Personal: Finding the Right Cap

As you can see from the below my hobby while not able to ride has become audiophilia. Rational audiophila, but audiophilia nonetheless.

I’ve decided to try my hand at developing an active triamped system using DEQX as a room correction system/electronic crossover (actually, the speaker is biamped and I’ll be running dual powered subs, so in essence it is a triamped system). I’ll write more about what this means, why I’m interested, and how I chose DEQX in a later post–in fact, I plan to document the entire process so that it can be replicated by someone equally naive. It should be interesting, as the hardware comes with a 178-page manual, and I also have to learn about DACs and a lot more details about amplifiers and speakers than my current “that sounds nice.”

I reserve the right to go back and edit these posts, as I’m likely to convey a lot of misinformation as I go along. In fact, I definitely will be coming back to these and cleaning them up so that it becomes an effective guide.

IMPORTANT NOTE: Before I scare you off DEQX forever, you should be aware that a) basic setup, which provides truly astonishing sound, is simple even for the technically disinclined; b) a dealer can set it up for you; and c) you can get a free 3-hour setup session via Skype if you prefer to take a middle route. I am just going the absolute hardest route–setting it up myself and using all of the most advanced features–because I find it entertaining.

I also plan to give some plugs to the various techs and businesses that are helping me figure this out and suffering through my dumb questions.

For now, though, I need a capacitor. Why? Regular passive speakers have a crossover that divides the signal from the amplifier so that the high frequencies go to the tweeter and the low frequencies go to woofer. The crossover acts as a natural protection against the amp frying the tweeter (the woofers are, apparently, more tolerant). Because active amplification means that I’m connecting the tweeter and the woofer directly to the amp, the tweeter needs some form of protection. And that protection takes the form of a capacitor.

So, capacitors (which store electric charge) come in all different values, measured in microFarads. I needed to figure out what size to get. According to the DEQX manual, I needed one that falls 2 octaves below my crossover point (the point where the frequencies are divided into high and low for the tweeter and the woofer), which in this case is 2800 Hz. Octaves are not linear, instead there is a 2:1 relationship. So 2 octaves below 2800 Hz is 700 Hz (divide 2800 by 4). (One octave below would be 2800/2 = 1400 Hz, and three octaves below would be 2800/8, or 350 Hz).

Then I had to get my tweeter data sheet to identify the resistance in Ohms at 2800 Hz–which turns out to be very close to 8 Ohms.

Now, plugging that into this handy formula:

C=1/(2*pi*f*R)

C=1/(2*3.14*700*8)= 2.84 x 10^(-5) = 28 uF

So I need a 28 uF capacitor. That value isn’t common, so I chose a 33 uF capacitor, which I purchased from the nice Canadians at PartsConnexion. Voila!

(Obviously this is a 6.8 uF capacitor, not 33 uF).

Alternatively, you can use this website to calculate the required capacitance:

http://www.claredot.net/en/sec-Sound/high-pass-cross-over-6dB.php

Good times were had by all. Now I have to learn to solder.

Executive Summary:

  • In an active biamped or triamped situation, a protective capacitor between the tweeter and binding post is desirable
  • The correct capacitance is determined in X steps: 1) Get your tweeter data sheet and find the resistance in Ohms at the crossover point you choose; 2) find the frequency that is 2 octaves below your crossover point by dividing the crossover frequency by 4
  • Plug it into the website above
  • Buy two from PartsConnexion (because they are lovely Canadians)

To be continued.

John

2 thoughts on “Personal: Finding the Right Cap

  1. David Higginbottom

    Hi John, I work with DEQX in Australia. I wanted to make sure you have seen our new and improved user manual – now only 172 pages…..

    Cheers David

    Reply

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