opamp sine wave oscillator

No. You also need to ad some Angostura bitters.

One method is to assume that the microphone is time-invariant at small amplitudes, pick a repetitive signal, and do a lot of signal averaging. By attenuating the sound with distance, the distortion polynomial (i.e. the Nth harmonic goes as amplitude**N for small amplitudes) allows you to distinguish between microphone and speaker distortion.

Not too much fun, but conceptually not too bad.

I believe that some are _advertised_ to be six nines...of course that's before they went through the drawing die, not to mention the oxidation due to using thin strands...even assuming that people who would stoop to fleecing audiophools have the elementary honesty to _try_ to deliver six nines wire.

Cheers,

Phil Hobbs

There really was at least one wire maker that really did deliver on the 6 nines pure claim.

I think they used ceramic or copper in all of the wire pulling hardware. Unlike the Phelpsdodge wire it didn't have to be pickled before coating to remove the outer layer.

Yeah, that sounds labor-intensive.

It's all Belden.

John

This varies wildly. Not only does THD vary with signal amplitude, but frequency complicates it. THD gets closer to a function of cone movement and of voice coil current.

Generally, loudspeaker THD ends up mostly being second and third harmonics, which are generally less audible as distortion products than the higher ones. (Obviously, there is an exception for harmonics at frequencies past the range of human hearing!)

Audibility of lower harmonics varies with frequency. I remember many years ago reading somewhere a graph showing % THD (in second harmonic only) required to hear the distortion as a function of frequency, and IIRC that was about 20% or 40% at 20 or 25 Hz or so. That means a 20 Hz sinewave with 20% THD entirely in second harmonic sounds like a pure 20 Hz sine wave. And a few percent of third harmonic (in lieu of a lot of second) is also inaudible here.

As poor as most loudspeakers are at radiating acoutstic energy at frwequencies in the lowest octave, they can easily produce very high THD at such frequencies due to being better able to radiate the harmonics. So it gets easy for a loudspeaker to make a 20 Hz sinewave sound distorted, whether from a lot of harmonics 2-4 or a little bit of higher ones or both.

Meanwhile, distortion produced by a "gentle" distortion device, perhaps a "diode clipper" (especially one made with germanium diodes), is generally not nearly as audible as that from hard clipping.

- Don Klipstein ( snipped-for-privacy@misty.com)

I know some do.

The electric field applies a force that squishes the dielectric, and that will change the capacitor's geometry to some extent. I have heard some change shape enough to audibly vibrate when AC is applied. I exect the capacitance to be not perfectly constant through a cycle of AC.

I expect that use of capacitors with a higher working voltage (thicker dielectric) will mitigate that one. I expect this to be worse with more compressible dielectrics - such as paper soaked with oil.

I also suspect that dielectric constant varies a bit with electric field due to electron orbits in the dielectric's molecules being squished out of shape. I expect that effect to be mitigated by use of a thicker dielectric (higher working voltage).

Some ceramic capacitors have dielectric constant so high that I would not trust the dielectric constant to be all that constant. This is probably worse with ones whose dielectric constant varies too much with temperature anyway for the capacitor to be useful to make a good oscilltor.

Not that electrolytics are used to make a good oscillator, but their capacitance often varies with voltage. As I understand it, discharging slightly erodes the dielectric and charging restores it. That would make the capacitance run high at lower voltages. Most of the few times I checked into that many years ago, I found this to be true.

- Don Klipstein ( snipped-for-privacy@misty.com)

On Feb 5, 1:23=A0pm, John Larkin

Obvious, but not nearly as linear as a cheap photocell or light bulb :-). And nowhere near as economical!

The fact that the gain of a light bult or photocell can vary by a factor of a few depending on room temperature or recent light history etc. is not all that important if you're searching for ultra-low distortion.

Tim.

A couple of others have been missed:

A resistively heated thermistor can be made to have a very long time constant. It has the advantage over the light bulb that the resistance value can be higher and the life expectancy very long.

If mechanically varied resistances weren't so microphonic, it would have an advantage of making very long time constants easy to do.

On Feb 10, 2:54 pm, snipped-for-privacy@manx.misty.com (Don Klipstein) wrote: [...]

All of the high K ceramic materials are piezo electric and nonlinear. The internal structure distorts under bias.

You can make a parametric amplifier using ceramic capacitors. Unfortunately the capacitor makers always try to reduce the effect that the parametric amplification needs so unless you get into some sort of a relationship with the maker they may quit making ones that work well.

Don't LDRs also have a nonlinear voltage effect? Light bulbs do at low frequency. If you're hunting ppm distortion, may as well do it right.

Or use a 20-bit DAC with feedforward distortion tweaking.

John

Any digital AGC must, by definition, hunt.

And Ann Coulter has lost her sense of humor... thus, by definition, has become a Democrat ;-)

...Jim Thompson

The dac don't need none!

Right, she's trying way too hard to be outrageous. The Hillary-before-McCain thing is just plain idiotic.

Or a high-voltage shock line.

John

Show me... the gain can NEVER be exact.

Yep. For shame :-(

...Jim Thompson

Heck, "regular" resistors also have nonlinear effects! Ohm's law is more of a "serving suggestion" than a "law" if you are trying to do things at the ppm level :-).

But the $10-or-so analog multipliers I have worked with have a spec of

The parts I'm familiar with are the MPY634 (0.01% linearity) for $15 or so, AD734 (0.025% linearity) for $20.

Is there a new class of analog multipliers with 0.001%, 0.0001% linearities that won't break the bank? I admit that the parts I know are as old as the hills!

Ah, at that point we just go DDS... consumer 20-bit DAC's are just a dollar or two and pretty good through the whole audio range!

Tim.

Just digitally generate some sinewave numbers and pump them into a 16 or 20-bit DAC. Follow with an undemanding lowpass filter.

Add a second DAC that's the distortion stomper. Sum it in, scaled down by, say, 1000:1, and load it with distortion correction signals. Once (somehow) you can measure the distortion of the main dac, it's simple to program the stomper.

Amplitude is digitally adjustable, by programming the dac numbers, or the dac reference, with the aid of downstream passive attenuators.

So now we have a digitally-programmable sinewave source with ppm-accurate frequency and amplitude setability and ppm distortion, down to milliHertz. May as well toss in other waveforms, and programmable distortion, noise, and modulation, while you're in there.

I'll email her and tell her we said so.

John

It works only if the parameters of the first DAC are stable to the same

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I'd imagine that harmonic distortion would be pretty stable over time and reasonable temperature. THD would be very low from a good 20-bit dac, so the stomper doesn't have to tweak things a lot.

At higher frequencies, the output amps become a serious problem.

John

I think that a thermistor heater would draw more power than a little lamp that I don't notice any temperature rise of.

As for life expectancy - in Wien bridge oscillators, the lamp is usually operated with the filament running a dim red-orange or close to that. I consider such a dim brightness and color typical of a 750 or 1,000 hour lamp being operated at 10% or less of rated voltage. (More than that only for gas-filled lamps with design current around or less than twice the minimum for gas to overall be of benefit.) The usual "1-size-fits-most" rules say that life expectancy of an incandescent is inversely proportional to voltage to the 12th power. I would not get too worried about any inaccuracies here at this rate!

Although a thermistor with a heater sounds good for longer time constant, I would go for the simplicity of using incandescent lamps since they do have a good track record.

- Don Klipstein ( snipped-for-privacy@misty.com)