Q&A # 138 - September 13, 1999

Staff

Q I recently purchased a JVC 884 Dolby Digital amplifier, and I wanted to try my optical output from my CD player, a Technics SLP-50 (no big deal, it's a 4-DAC 1 bit MASH technology). However, I was surprised that at high volume, with the optical connection, I can hear a strong hiss on the speakers (or headphones), even with no disc playing, and that's not the case in analog mode (practically nothing is heard at max volume). If I disconnect the optical output (or input on the amplifier), the hiss disappears. I thought a digital signal would be much cleaner than an analog one. I tried with a high quality optical cable too, and no improvement.  Also, I have the feeling that when it's my CD player that decodes, the sound is much richer in details, sharper, and the bass cleaner. I can tell the difference with closed eyes between the JVC decoder and the Technics one. I hoped the DAC on the JVC would be a better one than the one in the CD player, especially when it's only a 4-DAC in this technology. One last thing: the volume on the JVC amplifier is an 'electronic' one, between 0 and 90, and it's supposed to be a 2 x 130W rms in stereo mode. But, I used to have a Technics (the 500 model) with 2 x 80W rms, and when the volume was at not quite halfway, the sound was already very strong. With the JVC, even at 65 out of 90, it's just ok. Where's the power?

A When you are using the optical connection, you are using a different circuit both in the CD player and in the receiver than when you use the analog connections, so the hiss is coming from one, or both, of those places. It is not in the optical cable itself. In your setup, the DAC in the CD player is the better of the two, so use that one. Volume controls on different products are not in a one-to-one relationship. Depending on a variety of things, including how they are actually rated, the volume on a lower powered amplifier might be higher with the setting at halfway compared to a more powerful one set a little higher on the control. But, assuming both amplifiers are rated in the same way, setting the volume control near full level will most likely give you greater loudness with the amplifier that has the higher rating. I suspect that because both of the products you are referring to are mass market receivers, the power ratings may have been made quite differently between the two. The 0 - 90 is just for your convenience. The numbers don't have an exact reference.

Q Please help me to put this question to rest. Mr. average (me) walks into an audio store and asks a simple (?) question. As my current CD player needs replacing, and as I would possibly consider a DVD player in the near future, but would anticipate that a DVD player would spend 95% of its time playing Audio CDs . . . is the sound output/quality of an average DVD player as good as a decent CD player? There you have it: In the left hand a good CD player. In the right hand a DVD player costing 2-3 times as much but spending a lot of its time acting as a CD player. Which way should I go? (By the way, the score so far: 6 salespeople say CD, 8 vote for the DVD. Any wonder I have yet to purchase?)

A We have discussed this problem from time to time, with the answer being that the Sony DVD players, with their dual laser mechanism, will be as good as a nice CD player. However, the situation is now complicated by the fact that DVD Audio is about to descend upon us. The discs will only be playable on DVD players that have DVD Audio capability, with such players purported to be on the dealers' shelves sometime this Fall. Therefore, I would suggest waiting until then, and getting a DVD Audio player. They will be about $1,000, which is the same price as a top of the line current DVD player. With 24 bit 192 kHz sampling, DVD Audio will be far better than conventional CDs, so it will be almost moot as to whether or not these players will handle CDs as well as a regular CD player. My guess is that the DVD Audio players with dual lasers will probably be as good as the CD players.

Q I'm planning to replace my Pro Logic receiver with the Yamaha RX-V995. For speakers, I'm thinking of using the Mirage AVS series with their sub. Could the AVS handle the power of the V995, or should I save some more and get the Mirage floor standers? In your opinion, are the Mirage Omnipolars better in home theater applications compared to conventional speakers?

A Damaging speakers is more a result of not having enough power than having too much. When using smaller speakers, you just don't turn it up as high. For home theater, though, I would suggest getting the larger speakers, unless you have space constrictions. The Omnipolars give a huge soundstage, but they do so even with music. Their capability in this regard is not limited to movies. However, it all depends on your tastes. Go and listen to see if you like the effect.

Q In Q&A # 134, you advised a reader to use the analog inputs on his Yamaha RX-V2095 rather than the digital inputs if the CD player had a better DAC than the 2095. My understanding is that the 2095 like the 995 converts analog inputs to digital then converts them back to analog before they go to the power amplifiers. This means you would be using the DAC in the CD player as well as the receiver. A way around this is to use the 2095's right and left channel inputs in the 5.1 analog input set, which would bypass the DAC in the receiver yet still retain use of the volume control. Wouldn't this be the better solution?

A The reason analog signals are converted to digital ones in receivers is for the purpose of Digital Signal Processing (DSP). This is where the Stadium or Concert Hall sounds are added. If one is going to use any DSP mode, the signal has to go through these A/D - D/As. Using the analog out from the CD player to the pre-ins would indeed bypass these DACs, and could very well sound better than the two-channel mode in the DSP setup. However, the DSP array assumes you will be using one of the modes that entails surround sound rather than just two-channel stereo. In the case where the DAC in the receiver is better than the one in the CD player, then there is no choice but to use the digital input on the receiver. It all boils down to just trying the various combinations out and seeing which one you like the best.

Q Since most high quality power transistors in amplifiers are high voltage and high current rated, what benefit is there from having so many output devices in the circuit, compared to just one pair?

A The usual transistor ratings are basically paper numbers not useable in real design. For example, a 30 Amp transistor may be used for short time with 30 Amperes, but very low voltage. If you have some voltage, then the allowable current drops very fast. In many cases, if a designer wants any margin at all, they use a 30 Amp transistor at maybe 3 or 4 Amperes. It is all governed by the transistor so-called Safe Operating Area (SOA) curves plus the design margin they are comfortable with. So in order to achieve the required 20 A, or whatever, output current rating, they need to connect several transistors in parallel.

Q I know what causes hum, but what causes hiss? Is it just inexpensive transistors leaking white noise at random?

A There are MANY sources (I am not sure if you are talking about a particular product or just in general). Most often it is caused by the fundamental noise of the input devices. Inexpensive preamplifiers often have lots of hiss. Mass market receivers have quite a bit too. Tube products don't necessarily have much hiss.

Q I recently upgraded from an older Onkyo Pro Logic receiver to a Denon AVR-3300 receiver and a Toshiba DVD player (SD-2109). I was astounded and delighted by the difference between Pro Logic and DD. However, my center and surround speakers don't seem quite up to the task of reproducing the full-range signals DD provides. I have Polk RT16s in the front, a Polk CS-250 center, a pair of Polk M1s in the rear, and a Klipsch KSW-12 sub. I've been thinking about upgrading to Mirage OMR-2 surrounds and an OMC-2 center. I could also replace the KSW-12 with a Mirage BPS-150i. I like the sound of the Mirage speakers, but I'm somewhat concerned about matching. I don't plan on replacing the RT16s right away. I listen to about 1/2 music and 1/2 movies. I enjoy the 5-channel stereo capabilities of the Denon, but never use any DSP modes. My listening area is small and irregular (the usable space is roughly 9' wide by 11' deep), in a basement with a low ceiling (6.5'). My current rear speakers are placed behind the very small seating area, and are obscured by furniture. I could place the OMR-2s on side walls and hopefully achieve a better rear channel sound. In your opinion, considering my setup, how important is speaker matching? I have also considered trying to find some used RT16s so that I could have 4 or 5 of the same speaker, and I've looked at the newer Polk surrounds and centers.

A Speaker matching is important, but if your present speakers are neutral in sound, then other speakers that are also neutral, will match. Because DD is so demanding, I would suggest getting something as large as your RT16s for the rear. The center channel is always a tough call because large speakers just don't fit on top of the TV. In any case though, get as good a center speakers as you can afford. The center is VERY important.

Q First, what is your suggestion for in-wall speaker cable? I am installing speakers in several rooms of my house to be driven from the rack at the opposite end of the house. The longest run will be about 40 - 50 feet. The second question is the iffy one. Is it possible to use a line doubler with a direct view TV? I currently have a 32"  Hitachi Ultravision but will be building a theater within the next two years. I know that I will want a doubler when I put in the front projector and would like to break up the money-spending frenzy as much as possible. My thought is if I can use the doubler now, I might as well benefit from a sharper picture until then. If the line doubler will work (and even if it won't since I will be getting one eventually), what should I be looking for? Is the DVDO iScan worth a look? Thank you for taking the time to look over my questions. Your site is the only one I check religiously and have been extremely pleased with all the advice and information I have read.

A If you are putting the speaker cable inside the wall, then it does not really matter what its shape is. For 50 foot runs, you should use something pretty big in caliber, such as 13 gauge zip cord. If you are going to put the cable just inside the molding around the base of the wall, then try one of the flat cables that are made by several companies, including Nordost (2-Flat). You can run them under the molding, then bend them so they go upward and make a slot in the sheet rock to slide the cable into. Then cover the slot with joint compound. Regardless of whether the TV is direct view, front projection, or rear projection, it will need to be able to handle a 31.5 kHz scanning rate to do line doubling. A typical direct view TV or rear projection TV won't do that. Interestingly, for the new TVs that will handle 31.5 kHz scanning, many of them already have line doublers built-in. The DVDO iScan is a very cool product, but wait until you have your new front projector (most have high scanning rate capability) before you get one.

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