amplifier output impedance measurement
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 31st January 2013, 12:20 PM #11 DF96   diyAudio Member   Join Date: May 2007 An important part of measurement is interpretation: what does the number mean? That usually requires some understanding of what the measurement means, as well as how to actually do it. In the case of output impedance knowing what it means and knowing how to measure it are virtually the same thing. I suggest the OP does some Googling on Ohm's Law, potential dividers and the Maximum Power Transfer Theorem. He will then know what he is measuring and how to measure it. Without this there is little point in helping him to arrive at a meaningless number.
 31st January 2013, 07:12 PM #12 Hi_Q   diyAudio Member     Join Date: Oct 2007 Location: Watford,Herts I made many moons ago a bridge circuit with a low value variable resistor in one leg. The unknown impedance is connected to the other leg. A signal of say 1kHz is applied to the bridge and earphones connected to the bridge centre arms. I'm sure you know the set-up, adjusting the variable resistor for a null in the headphones. The variable resistor had a pointer calibrated against known fixed values. Thus you could find the impedance of speakers or transformers at different frequencies if one wished to do so. Just another way of doing it.
 31st January 2013, 07:51 PM #13 Merlinb   diyAudio Member     Join Date: May 2006 Location: Lancashire This is a traditional method, but it's only practical if the output resistance is at least an ohm or more.
 3rd February 2013, 05:21 AM #14 midmoe   diyAudio Member   Join Date: Jan 2013 hello, thanks for the good feedback. Am waiting on a sextet of 6550' s JJ's (slovakian)from tempe, az to complete the unit, right now only have 4 westinghouse straight sided ones (amp set up for 6 outputs) ... I'm going to check impedance next week with the formulae when the new tubes get here, hoping I can get it down below 200 ohms with more tubes and adding more local neg feedback in the output stage. It would be nice to have a quick and easy method to assess impedance with some sort of test jig....if impedance matching is maximum transfer of power to the load, wouldn't this result in maximum heating of the load? Perhaps an accurate thermocouple reading would be indicative of max power transfer, without all the attenant reworking of formulae calculations. Physically attaching a thermocouple to the resistance, allowing the temp to stabilize for a few minutes after each change of load, then proceed up or down in resistance from a starting guestimate...until a peak is found. If that would be the theoretical impedance, with the real world situation of back-emf and whatnot, it wouldn't be perfect, but perhaps as accurate as the formulae. The heat method seemed to be promising as I hooked up a 1K, 10 watt pot and ran it through the 100-300 range and that seemed to be max heat with a whitenoise input at 200 ohms just feeling it with my hand.
 3rd February 2013, 05:32 AM #15 midmoe   diyAudio Member   Join Date: Jan 2013 oops, forgot to state that the amp in question is an output transformerless circlotron, which is why all my hub-bub about impedance questions... certainly not a standard topology....my first attempt to build one.
cnpope
diyAudio Member

Join Date: Aug 2009
Location: College Station, TX
Quote:
 Originally Posted by midmoe hello, thanks for the good feedback. Am waiting on a sextet of 6550' s JJ's (slovakian)from tempe, az to complete the unit, right now only have 4 westinghouse straight sided ones (amp set up for 6 outputs) ... I'm going to check impedance next week with the formulae when the new tubes get here, hoping I can get it down below 200 ohms with more tubes and adding more local neg feedback in the output stage. It would be nice to have a quick and easy method to assess impedance with some sort of test jig....if impedance matching is maximum transfer of power to the load, wouldn't this result in maximum heating of the load? Perhaps an accurate thermocouple reading would be indicative of max power transfer, without all the attenant reworking of formulae calculations. Physically attaching a thermocouple to the resistance, allowing the temp to stabilize for a few minutes after each change of load, then proceed up or down in resistance from a starting guestimate...until a peak is found. If that would be the theoretical impedance, with the real world situation of back-emf and whatnot, it wouldn't be perfect, but perhaps as accurate as the formulae. The heat method seemed to be promising as I hooked up a 1K, 10 watt pot and ran it through the 100-300 range and that seemed to be max heat with a whitenoise input at 200 ohms just feeling it with my hand.
Well yes, maximum power would also imply maximum heating, so indeed you could in principle do something along the lines you are suggesting. It would probably not be very precise, and you would have to wait quite a long time after each adjustment of the variable resistor, for thermal equilibrium to be re-established. And in the time you were waiting for the temperature to settle down you could much more accurately, and quickly, determine the power by measuring the voltage and the current and multiplying them together!

For fun, I tried out a variant on merlinb's method with the transformer (which is, itself, essentially the same method jcx was talking about). Rather than connect the primary to the mains supply, I connected it to a signal generator. (Amongst other things, it has the advantage one can test the impedance at other frequencies, and not just at 60Hz or 50Hz.) Within experimental accuracy, it gave the same answer I was getting from the resistor plus paralleled resistor method. (About 0.8 ohms for the OTL I was trying it with.)

By the way, I don't think you said what test equipment you have. Presumably you have a meter (DVM?) that can measure AC voltage? Do you have a signal generator? I think it would be much better to do your measurements using that, rather than white noise. If you don't have a signal generator, I am guessing that you can probably find sinewave signal sources as mp3 files on the web somewhere.

Chris

 3rd February 2013, 05:21 PM #17 midmoe   diyAudio Member   Join Date: Jan 2013 yes, I am a little short on equipment, although I have a good audio oscillator, I'm waiting till next week to drag it out and use it for a precise freq sweep. My FET volt/ohmmeter is acting strange, so I need to get a new one, am using the old standby V/O from when I had the tv shop, which is fine for nearly all needs. Never really liked the DVMs as they aren't useful in hunting intermittents....so far, it's working great with an incompletly tubed output stage, once having gotten control of distortion in the driver stage...took an extra 12BH7 to deliver enough voltage to the grid rails....the heat method is more of an intellectual excersize than any serious measurement, unless I spend alot of money getting research grade thermocouple components and spend another whole month getting all that figured out...but as a ballpark estimator, I think I found a neat trick.....at least new to me..maybe someone has done this before? The front end I'm using is a modified grommmes 260A design...DC coupled all the way to the output stages...one problem I had to overcome was that it had too much gain...you could have plugged in a mag phono and done alright, but for a basic amp, too high, so I switched the 100K plate supply to the 12AU7 from+250 to the plate of the driven phase splitter,which has about 190 volts, plus delivers huge amounts of neg feedback to the stage...I will probably modify this later, doing a partial pull-up to the regular +250 and increase the resistor value back to the 12BH7 plate. That will be a later on tweak to equalize the gains in the phase splitter sides...right now, the grounded grid side has more gain in the other..which is opposite of normal...and I've been thinking it's possible to do a completely DC to DC amp all the way to the speakers....by running the driver stage with a neg supply to cathodes and grounding the normal B+....I'm running the 6550 grid rails at about -24 volts, so this seems doable. Haven't plotted it out yet..so it's kinda speculative...pretty cool idea though...definitely would have to put in fuses to the speakers along with some kind of infrasonics-limiter...
 6th February 2013, 06:00 PM #19 midmoe   diyAudio Member   Join Date: Jan 2013 have hooked up both subs in series (16 ohms) and used a low pass inductor in series with a 120 ohm resistor, giving a net resistance of 136 ohms and hooked this up to the plate rails...elimnating the 500 ohm transformer, getting to a pure OTL system. You would think that having the 120 ohm in series with the subs would kill output, put strangely, doesn't...I guess as the closer impedance matching somewhat compensates for power loss...anyway, the real intresting thing is that there appears to be some natural crossover qualities of the OTL....as I removed bass information from the plate rails, it seems to dissaapear in direct proportion from the cathode rails....I havent' verified this yet, but will attempt to do so shortly. This would mean, if a real effect, the boses can be alone on the cathodes with no high pass filter required, the selective loading on the plates with the low pass load seems to elimenate this need...leaving only the low pass inductor to funtion as a complete crossover. Even if the "natural crossover" effect doesn't pan out, removing the xfmr has vastly improved the bass detail. Only drawback now is that sub feed is at +170.
 7th February 2013, 02:21 AM #20 midmoe   diyAudio Member   Join Date: Jan 2013 today I worked to get more neg. feedback in the output stage. The diagrams I've seen usually have a 100K resistor between plates and grids, I hadn't tried that yet, waiting to get all the other basics done. Upon adding the 100Ks immediately the bias swung way too positive, beyond the range of correction with the 5K bias contol. Further trials hit upon a 2.2 meg being high enough to allow the controls to give the minus 16 neg bias. This seems to have stabilized the output even more, as now the bias control voltage range seems expanded....before there was a rather limited range of bias voltages between cutoff and saturation distortions...now the cutoff distortion seems to occur with more reverse bias than previous, and the foward bias distortion seems to have dissapeared at the previous voltage observed without the 2.2 meggers. I didn't leave in foward bias for long so as to protect the tubes, but the functional range of bias voltages seems to have increased, making the output stage much less subject to "under and over" distortion, the control setting is much less critical. I've read about the bias matching problem some bloggers have reported, to the extent of placing center-null meters on the front to keep an eye on that. With so many of those giant russian triodes in their amps, probably not a bad idea...but with 6 standard outputs, that problem doesn't exist. I zero the voltage at the cathodes in the morning, that evening it reads within a few tenths of zero. I will go back into the bias circut later with the goal of being able to lower the 2.2megs down toward the 100K shown online...will have to go back in an re-jigger the bias feed resistors, to compensate for more + coming in from the plates...will report on that tommorow. Another good thing is that the tubes run suprisingly cool...I can touch them and they're not overly hot. In fact, they run cooler than if they were in the macintosh 275. A friend is bringing down some sa-cds to see how they do. I don't have any sa-cds yet, but have noticed some cds have a "fringey" high end, I guess because of lower sampling standards....kinda like mp3s which I never go into. I still like vinys as the high end seems a little sweeter. Another thing I've noticed, running the preamp in mono mode, is a shrillnes in some cd's midrange, that isn't there when set back to stereo. On dylan's "Modern Times" cd I noticed this more than others...at first I thought it was the amp. Upon flipping the preamp back to stereo, the harshness went away...his voice is getting pretty harsh anyway, so I,m happy to hear he's doing better again...haha...

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