Santa, the slave driver (and misc tube news)


Since May, the sun has risen and set on my beautiful baby girl. Daddy does not resent any of it for a second, but babies and the holidays make for slow progress on tube projects. I think my New Year’s resolution will be weekly posts, even if they aren’t all in-depth technical posts or finished designs.

I’m starting early because something that is [sadly] unusual has just occurred. Someone released a new tube audio kit/board:

I’ve used Boozhound Lab’s products in the past, but this is the first kit Jason has released for tubes. It’s a push-pull 6C45Pi amplifier that puts out about 6W. With just a pair of triodes sandwiched between input and output transformers, it’s also a minimalist’s wet dream (and similar to what I did with the Bad Hombre Mk 1 for headphones). I love it already and I hope it encourages people to pick up their soldering iron and bite the Edcor lead time bullet.

Jason has a great discussion of the design and building the amp here.

WTF Updates:

Chassis work for a TubeCAD headphone amp build is done: this will be a review and test of a circuit hack JB suggested (see SRCFPP), pretty paduak wood

Chassis work for a small SET amp is nearly done: this will be a published design, kind of a study in traditional cap-coupled single-ended amplifier design, goal of making this write-up very beginner friendly with a focus on applying fundamental concepts


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!