
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

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 
16th September 2003, 01:40 PM  #1 
diyAudio Member
Join Date: Sep 2002
Location: Oregon, USA

How do I calculate (or measure) my amp's output impedance?
Not much more to add here. I guess I could measure it by putting different load resistors on and seeing how much the output dropped, and then figuring out the output resistance (assuming a simple model with a perfect voltage generator and a series resistor, ignoring frequencyrelated effects). There must be some way to calculate it too though, right? Does anyone have a readymade formula for this? The output stage is a SE 2A3, cathode biased (and bypassed), through an OPT.
Thanks, Saurav 
16th September 2003, 01:47 PM  #2 
diyAudio Member

find out what operating point the 2A3 is running at, and then the impedance ratio of the OPT..

16th September 2003, 02:02 PM  #3 
diyAudio Member
Join Date: Sep 2002
Location: Oregon, USA

So the output impedance will get divided by the square of the turns ratio, just like the load does. My OPT is 2500:8, which is a ratio of 312.5, so my 2A3's output impedance will also be divided by 312.5. Is that correct?
Then, how do I calculate the output impedance from the operating point? Let's say I'm at the 'classic' 250V/60mA operating point (I'm not, but I haven't measured it since my latest changes, beyond making sure none of the voltages were too high)  how do I calculate the output impedance from that? Is it just the slope of the plate curve at the operating point? 
16th September 2003, 03:08 PM  #4  
On Hiatus

Re: How do I calculate (or measure) my amp's output impedance?
Quote:
The frequency variation of source Z is an interesting and useful thing to know, too, and is trivial to get. The simpler formulas won't be accurate for that.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." 

16th September 2003, 03:30 PM  #5 
diyAudio Member
Join Date: Sep 2002
Location: Oregon, USA

OK, that should be pretty easy to do. Just for curiosity's sake though, how would I go about calculating it?

16th September 2003, 03:34 PM  #6 
On Hiatus

As Colt says, determine the source impedance of your output tubes and divide it by the turns ratio. That will be rather inaccurate.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." 
16th September 2003, 03:42 PM  #7 
diyAudio Member
Join Date: Sep 2002
Location: Oregon, USA

Thanks.

16th September 2003, 05:26 PM  #8 
diyAudio Member

Yeah. Zo is plate resistance in parallel with any other resistance (for instance if you used a resistancecoupled stage, but obviously that doesn't apply here).
Typical Rp for 2A3 is what, 800 ohms? Zo should be around 3 ohms then (don't forget to add in the resistance of the OPT). Tim
__________________
Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation 
16th September 2003, 05:41 PM  #9 
diyAudio Member

Measuring Zout.
Take a signal generator and add a series resistor to its output so that Z out of generator plus external resistance is 1k ohm. Set the voltage to 1 volts rms ( say 1 KHz) . You now get a current source of 1mA.
Connect the free end to your 8 ohm load which is connected to the OPT. Measure the voltage across the 8 ohm load . It will be in mV. So if you get 0.3mV the output impedance with 8 ohms parallel with it is 0.3mV/1mA = 0.3 ohms. It is not absolutely correct but pretty close. The actual output impedance will be = (8x0.3)/(80.3) = 0.3117 ohm. You should try this at other frequencies also. Note : Do not connect any input signal to the amp ! It is best to short the input / plug in a 600 ohm load / turn the volume down to zero etc. but " no input signal". The signal generator and the mvMeter can be obtained free on the web as software used in conjunction with your sound card. Try www.dazyweblabs.com and www.rightmark.com amongst others. Cheers.
__________________
AM 
16th September 2003, 05:41 PM  #10 
diyAudio Member
Join Date: Sep 2002
Location: Oregon, USA

> don't forget to add in the resistance of the OPT
That would be the DCR of the secondary winding? 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
How to calculate gain and output impedance?  RogerGustavsson  Solid State  1  7th June 2008 05:09 PM 
How to calculate Tube Preamp output impedance?  john65b  Tubes / Valves  20  20th November 2007 02:44 PM 
Calculate PSU output impedance?  jayme  Tubes / Valves  1  19th May 2007 07:27 PM 
How to measure the output impedance of the DAC chip  ergo  Solid State  2  9th June 2004 07:07 AM 
Formula to calculate speaker impedance  Perry Babin  MultiWay  5  21st November 2003 06:19 AM 
New To Site?  Need Help? 