I've just posted Part 4 of the video series on building our Music Room and installing the Infinity IRS. This latest video discusses in some detail Helmholtz Resonators and how they work. You can watch the video here. If you'd like to know when I post a new video, be sure and subscribe to my channel on YouTube. If we have a choice in our designs we'd prefer to have as little in the signal path as we can - the proverbial straight wire with gain would be ideal - but of course that's just a myth. It's been pointed out that perhaps this idea of minimal obstructions in the music's path doesn't matter since the recording chain we listen to is littered with many components and devices in that same path. However, I would argue that regardless of how many barriers are in the music's path, adding more only serves to make things worse. Truly this is a case of less is more. Direct coupling of an amplification circuit means we have less in the path, specifically the lack of a coupling capacitor at either the input or the output of the device. But many designs today still rely on these coupling capacitors and the music we're so interested in preserving is forced to pass through them. While we don't subscribe to their use (and I'll explain the alternatives later) I think it's instructive to understand the different types of coupling capacitors available to audio designers - because some designs (in particular tubes) require a coupling device both on their input as well as their outputs - but their use is certainly not restricted to tubes. There are many types of capacitors on the market but I would suggest the two most common coupling capacitors are electrolytic and film. If you'll remember our post on what capacitors are we have two conductors and an insulator between them. When you put a signal into one conductor it develops an airborne field that jumps across the insulator and transfers over to the second conductor. The good news here is there's no physical connection between the two conductors - imagine a wire that you cut in half and the musical signal simply jumps across the air gap you've created in the wire- now that's freaking magic! The cheapest and most common capacitor of the two is the electrolytic. Pretty much all power supplies use these capacitors to smooth out the AC in DC but they are also used a lot as coupling capacitors. These work by using some chemical goo (called an electrolyte) as the conductor and a thin strip of anodized aluminum for the insulator. The goo and the aluminum are rolled up like a jelly roll and leads attached for the signal. Electrolytics work best at lower frequencies and aren't particularly linear when it comes to passing music - yet their use in audio circuits as coupling capacitors isubiquitousbecause of their very low cost. Their performance can be helped if you place another type of capacitor in parallel with them - or replace them with - a film capacitor. Film capacitors use a thin film (plastic sheet) as the insulator with an equally thin sheet of metal between the film, forming the conductor. The plastic film and the metal sheet are rolled up together to make the capacitor. Depending on the type of film used - mylar, polypropylene, polystyrene, Teflon - very linear audio performance that extends out to high frequencies can be achieved. You might notice that the names of the insulating plastic material are also the names of the film caps. Their downside is they can't be made very large - thus limiting their low frequency (bass) response - and they are relatively expensive. So if less is more - and it is in audio - then how do we move away from these intrusive rolled sandwiches of goo, metal and plastic to couple our audio kit to the outside world? Ahhh, let's wait until tomorrow.
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