Spectrum evaluation ECC82

[karsten21]

Dear Folks,

I measured the output spectrum of my tube DI-Box using a sine 1Khz 0dB input signal. The K2 seemed to me nice and aligned to a typical tube stage. But I´m not able to interprete the K3 and all other odd harmonic and thier levels in relationship to the K2.

Are the odd harmonics in this spectrum "typical" for an tube stage?

For my ears the DI-Box sound realy nice but I also want to know If I´m on the right way with the design..

Thanks a lot for your suggestions.

Best regards
Karsten

[DF96]

Any active stage will give all harmonics. The relative level depends on device and circuit details, as well as signal level.

[karsten21]

Hey DF96,

Yes, that´s clear. The question about the displayed sectrum is if it is a "normal" spectrum which I can expect from a triode standard gain stage using a ECC82? Are the odd harmonic to high in relationship to the others?

It may be that you right that a spectrum for itself is absolutly not usable. But to get a "better feeling" if the gain stage works as expected for an tride gain stage I think a spectrum is helpful.

Regards
Karsten

[DF96]

The only slightly unusual thing about the spectrum is that there appears to be some cancellation of 5th harmonic. This suggests that the signal level was sufficiently high that 7th or 9th order non-linearity is generating enough 5th harmonic to cancel that produced by 5th order non-linearity. You can learn more from how the spectrum changes with signal level.

Whether odd harmoncs are too high depends on what you are trying to achieve.

[ruffrecords]

Your second harmonic is a little over 40dB down (representing about 1% distortion). All the other harmonics are at least another 40dB down and can effectively be ignored. They my well be simply artefacts of the oscillator.

Whether this is the expected spectrum of an ECC82 depends on the circuit configuration and the oscillator signal level. You say the input level is 0dB but what we really need to know is the rms input signal level and see the schematic.

Cheers

Ian

[artosalo]

The question about the displayed sectrum is if it is a "normal" spectrum which I can expect from a triode standard gain stage using a ECC82? Are the odd harmonic to high in relationship to the others?

Your spectrum looks very typical for single end triode stage, i.e. high 2nd harmonic and much lower higer order harmonics. But when you increase the input level, higher order harmonics will grow rapidly.

You should analyze your signal source too.
A typical audio generator has harmonics about 80 dB (0.01 %) below main signal.
So, quite obviously the higher order harmonics are from the generator.