Current drive of speakers and
speaker distortion
1. Intro
2. Measurement of distortion of speaker current
3. Measurement of speaker distortion with voltage and current drive
4. Measurement of tweeters at acoustical side
4.1. Tweeters
Large set of measurements was done with 2 types of metal dome tweeters, with both voltage and current drive, in 2015. However, the new set of measurements, performed in December 2018, has shown that it is especially revealing to measure twin-tone intermodulation distortion. Measurements with 13+14 kHz twin tone and 4800+5000Hz twin tones were performed on Beyma T2030 1" metal dome tweeter, both in voltage drive and current drive mode. It was observed that 3rd harmonic and further odd harmonics have decreased considerably with a current drive, 3rd harmonic was reduced of almost 20dB. Second harmonic and even harmonics were not affected much by a driving mode. So, it seems that the current drive mode has reduced mostly the symmetrical distortion, leaving asymmetrical distortion almost unaffected. However, reduction of the 3rd harmonic of some 20dB is an interesting result.
13 + 14 kHz - voltage drive
Measurement of tweeter distortion at acoustical side, 13+14kHz twin tone, 1Vrms, voltage drive.
13 + 14 kHz - current drive
Measurement of tweeter distortion at acoustical side, 13+14kHz twin tone, 1Vrms, current drive.
We can see dramatic decrease of odd harmonics ("skirts" around 2 base tones)
13+14kHz - amplifier distortion
This measurement shows distortion of the amplifier used in the test. It was measured simultaneously with acoustical measurements.
4800 + 5000 Hz - voltage drive
Measurement of tweeter distortion at acoustical side, 4800+5000 Hz twin tone, 1Vrms.
4800 + 5000 Hz - current drive
Measurement of tweeter distortion at acoustical side, 4800+5000 Hz twin tone, 1Vrms.
We can see again dramatic reduction of odd harmonics.
4800 + 5000 Hz - amplifier distortion
Measurement of amplifier distortion, 4800 + 5000 Hz twin tone, 1Vrms, simultaneously with acoustical measurements.
Measurement of THD, H2 and H3 under voltage and current drive
Measurement results
Though not much improvement was detected in even harmonics under current drive, odd harmonics were reduced considerably, almost of 20dB, above 4kHz, as can be seen from H2/H3 vs. frequency plots and from IMD plots. It seems that the current drive might be an effective method to reduce odd harmonic distortion of some tweeters, provided that it is used high enough above the tweeter resonance. Or we have to use a pre-filter to compensate for impedance plot, as mentioned in the Intro.
Pavel Macura, 01/2015 and 12/2018