Theory question, what limits the frequency range?

mcandmar

New member
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.
 
Sounds like your scope might be losing sync with the signal above 20khz
 
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!
 

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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.
 
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..
 
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