Easy ways of measuring output impedance of tube stages
 User Name Stay logged in? Password
 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
 15th May 2003, 04:55 PM #1 stigla   diyAudio Member   Join Date: Nov 2001 Location: Trondheim Easy ways of measuring output impedance of tube stages Hi all! Going to start a tread to shed some light about measuring the dynamic output impedance of tube stages. The general idea is to get you guys to teach me this, since I have no clue of how to do it, just vague ideas. (AC measurments?) There are of course ways to calculate this, but sometimes it's nice to have a more precise (and real) number to work with (with RIAA's for example.) So how's it done? What gear is needed? Is there any nice "tricks" to easily do this? How accurate can I get it?
 15th May 2003, 05:24 PM #2 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland It's exceptionally easy. Make a wild estimate of the output Z. Let's say it's 10K ohms. Choose a resistor of 4-5 times greater, say 47K ohms in this example, as a load resistor. Inject a signal into the input and measure the output voltage without the load attached. Now attach the load resistor and measure the new output voltage. Use the voltage divider equation to solve for the output Z. You can also do this by injecting a signal into the output of the stage, but that can be a bit trickier. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 16th May 2003, 12:22 AM #3 Tom Bavis diyAudio Member     Join Date: Jun 2002 Location: Macedon NY And if you don't like doin' math... measure output voltage, connect a variable resistor to output and adjust resistance until out drops in half. Disconnect the pot and measure it - there's you're output impedance.
 16th May 2003, 11:04 AM #4 stigla   diyAudio Member   Join Date: Nov 2001 Location: Trondheim Thanks for the input! Hmmm, that was really more easy than I had imagined! One question more: Will the output Z be the same for all frequencies? Does it matter what frequency I make the measurments on? My guess is that this will of course vary from one type of tube-stage to another, but It would be nice to do measurments under 1K since I can use only a multimeter then... stigla
 16th May 2003, 12:37 PM #5 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK Ah, now the tricky questions. No, it won't necessarily be the same at all frequencies. The most usual reason is that the coupling capacitor in series with the output causes the impedance to rise at low frequencies. If the amplifier is transformer coupled, then there's a whole selection of reasons for changing impedance with frequency. Amplifiers with feedback are also likely to have a non-constant output impedance. I use the variable resistor method to measure output impedance, and temporarily fit a much larger coupling capacitor (>3uF), but you need to be careful about level. Stages like mu-followers have low output resistance, but it doesn't mean they can drive any significant voltage into a load of equal value. I monitor distortion at the same time. If distortion <1% into the load, then the output impedance measurement is likely to be valid. Unless your DVM is very posh, it will be even worse accuracy at 1kHz than it is at 50Hz or 60Hz (and they're none too hot there). An oscilloscope or dedicated AC test set is a better bet. Using an oscilloscope, if you can't see distortion on the waveform, then the measurement is probably valid. Old electronic saying, "If you don't know what the answer from the measurement should be, you shouldn't be making it." __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post korneluk Tubes / Valves 21 27th December 2010 10:58 PM aldovan Tubes / Valves 10 4th March 2009 12:11 AM davidallancole Multi-Way 29 19th November 2007 03:49 PM ray_moth Tubes / Valves 14 4th July 2007 04:09 AM aliwakil Tubes / Valves 2 15th March 2005 07:14 AM

 New To Site? Need Help?

All times are GMT. The time now is 12:51 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.