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In Reply to: RE: Tube matchin posted by neolith on November 20, 2015 at 22:10:13
Your signal generator and 'scope method is sound. To check symmetry of calibrations, first reverse the connectors at the scope to confirm the scope is calibrated equal on both channels. Then repeat with the inputs to confirm the difference is really in the tube under test, and not in the signal source.
Follow Ups:
Great . I did as you suggested and things looked good. I then rigged up a mono y-splitter and hooked the signal from the single channel of computer function generator to both channels. I then measured the output on the both channels using X-Y alt and voila, the signals superimposed exactly.
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The results for the line stage and phono stage look the same :).
Any suggestions on how to evaluate tube noise. The pre-amp is dead silent when hooked up so audible noise is not a problem. I just like to know how to do the evaluation, if possible. Is a computer function generator (I am using REW) adequate for this purpose?
Noise is always difficult to measure accurately, because there are so many possible sources which may confound the measurement.
First, the test rig must not contribute significantly if you want the tube noise. That means a quite power supply, and checking the noise from bias and plate load resistances to be sure they are all small relative to the expected noise.
Second, the signal level is small; you need a really quiet amplifier with a gain around 1000 (60dB). This raises the tube noise to a level that can be seen on a measuring device such as an oscilloscope.
Third, you really need a spectrum analyzer, since tube noise has a white noise component plus a "1/f noise" component. The spectrum analyzer is also good for detecting small levels of hum and other interferences, which would otherwise distort or dominate the measurement.
Here's an interesting thread on triode noises:
Paul, just to add to what you've said, the external 60 dB amplifier is really the key to this. It's also relatively simple to construct. If one selects wide bandwidth opamps with very low noise specifications (nV/hz), two can be connected in series, 30dB gain each, in order to produce a very high quality amplifier. In real world terms, it's possible to construct an amplifier like this with a simple regulated supply (or just a bipolar "cleanup" circuit) that will enable viewing a noise floor below 100 nanovolts. Input this to a PC soundcard with FFT (spectrum analysis) software, and your system will be orders of magnitude more sensitive than what's needed to analyze tubes.
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Buy Chinese. Bury freedom.
Agreed. I would use batteries for the preamp, to ensure against power supply noises.
"I would use batteries"
Yes, especially if the preamp will only see occasional use. Also, if one desires to make measurements down to DC, a split voltage source is required. Two 9V batteries are much less expensive than two regulated supplies. BTW, if two opamps with +30dB gain each are chained, only the first needs to be low noise. The second (and whatever follows) will have virtually no impact on the noise floor measurement.
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Buy Chinese. Bury freedom.
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