Q & A

Q&A # 77 - August 18, 1998

Staff

Q How can a speaker be bright or have a heavy bass response when its specifications say it has a response of say 30 to 20,000 Hz + or - 3 dB? Doesn’t this mean it should have a neutral response? I see so many reviews, and have myself auditioned speakers that were bright or had a heavy bass response. Nevertheless, the specifications say that it is flat from 30 to 20,000 Hz. How is this so? It seems to me that if a speaker is bright it will be out of the 3 dB range in the treble area. Could you please explain this to me?

A The specified response is tested in an anechoic chamber where there are no reflections from the walls, ceiling, or floor (in other words, no room effect). This is usually what causes heavy bass. As to the brightness, this is often the case with new speakers that need to be broken in. Other than that, brightness can be caused by harmonic distortion. Depending on how the response measurement is made, the output can include the harmonics when a single frequency sinewave is input. Also, standing waves in the room can add some to the response at any frequency, depending on where the microphones are placed, if other than the anechoic chamber is used for the measurements.

Q I do not understand how manufacturers of CD players and DACs can claim 20-bit resolution from 16-bit sources. Is this possible? I do not think that it is, and what we are hearing is not true 20 bits of resolution, but instead an "interpolation" of 20 bits (similar to line doublers and quaddruplers). Could you please explain?

A The 20 bit resolution applies to 44.1 kHz 16 bit CD recordings made with such technology as High Definition Compatible Digital (HDCD), where the player interpolates the 20 bits, and also to players that interpolate up to 20 bits using technology other than HDCD. It is, indeed, an interpolation, as you said. DD and DTS use 48 kHz and 20 bits in the original bitstream. It is not interpolated. The new DADs use 96 kHz and 24 bits in the original bitstream. No interpolation. Although interpolation does improve the sound marginally, it cannot compete with bitstreams that have the higher bit data to begin with.

Q What is your opinion on passive preamps? I am looking at the $200 remote-controlled Creek OBH-10 passive unit. As you can see, the unit is a bargain, and I wonder whether it offers any advantage (or disadvantage) over an active unit. I have only one source, and the Audio Alchemy DDE v3.0 DAC's analog output stage is powerful enough to drive the passive control unit. I can use the DAC to drive my power amp directly, but its volume control is digital. I'm not a fan of digital volume control, and I want to have all 16 bits at the output (the DAC's 6 dB digital domain attenuation for non-HDCD discs has been defeated in my case).

A The idea behind passive preamplifiers is that you eliminate a lot of circuits that would otherwise be in the signal path. You can put a simple ganged potentiometer in between the CD player and power amplifier, and it does work. In fact, many passive preamplifiers have the option of just using the volume control potentiometer in the signal path. But here are the problems: (1) the potentiometer varies its impedance as you change the volume (that's how it works). By changing the output impedance of your passive preamplifier, the tonality of the sound will change too. One of the things a regular preamplifier does is (hopefully) keep the output impedance as constant as possible as you change the volume. (2) If the input impedance of the power amplifier is not very high, the output voltage of the CD player could drop (clip) as you try to turn up the volume with the passive control, because the power amplifier input will attempt to draw more current than the player can supply. CD players do not have huge power supplies. A regular preamplifier has a power supply that can maintain the output voltage even when the input impedance is not very high. Somewhere in between is the passive preamplifier with a buffer at the input and output so that a minimum of circuitry is in the signal path, but constant impedance is maintained. I have both types of preamplifiers here in the lab (passive and active). I use the buffered output of the passive preamp, and it works very well. One of the nice things about it is that the volume control is usable through most of its range, because it is not changing (increasing) the voltage from what it is at the source (CD player). It's a very clean sound. However, I prefer the tonality of our active preamplifiers, which do not vary from source to source.

Q I have to run very long XLR cables from my surround processor to my power amp. I have to run three channels for main left, main right and the center channel. The length is 15-17 meters. What I would like to know is (1) Is this too long for an XLR cables to cope with? (2) Do I have to use expensive brandname cables? At $40/meter/pair, they would cost me some $1000. (3) What is the least expensive XLR cables that still give me a good performance?

A XLR (balanced) cables are for use exactly in your type of situation. They are used in professional sound systems up to 1000 feet long. Unfortunately, they are all expensive because they are not commonplace in mass market products, and therefore, not made in the kind of numbers that would keep the price low. You might be able to make them yourself using three solid core conductors wound together, and XLR connectors on the ends. However, impedance is an issue, so you will have to experiment.

Q I have a Sherwood Newcastle R-945 receiver with 5 (5 year old) Klipsch bookshelf speakers and a 10" Klipsch powered sub. I recently inquired of you what your upgrade priorities might be to enhance my system. You recommended buying a new sub (Sunfire), which I have done and am very happy with. And you also recommended an outboard 5 channel amp to add power. This is where the problem lies. While shopping for an amp I have been bitten by the hi-fi bug. I am now considering using the resources I was planning for a 5 channel amp (ATI, Sunfire, Parasound) to purchase a higher quality stereo amp (Krell, Proceed, etc.) and buy a matching 3 channel amp later. After which I will buy new speakers in the $3,000-5,000 range for a set of 5. My question is, as I go through this process and continually want to spend more money on higher quality gear, what is a good formula for the "Theory of Diminishing Return"? What I mean is, what is the audible difference between a good receiver and 5 channel amp ($3,000-$4,000) and a higher quality set of 2 and 3 channel amps with a pre/pro ($7,000-8,000 or more) driving the same set of speakers I may end up with (B&W, PSB, Sonus-Faber, Snell, Krix, etc.). If I sound confused, it's only because I am. However, it is great fun learning these things. I am thoroughly enjoying strolling through the better audio shops in my area, listening to all the different gear.

A What I have found is that for any particular component, the audible improvement is easily apparent as you go up to about $1,000. After that, the curve begins to flatten out, and the improvements, while still there, are more subtle. Your plan to start with two channel stereo, and then add more of the same amplifiers to give you 5.1 is a good one. That way, you won't have to put any of your previous components in the closet. It's a smart way to build an expensive system.

Q I have a question about the recommended speaker setup for DTS. I understand that DTS recommends five full range direct radiating (monopole) speakers and a subwoofer. What exactly do they mean by full range speakers, i.e., what is the minimum frequency response DTS recommends for a full range speaker? Also, how do they recommend that a sub is integrated into this full range system? I am assuming DTS recommends five full range speakers and a sub for the low frequency effects channel. Would this speaker setup not give a bass heavy response, since full range speakers by definition go much lower that 80 Hz?

A Theoretically, a full range speaker would respond reasonably flat from 20 Hz to 20 kHz. Only a very small number of (very expensive) speakers do this. The term is rather a loose one used in the audio industry, and refers to speakers having a flat bass response down to perhaps 40 Hz. The lowest octave (20 Hz - 40 Hz) is where the subwoofer comes in. Many floor standing speakers will give you a good response to 40 Hz. Bookshelf and mini-monitors are less likely to do this, but there are a few that will. Since DTS and DD have full range in all channels, except the LFE, the idea is to have bass response down to about 50 Hz, where directionality is still reasonbly there. If they responded only to 70 Hz - 90 Hz, the 50 Hz - 90 Hz info would all be coming from the subwoofer, and is directional, so this would mess up the sound stage.

Q I have a Zenith pro851x front projector. I just bought a DVD player, so is there any gizmo that I can add to my projector for it to support Super VHS (S-Video)?

A If the projector does not have an S-Video input or component RGB input, then no, you cannot, because the signal is always passing through the projector's comb filter.

Q My question is about my new CD player. I replaced my HK 8450 with a Parasound CD 1500. The Parasound has a smother sound when listing to a woman’s voice and highs, but it seems to me that I am losing some detail too. Is this me or could it be the differences in the two CD players?

A It is a difference between the two players. However, one player can sound smoother without losing detail. The lost detail could be just an auditory illusion. In fact, when one moves up to high performance audio, it can often sound a little lifeless at first. Then, you realize that what is missing is the distortion. Distortion can occasionally make it sound like there is more body or more crispness. However, once you get used to music that is clean, there is no going back. On the other hand, some CD players have more detail than others. It is one of those things you have to sit down and listen to for a good long session, to discover what the differences really are.