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hey unlunf.
cool article ! you refer to mostly tubes though, does this generally apply to
solid state as well ?
aso what i was kinda wondering about was i have a solid state crate amp with
a 4 om speaker in it. it also has an external speaker jack, but its labeled ' 8
ohm min'.
i was inquiring about plugging it into my peavey bass cab that has 2 15 inch
speakers ( not sure about the ohms of each one ) but the cabinent is labeled '4
ohm' by peavey
unlunf <> wrote: Hmmm, the whole idea of an article
that is tech oriented,
and completely off topic (surf music).... is this a good
thing? So let's cut to the chase, and eliminate all the
blathering. We'll also eliminate the math portion of our
curriculum. <g> Since we started talking about impedance
in the power amp section, we'll stay there for now. In a
few paragraphs we'll deal with low signal level impedance.
First, every circuit in electronics has two impedance
factors, the input and the output. Some components
in those circuits are impedance-neutral, such as resistors,
diodes, batteries, etc. Others are very impedance critical,
these would be capacitors, inductors such as transformers, etc.
Impedance is frequency sensitive.
Let's just take it for granted that a tube has fairly high
input and output impedances. OTOH, a speaker's impedance
is low. We use a transformer to convert from one to the
other. So, what happens if we don't match the speaker to
the transformer?
Well, the transformer doesn't do anything in and of itself,
it's not an active component. What comes in, goes right
back out, after the conversion. If we hook up a 4 ohm
speaker to a 4 ohm output transformer, then the tubes will
be happy. If we hook up an 8 ohm speaker to that same
transformer, what happens to the tubes? They 'see' twice
the number of ohms of impedance. Is this a bad thing?
Depends on the tube, but for the most part, no. Lots of
tolerance is built into both tubes and transformers, and
even most speakers can handle some mis-matching of
impedance values.
Some guidelines that have been accepted by the amp design
and repair industries over the years:
You can usually feed a lower output impedance into a higher
speaker impedance without causing grief to the tubes, but
there are limits. You shouldn't go over twice the rated
number of ohms. Remember, as the ohms went up, the tubes
tried to overcome that, and tried to draw more power from
the power supply. Go too high, and the power supply will
crap out on you. You'll end up with less power output at
the jack, and probably a muddy sound.
Pretty much the same holds true in reverse, but that way
is a bit more dangerous. All the good amp designs, not
just Fender, can handle a halving of the impedance (2 ohms
instead of 4), and the tubes should survive. As has been
pointed out before, always look at your tubes for awhile
after doing this. If they start glowing red, STOP! The
tube is not sufficiently loaded down (too few ohms), and
is drawing too much power. This is something that can't
be fixed by twiddling the bias, you'd have to reduce the
power supply voltage (the B+). Moreover, the tubes will
very likely clip in the second harmonic range, and that
will pass right through the transformer and take the
speaker out like a snuffed candle. Too much of this,
and the transformer will also buy the farm.
Final rule of thumb: You can go one step up or down, but
no further. If you're going down (speaker is lower than
what the amp is rated for), then always observe the tubes
for at least 5 minutes after warming up. And re-check them
from time to time, they may not get stressed out until
later in their lives.
in re: low level signals......
The same rule of thumb holds true here, except that we don't
have to worry about blowing tubes or speakers. In fact,
what we now worry about is fidelity. You can always go
from a low impedance source to a high impedance load, and
it's very likely that you will suffer little or no signal
degradation. The reverse is -not- true, you will definitely
suffer degradation if you do more than halve the load's
input impedance compared to the source's output impedance.
And that's it in a nutshell. Without any math, even!
The rest of the lesson is best taught by example, and that's
where you come in. Ask your questions, and we'll see what
would happen (good or bad) if we do this or that or the
other thing.
unlunf
.
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