Monday, 22 March 2010

Active versus Passive Speakers THD.

In my previous blog I explained the various different distortions that occur in passive speakers and which made active ones so much better. In this one I will present measurements made of both an active ADM9.1 and a passive version driven by a low distortion Class A test amp.

Both speakers were measured at a nominal 86 dBa at 0.5 Metres. The Mike was a Bruel & Kjaer type 4165 measuring microphone and the source was the low distortion output of an HP339.

The above is a distortion plot of the active ADM9.1 at 100 Hz and it shows the second Harmonic at -48 dB and third Harmonic at -50dB just visible above the noise level in the factory.

The above shows a passive ADM9 at 100 Hz where the second harmonic has remained at the same level as the active version but surprisingly the third Harmonic has increased by 12 dB and the fifth Harmonic, absent from the active version, is at -45 dB.

 The above shows that at 3 kHz (in the region of the crossover) second and third Hamonics are almost lost in the noise at -55 dB.

The above shows the passive version at 3 kHz, the second Harmonic is now -46 dB and the third Harmonic at - 48 dB.


  1. How do those measurements (-48dB vs -55dB) to each other in terms of how much distortion. i.e. how big is a 7dB difference?

  2. As a rough guide 10dB is ten times as much.


  3. Excellent stuff, Ash; thanks for showing this. It is what I wanted to see all along. Those 3rd and 5th harmonics are startling! One only imagine what poorly designed passive crossovers do with tubeamps....