A U D I O... P A P E R
THE EX MODS
2004 by Steve Deckert
our online forums it is not hard to get a little confused or miss-guided
by the simple fact that there is so much information there.
topic that still seems to be misunderstood is the Zen EXTREME mod's
that I've mentioned in several places.
will describe exactly what they are in a way that makes sense so
you can know if you should pursue them or not.
The EX modifications
are specific to only two of our amplifiers. These are the
SE84C and or SE84CS.
involves changing the output transformers from a 9800 ohm primary
impedance to a 3300 ohm primary impedance. What does that
do? Well, in short, it re-focuses the amplifiers performance
from low impedance loudspeakers to very high impedance loudspeakers.
Please read on!
As many of
you know these amplifiers in their stock form were designed to drive
very difficult loads all the way down to an almost short circuit
if need be. For example, you could hook several speakers in
parallel to one of these amps bringing the total speaker load down
to around 1/2 ohm and the amp will actually LIKE it! For sure
it is the ONLY SET (Single Ended Triode) amplifier in the world
that can do this! It is also THE reason why these little 2
watt amplifiers kick-ass the way they do!
THERE IS A
REASON why I designed the amps to handle such low impedances!
With few exceptions,
most loudspeakers today carry a nominal impedance rating of 6 or
8 ohms. However, many people do not realize that "impedance"
is a combination of "resistance", "inductance"
and "capacitance". They also do not realize that
when a speaker claims to be 8 ohms that it really is NOT. A
speaker will present a different impedance to an amplifier at every
frequency it plays. This phenomenon is greatly effected
by the cabinet design, ie., cabinet size and tuning frequency.
typically will go up and down across the frequency spectrum. On
an typical 8 ohm loudspeaker it will usually drop in places down
to around 4 ohms and can rise as high as 50 ohms or more in other
places. In fact the popularity of many of the great sounding
loudspeakers of the last 20 years was offset by the fact that very
few amplifiers could properly drive them. Reason? They
had dips in the impedance curve that dropped all the way down to
2 ohms. Most amplifiers, especially solid state, went into
cardiac arrest when used on these speakers.
the SE84C and CS amplifiers to cope with this reality. This
has lead to a common assumption that you need low impedance speakers
to have the "best match" with these amps. This is
not true. The SE84C and CS amplifiers are designed to work
well with conventional speakers rated at 4, 6 or 8 ohms.
you run into a speaker that is rated at 8 ohms, and actually presents
a load to the amplifier between 6 and 50 ohms. This would
be like our Fostex single driver crossoverless speaker, Lowthers,
and similar crossoverless speakers. Of course there are also
many speakers with a 16 ohm rating that present the amp with loads
that seldom drop lower then 10 or 12 ohms. It is these type
of speakers that motivated the EX mods. Why? Because
in a stock SE84C or CS amplifier the power tends to drop as the
impedance rises. That means that it's 2 watts of power may
drop to 1 watt or less when coupled to these high impedance loudspeakers.
While many of our customers use these amps with these speakers
and think it sounds just fine, it is only because the speakers just
happen to also be very high efficiency. High enough to where
a 1/2 watt of power is plenty.
the EX mod specifically for HIGH IMPEDANCE speakers which include
8 ohm single driver speakers with no crossovers, and or conventional
16 ohm speakers. The reason behind the mod was to get more
TONE and WEIGHT to the music... a more complimentary signature if
So, if you've
read about the EX mod's on forums and own conventional 8 ohm speakers
with crossovers, the mod is not for you because your speakers most
likely have dips in the impedance that fall around 4 ohms. IF
you have a conventional speaker that claims to have a very "easy
to drive" flat impedance curve with minimal dips and peaks
in it, you can be certain that the speaker has a COMPLEX crossover
network with Zobel impedance networks in it. Complex crossovers
and SET amplifiers are generally a poor combination because complex
crossovers spend much of the first watt or two as heat making them
too inefficient to work right with low power amplifiers.
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