Theory question, what limits the frequency range?

[mcandmar]

I was playing around with my scope last night and found something interesting, whenever i play a 22khz 0db sine wave the output looks like it has 3-4 different traces at different amplitudes on top of each other.  But at 20khz or below i get a crystal clean symmetrical sine wave right down to 10-12hz, even at silly volume levels.

Question is, what limits the frequency range in a tube amp, is it the size of the capacitors in the signal path or the tube itself?

I understand this is a purely academic question as the only one in the house who can hear that high is my cat, i'm just curious to know what is happening.

[Doc B.]

Sounds like your scope might be losing sync with the signal above 20khz

[mcandmar]

Hadn't thought of that, will check it on another amp and see what happens.

Thanks,

Mark

[mcandmar]

You were right, the issue is the scope.  Is this a known phenomenon or has it just drifted out of spec with age?

For the test i used an Objective2 amp and omg is it horrible, i attached a screen shot of 20khz on the O2 and SEX amp as i couldn't believe what i was seeing.  Objective my ass!

[Paul Joppa]

Looks to me like you are using a digital source at 44kHz sample rate, with an inadequate (possibly non-existent?) reconstruction filter. The jaggies are the quantization; the SEX acts as a moderate reconstruction filter but the solid state amp has a much more extended ultrasonic response. At a 44kHz sample rate, actually 44100 samples, the Nyquist frequency is 22050, so with a 22kHz signal, every 50 cycles the sampling goes in and out of phase with the sine wave - leading to an apparent fluctuating level after moderate filtering. A perfect reconstruction filter has an impulse response of infinite length, so it picks up information from a wider time frame to solve that problem.

[mcandmar]

Your posts are always pure gold Paul.  You are absolutely right that the source was a DAC playing a 44.1khz sample, and the jaggedness was the DAC itself not the Objective2 amp.

Using the same O2 amp i plugged in a generic Chinese dac kit i built which had none of the quantization jaggedness, but over 15k something odd is going on which looks like two traces slightly offset from each other. For the second test i dusted off my old ODAC and found its perfect up to 20k, not a hint of quantization or noise anywhere.  They all do the same @22k though.

The problem DAC is one of these http://www.ciaudio.com/products/VDA2 which is unusual in that it uses a discrete I/V stage instead of an opamp. Glad i only paid $100 for it, not impressed with that at all..