and becomes Rin = Rup + Rlo//Zin = Rup + {(Rlo*Zin)/(Rlo+Zin)} :1 I can visualise where each term comes from. It's not important that we can decipher your naming convention, but think about yourself looking up this spreadsheet in 6years time. Yes at high attenuation the pot provides almost the whole input impedance seen by the source. At near full volume the Zin of the amplifier reduces the apparent Rin of the pot. This is particularly important when the ratio of pot:Zin becomes closer to 1:1, i.e. a high pot value followed by a low Zin becomes worthless at near full volume. I have not converted your second formula so can't comment on it's accuracy. However, that 6dB peak is exactly what we expect for a worst case source impedance seen by the receiver, when the volume pot is set to half maximum.
__________________
regards Andrew T. Last edited by AndrewT; 22nd March 2011 at 12:17 PM. 
22nd March 2011, 01:16 PM  #28 
diyAudio Member
Join Date: Mar 2011

Sorry to cause confusion. I'm new to all this, and obviously not yet familiar with all the nomenclature.
My second formula, according to your labels is: Rout = ((Rlo x (Rup + Rs)) / (Rlo + Rup + Rs) From now on I'll adopt the nomenclature you've described. You've given me confidence that my calcs are correct though, so thanks very much. It's a reassuring find, since my Zin is 33 times my Rout at worst case, and my Rin is 50 times my Zout at worst case too! So I've nailed the "at least 10 times" rule. Awesome, thanks very much. I still don't quite get why the Rout peaks at 6 dB though, but I'm not particularly bothered why, it's probably over my head. Just as long as that's normal I'm a happy bloke. Gonna get these parts bought now and get soldering! Thanks for all the help everyone, last week I knew nothing about this, and now I'm in a position to start building. Leon 
22nd March 2011, 07:22 PM  #29 
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders

Hi,
don't simply accept my suggestions for term names. Use names that mean something to you. Something that will be easy to remember and can be applied consistently in different situations. This was/is particularly important when programming. After you have written half a dozen programmes each with over a couple of hundred lines. When you look back will it still make sense, even years later? Try to check the current capability of your 32ohm source. Low impedance does not equal high current capability. Check the open circuit voltage, into a 10M voltmeter, try stepped test loads, 100k 10k 2k 1k. Does the output impedance appear to hold steady for all current outputs? Alternatively, test output distortion into a variety of test Loads. Is there a pronounced change in slope of distortion vs output current?
__________________
regards Andrew T. Last edited by AndrewT; 22nd March 2011 at 07:26 PM. 
18th October 2017, 03:43 PM  #30 
diyAudio Member
Join Date: May 2007

No. For a potential divider with total resistance Rp fed from source resistance Rs the maximum output impedance is (Rp+Rs)/4.

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
stepped attenuator  onno  Pass Labs  20  25th September 2007 09:04 PM 
stepped attenuator impedance  troublecomes  Chip Amps  7  9th January 2006 11:03 AM 
stepped attenuator  ivegotmono  Tubes / Valves  4  7th January 2006 02:49 PM 
Stepped Attenuator  officeboy  Parts  3  6th May 2004 03:32 PM 
New To Site?  Need Help? 