Q&A

Q&A # 62 - May 5, 1998

Staff

Q I have seen you refer to DC offset several times, and you measured the amount of it in your latest reviews. How much DC offset is acceptable in amplifiers?

A DC offset is the amount of DC voltage at the outputs when the amplifier is at idle (no signal passing through). When it is present, it "offsets" or biases the speakers by the amount of the voltage. It can be positive or negative DC offset voltage. Let's say it is 100 millivolts. This will cause the speaker cones to "rest" slightly away from the spot they would rest if there were no connection to the amplifier, and can result in distortion when music is playing. If DC offset is in the preamplifier outputs, this affects power amplifier performance. In general, if DC offset is 50 millivolts or less, this is not a problem. Manufacturers limit DC offset by several methods. One way is to put a capacitor in the + lead of the output (called a DC coupling capacitor). A second method is to use DC servo feedback from the output. Coupling capacitors are found in many designs, but DC servo is usually found in the most expensive products.

Q I have a powered subwoofer connected to the subwoofer jack of a Yamaha RXV992 receiver. This sounds pretty good, but I have two questions. When playing music, the subwoofer seem to lag slightly behind the rest of the speakers. This is so slight that I don't notice when watching moves (except if there is a bass heavy musical score), but with some types of music, it's annoying. I can hear the difference even when sitting close to the subwoofer, so I don't think it's a placement problem. I have the subwoofer located to the left of the TV, next to the left main speaker. The other problem is that the subwoofer seem to be extremely sensitive. I had to turn the subwoofer output from the receiver down to -15 dB, and the LFE setting to -15 dB to prevent the sub from going completely boinkers. Now, here is the unusual part of this setup. I'm from Europe, but live in the US. Most of my system is bought in the US and is running on 110V, but the subwoofer I brought with me from Europe and is running 220V through a 110-220 power converter from Radio Shack. I don't know if this matters, as the sub was just as sensitive when I used it in Europe.

A This is a common problem and is a result of phase delay (shift) caused by the crossover network. Subwoofer outputs have their own internal active crossover, which has phase delay. The subwoofer has a crossover network that also has phase delay. The two crossovers in sequence result in misalignment of the low frequency phase with the rest of the music coming from the other speakers. If there is a phase adjustment switch on the subwoofer, try changing it. THX subwoofers have the option of bypassing the crossover entirely. Otherwise, you may need to move the subwoofer farther away from the main speakers.

Q I just bought a Yamaha DSP-A1 in anticipation of the new DTS DVD players being released. What companies are going to be using this format and when can we expect them?

A The first DTS-compatible DVD players are available now. They will have a logo on the front that says "DTS Digital Out" or something to that effect. DTS DVDs are scheduled for release around June of this year. There may be an upgrade chip available that will make some of the earlier DVD players DTS compatible. This includes the Sony DVD-S7000 and the Toshiba SD-3107. You just remove the old chip and put in the new one for DTS DVDs. Most of the early DVD players will not be upgradable unfortunately. Since some of the DVD player manufacturers are into the DTS arena now, I think they all will be into it eventually, especially since there are decoding chips getting ready for mass production that decode DD and DTS all in one chip. The next models of receivers should have multiformat chips too.

Q I have a Denon AVR-3600 Receiver, with a pair of B&W 604s, CC6 for the center. I am in the market for a pair of surrounds. My question is should I buy a pair of dipoles or the regular front firing speakers? I am using DVD as the source for Dolby Digital. Should I stick with B&W or go with something else?

A Since digital surround is now in full swing (HDTV is going to have it too), get another pair of 604s for the rear. It is best not only to use the same size speakers for the surrounds, but the same brand too.

Q Recently, I purchased a new receiver, DVD player and speaker system to accompany my existing 50" TV. My system now consists of a Yamaha RX-V992 (as recommended), Toshiba SD3107 DVD Player (as recommended), Mitsubishi 4043 - 50" Rear projection TV, Mitsubishi VCR - (hifi/no S-Video), Front Speakers - NHT 1.5s, Rear - NHT 1.5s, Center - NHT AC-1, Sub - NHT SW2Pi. My first question concerns the connection of the DVD, TV, and VCR for video. My 992 and DVD are located approximately 20 ft. from the TV because of space limitations around the TV. I put my VCR on top of the TV for ease of wiring PIP (the TV only has one tuner). My problem is that, I was told that I should use the composite video output from the DVD directly into the DVD input on my receiver, then send the video output from my receiver to input 2 on my TV and then use input 1 on the TV for the VCR. The rational was that if I use S-Video out from the DVD into the single S-video on the TV then input 1 would be forced to S-Video/DVD and that PIP would not be user friendly. Your Q&A # 33 says to never use composite out on your DVD player. Should I definitely change to S-Video? How would you set this system up? I have ordered Video Essentials to help with further adjustments.

My next question concerns the viewing format of DVD discs. Why are some of the titles being released with only the widescreen format and not the standard format? I have not decided which I like the best, but I would like the option. Will they all eventually be released in only widescreen? My next question is open-ended, however your experience with the Yamaha 992 and the Toshiba 3107 is of great value. Are there any "tuning" tips/hints that you would recommend in the configuration of my system? I have never had a "good" system before, so my ears are overwhelmed by all the new sounds. Consequently, I am not sure what to adjust to make the speakers/system sound even better. Should I be adjusting levels, crossovers, ..? these are all terms I have heard but am somewhat unfamiliar with.

A Definitely use the S-Video from your DVD player directly to the TV's S-Video input. It is difficult to say whether your PIP will work with this setup, but the picture from DVD is so much better with S-Video than with composite, that it would be worth giving up PIP. The composite output of your VCR can be connected to the composite input on your receiver for switchthrough and use of the receiver menu as seen on the TV, or you can connect the composite output of the VCR directly to input number 2 on the TV. Your TV should be able to have one video input on the main screen and a second input on the PIP, and be switchable between all the inputs. This could be Broadcast/DVD, Broadcast/VCR, or DVD/VCR. This may require setting up the source that you want on the PIP by putting it on the main screen first, then pressing "Exchange" or whatever button your remote uses. This puts the source that was on the main screen into PIP, and then you select a new source for the main screen. Most receivers duplicate the on-screen menu on the front of the receiver, so you don't really need to see the receiver menu on the TV screen. In fact, I find that the receiver on-screen menu sometimes gets mixed up with the TV's own menu. Just connect the components up in the various ways I mentioned, and see which one gives you the PIP, but be sure to use the S-Video from your DVD player directly to the TV in any case.

Many DVDs are now being released in only the widescreen version for several reasons. One is that HDTV will be a widescreen format in itself, and there would be no point in making a pan & scan version of the movie since it would simply occupy the center of the screen and be cropped on the sides compared to the widescreen version. Secondly, it is very expensive to produce both formats, so by having just the one format, costs are greatly reduced. There is some consideration now for producing separate widescreen and pan & scan versions for DVD, again, to reduce costs of making double sided DVDs (although I don't see why they couldn't use the Reverse Spiral Double Layer - RSDL - method for having both versions on one side).

Q I need to know if the Rotel RB-985 is rated at 4 ohms. I have a PSB Stratus Gold and I intend to use it as my front channel speakers but its impedance is listed at 4 ohms.

A The RB-985 is a THX certified amplifier meaning it has some headroom. However, its power supply will limit performance at 4 Ohms, and you are likely to encounter clipping if you watch action movies. If you already have the RB-985, then lower the volume control for the front left/right channels with respect to the other channels (assuming you have 8 Ohm speakers in the other channels). Otherwise, get a larger two channel amplifier to drive the front left/right and a three channel amp to drive the center - rear left/right. If all your speakers are 4 Ohms, then get an amplifier designed to handle that type of load at high output, like the Sunfire or CinePro.

Q Does the 24-bit/96 kHz DAC make a difference with the conventional 16-bit/44.1 kHz CDs? I thought this feature can only be taken advantage of when playing the new DADs which are recorded in 24-bit/96 kHz sampling. I see some forums say the 24-bit DAC sounds better but doubt what they are saying as they listened to conventional CDs. Well, I know 24-bit should sound better but only if listened on the right medium. Comments?

A With today's technology, a 24/96 DAC won't do any better with a 16/44.1 CD than a 16/44.1 DAC. The complication arises when discussing some of the CDs that have special codes which attempt to recreate the 20 or 22 bit original recording. This is the case with HDCD for example. In these situations, a higher resolution DAC could possibly make a difference. But with a conventional CD that has had no special tricks applied to it, there will be no difference between the DACs.

Q With the advent of music in the DTS format, what would be the recommended frequency response for the center channel and surround speakers? Should these speakers be able to go very deep?

A DD and DTS are full spectrum sound in all channels (except for the LFE). All speakers have good high frequency response because the tweeters are small and can go in even the smallest enclosures. The low end, as you mentioned, is the problem. Although it is simple to get rear surround speakers that go down to 30 Hz or so (by using floor standers the same as you would in the front), the center channel speaker is a problem because it sits on top or underneath the TV. If you use tiny speakers in the rear, you will miss some of the deep sound effects (like a jet flying from front to back or back to front), and, at 80 Hz and below to the subwoofer, you will pick up some of the low frequency directionality that should have been from the rear. Also, up around 80 Hz - 90 Hz, a subwoofer begins to sound boomy. This is because subwoofers don't generally have any damping material in the enclosure. So, I like to set the crossover for the subwoofer at no more than 60 Hz - 70 Hz. Digital surround has changed all the rules, in my opinion. I would suggest having speakers all the way around that can go to at least 50 Hz - 60 Hz and using a good subwoofer to handle the lowest octave. They don't have to be large to handle 50 Hz, but they do have to be well designed.