Q&A # 60 - April 21, 1998

Staff

Q I would like to know if mounting a set of rear speakers that are bipolar in front of the listening area would work out alright since in my new home I do not have walls to place them in the rear or on the side. Would inwalls in the ceiling just behind or even with the couch be better?

A For bipolar speakers, it would be best to have walls. Therefore, in your case, get monopolar speakers and put them to the side of the couch, aiming them at the listening position or slightly towards the TV. If this does not fit into your room decor, then get ceiling mount speakers. Have a couple of friends stand (carefully) on a stepladder and hold the speakers at the potential spot where you plan to mount them, and you sit in the listening position with surround sound playing. That way you can adjust the position and angle for the best results without having to actually drill holes in the ceiling until you are satisfied with the position.

Q What are the the merits and drawbacks of Class D amps? I'm trying to learn more about them and get a sense for how big a trend (if any) they represent.

A Class D represents digital switching amplifiers. A switcher turns the output transistors on and off at different frequencies to represent the waveform. The output transistors are fully on or off when switched, but If the switcher turns them on and off at a higher number of times at any particular point, this will produce larger amplitude (higher voltage or volume). These products are definitely worth keeping an eye on for the future. However, they represent new technology and certainly are not perfected yet. One problem is that they don't necessarily switch on and off as fast as they are supposed to. Secondly, an LC network (inductor and capacitor) "ladder" is necessary at the output to keep RF from being sent to the speakers. A third problem is that the two legs (+ and -) of the output transistors cannot be on at the same time. Otherwise, the amplifier would burn up. So, in order to make sure that the two legs are not on at the same time, there is significant "dead time" where they cross over, and that produces distortion. However, Class D amplifiers are currently doing very well in subwoofers, since the switching frequency only has to deal with about 150 Hz and less. For full spectrum audio, however, there are only a couple of good products on the shelf right now.

Q What is the preferred digital input, Toslink or coaxial? Is one sonically better than the other?

A Toslink eliminates the possibility of ground loop hum through that particular connection. When I have seen the topic discussed in various places, the general opinions have been either no difference in sound quality, or a preference for coaxial.

Q I have a Yamaha 2092 receiver. I am interested in purchasing the new Yamaha DVD player that is capable of playing DTS DVDs   (i.e., S-700). In order to play DTS, what will I have to do with the receiver? How much would I have to spend on a decoder or an upgrade module if I require one? Finally, if these upgrades will be costly would I be better off selling my 2092 and getting a receiver that is compatible with DD and DTS?

A The 2092 is an excellent receiver. I would suggest having MSB Technology add a set of 5.1 inputs for you. You also need a DTS decoder. There are several good ones on the market, such as the Millennium 5.1, Rotel, and they are not very expensive. The DVD player will recognize whether the data are DD or DTS and output the digital bitstream accordingly. The digital output of the DVD player is connected to the digital input on the DTS decoder. You need to make sure that whatever DTS decoder you purchase has a digital throughput that will send DD data from the DVD player on to the DD decoder in your receiver through the digital connection. In other words, the DTS decoder needs a digital input and a digital output. Since new DSP chips have been announced that will decode HDCD, DD, DTS, and other things, receivers capable of handling all the formats using their own built-in circuitry will probably hit the market by Christmas, 1998 (if we are lucky). Otherwise, in the Spring of 1999.

Q A few questions regarding amps and preamps:

3. I am considering purchasing a Rotel RTC-970 preamp processor. In reviewing their manual, I read that in all HT modes the tone controls are defeated and not amenable to be "turned on." I really like the sound of this preamp in stereo but am concerned that not having the ability to manipulate the tone controls will limit corrections for any room anamolies when playing movies or even surround CDs. Are these limitations typical?

A Preamplifiers and power amplifiers have equal effect on the sound quality. However, it is much more expensive to build a good power amplifier than it is to build a good preamplifier, the reason being that a preamplifier does not require such a big power supply to be good. There are some preamplifiers with big power supplies, and they are expensive.

The differences between a $1,000 DD receiver and a $5,000 DD processor/preamplifier are, indeed, not very large. The reasons for this are several. One is that the receiver is built in huge numbers, making it a good value. The high performance processors are made in very small numbers, and this increases the ultimate cost. If the high performance processors could be sold in the same numbers as the mass market receivers, they would cost $2,000 instead of $5,000. However, the $5,000 item still is noticeably better than the mass market receiver and is worth the price to many enthusiasts.

Receivers in general have the tone controls only for the front left and right channels, so that is all you are losing with the home theater modes. Secondly, tone controls alter phase relationships which can mess up Pro Logic decoding if the tone manipulation occurs before decoding. The best way around this is to use a good home theater equalizer like the Audio Control Rialto or Bijou. You need an outboard power amplifier for this, and you put the EQ between the pre-outs of the receiver or preamp/processor and the power amplifier.

Q When I connect a receiver with another preamp for music listening and home theater watching according to your method, which one will have the master volume control? The receiver or the other preamp? It seems to me that the preamp will have the master volume control, but if this is the case, then how to adjust the volume to match front left righ to the others?

A They both will affect the volume. When using the tape loop or processor loop on the outboard preamplifier, the signal is sent to the receiver, which decodes it into surround sound, and the front left/right signal is sent back to the preamp loop. You should set the volume control on the outboard preamplifier and receiver to a low level when switching over to home theater and the loop. Then, adjust the receiver volume so that the center and rear surrounds are the volume you want, and finally, adjust the volume control on the outboard preamplifier which will change the volume of the front left/right channels.

Q I have a Pioneer DVL 700. I now understand that a newer model is available, DVL 909. Do you know the differences in performance, etc., and can I get an upgrade? Also, the DVD " Fleetwood Mac, The Dance" totally refuses to play on the DVL 700. I am curious if you know why.

A The DVL-909 is their new model that uses 10-bit video DACs as compared to the 9-bit ones used by the DVL-700. The DVL-700 is also known to produce artifacts that are non existent on other DVD players. The Pioneer second generation DVD players are much better than their first generation.   No, there aren't any upgrades available. You might want to wait for a DTS compatible player. I am not sure whether the 909 is DTS compatible or not. There is supposed to be a DTS logo on the players that are compatible. Try your Fleetwood Mac DVD in a different DVD player and see if it will play there. It might be the disc.

Q I have a 52" rear projection TV. What is your opinion of protective screens, both smoked & clear? My dilemma comes from the question, Off or On? Obviously the calibration (ISF) will be different depending on whether the screen is in place or not. The screen (smoked) appears to increase contrast and color saturation, but renders whites somewhat brown and therefore has to be affecting colors as well. The other drawback is of course reflectivity during ambient daylight viewing. I realize that it is somewhat subjective as to whether or not to use it, and I do not require it for protection in that I have no children running around with crayons, etc. It is an option that happened to come with the set and is easily removable. I have read nothing re: this question, and even the ISF people do not have a definitive answer. HELP Please!

A Remove the screen. It actually cuts down on the overall light output and is very reflective. I recently removed the screen from my Toshiba TW40F80 and I am much happier now. If you have an ISF calibration done, do it after you have taken the screen off.

� Copyright 1998 Secrets of Home Theater & High Fidelity
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