Speaker sensitivity ratings and amplifier power

10 * log (power) = decibel

10 ^ (decibel / 10) = power

10db increase (10x the power) is perceived as twice as loud

Most desktop-size speakers are in the mid 80s db/W @ 1m sensitivity wise. We’ll call it 85db for the sake of calculating stuff. The sensitivity rating means that with one watt of power, you’ll get 85db of sound at one meter away. For reference, 80db is pretty loud. It’s about the level of a running garbage disposal or an alarm clock. You can listen at 85db for eight hours before you start risking hearing loss; this is also the sound level at which OSHA will fuck your shit up.

For nearfield listening, there may be less than a meter between you and the speakers. If you halve the distance, you can add 6db to the sensitivity rating. Now with the same speakers you’re getting 91db at half a meter with one watt of power. You should probably turn it down a touch to protect your hearing (2 hours at 91db is the maximum recommended duration). Every halving of the power deducts 3db, so one quarter of the power (0.25W) gets you back down 6db to a non-litigious 85db. If you want to listen at 80db (which is comfortably loud, believe me) you only need around 100mW.

Aren’t decibels fun?

This goes to say that you do not need a whole bunch of power for nearfield listening, even if the speakers have a low sensitivity rating. And if you have high sensitivity speakers in your “main rig”, a single-ended low wattage amplifier works there, too. Say you have speakers rated at 95 db/W @ 1m and like to listen around 80db. If you listen at one meter, you only need 32mW. If you listen at two meters, you need just 125mW of power.

The above discussion of power and decibels does not take into account dynamic headroom. It’s always good to have some power in reserve for music dynamics. Or for cranking it when OSHA isn’t paying attention. I try to have at least 10db to spare (10x the power) over what I expect my average listening levels to be. If you didn’t fall asleep while I fapped around with decibels and logarithmic math, you noticed that average, safe listening levels (80-85db) need only a fraction of a watt with average sensitivity speakers nearfield or high sensitivity speakers at a regular distance.  Ten times more power is just a couple watts and will often get you pretty comfortable listening levels with headroom to spare.

Just make them some high quality watts.

Back at it finally!  This is an excerpt from the first speaker amp write-up I’m doing for the site.  Happy Cinco de Mayo!

Something for beginners

Pete Millett’s Starving Student was one of the first amps I ever built completely from scratch. Unfortunately, the 19J6 tubes have become rare (or at least no longer dirt cheap) due to all the bright eyed DIYers scooping them up to build amps. I think the world needs another <50V tube amp for beginners, so I’m designing one. Like the original, it’s an oddball tube with a MOSFET buffer and an off-the-shelf power brick (same brick, in fact).

Millett is one of my personal tube heroes. This is a tribute.  Full write up coming soon (and parts values subject to change once tested).

 

I love you litte Weller choo choo

IMG_20170110_195626453.jpg

Sometimes people ask about how much money they need to invest to start playing around with DIY tubes and audio. The answer is probably less that you think.

I have been using the puny 23W Weller pencil iron for several years. You can buy this thing at literally every hardware store on the face of the earth. The tip is replaceable. Sometimes the little iron struggles with larger buss bars or binding posts, but its never given up. It’s my little orange choo choo iron.

Why am I thinking about this now?  I bought a 45W iron recently and immediately found it to be almost too hot for small sockets and parts. Little choo choo is not going to the tool box graveyard yet.

Why DIY?

 

If someone told me they hiked to the summit of Mt Everest, I’d think they’re pretty amazing. If they told me that their first thought when they got to the top was how much money they spent to get there, I’d probably think they’re not quite so amazing. If they told me they actually took a helicopter to the top instead because it was cheaper, I’d be sure that they’re an idiot.

DIY requires an investment in tools and parts, but more importantly, it requires an investment in time and patience. Your first project is not going to be the Mt Everest of projects, just like a hiker’s first backpacking trip shouldn’t be up the side of the Himalayas.  Like every hobby, excellence takes patience and practice.

It’s very tempting to compare the cost of building something yourself to the cost of buying something that’s commercially made. As you’re starting out in DIY, you won’t win this battle. The more you practice, the more you’ll refine your finish and design theory, and the more of a return you’ll see on the time you’ve invested. But price-points still do not capture what DIY is about.

Craftsmanship is what you pay for with high-end, high-dollar, boutique design. The craftsman that wields it didn’t buy his or her experience with money. He or she built experience and knowledge by making mistakes and a lot of crappy products first. When you buy commercial, you are paying for the mistakes it took for the craftsman to become exceptional.  DIY is about having the courage to overcome those mistakes yourself instead.

P.S. Yes, I know helicopters don’t fly to the top of Mt Everest.