Capacitor Biasing in Crossovers

[mah]

It appears that JBL has applied DC biasing to crossovers in high end consumer speakers sold in Japan. I tried to find a technical paper on the subject but could only find the usual recycled hearsay or revellations regarding yet another great leap forward posted on various forums.

Is there a serious look at this subject available?

Cheers, Marshall.

p.s. whilst Googling this topic I came across a convert who also DC biases audio cables in order to "stabilise the dielectric".

[bfish]

One of the more in-depth JBL explanations of the CC method can be found in pages 10 thru 12 here;

1993 K2-S5500

[bowtie427ss]

Yuck! A "monster" plug in JBL literature, wonder if they had to sue to get that too.

[bfish]

Consider the context; 1993 sales literature.

[bfish]

I have a tendency to disregard any technical explanation that contradicts the basics, such as "Current flows through the capacitor..." As anyone with even elementary electronics knows, no current flows through any capacitor, save for an infantessimally small leakage current. In this case though, I suppose I should heed my own advice;

Consider the context; 1993 sales literature.

[GM]

OK, time to show my ignorance...... Sadly, I have zero formal education on the subject, only some technical references I consider beyond reproach that in my ignorance I could very easily be misinterpreting, so by leakage current, are you referring to the cap's conductance due to the electrical circuit's capacitive reactance WRT frequency or......?

I've always taken for granted that 'conductance' means just that, (current/whatever) flow that can be a considerable percentage of the whole in a tweeter's XO BW if I'm understanding what I'm reading correctly.

GM

[mah]

Conventional electron current does not flow through a capacitor, except for a small current due to leakage impedance. However, charge transfer occurs from plate to plate enabling current to flow in the external circuit.

Also, current does not flow through a transformer with separate windings.

[mah]

Thanks, bfish,

Interesting, especially the bit about capacitor transient response. I would like to see the math or physics behind the claims.

Marshall.

[bfish]

...so by leakage current, are you referring to the cap's conductance due to the electrical circuit's capacitive reactance WRT frequency or......? ...

Consider a caps' construction. Two conductive 'plates' are separated by a very thin, non-conductive dielectric material. No way for current to flow unless the breakdown voltage of the dielectric is exceeded, or the dielectric is damaged in some other way, such as deterioration of the dielectric oxide in electrolytic caps. Leakage current is undesirable, it's the result of the dielectric material being a less-than-perfect insulator and passing a negligable current flow. Leakage is also the reason a charged, out-of-circuit cap will eventually discharge itself.

The mechanical analogy I was taught was that of a thin rubber diaphragm stretched across a water pipe, where the water pressure represents voltage. As the pressure (voltage) on one side of the diaphragm increases, the diaphragm stretches towards the side of lower pressure, forcing water movement in that direction. As the pressure decreases, the stored energy is discharged, and the cycle reverses in direction, following the alternating pressure source. Energy is transferred, but no current actually passes thru the diaphragm, save for a tiny bit of leakage through a pinhole (imperfect insulator). Exceed the pressure the diaphragm can withstand, (breakdown voltage) and it bursts, allowing flow (a short).

Another telltale sign is that caps have no current or dissipation rating as do all conductors or resistive devices that actually carry current. Nor do they produce the heat indicative of power dissipation.

I found this animated gif that shows a similar analogy in a smoothing application, as used in a power supply;

[bfish]

Thanks, bfish,

Interesting, especially the bit about capacitor transient response. I would like to see the math or physics behind the claims.

Marshall.

You're welcome.

Well, zero-crossover distortion is well documented. Various amp topologies address it (or not) in different ways. By biasing the signal with a DC offset, you can elevate it so it never crosses zero. The CC method definitely achieves this, at least as long as the circuit voltages don't exceed the bias level. The only real question...is it audible, or at least enough to warrant doubling the size and cost of the crossover capacative elements.