Q&A # 125 - July 13, 1999

Staff

Q Can you tell me the significance of 1st order pass, 2nd order pass etc. crossovers when auditioning loudspeakers please? Also, in layman's terms if possible, what exactly is meant when people refer to timbre when matching loudspeakers?

A Crossover networks have a capacitor, an inductor, and sometimes a resistor. When it is just one set of these parts, the crossover allows a first order low-pass/high-pass signal to the woofer and tweeter, meaning that, at the crossover frequency (determined by the value of the capacitor and inductor), the signal will attenuate (decline) at the rate of 6 dB per octave above the crossover frequency to the subwoofer and the same amount below the crossover frequency to the tweeter. So, if the chosen crossover frequency is 2 kHz, the signal to the woofer is flat as it approaches 2 kHz (from below), and then it is reduced by 6 dB at 4 kHz, 12 dB at 8 kHz, and so on. For the tweeter, the signal is flat as it approaches 2 kHz (from above), and then it is reduced by 6 dB at 1 kHz, 12 dB at 500 Hz, and so on. For a second order crossover, there are two capacitors and inductors, giving a 12 dB per octave above and below the crossover frequency. For a third order crossover, there are three sets of parts and an 18 dB per octave reduction. The rate of reduction, whether it is 6 dB per octave, 12 dB per octave, or whatever, is called the "slope". Design of the crossover is a difficult task, mostly done with computer programs these days. The idea is to have the slopes overlap in such a way that the amount of signal produced by one driver is balanced by the other drivers so that the sum of the drivers' outputs is a flat frequency response. Also, there are two-way crossovers, for a woofer and tweeter, three-way crossovers for woofer, mid-range, and tweeter, and other designs. The frequency response is never perfectly flat, because the crossovers, drivers, and enclosures are not perfect either. So, depending on the curve of the frequency response, the "timbre" will be different from speaker to speaker, which also includes the effects of the enclosure as well as how much harmonic distortion the speaker has. The timbre might be a laid back mid-range, or a bit boomy, perhaps some nasality or chestiness. Some people like the various characteristics, that are actually a distortion compared to the original signal, but the best speakers are the ones that are neutral. They don't add sound characteristics of their own. They don't sound boomy, chesty, nasal, or tinny. So, when you are shopping for speakers, take some CDs with piano, orchestra, male and female voices, violin solos, movie sound tracks, and any other music that might give you an idea of how "natural" the speakers sound.

Q What do you call the speaker connection that has the red and black spring loaded connections, the type that you push down on with your thumb, then insert cable and release. Also with all this talk of Monster Cable, the cable is out there, but connection hardware seems to be lacking, and your system is only as good as your weakest link.

A Those are called "Spring Clips" (for connecting speaker cables), and nowadays, they are found only on budget receivers and speakers. Actually, they can provide satisfactory contact between the connector and cable, but they limit the type of connector on the cables that can be used, although some of the newer ones are flexible here. It used to be that only bare wire could be inserted, but I have seen some that now have a hole for inserting banana plugs. Most receivers now have three-way or five-way binding posts, usually with plastic nuts to tighten down. All of them will accept bare wire and banana plugs, although European electrical codes prohibit some types of banana plug capability due to the possibility of inserting the AC connector accidentally. To sum up, both will make good contact if you are careful, but binding posts are best for a secure connection.

Q When using a Dolby Pro Logic preamp, how is it possible that one can add a Dolby Digital or DTS decoder? How does it work? How much sense does it make to pick up an old (2- 3 years) high end THX Pro Logic preamp for about $500 and then add and outboard Dolby Digital decoder?

A In order to add the DD and/or DTS decoder to a Pro Logic preamp/receiver, the preamp must have a set of analog inputs for five channels (front left/center/right and rear left/right). Such products are called "Dolby Digital Ready" or some other marketing term. This lets you connect the five outputs from the DD/DTS decoder to the preamp, with the subwoofer or LFE channel going directly to the subwoofer. Outboard decoders are getting hard to find (we will be putting one - a Yamaha DDP-2 - in the prizes section in September, if you are interested), so buying a DD/DTS-ready preamp is becoming risky. You can also have a third party company install the decoder in the preamp, but since receivers with Pro Logic, DD, and DTS decoding on-board can now be had for $600, that choice is not economical. My suggestion is just to get a new receiver with all the decoding already there.

Q In my living room I have a home theater setup consisting of Parasound amplifiers and a Parasound preamp. For movies, I enjoy this setup very much. However, when listening to music, I use a different preamp and a tube amp hooked up to the same speakers. To accomplish this, I connected the speaker wire from the Parasound to one set of banana plugs and the speaker wire from the tube amp to another set of banana plugs. Depending on which source I listen to, I attach the appropriate set of plugs to the speakers. I would like to avoid the constant switching of speaker hookups by leaving both amps hooked up to the speakers at the same time (the speakers have two sets of inputs, so I would hook one amp to the lower and one to the upper and leave the jumpers in place). If I do this, then I can simply turn on the appropriate preamp and amp and leave the power switches on the others off. Is there any risk of damage to any of my equipment if I use this setup? Please consider the assumption that I would never turn on both amplifiers or both preamps at the same time, and that I do not want to use any kind of switching device to select the appropriate amplifier output (if such a devices exists).

A The two sets of binding posts on your speakers are for the woofer and tweeter. The jumper is removed when the speakers are bi-amped (one amp going to the woofer and another amp going to the tweeter). You should not leave both of your amplifiers connected to the speakers at the same time, even if only one is on at any one time. The voltage produced by one amplifier will go to the speaker outputs on your other amplifier and damage it. There are switching devices for this kind of thing, and they use a "break before make" circuit that disconnects one set before connecting the other set. It is VERY important that you do not connect the outputs of your two amplifiers together through the speakers.

Q I recently bought a home and am looking to set up a sound system throughout my house as well as outdoors. I need to buy speakers and I can take care of that. My dilemma is this: without a tremendous amount of money to spend on it, I'm looking for a device (I've seen it before) that is much like a splitter. From a central stereo, wires go to this splitter, and from the splitter to different sets of speakers (including, if wanted, outdoor speakers). This device is able to send sound to any one, two, or all sets of speakers, as you would need for entertaining, at the touch of a button or buttons. I would ask the owner of this unit, but, he's my EX-father-in-law and we are not on speaking terms. Are units like this still being manufactured? And, if so, where could I find it? I live in the Cherry Hill / Pennsauken / Marlton area of New Jersey. I am willing to travel to pick up this piece if need be. Please help.

A There are lots of companies that make these "switchers". The least expensive use mechanical push button switches, and the better ones use electrical switches that activate solenoids. They are usually the kind of thing that professional installers use, but you can do it yourself. Radio Shack has the mechanical ones, and the rest are supplied by numerous companies. Here are some links to get you started: http://209.150.0.195/xan_remote_control.html,    http://www.russound.com/.

Q I have a Sansui RZ-6100AV Pro Logic Receiver, a CLD-S180 LD, and a DVD-ROM in my computer. Because my receiver does not have AC-3, I had to connect it to the RCA analog outputs of the DVD card instead of the digital output. That caused a stereo effect instead of a surround effect. Is there any way I can alter my receiver to decode AC-3 or install an AC-3 input socket on my receiver? Another question - does my receiver need a special jack for a subwoofer? (I have only L,R,Center and Rear outputs.)

A You can have your receiver modified to decode Dolby Digital (AC-3), but it is the same price that a complete DD/DTS receiver can be purchased for (about $600). You are getting surround sound even with the stereo signal though, since that is what Pro Logic uses to give its version of surround audio. So, just spend the money on a receiver with everything. Use the Sansui in a different room.

Q The following is an excerpt from a Dolby paper: "Creating a ".1" channel. While 5.1-channel film sound mixes are created to take proper advantage of the ".1," or low-frequency effects (LFE) channel, you should think twice before creating such a channel for material originally produced without one. Keep in mind that Dolby Digital's five main channels (L, C, R, LS, RS) are all full-range, and the LFE channel does not extend frequency response. It exists primarily to enable more powerful low-frequency special effects for film soundtracks. Also, remember that all Dolby Digital decoders offer bass management, a feature that enables the consumer to direct extreme low frequencies to a subwoofer, or to any of the speakers in the system (such as L and R) that can reproduce low bass. When bass management is used, a ".1" track derived from a four or five-channel mix may cause unwanted summing or cancellation of low frequencies in a subwoofer. Delivering a quality track with five full range main channels, which allows the end user to optimize bass management to his specific playback system, is the best way to assure accurate delivery of the sound intended by the program's producers. A final factor to consider is that the ".1 track" is ignored by Dolby Digital's downmixing process, so it should include no vital information that would be missed in mono, stereo, or Pro Logic playback."

This info is obviously intended for audio production staff, but I thought the Secrets readership might be interested, seeing how many questions you receive about subwoofer connections, subwoofer outputs, and the LFE channel.

A Thanks. We appreciate this interesting info, and I am sure our readers do too.

Q I have a pair of EPI 320 front speakers. Any suggestions for a compatible center speaker? I understand a good match of timbre/tone is desired, and I'm finding this difficult to determine (short of taking the speakers with me into the sound rooms). Most retailers here have a no-return policy on speakers. I have listened to the Bose 245 (I believe it was), Energy E:XL C, and a JBL (all around the same price). Any other advice would be appreciated.

A What you need to look for in the center channel speaker (with any speaker for that matter) is neutrality, meaning that it does not emphasize or de-emphasize any particular area in the audio spectrum. Even if your front left/right speakers are somewhat non-neutral, it would be better to go ahead with a neutral center speaker, because voices go to the center, and any unnatural character to the sound of the human voice is very noticeable. Mirage and B&W make some of the best, most neutral, center speakers I have encountered.

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