Questions about some simple power/volt/current...
Apr 30, 2009 at 3:45 PM Thread Starter

#### odigg

##### 500+ Head-Fier
I've been thinking about amps a little (not building them, just thinking) and I've been running some calculations.

It seems like, as a general rule of thumb 30mw should be just enough to destroy your ears with most headphones.

From that I came up with the following.

With a 25 ohm headphone you need about 0.85 volts and .34 amps.
With a 300 ohm headphone you need about 3 volts and .01 amps.
With a 600 ohm headphone you need about 4.25 volts and .007 amps.

Three AA batteries could supply this much or even two batteries if you value your hearing. Assuming 30mw is very loud on something like a AKG 240M (88dbl/mw), three AA batteries is enough even for those headphones.

So why in the world are there headphone amps with 12volt, 15volts, 24 volts supplies, etc?

Is it simply a matter of headroom?

Apr 30, 2009 at 3:57 PM

#### kuroguy

##### 100+ Head-Fier
Because we are talking about an average of 30 mW instead of a maximum of 30mW. there are transients that can far exceed 30mW such as drums, cymbols, etc. and you need the added voltage (headroom) to handle those higher power transients.

Apr 30, 2009 at 4:02 PM

#### tomb

##### Member of the Trade: Beezar.com
Quote:

 Originally Posted by odigg /img/forum/go_quote.gif I've been thinking about amps a little (not building them, just thinking) and I've been running some calculations. It seems like, as a general rule of thumb 30mw should be just enough to destroy your ears with most headphones. From that I came up with the following. With a 25 ohm headphone you need about 0.85 volts and .34 amps. With a 300 ohm headphone you need about 3 volts and .01 amps. With a 600 ohm headphone you need about 4.25 volts and .007 amps. Three AA batteries could supply this much or even two batteries if you value your hearing. Assuming 30mw is very loud on something like a AKG 240M (88dbl/mw), three AA batteries is enough even for those headphones. So why in the world are there headphone amps with 12volt, 15volts, 24 volts supplies, etc? Is it simply a matter of headroom?

Round numbers, a Corvette needs about 27 horsepower to maintain a speed of 60mph - that includes estimates of drag, tire friction, and a weight of ~3200lbs.

So, why do we want hundreds of horses under that hood? Wouldn't 27 horsepower be enough?

Apr 30, 2009 at 4:12 PM

#### Iniamyen

##### 100+ Head-Fier
Another reason is that you generally get much better voltage slew rates and more accurate reproduction when you aren't operating the amp to its rail limits. This makes for a truer reproduction of the input to the amp, and makes it sound "faster" I think. And remember the headphone impedance is frequency dependent, the number quoted isn't necessarily the same across all frequencies.

Apr 30, 2009 at 5:48 PM

#### odigg

##### 500+ Head-Fier
Quote:

 Originally Posted by tomb /img/forum/go_quote.gif Round numbers, a Corvette needs about 27 horsepower to maintain a speed of 60mph - that includes estimates of drag, tire friction, and a weight of ~3200lbs. So, why do we want hundreds of horses under that hood? Wouldn't 27 horsepower be enough?

As much as I'd like you analogy to fit, it's not a very good argument. As stated by the other posters, there are practical reasons having a more powerful voltage supply, but there are limits where moving beyond these limits becomes fantasy as far as audible improvements.

Having a 400HP corvette does a lot of things. Having a 2000 HP corvette is pointless because it's unlikely that power can transfer to the ground in anything approximating a safe or sane process if you are using a factory suspension. By your argument we might as well build an amp with a 100V supply. Why not?

There are also far more variables in how much power a car needs and how it uses than what is required when you are developing a headphone amp. I don't want to get into the positives and negatives of a high horsepower car because this isn't a car forum and such points aren't relevant to this thread.

kuroguy and Iniamyen - thanks for your answers. They make sense.

One more question. Isn't headroom a way to avoid clipping? If I've got a 4.5V (three AA batteries) amp powering something like a 300 ohm (constant throughout frequency range) with 20mw and I can't detect any clipping and distortion, wouldn't this mean that in my particular case, with my particular music, a 4.5V supply is enough?

I realize that for highly dynamic music my statement above may be wrong.

Apr 30, 2009 at 8:44 PM

#### NelsonVandal

##### 1000+ Head-Fier
I agree regarding listening levels. You don't need more than 1 V for low impedance and maybe 3 V for high impedance phones. Still there are several reasons for using higher supply. The distortion generally gets lower if the supply voltage is increased. A lot of amps don't swing from rail-to-rail like many of the monolithic opamps do. If you eg use double or triple emitter followers, or cascoding, the voltage swing is reduced and a higher supply voltage is needed.

I don't think headroom is a major issue in the digital world, but some people claim that vinyl needs large headroom, like 100 V for a RIAA-stage, to sound it's best.

Apr 30, 2009 at 10:23 PM

#### tomb

##### Member of the Trade: Beezar.com
Quote:

 Originally Posted by odigg /img/forum/go_quote.gif As much as I'd like you analogy to fit, it's not a very good argument. As stated by the other posters, there are practical reasons having a more powerful voltage supply, but there are limits where moving beyond these limits becomes fantasy as far as audible improvements. Having a 400HP corvette does a lot of things. Having a 2000 HP corvette is pointless because it's unlikely that power can transfer to the ground in anything approximating a safe or sane process if you are using a factory suspension. By your argument we might as well build an amp with a 100V supply. Why not?

You know quite well why you wouldn't do that. It's extremely impractical. Nevertheless, there are instances where power for power's sake is pursued. I would've thought the leveraged multiplier of 400 to 27 would've been obvious to you. It's no more of a magnitude difference than the leverage in power supplies you cited.

Quote:

 There are also far more variables in how much power a car needs and how it uses than what is required when you are developing a headphone amp. I don't want to get into the positives and negatives of a high horsepower car because this isn't a car forum and such points aren't relevant to this thread.

It's perfectly analogous - even though I'm well aware of what kind of forum this is.
Acceleration is everything - and power reserves and the ability to apply those power reserves is what dictates that acceleration. Whether its measured in weight, distance, or velocity, or load, amplitude, slew rate or current impulse - it's analogous. Good analogies are often a great instructional tool. So sorry you didn't think so.