big sound for the barn - part 3

One thing that stands out with this pair of speakers is they are power hungry. I need to start with volume of 30 with the Sony receiver. With the Holman Preamplifier I set the analog volume to 12 o'clock and that is as loud as I would typically set it for a in person life-like performance. At this level the headphone SPL is too high to be healthy which is a testament the output wattage to the speakers is relatively high.

Human sensory is extremely unreliable in absolute level. We are very good in perceiving relative change, except temperature. I believe we are good at absolute temperature, in the absence of wind, is because we have very precise built in temperature regulation.

As a sanity check to how loud I am driving the speakers I set up the sound pressure meter. Its reading agree closely with that of the Apple Watch which is a good validation for both. For long duration monitoring the Apple Watch is a PITA to use as it would stop taking measurement to conserve battery charge.

this is a nice instrument to have for the money

The conjecture that I have is the midrange distortion could be cause by the 8" woofers back EMF feeding energy into the midrange drivers. This back EMF energy is out of phase with the audio information which leads to destructive superposition of the audio signal the midrange drivers receive. They are likely the harmonics of the woofer signal.

There are two approaches to address this. One is to have dedicated amplifier for the woofers. The other more cost effective method is to bi-wire the woofer separately from that for the midrange and tweeter.

I have a pair of duplex speaker wires that I haven't use for decades. I simply cannot locate them literally turning the house upside down. I resorted to jerry-rig bi-wired cable with another pair as a bench test. I just add another pare of speaker cable of mismatched lengths to test.  

These speakers has two sets of binding posts. The pairs are connected together with metal cross bars. For bi-wire or bi-amp one remove the cross bars and supply the woofers separately from that for the midrange and tweeter. 

By bi-wiring the speakers the midrange distortion is all but gone to the loudest volume I would listen to.

One thing I always notice. The potentiometers on audio amplifiers tends to develop the scratchy noise with age. I used to think is the deterioration of the carbon resistive film. But with this preamp all the switches exhibit similar problem. Because of this I believe the problem is not the carbon film, but due to oxidation of the metal contacts. Some contacts are to the point of intermittent contact with conductivity dropouts. Until I have time to open up the amplifier and clean off the oxidation the work around is to wiggle the control until contacts are made.

I will need to fabricate the matched length of bi-wire cables - mostly for physical tidiness