Power Amplifiers

Mystere pa21 Stereo Tube Power Amplifier

ARTICLE INDEX

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.

mystere-pa-21-power-amplifier-1-khz-5-volts

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.

mystere-pa-21-power-amplifier-1-khz-15-volts

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.

solid-state-balanced-push-pull-power-amplifier-1-khz-20-volts-8-ohms-thd-plus-n

With 60 hz and 7 kHz sine waves, and 5 volts output, IMD measured 0.77%.

mystere-pa-21-power-amplifier-imd-5-volts

At 15 volts output, IMD rose to 1.47%. This appears to be a high number, but it's often seen in tube components.

mystere-pa-21-power-amplifier-imd-15-volts

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.

mystere-pa-21-power-amplifier-19-khz-20-khz-5-volts

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.

mystere-pa-21-power-amplifier-19-khz-20-khz-15-volts

THD+N vs. Frequency indicated less than 1% distortion at output levels up to 15 volts into 8 ohms.

mystere-pa-21-power-amplifier-thd-plus-n-vs-fr

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.

mystere-pa-21-power-amplifier-thd-plus-n-vs-power-output

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.

mystere-pa-21-power-amplifier-fr