Q&A # 146 - November 2, 1999

Staff

Q I just purchased a Sunfire MK II sub, and when I went to connect it, it emitted a loud humming noise and caused interference from my main speakers. Why is this and how can I fix it?

A First, wait until the system is humming and producing noise, and then disconnect the RCA cable from the Sunfire input and see if it still hums. If it does, then the Sunfire is defective and should be returned for replacement. If it does not hum with the cable removed, then it could be a bad cable, a ground loop, or a bad subwoofer output jack on your receiver. Try connecting the subwoofer to one of the other pre-out jacks on the receiver. If it still hums, then try using a different interconnect cable. It that cures it, then it was the cable. Otherwise, it may be necessary to use a three pronged adapter for the subwoofer AC connection. This will let you use the Sunfire without the ground prong and also let you reverse the plug so that the two AC prongs are in the opposite sockets from where they were before. Keep in mind that eliminating the ground prong reduces the safety factor.

Q Not taking into account the power amplifier sections of the Denon AVR-5700 (THX) and the AVR-3300, are these essentially the same processor/preamps? If not, what are the major differences. Background: I would consider purchasing either item for personal use. I already have a sufficient number of outboard amplifiers, so it's really the comparative processors I'm interested in. You might well have a better idea about spending the money (US$2500) on a 'dedicated' preamp/processor. I would welcome your comments and/or research suggestion list.

A In our experience, the upper level receivers have a wider bandwidth than the lower priced ones (the point at which the frequency response is down 3 dB from where it was at 10 kHz). This suggests that the preamp design has more differences than just additional DSP modes. However, that does not mean the lower priced receiver used as a processor will sound bad in comparison to the expensive ones. Something like a $799 receiver with pre-outs, used with a $1,000 outboard power amplifier could result in a home theater package with slightly less performance in the preamp but lots more dynamics in the power section, than a $1,800 receiver. As to dedicated pre/pros, the problem is that these are made in much smaller quantities than receivers, so you cannot automatically assume that a $2,500 pre/pro is going to be better than a $2,500 receiver made in large numbers. However, I do suggest that you consider pre/pros that have upgradable chips and downloadable software upgrades. This is going to be important with all the new audio formats emerging. Bug fixes will be easy with the Internet, just as they are with our computer programs.

Q I need your opinion on amplifier specs with mid to high quality amps. When an amp is rated at xxx watts per channel from 20 Hz - 20 kHz all channels driven, does this mean that the amp is capable of driving all its channels simultaneously at xxx watts throughout this frequency range? I have seen reviews of these amps and the measurements taken are lower than the amp's advertised specs. Also, will plugging an amp into a line conditioner affect its ability to achieve its rated output?

A Specifications are a marketing person's dream, and an engineer's nightmare. They all want their products to look their best on paper. You can't blame them for that, but you have to pay attention when shopping. The spec you refer to means that the amplifier is tested with individual sine waves at all frequencies between 20 Hz and 20 kHz, with all channels being driven at the same time. The sine waves are fed in for short periods of time and the output waveform is measured for the presence of distortion. Some manufacturers will only make a measurement at 1 kHz, which gives a better number than if all frequencies are tested. The reading can be taken anywhere along the distortion curve. It might be 100 watts output with 0.01% harmonic distortion, 110 watts with 0.1%, and 115 watts with 1%. I saw a boom box that was rated at 100 watts rms amplifiers at 10% distortion. The best test would probably be a complex waveform with multiple frequencies, but that would not produce a very impressive specification. All amplifiers sound pretty good when they output half a watt. Even a factory car radio. What I want to know is how they do when the pedal is to the metal. Using a line conditioner that has a voltage regulator might help with big amplifiers, but probably not small ones.

Q After 10 years, I'm putting together the last system I intend to buy (I've said that before :) Its use will be 80/20 audio-home theater respectively. I'm no pro and wouldn't know a crossover from a crossword puzzle. Here's what I've acquired so far. Main speakers - Infinity Preludes, Receiver - Nakamichi AV-7, TV - Hitachi 61SBX59B. First, what is the big difference between Pro Logic and DD? Is there a major sound difference? If so, what do I need in a DVD player to be compatible with the AV-7 to get that great sound since it is only Pro Logic capable? Secondly, the Preludes have 12" powered subwoofers, so do I really need a separate subwoofer? I was thinking of adding the Infinity QPS-1 as the rear surround speakers, but wouldn't I get a better sound by adding a pair of decent floor standing speakers? Since the Preludes are 6 Ohms, is there anything I should be aware of when obtaining the rest of the speakers for home theater? Third, naturally I want to add a VCR, and when the ads say HiFi, does that mean stereo? Fourth, since I'm only going to be playing CDs, DVD movies, and MP3s, is there any need for a graphic equalizer? Last, what do I need in regard to cables? I don't need the most expensive ones, just some that will do the job correctly. Can you offer a couple of brand names and things to be aware of that I should consider? You guys are more informative than any print magazine, Hands Down!

A Yes, Pro Logic is quite a bit different than DD, and you will eventually want to be able to have DD decoding because it is wonderful. If your receiver has a set of 5.1 analog inputs, then get a DVD player that decodes the DD and has the 5.1 output jacks to connect to your receiver. Otherwise, you will only be able to use the stereo analog outs from the DVD player, into your receiver, for Pro Logic decoding. Built-in powered subs can be very good, but you need to test them to see if they can output down to at least 30 Hz without problems. Get a test CD that has low frequency test signals, along with a Radio Shack SPL meter and see if the subs respond with about the same output at 30 Hz as they do at 40 Hz. If they are 3 dB less at 30 Hz, that will still be OK, but if they are down 6 dB or more, then you should get a separate powered subwoofer, one that will do the job in the lowest octave (20 Hz  - 40 Hz). You can use another pair of Preludes in the rear. Using a receiver with 6 Ohm loads is not the best idea, as mass market receivers do better with 8 Ohm speakers, but you already have a pair of the Preludes, and it would not be a good idea to mix brands for the rear channel. An EQ is not necessary. Use 16 gauge (or larger) zip cord for speaker cable. Nordost 2-Flat is also excellent.

Q I am going to begin building some speakers in the upcoming months with the plan of someday having a home theater system that also sounds good when I play CDs that are only stereo. I have two options, and I was wondering what you suggest for home theater use. Should I build the two front speakers to be two-ways with a 6.5" woofer, or make them three ways? I want to be able to get deep bass when playing CDs, and I am concerned that with the two-way approach this will be impossible.

A Three-way designs add more complexity with the crossover design, and I have not heard a three-way that can achieve the deep bass of a separate powered subwoofer. Therefore, since deep bass is important to you, go for the two-way design and have a powered sub for the 20 Hz - 60 Hz region.

Q Good Guys, a local retailer of Energy speakers, is having a sale on the APS5+2. They have them priced at $1,400/pair (the full retail price is $2,500/pair). I read your review of the A3+2s and was very impressed with your findings. Do you have an opinion on the ASP5+2 model? The dealer informed me that this entire speaker line will end production soon. Also, the RVSS and the A2+2 speakers are selling for about the same price. I would use them in the rear, and I have space for either pair. Which to you recommend? Finally, I am going to be comparing the Overture 3s and ASP5+2s. The Overture 3s will arrive this week and I have 30 days to review them. I have also purchased the ASP5+2 with plans to make comparisons between the two. What should I look for? Which do you recommend?

A Since you will have both models in your home, this should be a relatively easy choice to make. Just compare the sound, and decide which you like. Try them out at low levels and relatively high levels. Use an SPL meter to match the loudness. Play some classical music, with piano and strings. Piano is hard to reproduce naturally. Also play some music with voices, particularly female. Listen for chesty qualities that indicate peaks in the 100 Hz area. Nasality would indicate problems at 800 Hz. Listen for too much sibilance, where "s" sounds crackly.

Q Recently, I came across two new speaker input ratings on a Panasonic SC-AK15 mini system. 1. 100W (Music) 2. 50W (DIN). I am used to RMS and PMPO. What do the above terms mean and what is the relationship to RMS and PMPO? The amplifier rating is 700W PMPO, 30W/ch RMS, 6 Ohms impedance.

A The rating at rms output into 8 Ohms is the spec to look for. Peak Music Power Output (PMPO) is another one of those marketing numbers to make specs look good. I doubt if it is 700 watts PMPO. If it says that, it is probably a typo, and more likely 70 watts. The 30 watts rms at 6 Ohms means it is about 25 watts at 8 Ohms. Isn't the specification game fun?

Q I want to build a modest HT system from scratch. I have only a pair of bookshelf KEF Coda 8 (6 Ohm, 100 W) speakers for the rear. I am considering two options for front speakers: B&Ws DM603 2S (8 Ohm) and KEF Q55 (6 Ohm), both sounding good to me and both rated at 150 watts rms, with matching center. Is it good to mix brands and impedances? The price is similar. For the receiver, I am considering the Denon AVR2800 or HK AVR65, which are around 70-80 watts rms x 5. I will be using the system primarily for music.
A Mixing brands or impedances can be problematic. However, if the speakers sound neutral, then mixing brands is not so important. However, since the KEF are 6 Ohms, and you will be using a mass market receiver, I would suggest going for the B&Ws, which are 8 Ohms.

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