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Q I just purchased a new preamp ($2,500) from a very reputable manufacturer. Everytime the preamp is switched on, several of my on-air (antenna only - no cable or satellite) TV channels develop interference. I tried a battery operated TV with rabbit ears (to avoid any possible physical connection between the TV and preamp). Even at a distance of 15 - 20 feet from the room where the preamp is located, the TV picks up interference. As the TV is moved closer to the preamp, the interference increases. This occurs whether or not any components are plugged into the unit. All of my existing equipment has been in use with another receiver used as a preamp without any problems. The unit is being shipped back to the manfacturer for testing. Does this sound abnormal for a preamp of this caliber?
A Your
preamplifier is oscillating at radio frequencies, and it is not
normal for this preamp or any preamp to do so. You probably remember
some of the old movies where someone who crash lands in the jungle
converts a radio receiver to a radio transmitter and taps out
some Morse code to get the rescue party to come and get them.
They do it by making the circuit oscillate at radio frequencies.
Even though you can't hear the oscillation in your preamp (the
oscillation is at frequencies beyond human audibility), it definitely
degrades the performance. One of the parts in the preamp may have
a short, and it is usually just a minor repair.
Q I have a query regarding speakers for the Denon AVR-3200 DD receiver which I have recently purchased, and you have recently reviewed. I am thinking of purchasing some kit speakers from Vaf Research (http://www.vaf.com.au) on the basis of the outstanding reviews they have received from Australian magazines such as Audio Yearbook, Best Buys and Audio & Video Lifestyle. My concern is with the impedances of the speakers. I am thinking of DC-X's (6 Ohm nominal, 95dB/W at 1m) as fronts, a centre DC-6 (?impedance, 95dB/W at 1m) and DC-7's (6 Ohm nominal, 93dB/W at 1m).
My hope is that the high sensitivity of
the speakers will allow me to drive them at sufficiently high
volume levels without damaging the
receiver. Given the high sensitivity of the speakers, a nominal
6 Ohm impedance looks ok, but the impedance/frequency plots available
at Vaf's WWW site show the impedance dropping dangerously close
to 4 Ohms for some parts of the frequency spectrum. I do value
Vaf giving out this type of information, because I feel I could
end up buying 8 Ohm "nominal" speakers only to find
that they have a << 8 Ohm impedance for a large part of
their frequency range.
So my questions are:
1) In your opinion, if used at sensible volume levels, would I be able to get away with using the above Vaf speakers, despite them having impedances less than Denon's specified 8 Ohms?
2) Would the protector circuit in the receiver
give any protection with low impedance speakers being driven too
hard, or is it just to protect
against short-circuits?
3) I know that a low powered amp can damage
tweeters if driven into clipping, and a too higher powered amp
can simply blow speakers up if
turned up to high. What are the symptoms/damage caused by using
speakers with lower than specified impedance at too high volume?
4) Would a cooling/ventilation fan for the receiver help at all?
5) I've heard you describe dynamic power ratings as meaning squat, but I've seen a fairly reputable magazine use them to approximate the amount of power available at non-rated impedances, e.g., 80W rated into 8 Ohms, with 105W dynamic power at 8 Ohms, 215W dynamic power at 4 Ohms indicates that approximately 215*80/105=164W could be safely supplied to a 4 Ohm speaker. Does this type of calculation have any factual basis at all?
And yes, as per your recommendations, I am planning to buy a 3 channel power amp as soon as my budget allows.
A Because
the speakers you intend to buy have a high sensitivity, you should
be just fine when using the Denon with them. But, as you say,
be sensible with the volume control. In fact, even after you get
the three channel power amplifier, still be sensible about the
volume. Your hearing is at risk if you play the system really
loud, whether the amplifier is powerful or not. The value of big
power amplifiers is that they will sound clean all the time, and
will have excellent dynamics for transient demands. Remember that
our ears were not designed for listening to the sound of a .357
Magnum at realistic levels.
Most high-performance components have protection circuits for shorts and overheating. Some mass market products have these also. But I would not suggest using a fan just to cool an overdriven amplifier down. Better to operate it within safe limits.
Symptoms of clipping damage to speakers would be a tweeter that does not work at all, and/or a woofer that makes scraping sounds (the voice coil rubbing against the magnet).
Power ratings have always been a problem
with mass market components. The Marketing Department wants the
specs to say "X", and the Engineering Department is
restrained by budgets. Thus, all the confusing numbers. What I
look for is the rating into 8 Ohms in rms, 20 Hz - 20 kHz, with
all channels driven, and the rating into 4 Ohms. If the product
is not rated into 4 Ohms, or is rated only at 1 kHz, I am more
cautious about its capability.
Q I
have a system with following: DSP-A1, Sunfire Subwoofer, five
identical B&W 603s and 2 small JBLs for front effects, Pioneer
DL-909 combo LD/DVD player. When I set the front main speakers
to LARGE, I don't hear much bass from a music source such as CD.
Obviously the DSP-A1 doesn't send any low frequencies to the subwoofer.
So I set the front main speakers to SMALL and the crossover frequency
on the sub to about 70 Hz, the volume to 0, the the phase to normal,
and it sounds much better when listening to music. However, I
still leave the center speaker (which is identical to the main
ones) to LARGE, and when I watch actions movies with intense explosions
(such as "Starship Troopers"), then it seems that the
center can't handle the sound or was I experiencing clipping?
Here are my questions to
you:
1. Should I set the center speaker to SMALL just like the main ones so that low frequencies can be directed to the sub?
2. If the answer to (1) is YES, then what
should I set the crossover frequency and volume on the subwoofer
and what should I set the EQ level for the center speaker? (The
DSP-A1 provides this as you know from your own review of this
unit.) Also, does this affect the dialog in any way? (i.e.
some low voice may come from the sub and it may sound funny?)
3. If the answer to (1) is NO, then what
should I do to eliminate the problem that I described above and
reduce some stresses from the center
speaker? Should I buy a different center speaker? I understand
DD or DTS specifies identical speakers all around and that's what
I have.
A Even
with all the speakers set to "Large", there will still
be some bass going to the subwoofer. If you set one or more of
the speakers to "Small", you just get more of the bass
to the subwoofer. The problem with using the "Small"
setting on receivers, is that they use 90 Hz as the standard crossover
frequency. This is too high, in my opinion. Bookshelf speakers
can all respond to about 70 Hz, and floorstanding speakers can
all respond to about 50 Hz, with no problem. It's just the tiny
modular speakers that need a 90 Hz crossover. At 90 Hz, just about
every subwoofer sounds boomy. Also, the bass at 90 Hz is directional
(you can tell where it's coming from). This is why I suggested
that the future models of receivers and processor/amplifiers offer
a selection of crossover frequencies. The DSP-A1 allows you to
direct the bass anywhere, but it still uses the 90 Hz as the crossover.
The center channel gets a lion's share of the audio action, so
you could try setting the speaker to "Small", with the
low pass on your subwoofer set to about 60 Hz. This might keep
the boominess down. If not, you should get a three channel power
amplifier for the front left/center/right and use the pre-outs
on the DSP-A1. A new center channel speaker is not necessary,
and in fact, you are much better off keeping the center the same
as your other front speakers.
Q Toshiba
is reportedly going to offer a DVD player with progressive scan
(480p). What are the advantages of progressive scan when playing
DVDs over a top of the line transport like the Faroudja or Theta
(which I am contemplating purchasing)?
A Progressive
scan images will look like your computer monitor. You won't see
the scan lines, or at least, they will be a lot less visible.
DVD stores the data in progressive scan format. Current players
convert the data to interlaced information that our NTSC TVs can
use. The new progressive scan DVD players will output the data
as non-interlaced info, and HDTVs will be able to show the video
in non-interlaced (progressive scan) form. Some current projection
TVs, capable of higher scan rates, should also be able to show
the images from these new DVD players. Line doublers take an interlaced
image and create the lines in between, giving a reasonable facsimile
of the progressive scan image. The difference is that progressive
scan data is represented by original video info, while the line
doubler uses mathematical algorithms to "guess" what
the video info would have been, in between the interlaced lines.
Personally, I would rather have real video data rather than synthesized
data, but line doublers are superb at what they do. Faroudja and
Theta are marvelous products. You might ask if progressive scan
upgrades will be available on their players.
Q Do
you know if the Yamaha DSP-A1 will need an upgrade to play the
new DTS DVDs? It was released before DTS DVDs. I would hate to
buy one and when DTS DVDs come out, find out it won't decode the
DTS bitstream properly.
A The
DSP-A1 plays the DTS DVD that we have in our lab correctly. It
will not require an upgrade. The Millennium 5.1 DTS decoder will
need upgrade chips if you bought one before April, 1998. The cost
is $20. We received an E-Mail from someone who has a Sherwood
receiver with DD and DTS decoding, and he said it works fine as
is.
Q I've
set the cut-off on my subwoofer at 70 Hz, and use it with music.
I tried, at your advice, to set the cut-off at 50 Hz, but it didn't
work well. The bass sounded slow and "fat". I heard
it on "Sorrow" from the Pulse album, Pink Floyd and
some other songs also. This disappeared when I set the cut-off
back to 70 Hz. Now it sounds clean and "right". How
come? My mains go to 35 Hz, and the sub to 25 Hz on paper, but
plays test tones at 21.5 Hz.
A This
may be due to phase shift. By adjusting the low pass on your subwoofer
to 50 Hz, the phase shift at 50 Hz was altered from where it was
when the low pass was set to 70 Hz. The phase shifted 50 Hz then
interacts with the 50 Hz from your main speakers and gives you
the different sounding bass. If you like your current results,
there is no reason to alter the settings. But, if you want to
experiment further, try setting it to 50 Hz again and moving the
subwoofer to some different locations in the room.
Q I
have read that VCRs have horizontal resolution of 210~240 lines,
broadcast TV 330 lines and LD/DVD of 480 lines. This is a bit
confusing. Any TV set will have a fixed number of columns of RGB
pixels say 800 odd. So when a VCR conveys information for 210
vertical lines, how are the balance vertical lines filled up?
A All
the pixels are illuminated, regardless of the resolution. With
lower resolution video, less of the pixels will have discrete
illumination. That is, with low resolution images, if you looked
at any two pixels next to one another, they would more likely
appear illuminated with the same luminance and chroma, than if
the image resolution were higher. Basically, it just means that
less information is spread out over the 800 pixels.
Q Do
all inputs on a preamp sound the same or not? I have read in a
few articles that a "tape" input on a preamp often sounds
better than other inputs due to a shorter signal path. Is this
correct? Since I listen exclusively to CDs, perhaps I can use
the preamp's tape input instead of
the cd/aux input.
A Except
for the RIAA equalized phono cartridge input, the preamplifier
inputs (line-level) should all be the same, including the tape
playback input. What you may be referring to is the tape loop,
which provides output that bypasses the volume control. Tape inputs
still go through the volume control. In any case, try the two
inputs to decide for yourself.