Q&A # 195 - August 16, 2000

Staff

Q You have mentioned that you prefer discrete output circuits rather than op-amps. Why?

A Some years ago, an article was published on this subject. It is entitled "Tubes vs. Transistors: Is There an Audible Difference?", Russell O. Hamm, Journal of the Audio Engineering Society, 43rd Convention, New York, New York, 1972 (http://www.milbert.com/tstxt.htm). In that article, he presented measurements taken on the type of distortion produced by tubes, transistors, and op-amps (chips containing transistors). He found that with triode tubes, the second harmonic was dominant, whether single-ended or push-pull. With pentode tubes, the third harmonic was dominant. With transistors of all types, the third harmonic was dominant. With op-amps (operational amplifiers), the third harmonic was dominant, followed by strong fifth and seventh harmonics. All even-ordered harmonics were suppressed. The key to understanding the significance of this is that even-ordered harmonics (second, fourth, etc.) are consonant, that is, they are easy to listen to, and are not unpleasant. Odd-ordered harmonics (third, fifth, etc.) are dissonant, meaning they are unpleasant to hear. With tube amplifiers, particularly triodes, distortion is not objectionable to our hearing. In fact, it can enrich the sound, even though it is distortion. Sort of like the harmony parts in a barber shop quartet singing louder compared to the main voice. This is why tube amplifier specs often list the distortion as 1% or even more, and why transistor aficionados say that tubes are not as accurate as transistors. Tube manufacturers just accept more distortion into the performance because it does not affect the enjoyment. They could rate the amplifier as having a lower output, taking the measurement on a lower part of the graph where the distortion is lower, but there is no point to that. The amplifier sounds great at higher output too, where the distortion is higher. For transistor amplifiers, the output spec is taken from the more linear part of the graph, because higher on the output vs. distortion graph, where distortion is higher, the sound is harsh.

Op-amps are used very often at the output stage of preamplifiers, because they are less expensive than discrete output stage circuitry. Since the power amplifier amplifies the bad with the good, I feel that op-amps should not be in this critical area, because they tend to have only the bad distortion (odd-ordered harmonics) rather than the "good distortion" (even-ordered harmonics).

So, what I would like to see is more product using discrete output stages, and in particular, I would like to see triodes used, driven in Class A. The 12AX7 is popular, but it is not as linear as the 6SN7. The 6SN7 is used as a driver in power amplifiers, and it requires lots of power, and gets hot. But, it is a dynamite triode, and it is used in a few high-performance preamplifiers. What I am hoping for is a surround sound processor/preamplifier with a huge power supply and something like the 6SN7 as the output right to the jack on the back of the processor. This would drive any power amplifier, and any distortion would have as its primary component, second-order harmonics. Since the 6SN7 is a dual triode, I imagine four of them could be use for eight channels. Or each tube could handle a balanced signal for one channel, requiring more tubes in the chassis.

Now, this story does not mean that op-amps are bad. No one would use them if they were. It only means that, in my opinion, discrete output circuits using a triode, run in Class A, would be better. Of course, it would be considerably more expensive. You won't see it in a mass market product. But, I would like to see at least a couple of high-performance processors available with this type of design.

Q I am considering some new speakers that have metal cones. Are there any problems with these designs?

A Aluminum cones are very good. There are several problems though. One is that they are more expensive to manufacture than paper or polypropylene. Part of this is due to having the aluminum crack during the process of forming it. This is made worse by the addition of magnesium to the aluminum in order to reduce ringing. Remember, the cone is shaped like a bell, and having the cone made of metal is asking for trouble. So, adding magnesium, which is soft, reduces the tendency to ring. But, if the ringing is properly tended to in the design, metal coned speakers can be superb.

Q While I've enjoyed my home theater for several years, we just this year upgraded to DD and DTS with a new Yamaha  receiver and a Panasonic DVD player. The system sounds great, but something I don't understand is why, with the package labels on both video tapes and DVDs indicating that sound tracks are encoded with DD and/or AC-3, why (in the case of tapes) I'm only getting Pro Logic out, and with DVDs, getting DD but not DTS. Is this a hardware issue? A connection issue? Or is it a non problem?

A Well, it's certainly not a non-problem because you are having difficulty with it. In the case of tapes, I think you are reading the production notes that are advertised with the movie, in other words, what it says on the marquee at the theater, rather than how the sound is stored on the tape. In fact, tape cannot hold DD or DTS (AC-3 is the old name for it). Only the digital media, such as laserdisc and DVD can actually have DD or DTS on them (some CDs have them too). For tape, the most you can get is stereo analog sound, and this gets decoded into Pro Logic by your receiver. With DVDs, you often have to go into the menu before you start the movie and select DD or DTS. It can very well default to two-channel sound (still DD), and your receiver will again decode this as Pro Logic. It can also default to DD even if there is DTS on the disc, so you have to select this if you want it to play with the DTS soundtrack.

Q I have an older Yamaha receiver rated at 100wpc hooked up to a pair of Definitive Technology BP2000 speakers. I'm sure you know this, but each speaker has its own built-in 15'' 300 watt powered subwoofer. When I listen to music and increase the volume up towards 11 o'clock and higher, there comes a point where the bass volume stops increasing and everything else keeps getting louder and actually flattens out. Why does this happen, and what are some solutions? Do I need higher power subs, a higher power amp, or both? Any advice you could give would be great.

A The Def Techs use an amplifier built-into the enclosure that powers the 15" driver. It downconverts the incoming speaker level signal to a preamplifier level signal that drives the Def Tech amplifier. The midrange driver and tweeter are powered by the amplifier in your Yamaha. At a certain point, the Def Tech amplifier runs out of power, and there is probably a limiter in there to keep the volume to the subwoofer from getting any louder. In the meantime, however, your Yamaha can further increase the volume to the midrange and tweeter. If you want more bass, then get another subwoofer, probably something like a Velodyne HGS-18. It has 1,200 watts and the 18" driver will move enough air to satisfy you.

Q I have a Sony STR-DE315 stereo receiver. After purchasing it I found that it has no phonograph inputs. My turntable has no preamp. I have asked Sony for suggestions, and they only want to sell me a new turntable.

A What you need is a phono-preamp. This is because phono cartridges don't have enough voltage to use with regular inputs on a preamp. Most phono preamps will handle moving magnet (MM) cartridges (output several millivolts), but if you have a moving coil (MC) cartridge (output less than 1 millivolt), the output may not be enough for some phono preamps. There are lots of phono preamps out there. It all depends on the quality you are looking for. The Musical Fidelity XLPS ($270) will handle MM and MC, but you should check out several before deciding which one to buy.

Q Recently I have been thinking of upgrading from my old Technics DD receiver to a receiver that has pre-outs so I can do outboard amplification on at least the front three channels. I am considering buying a receiver in the $1K range. I have in mind the Marantz SR-7000, or the NAD-T760 (both for around $800 each, in my area). Although the NAD only has 60W/ch, it has been recommended by our local retailer over Marantz, saying that its is a superior product compared to the Marantz. My speakers are Infinitys with around 89 dB/w/m sensitivity. I couldn't really tell much difference in sound between these two receivers. Also some time in the near future I am planning on spending around $2K for outboard amplification. Here again I am considering three options. 1) using the Adcom 5503 for my fronts + Adcom 5500 for the rears ($1900 total for 200Wx5). 2) Rotel RMB-1095 ($1800 for 200Wx5). 3) Parasound HCA2205 ($1999 for 220Wx5). The specs on the Adcom say a THD of 0.18% which bothers me, although I like the sound of MOSFET amps. The other two I believe have Bipolars in their output stages. Although I haven't heard the Rotel and the Parasound, I have compared bipolar and MOSFET Adcoms, and I like the MOSFETs better. All three amps have received raving review on your site as well as other sites. This has made my decision even more difficult.

A I think you already answered your own question. Get the Adcoms to give you 200 watts in all channels. Don't worry about the 0.18% THD spec. Lots of things affect this specification, including the overuse of negative feedback, which actually makes it sound worse, even though the THD spec is better.

Q I have a pair of Paradigm ADP-150s and I am wondering how they will work in the new house that I am building. Also, I was wondering whether or not I should replace them with a pair of direct radiating speakers. My question relates to the performance of my rear surround speakers in a three-wall listening room. The layout of the living area is much different from my current residence. The new living area is basically a perfect square with a front wall and two side walls, but the rear of the room opens into the kitchen area, therefore, no rear wall. In fact, the ceiling is vaulted as it enters the kitchen. Will the dipoles be able to fill the rear with sound without a rear wall to reflect the sound towards the listening area?

A Dipoles can perform very well in an open area, because, don't forget, the front of the speaker is still radiating directly. It is when the dipoles are too close to a wall that they have real problems. So, just try them out. I suspect you will be fine.

Q 1) How do I balance the volume level of a home theater setup with four powered subwoofers? Do I still balance them equal to the rest of the speakers? (It seems this defeats the purpose of the extra subs as the levels are so low to keep them balanced.) 2) Can I split the line level sub-out from my receiver to all four subs without degradation of signal or sound? (I hear of multiple sub setups but have never heard how they are wired.)

A If you have all the subwoofers being driven by one output channel, then you should probably just stack them together in one spot, in a set of two and two side-by-side rather than all four one on top of another (because of the danger of the stack toppling over). If you have them wired as two sets of two, with each set being driven by one of the front left and right channels, then put them on the right and left sides of the front wall. If you have each sub being driven by a different channel, say front left/right, center, and rear, I would suggest still keeping all the subs in the front, because if you put one in the rear, you will get significant cancellation. Just use a low-pass frequency of 50 Hz, and there will be no directionality. With all the subs connected to one output, you could very well have a problem, as the impedance that the output sees will drop considerably, and this taxes the driver stage. You might end up having to use another preamplifier in between the sub-out on your receiver and the subwoofers, to boost the voltage.

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