On my last blog post (http://www.fivefishaudio.com/blog/part-2-impedances-loading-adventures-in-building-a-tube-based-mic-preamp/), I added a buffer to the output of our vacuum tube stage. This additional buffer stage lowered the output impedance, while presenting a very high input impedance to our vacuum tube stage. This will allow us to couple to the next stage without losing a lot of signal due to loading.
Today, we’re going to scope the output waveforms after the buffer stage.
But first let’s talk about amplifiers. An “ideal” amplifier should do nothing but amplify the input signal by some factor of voltage gain. Therefore, the output should match the input linearly if graphed, where the slope of the line is the constant voltage gain.
But in the real world, the electronic devices we use are non-linear. (FET, transistors, tubes)
The input-output characteristics of a transistor or a tube is not a perfect straight line but a curved line (indicating a varying slope, i.e. a varying gain), which means the gain of the amplifier isn’t constant for the whole range of input signals. At higher input signal levels for example, the amplifier’s output voltage isn’t producing anymore a 1:1 correspondence to the input voltage.
This non-linear behavior is what produces harmonic distortions. And harmonic distortions cause our amplifier to produce signals that weren’t there originally! At the output, beside the fundamental frequency, you may also see some 2nd, 3rd, 4th or even 5th harmonics!
At first thought, that seems bad! Imagine… making up signals that weren’t there originally.
But harmonics themselves aren’t necesssarily a bad thing. In the real world, all musical instruments, even our voices, have harmonics. These are sometimes called overtones. In fact, certain harmonics may even be pleasing to the listener and make listening more pleasurable.
But certain harmonics (usually odd harmonics) are unpleasing and higher levels of harmonics (7th and above) can even be considered dissonant (i.e. sounding harsh, out of harmony) if their magnitude levels are high enough compared to the fundamental frequency’s levels.
Back to our project… I applied a sine wave signal to the our vacuum tube preamp, followed by a buffer, then connected it to a scope.
As you can see, at low signal levels, our amp can produce nice clean sine wave output. The FFT graph (in purple color) shows mostly the fundamental frequency and even harmonics (2nd, 4th, 6th). There’s probably some 3rd harmonics there, but the levels are very, very low. We’re operating mostly in the linear region of our amplifier device.
As we increase the signal, we start seeing 2nd harmonics building up and increasing in levels. Remember, these 2kHz signals (1kHz x 2) are not in our original input signal… and yet, we’re seeing them at the outputs.
Further increasing the signal, you’ll notice that we’re not getting any “hard clipping”, but instead a soft/rounding of the top and bottom of the waveform. Also notice, the distortion is not symmetrical. This is how vacuum tube circuits behave when driven hard. The FFT waveform still shows our fundamental frequency, a high 2nd harmonic content and increasing levels of 3rd harmonics.
And if we continue increasing the levels furthermore, we get large amount of 2nd and 4th harmonics, and now have odd 3rd and 5th harmonics in our output signal.
Here’s a nice animation of the waveforms, where you can see how the even and odd harmonics behave as the signal is increased up to the point of slight distortion, and then overdriven real hard.
Now you may be asking, what does a solid-state amplifier output looks like? On the next screenshot, we have a modern, fast, clean Balanced Driver chip. Notice how the output is mostly the fundamental frequency, with a noticeable absence of 2nd harmonics (it’s somewhat still there but not in the same large magnitude we saw with our vacuum tube amplifier).
Very obvious difference in output vs. a vacuum tube stage, i.e. the lack of 2nd harmonics. Just a perfectly clean, pure signal.
And this is a photo of how a solid-state amplifier clips when driven hard/overloaded.
Notice, unlike vacuum tube clipping, we’re getting a hard chopped signal. This is our output signal slamming against the power supply voltage rails (or the maximum output capability of the opamp). Notice also how symmetrical it is, it almost looks like a square wave. (And square waves are mostly the fundamental frequency + odd harmonics). So this wouldn’t sound good when you’re clipping this hard.
So there you have it… we have visible proof that our “low-voltage” vacuum tube stage gives us a lot of nice 2nd harmonics, with soft/rounded clipping, and assymetrical waveforms — just like what it’s “bigger brother” does, i.e. it exhibits the same behavior as a high-voltage powered vacuum tube circuits.
So now, it looks like the only problem we have left is giving our preamp sufficient gain so it can work with microphone level signals.
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