Q&A - #5 - March 24, 1997

Q Is it possible to add a pair of powered subwoofers (stereo subs) in a DD or DTS setup to achieve the same bass output like the Velodyne 1800R, M&K 5000THX, or Defintive Tech. PF1800 or would it be better to go with one of the big three subs that I have above?

If you could choose a pair of subs at around $1000-$1500, which subs would you choose or recommend for especially accurate music and homethater reproduction?:

2 - Polk PSW300 or 150
2 - Velodyne VA-1512
2 - Velodyne VA -1210
2 - M&K V-125THX
2 - Antlantic 352THX PBM
2 - NHT SW2Pi
2 - DCM KX Sub2

A Because the large subs you mentioned can go very deep, I would go with one of them rather than two of the others that you listed in the column. The Velodyne F-1800R can go almost flat down to 10 Hz, and the M&K MX-5000THX has astonishing punch. If your significant other has a problem with the size of the sub sitting in the room, you should also consider the Sunfire sub. There is no comparison in output for its size. Later on, you can add another sub. Even with the big ones, addtional subs make a difference, because each one has to do less and less work as you add more subs.

Q I recently visited a friend's home that had a surround sound system. The stereo sounded great and the movies were fantastic. He had hired a professional, bought the whole system at once, and had the professional set it up. I can't afford that.

At present I need to buy a new power amp, pre amp, and can possibly afford a Pro Logic tuner. I am a newbie and am looking for a web site, or any other reference for that matter, that can help me move in the surround sound direction.

A You have come to the right place! Read our past Q&A for suggestions as to a receiver that is flexible and forward compatible.

Q Do all Amps, given similar power ratings and similar specs, sound the same in normal operating conditions? I know this issue has been debated for years, but I would like some input from Secrets. I think there is a difference, but I have a friend Mr. Knowitall who says you cannot tell the difference doing a blind test!

A There is an audible difference between some amplifiers, but you have to go beyond the published specifications to see why. Take a look at the following three photos of the 10 kHz, ± 10V square wave response from three different amplifiers. The first is from the MOSFET 32B which uses MOSFET output transistors and is rated at 250 watts per channel into 8 Ohms [click here for photo]. Notice that the vertical portion of the wave is almost straight up and down (a high slew rate) and the corners are sharp. Now look at the waveform of the Carver Silver 9t which uses bipolar output transistors and is rated at 575 watts/ch into 8 Ohms [click here for photo]. The vertical portion is more sloped (a lower slew rate), and the leading corners are rounded. Finally, look at the waveform from an SE-1 Single-Ended Class A Triode tube amplifier, rated at 7 watts/ch into 8 Ohms [click here for photo]. The vertical lines are quite sloped (a low slew rate), and irregular. The MOSFET 32B operates in Class A/B and is very fast. That is why the waveform is straight. The Silver 9t operates in class H, so it is slower. All the amplifiers produce essentially identical waveforms when a 10 kHz sine wave is put through them. But music does not consist of single sine waves. It has complex waveforms, usually made up of numerous harmonics. When harmonics from an entire orchestra of instruments pile up together, the waveform can look close to a square wave (square waves consist of the fundamental and all of its harmonics, with the odd ordered ones producing the sharp corners). Square waves sound harsh. An amp that has a square wave response like the one shown for the MOSFET 32B will reproduce that harsnhess more accurately (if it is in the original waveform), while an amp that has a rounded edge to its square wave reponse (like the Silver 9t) is more likely to have a smoother sound to it. The 300B triode is a special case. Even though it is operating in Class A, the waveform is rounded. That is what tubeophiles love. Even though it is not accurate, it sounds good, and it is the sensory experience that counts. The difference between solid state amps and tube amps is probably the most noticeable difference that can be heard in audio amplifiers.

In my opinion, as long as the amplifiers are well built, two amplifiers of different make will sound very similar, perhaps to the extent that no difference can be detected, if the square wave response is similar. However, the differences between amps with different square wave responses will still be subtle ones. You have to listen to a lot of amplifiers before you can begin to pick up on what these differences sound like, and even then, it is hard to describe what the differences are. The answer to your question is that there are very clear differences between many amplifiers that can be measured with electronic instruments, but that audible differences are not always heard. It is the same with audio cables.

Q I am thinking about purchasing MB Quart CTR center channel accompanied by MB Quart One's left & right speakers complimented by the MB Quart Domain Balcony surround speakers with a Sonance DL 15 subwoofer. Is this setup close to tonal accuracy and timbre matching? If not, what can I do or buy to get it as close as possible?

A For home theater, it is usually best to get the same brand of speakers all the way around since it is more likely that they are tonally balanced. For subwoofers, any brand will work because tonality is not noticeable in the low frequencies (< 60 Hz). The reason I mention this is that the subwoofer is absolutely critical for the best home theater experience.

Q I have not been able to find a layman's description of what oxygen free copper is, how its manufacturing process is different and the advantages of OFC over regular copper. Can you either describe the above or point me to a site or book that may have a decent explanation?

A Oxygen free copper means that it is manufactured in such a way that it does not contain oxygen among the copper atoms inside its bulk. The reason this is important is that oxygen is very reactive, producing copper oxide. This disrupts the copper crystals, which is thought to reduce the efficiency by which copper conducts the electrical signal.

Q I saw the new Yamaha DDP-2 AC-3 decoder mentioned in the Secrets 1997 WCES Report - January, 1997, and I would like to know if there will be a be big difference in sound quality compared to the DDP-1?

A In terms of AC-3 decoding, I think the sound quality will be the same. However, the DDP-2 has added some DSP so that the AC-3 sound can be enhanced, like it can with Pro Logic.

Q I've just started getting into home theater and audio as a hobby and this e-mail comes all the way from Hong Kong. In fact, I've just read my first issue of Stereo Review and Audio magazines. They are very informative magazines but mostly deal with issues only seasoned audiophiles understand. Which is why I find your magazine really great. You publish/answer highly technical questions as well as amateur ones (for amateurs like me). Now, I know that you have dealt with matching amplifier and speaker impedances. However, I still find some things confusing. In your Q&A # 2 - March 7, 1997, you answered somebody's question about matching 4 ohm speakers and an 8 ohm amplifier/receiver by saying that "When you use an amplifier with a 4 Ohm load, twice as much current, and half the voltage, is required to deliver the same amount of watts as into 8 Ohms, as per Ohm's Law (Voltage=Current x Resistance). Most amplifiers can handle a 4 Ohm speaker, but you have to be careful not to turn the volume up too loud. Otherwise the amp can overheat because it is attempting to deliver more current than it is designed for. You should have no problem with your 4 Ohm speakers if you are cautious." If that is the case then to deliver 8 watts of power through an 8-ohm impedance requires an 8 volt, 1 ampere current whereas to deliver 8 watts of power through a 4 ohm impedance requires a 4 volt, 2 ampere current. However, if you compute (Voltage=Current x Resistance) then you come up with 8 volts = 1 ampere x 8 ohms which is correct but also 4 volts = 2 amperes x 4 ohms which is incorrect. Also, if a lower voltage is required by the 4 ohm speaker from the 8 ohm amplifier/receiver, why doesn't the 4 ohm speaker get blown by too much voltage from the amplifier/receiver? You wouldn't plug a 110v appliance to a 220v socket. I know this is getting mathematical but I need to know if I am getting my math right and because I am matching a 4 ohm, 100 watt amplifier with an 8 ohm, 100 watt speaker. Does this mean I cannot maximize the power of the amplifier (although I have no intention of turning the volume to the maximum) because the voltage delivered by the amplifier is less than what the speaker can take and that the speaker will draw fewer amperes than the amplifier can deliver? I assume that an amplifer maintains a constant voltage and increases the ampere output as you increase the volume. If so, is there a standard voltage output to speakers by amplifiers?

A This is a case where, in my wish to keep it simple, I made things more confusing. When you turn up the volume, higher voltage is made available across the speaker terminals. No matter what the impedance of the speaker is, more current will be drawn, as per E=IR, or I=E/R. With the same voltage, twice as much current will be drawn by a 4 Ohm speaker as with an 8 Ohm speaker. And that is the problem. An amplifier has a rail voltage that is either fixed (e.g., 48 volts) for class A and class A/B, or variable (e.g., 20, 40, 60) for class H. Depending on the volume setting, a fraction of the rail voltage appears across the speaker terminals, all the way up to the maximum of the rail voltage if the volume is turned all the way up. With a modest amplifier, it cannot deliver the current demanded by a 4 Ohm speaker when that high voltage is placed across the speaker terminals, whereas the amp has no problem delivering the lower current when that same voltage is placed across the terminals of an 8 Ohm speaker. All amps are designed with 8 Ohms in mind, and many have no problem with 4 Ohm loads. Few are rated into 2 Ohms, and a handful can handle 1 Ohm for a short period.

Insofar as comparing current and voltage at the same power output (watts) between an 8 Ohm load and a 4 Ohm load, the square root of 2 (1.414) is used. So, if the voltage across an 8 Ohm speaker is 30 Volts, I = 30/8 = 3.75 Amperes. The power is voltage x amperes, which is 30 x 3.75 = 112.5 watts. For a 112 watt output at 4 Ohms, divide the 30 Volts by 1.414 = 21.22 Volts. I = 21.22/4 = 5.3 Amperes. Power = 21.22 x 5.3 = 112.5 watts. The point here is that for a 4 Ohm load, and the same wattage into the speaker, more current is demanded. Mass consumer amplifiers are the least likely to be able to handle this problem. The safest configuration is to use an 8 Ohm high efficiency (= or > 88 dB/w/m) speaker. The speaker that sounds the best, unfortunately, is not always like this. So, if your favorite speakers are 4 Ohms and low efficiency, get a good amplifier with a potent power supply. Other than this, any amp will drive a 4 Ohm load, but the volume control has to be used judiciously.

Q Can you please explain how to match pre-amps, power amps and speakers? Can you also give some example systems and their specs and explain why those systems are well matched?

A In general, the same brand of preamplifier and power amplifier provide good matches. However, part of the reason for separates is so that you can choose from all brands for the various components. For a preamplifier, look for a low OUTPUT impedance (e.g., 800 Ohms), and for the power amplifier, a high INPUT impedance (e.g., 100 kOhms). Don't be afraid to mix tubes with solid state. One of my favorite systems is a triode tube preamplifier and a solid state power amplifier. For speakers, 8 Ohms is safest for the amplifier, and the lower the OUTPUT impedance of the amplifier (e.g., 0.01 Ohm), the more control it will have over the bass and the less the amp will be reactive to changing impedance of the speaker across its frequency response. We have published a couple of recommended systems which are explained in their reviews.

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