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Re: A few questions about output capacitors on my headphone amp.

Posted by BrassTeacher (A) on April 30, 2007 at 07:28:42

In Reply to: A few questions about output capacitors on my headphone amp. posted by CheapEric on April 30, 2007 at 06:20:57:

(Friendly warning: If this post seems to get too long, scroll to the very last sentence for the boiled-down version, and if that catches your attention again, read the whole thing. Trust me, I won't take offense at all, I'm far into my adult years and still suffer from very pronounced ADHD - no kidding)

Well, I don't know much about headphones really, but:

Dynamics relates to the amount of variation present in the amplitude of the sound. In that case, it doesn't matter if you're talking about speakers, headphones, or the natural sound coming out of an acoustic instrument.

An interesting thing I've observed in audio electronics relates to teaching younger people how to play wind and percussion instruments. Since they are just starting out, their dynamic range will be limited, due to strength and experience. In audio electronics that are either really cheap, or lacking a bit in design, the effect is called "compression" (unlesss you are deliberately doing it in a pro audio situation).

Once they progress a bit, the next expansion in their dynamic range is at the top end. In other words, they usually go from "middling" to being able to play from "middling-to-loudling" (ok, so I'm making up words here, gimme a break). usually at first, their louder levels of sound tend to distort a bit until they gain the strength to control it. Audio gear the next step up the foodchain exhibits similar characteristics, with the exception of getting stronger with age!

The last, and hardest step to develop, is getting the young ones to be able to play SOFT well. It requires not only more strength than most people think, it requires a high level of skill and control. Their playing technique, as far as sound reproduction goes, has to be incredibly efficient. Audio gear,again, follows a VERY similar parallel.

So, how to get audio gear to have great dynamics? It not only has to be able to play louder without distorting, but it has to have the strength and sensitivity to be able to play SOFT without dropping the signal. In my home stereo system, my speakers have 90db sensitivity, so I'm pretty good on that end for the most part, since a 20W per channel amp is capable of running you out of the room without distorting. My current main amp is the power section of an old NAD 3150, which is fairly sensitive as far as input goes. I'm using a preamp of my own design, that has very few frills, the only thing that draws current off the power supply that ISN'T pushing sound out the back end is the power LED, runs off a +/- 15V supply, and is very very sensitive on the front end. It's almost too sensitive, it took me quite a few builds to get the noise out of the system, and I may still yet move to a battery-based supply. I've tried quite a few preamps with my system, and I've noticed that the better the preamp, the SOFTER the system can play at the lower end, even with the volume cranked up quite a bit. In other words, it hits you in the face with nice, clean sound at the louder moments, but it sensitive enough that it can really drop back down without dropping the signal. I hope this makes sense, I'm not sure what I just typed really makes sense to me as far as an explanation goes. :/

So, without knowing too much about your headphone amp, I'll make a couple assumptions based on what you've described so far:

9V battery. So as far as voltage is concerned, it looks like you are limited to a +/- 4.5 V voltage swing no matter what. What I would look at first is how much of that voltage swing is usable by your headphones. In other words, at the "loud end", will they be distorting if the signal goes past +/- 4V? or +/- 3.7V?

More important, how are the headphones at the soft end? How much signal to they need to have to be able to respond? Say there's a note or passage (now I'm just throwing these values out there, just for example) that is so soft it only goes from +/- 1mV, but your headphones need +/- 2mV to respond. You will hear nothing at all (except maybe "the ocean").

Now, headphones and speakers also are current-driven devices as well, and the parallels to voltage are somewhat similar, you need the headphones to be able to respond to variations in current, and the amp needs to be able to provide the necessary changes in current.

OK, now for more specifics to your situation. What are you using for a source? What chip are you using to drive the headphones, a small-signal opamp, or something heavy-duty like an LM3886? And probably just as important, what are you using for source material? As in what type of music as well as file format? That also makes more of a difference than a lot of people think.

Dang, I type too much when I get going. To make the long story above short, your amp may be just fine, we need to look at what's going on at each end of it.

BT

(I left out stuff related to noise floor too, but that's for another time)(maybe)

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