These were on sale not too long ago. I don't know if they ship overseas, however. Come to think of it, I don't know if they are entirely non-inductive either, being rather large wire-wound resistors...
8 OHM 120W POWER RESISTOR-MPJA, Inc.
I think in all cases you are best off to wire several higher value resistors in parallel. If they are inductive the inductance will be divided by the number of resistors used.
I use old Russian-made 25W and 50W wire-wound potentiometers (4.7 and 10 OHm) as dummy load. Their inductance is so small that it actually doesn't matter on audio frequencies. They serve as voltage dividers at the same time.
Another lesson learned the hard way...
Just repaired the blown power supply on my PP 6L6 MOSFET follower amp. Testing the outputs using 50W aluminum clad resistor. Hmmm, easier to clip the scope probes to the resistor leads than the speaker binding posts...
Several minutes of testing at just under clipping, hmm, what is that smoke?
It was the plastic on my (brand new) scope probes melting 🙁
Just repaired the blown power supply on my PP 6L6 MOSFET follower amp. Testing the outputs using 50W aluminum clad resistor. Hmmm, easier to clip the scope probes to the resistor leads than the speaker binding posts...
Several minutes of testing at just under clipping, hmm, what is that smoke?
It was the plastic on my (brand new) scope probes melting 🙁
Ha Ha, yes! Some lessons are best learned the hard way, or at least those learned the hard way are best remembered! Ask me why I ALWAYS use a bleeder resistor on my power supplies 😉
Unfortunately, in some areas, the hard lessons are not survivable, eg high voltage. I note a Bonanza in your avatar, are you a pilot? - aviation is one of those areas too!
Regards,
Chris
Unfortunately, in some areas, the hard lessons are not survivable, eg high voltage. I note a Bonanza in your avatar, are you a pilot? - aviation is one of those areas too!
Regards,
Chris
Ouch, not good! I have a solder fume exhaust fan. Melted right thru it with a dummy resistor on top. Same "what's that smell?"
Real Life Measurements on Power Amplifiers - Loadspeaker-Load Dummy instead Resistor
check out this article:
1) English
Stereophile: Real-Life Measurements
2) German (schematic page 56)
High-end Röhrenschaltungen - Google Bücher
check out this article:
1) English
Stereophile: Real-Life Measurements
2) German (schematic page 56)
High-end Röhrenschaltungen - Google Bücher
The article deals with SS amps with output stages not so frequency dependant when compared to Tube amps.
Every tube amp I've come across has the "rising thd with rising frequency" characteristic as the output tranny open loop response is deliberately tailored away by phase shift and the output transformer response when global feedback is added. This raises the effective output resistance of tube amps at both LF & HF extremes so the stiff impedance/load correction of a high octave crossover isn't liked by tube amps..
So, implying a tube amp performs the lowest HF distortion with only a simple passive 6dB/octave crossover network on the tweeter which gives the least loading effects. All agree to this ? This may suggest why tube amps are sonically better reproducing the top end.
The Kantor article gives only a partial picture of the lumped parasitics. The picture for tube amps is far more complicated than one imagines. The equivalent electrostatic speaker load is far more revealing and disturbing: my Dynaco goes in oscillation but the Williamson has no problems.
richy
Every tube amp I've come across has the "rising thd with rising frequency" characteristic as the output tranny open loop response is deliberately tailored away by phase shift and the output transformer response when global feedback is added. This raises the effective output resistance of tube amps at both LF & HF extremes so the stiff impedance/load correction of a high octave crossover isn't liked by tube amps..
So, implying a tube amp performs the lowest HF distortion with only a simple passive 6dB/octave crossover network on the tweeter which gives the least loading effects. All agree to this ? This may suggest why tube amps are sonically better reproducing the top end.
The Kantor article gives only a partial picture of the lumped parasitics. The picture for tube amps is far more complicated than one imagines. The equivalent electrostatic speaker load is far more revealing and disturbing: my Dynaco goes in oscillation but the Williamson has no problems.
richy
Thanks for those pictures! I was just going to recycle an old small form factor PC. Now I have a use for it!
If you use NFB over more than one stage this is an interesting experiment to do:
1) Create an A/B switch so you can switch between speaker and resistive load.
2) Connect a scope probe to a signal within the feedback loop
3) Setup a square wave (500Hz etc - to suit)
Then look at the wave shape change as you switch between the resistor and speaker!
1) Create an A/B switch so you can switch between speaker and resistive load.
2) Connect a scope probe to a signal within the feedback loop
3) Setup a square wave (500Hz etc - to suit)
Then look at the wave shape change as you switch between the resistor and speaker!
Chris: At the risk of stating the obvious, if your output transformers have 4 ohm or 8 ohm taps, then a 6R load will not reflect the proper load to the output tubes, it'll either be a little high or a little low, depending on the tap. Pretty much like a loudspeaker...😛
So your theoretical load line/bias point and what you measure on the scope are going to be a little different.....and you've got to divide by 6 instead of 4 or 8 to determine output power.
Since that post I managed to get some 8R 50 Watt resistors. I mounted each on a piece of heatsink I parallel them for 4R.
Well, have to restart this thread ...
Did one actually measure the inductance of these so called " non inductive resistors" ?
I put four of the common yellow 50W Dale's in series to get 8 Ohm and my cheap LCR meter ( ELC-131D )
measures 0.3uH @ 1kHz; near the measurement limit.
Any good ?
TX Gary
Did one actually measure the inductance of these so called " non inductive resistors" ?
I put four of the common yellow 50W Dale's in series to get 8 Ohm and my cheap LCR meter ( ELC-131D )
measures 0.3uH @ 1kHz; near the measurement limit.
Any good ?
TX Gary
As Artosalo said. That could be even less than the inductance of some leads/connecting-wires in a build.
For newbies: I generally find an overrated concern in audio with inductance etc., qualitatively mentioned instead of putting some figure to the same. In decades of practical experience I never had a problem with inductance, di-electric material, hookup wire resistance and such in audio circuits. Yes, these could have an effect in critically designed circuits, but such effects come in only at frequencies quite outside the audio spectrum and should have been taken care of by the designer to begin with.
For newbies: I generally find an overrated concern in audio with inductance etc., qualitatively mentioned instead of putting some figure to the same. In decades of practical experience I never had a problem with inductance, di-electric material, hookup wire resistance and such in audio circuits. Yes, these could have an effect in critically designed circuits, but such effects come in only at frequencies quite outside the audio spectrum and should have been taken care of by the designer to begin with.
Speaker voice coils have some inductance too!
As you say, people get far too excited about resistor inductance in audio circuits.
As you say, people get far too excited about resistor inductance in audio circuits.
No, it won't. A resistive dummy load is a pure resistance at audio frequencies for all practical purposes. AC into a pure resistance introduces no phase shift, but reactive loads do. Being a synchronous AC motor, a speaker won't always have a 100% power factor. Another difference is that an AC motor is designed for, and operates at, one specific frequency. Add in that your speaker needs to operate across a wide frequency band, and impedance can wary widely in both magnitude and phase.
You should always power up a new project for the first time -- especially if you include gNFB -- into a resistor. If Mr Murphy has wired your feedback positive rater than negative, you've just built yourself a high powered Royer oscillator. I had two projects that took off oscillating at ~20Hz because of that which could bring a 30W, 8R test resistor to red heat in under a minute if I let it run that long. That would blow your speeks, not to mention your ears, if you didn't use a dummy load.
Even if you get it under control, your neighbors are likely to complain if you run full power, 1.0KHz tests into your speeks. You can still learn quite a lot with a nice, resistive test load. If you're not stable into a test resistor, then you certainly will have major problems when you connect the speeks.
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