[sdiy] Re: pentode power amp circuit

[Magnus Danielson]

From: metasonix <metasonix at earthlink.net>
Subject: [sdiy] Re: pentode power amp circuit
Date: Sat, 16 Aug 2003 22:09:32 -0700

> >There are many things that contribute to a speaker driver's efficiency.
> >Size of the cone is one part of it, but so is the material the cone is made
> >of as well as the cabinet its mounted in, the biggest misconception in that
> >being that ported cabinets are more efficient than if sealed.
> 
> Hah?
> 
> >As far as driver itself though, boils down to the mass and tension of its
> >diaphragm in relation to the signal driving the transducer, and the use of a
> >ferrofluid lubricant or not.
> >But there are 15 inch drivers out there that are a hell of a lot more
> >efficient than headphone speakers.
> 
> Excuse me? Does the term Thiele-Small ring any bells?

Thiele-Small is a good start, but as you say, efficiency is a complex buisness.

What is especially overlooked by traditional speaker-designers is 3-dimensional
wavefront properties. Also, add to this the properties when signal has
different speeds of amplitude acceleration. An undamped ported box can behave
quite badly there... actually you can hear it once you've over the relative
good bump in the base... which doesn't bring you much joy in the long run.

> Efficiency is a complex business. You want efficiency, the 
> best way is STILL with a horn. Or headphones. The loudness of
> the wave hitting your eardrum is what matters! So, headphones
> are the ultimate. Even average ones need only 400 mW to achieve
> painful sound levels.

Horns is very efficient. Normal cone-speakers is really just a waste of energy
and much of it radiates off the coil, which is a real problem if the magnet
assembly has gotten warm (after 6 hours of "disco music" it is warm).

A horn-design which I used peaked at 139 dB(SPL) at one meter for the mid, and
136 dB(SPL) at one meter for the top. We never used less than four of those a
side. Amps would thermically eat the elements for breakfast before clipping,
and I made sure none of them clipped.

Cheers,
Magnus