crystal clear : was : RE: [sdiy] Simple discrete Unity-Gain Follower ?

[Magnus Danielson]

From: Tim Ressel <madhun2001 at yahoo.com>
Subject: Re: RE: crystal clear : was : RE: [sdiy] Simple discrete Unity-Gain Follower ?
Date: Wed, 7 May 2003 13:48:45 -0700 (PDT)

> Oh, folks, it gets better....
> 
> First, the case for capacitors on high-freq drivers.
> High-freq drivers tend to not like lots of DC across
> them. If your amp misbehaves, you are buying $300
> worth of diaphrams for your Altec 806Bs. A $2 cap will
> prevent this.

.. or make sure that the amp never missbehaves. Incorrect matching of amp and
driver isn't uncommon. A good amp should be able to handle the impedance and
especially the screwed up phase!!! Some amps doesn't handle of-resisitve loads
very well at all, including fatal breakdown at worst. A way to handle the
situation is first of all use a too big amp, so you don't clipp, but also make
sure you have headroom. Limiter to make sure to DONT cut the corners.
When you have headroom up to clip more amps is able to handle the reactance of
the element, since most amps consumes much more power for reactive loads and
may go into clipping for much lower levels.

It is easy to use a 100 W amp for a 120 W element, since you figure the element
is able to handle the power - not so! When you reach 100 W you are about to
clip, when you clip you starts to push out more and more power than amplitude
motivates, so you reach for 140 W which is what the voltage allows for
squarewave, so you toss more power than you thought, but to make things worse,
inappropriately amount of energy in overtones. The 120 W element should have at
least a 150-175 W amp, but also a limiter so that you don't push more than
80-100 W at most. My experience is that when you do things properly, you don't
burn elements and you don't burn amplifiers, and let me put it this way, this
then when not cheating on the price on either of them either.

... and we didn't have a DC-blocking cap!

All I am saying is that it can be done and I also advocate that it should be
done.

> Now, accurate passive crossovers are difficult to make
> for one simple reason: driver impedance is not
> constant over frequency. And the cabinet can also
> alter the impedance. Multi-amp setups eliminate this
> problem.

Indeed. Driver impedance isn't as easy as you would like. Also, acoustical
impedance blend into the mix of impedances. Now, usually not as great an
influence on the electrical impedance, but things like placement of speakers
will actually be visible in acoustical impedance and thus (due to the
reciprocal property of the speaker) also in the electrical impedance. This just
shows that the conditions for the passive filter is not only as easy to say,
put also indeed a changing condition. Heat-related changes is also there and
not to mention non-linear properties. For a propper filter design you really
need to know the source impedance (in it's fullblown complex impedance!) of the
amp, so then you lock your design to a specific amp (which people tend to
forget is really part of the system design, not just a part you may choose at
lust which still seems to be in vouge).

Cheers,
Magnus