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Q&A # 119 - June 8, 1999
Staff
Q My current systems consists of:
Yamaha RX-V2095
NHT VT-2
NHT VS-2
Yamaha DVD-S795
Sub to be purchased soon (Velodyne HGS-15 or Paradigm Ref Servo-15)My main question is regarding ampflication. While the receiver has respectable power for moderate listening levels, it seems like it could use a little "oomph" for some action flicks. Part of this is probably due to the speakers (6 Ohms, 88 db/w/m). Since they are bi-ampable, my question is what are the pros and cons of passive bi-amping? For example, does it make more sense to buy a 5 x 150, and bi-amp the front and use the fifth channel for the center (still using the receiver for the rear)? Or, cough up the extra to begin with, and use 5 x 250 for all five channels (not bi-amping)? At the moment, I can only see the financial benefits, and wonder if there are sonic benefits too. I've auditioned Rotel and ATI with my speakers, and a Sunfire Cinema Grand on some Mirages. The VT-2s are quite revealing, and perhaps a bit bright turned up with just the receiver powering it. With that in mind, can you suggest a "warmer" amplifier for this situation?
A You should be able to bi-amp the speakers using the receiver's amplifiers for one driver in each speaker, and the outboard amplifier for the other, utilizing the pre-outs on the receiver to drive the outboard power amplifier. This will not be as efficient as using an active crossover between the preamp and power amp of the receiver, but it will still give you more clean sound than just using the receiver by itself or the receiver as preamp and outboard power amplifiers. The 2095 has some pretty big power amplifiers, but like receivers in general, it does not have the kind of power supply you will find in a good outboard power amplifier. You might try setting up the combination two ways, one with the receiver driving the tweeters and a second way with the receiver driving the mid/bass. See which one gives you the sound you prefer. If brightness is your concern, then get a MOSFET outboard power amplifier, like the Adcom 5500, 5503 combination for five channels at 200 w/ch. That, plus the power amplifiers in your receiver, should give you all the oomph you want. However, it will be important to keep in mind that the input sensitivity of the 2095's amplifiers and the outboard power amplifier will likely be different, and it may (in fact, probably) be necessary to put some sort of a volume control in between the pre-outs of the receiver and the inputs of the outboard power amplifier, if you use the bi-amping method. Otherwise, you could experience having too much bass or too much treble, since the receiver's volume control will adjust the output of the receiver's amplifiers and the outboard power amplifier at the same time. This may limit you to one of the outboard power amplifiers that have built-in volume controls for each of the five channels. If you decide to use just an outboard power amplifier, and not the receiver's amplifiers, then the power amplifier volume controls are not necessary.
Q I have a Velodyne HGS-15 and it is indeed a powerful one. My first question is in reference to the switch for PHASE 0-180 on the rear panel. What does this mean and what is the difference between the two settings? Which is preferable for movies and which is better for music? There is also another switch for crossover in-out. What should I set this to? I connect my RX-V795 sub pre-out to the sub and the rest of the speakers from the speaker outputs on the receiver. The last question is: The sub is capable of going way below 20 Hz, but the receiver processing ability specification is between 20 Hz - 20 kHz. Does this mean that if a DVD contains audio frequencies below 20 Hz, let's say during scene of a shuttle take-off in "Armagedon" or a nuclear blast from "Broken Arrow", it is not passed to the sub? If that is the case, I need to have different A/V receiver or Processor?
A The phase setting has to do with the phase angle of the waveform coming out of the subwoofer in reference to the phase angle of the mids and highs coming out of your other speakers. The term "phase angle" is used for all sorts of things in physics, including electronics as well as astronomy. With respect to the audio waveform, take a look at the diagram on the right.For any particular frequency, the waveform starts at 0 voltage, or 00, then rises to a peak positive voltage, called 900, then decreases back to 0 volts, or 1800, then reaches a peak negative voltage, or 2700, then back to 0 again, which is 3600, or 00. Let's say that your subwoofer cone is at rest when the voltage is 0. At 900, the cone is extended out from the enclosure, resting at 1800, then retracted inward to the enclosure at 2700, and resting again at 3600. The 900 position represents positive air pressure, and 2700 represents negative air pressure. Your other speakers are reproducing the mid-bass, mids, and highs from the music or movie sound track. With a musical instrument or sound effect, the phase of the various frequencies are all aligned, meaning the 900 positions of the various frequencies are all occurring at the same time, as well as the other positions. Since your subwoofer is in a different part of the room, and possibly a different distance from your ears as the other speakers, the 900 position of the low frequencies from an instrument or sound effect might not arrive at your ears when the 900 position of the other frequencies produced by that instrument arrive from the other speakers. In this case, there is "Phase Shift" between the two sounds coming from the two speakers. Let's say it is 400, meaning that when the sound from one speaker is at 900 when it reaches your ears, the fundamental sound frequency from the subwoofer might be at 500. The result is sound that is not quite natural. So, the HGS-15 allows you to shift the phase by 1800. Some subwoofers let you vary the phase between 00 and 1800, but the problem is that, due to crossover networks, the phase shift varies depending on the frequency. So, no setting will give you complete recovery of the accurate phase angle. With very low frequencies, such as with a subwoofer, it is not really a huge problem, but with mids and highs, phase shift can cause large differences in the way sound is perceived. It is a major concern in the design of crossovers for two-way, three-way, four-way speakers, etc., so that the phase angles of sound from the various drivers in the enclosure line up with one another. With your subwoofer, which it appears you have connected correctly to your receiver, just switch the phase setting between the two options (00 vs. 1800) while listening to music and movies, and use the one that sounds the best (at your listening position, not while standing next to the subwoofer). If you set the crossover in/out switch to "out", this removes the variable low-pass feature, so you would have to make do with whatever low-pass frequency your receiver uses. They are often too high, so I would suggest leaving that switch to "in" and setting your subwoofer low-pass to about 60 Hz or so, depending on the low frequency capability of your other speakers.
Human hearing capability is usually defined as 20 Hz - 20 kHz, but many people can hear higher than 20 kHz and lower than 20 Hz. Your receiver will respond down to about 10 Hz and up to about 50 kHz in a linear way (no dips or peaks). So don't worry. Those room shuddering sounds in the movies will reach your HGS-15, which does nicely in the < 20 Hz region.
Q Back in 1997, largely based on your favorable review, I purchased a Toshiba TW40F80 TV, and followed on with a Toshiba 3107 DVD player for component video connections. I have been well served by these components. Toshiba then got me excited with their premature announcement of ColorStream PRO progressive scan matched TVs and DVD players. When I contacted Toshiba Customer Service about these upgrades and to inquire if there were going to be add-ons to preserve my almost three-thousand dollar Toshiba investment, they were unhelpful to the point of being brain-dead. When Hollywood forced withdrawal of the product - let's call it indefinite delay - I forgot about pursuing it any further. Now the PC DVD revolution is upon us and progressive scan (480p) extracts from DVD sources are alive again. These would output from the PC as RGB-out from an appropriate videocard/de-interlacer (see Mpact from Chromatic Research). My questions:
1. Will my TW40F80 detect and display such 480p from a computer DVD source given a conversion of the ColorStrean component inputs to accept RGB+sync?
2. What further modifications/add-ons might make this possible?
A It is not possible for the TW40F80 to display a progressive scan picture. The TV scans at the NTSC rate of 15.25 kHz. Progressive requires a scan rate of 31.5kHz. Only the current HDTV sets will be able to do this, as well as a few DTVs from Toshiba.However, you should not be in any hurry to upgrade your TV, since all of the current HDTVs have a few bugs. Of all that are available, only one coming from Panasonic will be able to handle the native 720p signal (native meaning that the incoming signal is 720p rather than some other mode and then being upconverted or downconverted). Others upconvert that to 1080i. Samsung upconverting 480p (progressive DVD) to 1080i.
Progressive DVD players will be coming soon, whether or not Hollywood is happy. They have been delayed a year to be kind to Hollywood, but that is about to change.
So you can still enjoy your Toshiba TV today. I have that model and am perfectly happy with it.
Q I own an Acoustic Research S12HO subwoofer. When in "stand-by" mode, a faint hum (not related to ground hum) can be heard if I am in very close proximity to the sub. In addition, when turning the sub power off when in "on" mode, a single pop sound can be heard. Depending on the preamp volume, when rotating through the various audio inputs, a clicking noise can be heard.
I have sent the amp section in for warranty work. A lot of parts were reportedly replaced. Upon return, the same noises can be heard. I sent the amp back to the warranty repair shop, and they indicated that on their test machines they could find nothing wrong and sent it back. What could possibly be going on? The warranty is good for another month!
A The faint hum is probably mechanical noise from the power supply transformer. This is pretty common, and you should not worry about it. Some rheostats for halogen lamps can cause the transformers to hum louder if they are nearby, and set to some spot in the middle of their brightness range. The clicks and pops are transients that are caused by small voltages that occur in the preamplifier output jacks when switching preamplfier inputs. As long as they are just small sounds, rather than loud cracks, you don't need to worry about them.
Q Which direction should I point the bass reflex port (if that's what it is) on my Klipsch KSW-200 subwoofer? I use two subs, one in each front corner of my room. This is probably a basic question but I cannot find any topic on it.
A In general, a subwoofer's main frequency range is above the port resonance (for the models that have ports) down to the resonant frequency. At that low frequency, it won't make a lot of difference which way it is pointing, unless it were up against a wall. Passive radiator designs work in a similar way, with the passive radiator serving as a "stiff" port (the cone of the passive radiator is harder to move than air in an open port). I would suggest trying the setup in several directions, including with the drivers facing sideways in the corner. Performance of subwoofers will depend on your room, rather so much with the direction that the drivers are facing, at least with the lowest part of their range.
Q I purchased a Sony Wega 36XBR200 TV, and I've been happy with this unit so far. It was built in March, 1999. My question is regarding the horizontal scan rate. I want to know if it's capable of 31.5kHz, so I can use a unit like the iScan from DVDO. A call to Sony proved fruitless. Why do they bother to staff the phone lines with folks who know little more, or less, than the average consumer?
A No, the 36XBR200 is a standard NTSC TV and will only scan at 15.25 kHz. You would need their new HDTV Wega to take advantage of the iScan. As to the consumer hotline, probably nobody had asked that question before, and they were not prepared for it. We have the luxury here of being able to think about the questions for awhile, and even look up the info on other websites.
Q I am about to purchase a five-channel amp. In fact, I am looking at two: The Sherbourn 5/1500 Five Channel @200watts per channel and the new Outlaw at 165 watts per channel. I am having trouble getting the power consumption numbers from either of these models' spec sheets. I live in a normal American house and don't want to blow fuses everytime I turn my system on. Could you give me some idea how much power these machines will consume? The Outlaw says something about 1,800 watts maximum. Listening to a DTS DVD movie can get up there in volume, but nowhere near maximum. How do I figure out how much it's using? Should I have the amp on a completely different power strip and outlet?
A Although it would be possible to blow a 20 amp fuse with a big amplifier, it is unlikely with the average five-channel amplifier, because it would be rare that all five channels are at full output, and even then, unlikely that they would be at full output for more than a split second. Fuses are designed to allow short term transients above their rating without tripping. I have never tripped a circuit breaker in my system even with high power equipment. If you go to something like several big Krells, then you might have to install separate lines. The average power being consumed is probably about 200 watts total for all the amplifier channels at any one time, regardless of how much the amplifier is capable of delivering, because we tend to watch the movies at a reasonable loudness, on average, and watts are watts, as they say. I don't think that you should necessarily have your power amplifiers on a separate power strip, but some sort of power protection strip is nice for an audio system. I use two AC line conditioners, but only because there aren't enough sockets on one strip. I have them both plugged into the same wall outlet because this helps to eliminate ground loops.
Q What kind of speaker stands should I buy? I own a pair of RBH Signature Series 661-SVs (www.rbhsound.com) which have two 6.5" woofers and one 1" silk dome tweeter in the D'Appolito configuration. The dimensions of these speakers are 22"H x 8.75"W x 11.25"D.
I know the answer is not as simple as the question. Words like wood, metal, height, sand, buck shot, weight, vibration, dampening, wire management, etc. seemed to shout from my monitor as I ran a few quick searches across the Internet. I don't want to run down to Circut City or Best Buy to pick up a pair of the $49.99 on sale special (none of these seem to have carpet spikes), but I don't want to spend $500 either.
I called RBH Sound to ask their opinion, and they suggested the Plateau K-30's, but is a 30" stand too tall for the dimensions of my speakers (the tweeter would be about 40" off the floor). Is there a standard guideline for the selection of speaker stands?
Ultimately, I would like to purchase stands that would provide proper height and dampening . . . with looks my wife would accept.
A It may not be as complicated as you might imagine. The idea of a speaker stand is to place the speaker at the correct height for listening, but not provide any vibration of its own. Nor should it provide a surface from which speaker sounds could reflect. I like wooden stands because they are less resonant than metal ones. That is why metal stands usually have a hole in which to dump some sand. Secondly, look for stands that have as little vertical surfaces facing out into the room as possible. This suggests the ones with thin support rods rather than planks. Thirdly, you don't really need speaker stands per se, at all. A couple of end tables will work fine, say one at either end of the couch, for rear surround speakers. Place the speaker on felt furniture pads, and put the speaker at the edge of the table so that there is no horizontal table surface in front of the speaker. Having a table cloth that drapes over the front edge of the table - especially a cloth with a rough weave - will also help. I have used this type of setup in the past, and my wife found it very acceptable. I have also used plant stands (stands made to hold flower vases), which are more attractive than speaker stands. For the front speakers, stands are more appropriate than tables, and there are lots of them out there with various heights to choose from. Pick ones that will put your tweeters at about ear height, when you are sitting in the listening position.
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