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Q I am considering purchasing a THX speaker system. The complete system has two subwoofers. I have the option of purchasing the system with just one subwoofer. I have an average size area for my home theater system (12x15x11) My questions are 1) Will buying one sub instead of two decrease my lower end response and punch given the size of my room, and 2) Is a subwoofer rated to 35 Hz good enough for the low end?
A Between
20 Hz and about 50 Hz, bass is non-directional (you can't tell
where it is coming from). However, using two subwoofers still
gives an increased sense of three-dimensional soundstage at those
low frequencies. Part of this is tactile (vibration sensors in
your skin) and part of it is auditory. However, in order for it
to work, you need to connect the two subs to two different output
signals (i.e., stereo subwoofers, with one to the left front channel
output and one to the right channel output). Two subs have to
be placed carefully, though. Otherwise, you might end up with
decreased bass at the listener's position. If done right, you
will get about 6 dB more bass with two subs. On the other hand,
there can be considerable signal below 35 Hz these days with the
intense sound tracks that some movies tend to have. Personally,
I would prefer to have one really fine subwoofer that is rated
down to 20 Hz (no more than 3 dB down at 20 Hz) rather than two
that are rated to 35 Hz. Then, later, I would get another sub
rated to 20 Hz if could not afford to get both at the same time.
Good subwoofing is absolutely critical to action movie home theater
experiences. The new designs with extremely powerful amplifiers
(1,000 watts rms or more) are the ones to consider if action movies
are your thing.
Q It seems that the more I learn, the less I realize that I know. I've witnessed home theater systems at retailers and know that I need / want one. However I recognize that there are competing systems out there - Pro Logic, Dolby Digital (AC3), THX, and now DTS. I'm not one for continually upgrading and need to find a setup that will set me right for say 5 years.
I see receivers that (at a price) offer
the ability to accept all of the above inputs - do I really need
this? DVD will, I suspect, make VHS tape
somewhat similar to 5 1/4 floppies, hence I'm inclined to accept
this as the way to go.
So what do I need? I've read your articles on receivers vs. discrete components and find myself confused.
I suspect that I'm not the only person in this position - i.e., someone who would appreciate the hightened experience of surround sound but doesn't quite know what path to take.
An article on this would, I suspect, satisfy
many potential buyers.
A Fortunately,
receivers with everything built-in (Pro Logic, DD, and DTS decoding)
are now available at mass market prices (e.g, Yamaha RX-V595 at
$799), and I would definitely suggest getting one of these since
you are headed towards DVD. RIght now, only a handful of receivers
have all three decoding formats, but by next year, probably most
of them will, even those in the $500 range and below. However,
depending on how much you like to play the system cranked up,
a set of pre-out jacks is still useful for connecting a more powerful
outboard five-channel power amplifier. Make sure the receiver
has at least one subwoofer output jack too (most seem to have
one). Also, the marketing info on receivers at stores can be confusing.
Stickers on the front of the receiver, or on the price/info tag,
might say "Dolby Digital Ready" rather than "Dolby
Digital". If the word "Ready" is there, it means
that the receiver has a set of pre-in jacks ("5.1 inputs"),
but that the receiver does not decode Dolby Digital itself. You
would need an outboard Dolby Digital decoder to plug into the
5.1 input jacks on the receiver. I am organizing a list of low-cost
receivers ($500 and less), with ratings on their features and
ergonomics. The list will say whether or not DD is built-in. I
will publish it in the next few weeks.
Q I
am currently in the market for a Dolby Digital-ready receiver.
I have looked at the Technics line SAAX 6 and the SAAX 920 in
the $400 - $500 range. I just found out that neither of these
receivers are high current. How important is it to get high current
vs. low current? I purchased my main speakers (Energy Pro 2.5),
my rear speakers (JBL M5 Bookshelves), and my center channel (Mission
70) and have yet to get the subwoofer. Am I going about this the
right way or should I spend more and get the JVC RX884VBK, or
do you have any better suggestions? The Technics receivers also
lack discrete amplification circuitry which is another concern
of mine. Any help in these areas would be greatly appreciated.
A I
have seen a few receivers in the $500 range that have DD built-in.
Wait for the list I mentioned above. High current is a marketing
term that means the amplifier will drive a 4 Ohm load. If you
use 8 Ohm speakers, it is not really an issue. Mass market products
tend not to be capable of driving 4 Ohm speakers very well. The
low-cost receivers may have a set of 5.1 input jacks for other
decoders, but they tend not to have a set of pre-out jacks for
outboard amplifiers that will handle 4 Ohm speakers. Judging from
all the recent questions on economical receivers and their features,
I should get that list published ASAP.
Q I
have heard that line level output impedance must be lower than
input impedance, otherwise high frequencies are disrupted. Can
you explain in layman's terms (or as simply as possible) why this
is so? Why does it not follow speaker level logic that it is better
to match
ouput (recommended) and input impedance level? Is this the same
as using 75 Ohm cables for RF and digital connection instead of
regular 50 Ohm (? not sure) RCA cables (as in Q&A #85)?
A Audio
signals are recorded and transferred as electrical voltage (force).
The greater the voltage at any given moment, the greater the amplitude
of the signal. For maximum voltage transfer, the output impedance
should be as low as possible, and the input impedance as high
as possible. Think of it this way. A pitcher can hit the catcher
with the ball harder if he doesn't have to hurl it through a bowl
of mud first. With line level components, input impedances generally
range from 10,000 -100,000 Ohms, while output impedances tend
to range from 10 Ohms -1,000 Ohms, so almost all equipment will
satisfy the requirement that the output impedance be lower than
the input impedance. When the output impedance gets very high
compared to the input impedance, the output voltage begins to
drop more and more across the output itself, instead of the desired
input. So long as this is done evenly across the frequency range,
it only results in a smaller but otherwise unperturbed signal.
The disruption in high frequencies is a rolloff caused by the
parallel capacitance of interconnect and some line level inputs.
It causes the impedance that the output sections "sees"
(as the input) to drop as frequency rises, creating a change in
input/output impedance ratio, resulting in a low-pass filter.
The lower the output impedance, the less effective the capacitve
portion of the load is at creating that filter. That's why the
theoretically perfect voltage source has an output impedance of
0. As to your reference to speaker level transfer, most solid-state
power amplifiers have output impedances in the fractions of Ohms
for that same reason. Loudspeakers are quite reactive, meaning
that their impedance changes with frequency. Any amplifier not
very near that ideal voltage source will become part of an elaborate
equalizer, boosting output into higher impedances, and lowering
it into lower impedances, in a manner identical to that of the
line level component with a high output impedance.
Matching impedances to specific levels becomes important with
RF and other ultrasonic frequencies (like digital bitstreams)
with wavelengths
much shorter than audio frequencies. Then it becomes important
that the output impedance and input impedance, as well as the
characteristic
impedance of the cable, are identical. A 75 Ohm cable, used for
single-ended digital transmission as well as video, within the
designed bandwidth, would look like 75 Ohms no matter how long
you ran it so long as you terminated it with a 75 Ohm load. If
that impedance matching isn't maintained, it may result in reflections
of the signal back down the line, and back up again, which create
distortion. Because audio frequencies are very long in wavelength,
and these reflections can only occur if the transmission line
is greater than 1/4 of the smallest wavelength, we simply try
to maintain the integrity of a voltage source. But a 75 Ohm cable,
such as a video cable, can make a great interconnect due to the
low inherent capacitance required to obtain that characteristic
impedance. Likewise, a 4 - 8 Ohm cable of sufficient conductance
can make a great speaker cable because of the low inductance inherent
in a cable of a 4 - 8 Ohm characteristic impedance.
Q I
have a B&K Ref 20 hooked to Adcom 200 watt X 3 and 250 watt
X 2 amplifiers along with a 50" projection TV, DVD/LD player,
and Velodyne 15" subwoofer. Except for the TV (although in
the same receptacle) the components are powered through a Panamax
surge protector. When I turn the system on, the room lights dim
for a second, and the Panamax LEDs are generally in the yellow
zone. I have been told that putting the components on their own
dedicated 20 amp circuit would have a positive impact on sound
performance particularly with Home Theater DD. My understanding
is the system gets starved for power and doesn't perform as it
should. This could be a little expensive to have an electrician
do. I spent enough on the system components and cables to spend
a bit more to fix this if it's really worthwhile. What's your
opinion?
A All
reasonably sized amplifiers cause the lights to dim when they
are turned on. This is the result of a large current drain as
the transformers and capacitors get charged up. A 20 amp circuit
would deliver more than 20 amps, but it is controlled by a circuit
breaker. The circuit breakers will allow more than 20 amps to
flow for a short time without tripping. It is when, say 22 amperes
try to flow continuously, that the breaker will trip. So, if your
circuit breaker is not tripping, you have no problem. In fact,
it is rare that most equipment would need more than 20 amperes
for a significant amount of time. I have seen systems with thousands
of watts of power that require dedicated lines, e.g., three 20
amp circuits, but most just don't need it. The bottom line is
that if your circuit breaker is not tripping, you are getting
sufficient current in your system.
Q I
plan on adding a second center speaker to lessen bass loading.
I know that by doing so I am overloading the amp. Can I
compensate for this by using the center channel trim level? That
is, can I lower it by 3 dB or thereabouts in relation to the other
channels so my
amp won't clip (also to match the actual sound level of the other
speakers)?
A I
assume you have the first center channel speaker on top of the
TV and are planning to put the second speaker underneath the TV.
You will need to put the lower speaker farther beneath the TV
rather than immediately beneath it. This will get the sound emergence
away from the TV screen (the vertical surface of the TV screen
is what causes the bass loading). I would suggest connecting the
two speakers in series rather than in parallel. This will increase
the impedance and cause no amplifier overloading compared to connecting
them in parallel. You might have to adjust the center trim up
a bit, or perhaps not at all, to match the output of the other
speakers.
Q I've
a fairly basic question I'm hoping you can answer. For television
viewing, I've heard that its best to watch TV programs in surround
mode only when the program is encoded in surround, and to watch
the rest of the programs in stereo mode because when watching
a program encoded in stero in surround mode, most of the stereo
effect is lost due to most of the audio being sent to the center
channel. Is this true? If it is, it makes watching television
a problem. Is there a way to tell in what format a program is
encoded other than trying to catch the first few
seconds to see if the Dolby surround logo appears?
A All
stereo sound will decode to some degree with surround sound processing.
Usually the music will go mostly to the front right and left channels,
and voices will go to the center because they are mono. A little
of the music goes to the rear for ambience. For programs encoded
specifically for surround, certain special effects are encoded
out of phase in the stereo channels, which means they will be
decoded to go to the rear with surround sound systems. So, watch
all your stereo programs in surround sound if you wish. The Surround
Sound programs will just give you a little more in the rear channels,
but the regular stereo programs will still give you some nice
surround sound.
Q I'm
building a home theater. So far I've decided on the Martin Logan
Requests for the front, the Logos center channel, and the Scripts
for the rears. My question is with amps. I'm thinking of auditioning
the Sunfire 2 channel amp for driving the requests, then using
an additional amp for the center and rears. I'm wondering how
much effect I'll see if I don't use a Sunfire amp for the center
and rears, or whether I should stick to the same brand for all
channels. I've also considered the Sunfire 5 channel amp but don't
think it can drive all 5 electrostatics...what's your opinion?
A Electrostatics
are great speakers, but they require lots of power and they drop
down to very low impedances at certain frequencies. I would suggest
waiting until the new Sunfire Signature five-channel amplifier
is available (coming shortly), which will deliver about 400 watts
per channel into each speaker. That will give you sufficient power.