Power Amplifiers
Mystere pa21 Stereo Tube Power Amplifier
- Written by John E. Johnson, Jr.
- Published on 21 June 2010
On the Bench
Distortion measurements were made within an 80 kHz bandwidth. Except where indicated, the load was 8 ohms.
Most of the time, we listen to our audio systems with just a few watts. Here is a 1 kHz sine wave test signal at 5 volts output (the left channel is represented by the yellow lines, and the right channel by red). There is 0.21% THD+N. That would be considered quite a bit in the solid state world. But for tubes, it is the norm. Notice that the 2nd order harmonic predominated.
At 15 volts output, distortion rose to 0.38%, but the 2nd order harmonic was still the biggest one. Again, seeing all those tall peaks makes it look like there is a lot of distortion, but the Y axis is exponential, and this is really just so we can see the peaks more easily.
Compare the above graph to that taken from a solid state balanced push-pull power amplifier, shown below. The total distortion figure is lower by a factor of about 30. All of the even-order distortion products have been cancelled, leaving only odd-order harmonics. The total amount of distortion is about 1/100th of a percent. This is right about at the level of controversy as to whether the distortion can be heard. It's likely not "heard", but perhaps "sensed" in some form.
With 60 hz and 7 kHz sine waves, and 5 volts output, IMD measured 0.77%.
At 15 volts output, IMD rose to 1.47%. This appears to be a high number, but it's often seen in tube components.
For 19 kHz and 20 kHz sine wave test signals, at 5 volts output, the B-A peak at 1 kHz was 52 dB below the fundamentals.
At 15 volts output, B-A was 36 dB below the fundamentals. Looks like a lot of distortion doesn't it? But don't critcize until you have listened.
THD+N vs. Frequency indicated less than 1% distortion at output levels up to 15 volts into 8 ohms.
Power output didn't quite make it to the specified output, but it was close. Because of these curves, I would suggest using the 8 ohm tap regardless of the speaker.
The measured frequency response was 20 Hz - 20 kHz, + 2 dB at 5 volts output. At 15 volts output tended to rise a bit at both the low end and high end.
