Q&A # 24 - August 12, 1997
Staff
Q I read your comments on using 8 Ohm or 4 Ohm speakers with an amplifier, which I now understand. Thank you. Now, could you explain how I can tell if an amplifier I'm considering for purchase will be able to handle 4 Ohm speakers?
A Look at the specification page and see if the rating at 4 Ohms is twice what it is at 8 Ohms, i.e., 200 watts/ch at 8 Ohms, and 400 watts/ch at 4 Ohms. If it is a mass market amplifier, it is not likely to be twice the power, while a high-end product may very well be double. This is due to the power supply. An amplifier has a rail voltage, let's say 40 volts. When you turn the power up all the way, 40 volts are across the speaker terminals. For an 8 Ohm speaker, this results in I=E/R or current=40/8, which equals 5 amperes. This is 40 x 5 = 200 watts. With a high-end amplifier, using a 4 Ohm speaker, current will be 40/4, or 10 amperes, and 40 x 10 = 400 watts. Mass market amplifiers won't deliver that kind of continuous current, but rather some smaller value. Let's say it is 9 amperes, which translates to E=IR = 9x4 = 36 volts at clipping; 36 volts x 9 amperes = 324 watts. So, the closer the watts rating at 4 Ohms is to twice the value as it is at 8 Ohms, the more capable the amplifier is in handling the 4 Ohm speakers.
Q I am drawing very close to purchasing new speakers for my home theater and would like some opinions, comments, suggestions, warnings, etc. The speakers are Infinity's Overture 3 and matching center, CC3. My reasons for considering these are they're sheilded and have built-in powered woofer sections. A couple of months ago I was this close to buying a sub (M&K V-125) for my system, but my better half didn't shine on the idea of having a two-foot cube in the corner of our modest home theater. This left me with the "powered tower" option. I considered these also for their close proximity to our television. My question is, do you consider these speakers a good purchase and have you had an opportunity to evaluate them? I would greatly appreciate any comments you might have. This is a substantial purchase for me and I want something that will last for some time.
My other question concerns a project with my center speaker. My
current receiver is rather cheap and underpowered (awaiting upgrade:-), but
I want to make the most of it until funds allow otherwise. It has a pre-out
for the center channel which I wanted to use for better sound. In its
present setup, the center speaker is very noisy (pink noise?) with the
volume up and any of my sources (LD, CD, VCR) off or paused. I assume this
is due to cheap amplifiers. My question is, will adding an amp for the
center alone remedy this problem? Also, will it help the remaining amps (in
the receiver) be more efficient? I'm considering using an old car amp for the power. It's bridged rating is 125 watts into 4 ohms. It's in good shape and well built. Of course, I would convert AC to DC to power it and wire it to come on with the receiver. Is this an advisable setup? Are there any
considerations I should take into account before attempting such an upgrade?
This would only be a temporary fix until I upgrade my receiver. Any
suggestions on a model that would work well with the Infinity's?
A There are so many speakers with shielding these days, you certainly should not choose one brand just for that factor. The powered tower speakers are fine, but I have never found any of them to substitute for a good self powered subwoofer. Plus, if your wife is very concious of the equipment size in the room, it might be better to get some regular, and smaller, tower speakers, and a heavy duty subwoofer. If the M&K is too big, then look at the Sunfire Subwoofer. It is only an 11" cube, but puts out incredible bass. Since you like Infinity, consider some of their other tower speakers that don't have the powered bass driver. I am particularly fond of towers with one or two 5" or 6 1/2" drivers for mid and bass. They keep the sound under maximum control, and leave the massive air-moving to the subwoofer. Secondly, they will be less expensive than the powered towers, and you can put the difference into the more expensive subwoofer.
As to your center channel, the noise you hear may be present in both channels of the pre-processing stereo, and in phase, so the Pro Logic processor sends it all the to the center channel. Make sure your interconnects are not near any AC power cords and are as short as possible. The car amp should work, and it will indeed take some of the load off the rest of the receiver amplifiers, especially since the center channel is where most of the audio action is. That will help, even if the noise in the center channel cannot be eliminated. However, you will need to make sure that the 12 Volt DC power supply for the car amplifier can deliver enough current. A simple 12 V "wall wart" won't do. This is because car amplifiers take the 12 Volt car battery input and convert it to a higher voltage so that more than 36 watts can be delivered to a 4 Ohm speaker (I=E/R, or Current=12 Volts/4 Ohms, = 3 Amperes; 12 Volts x 3 Amperes = 36 watts). Doing this places large demands on the power supply for a high wattage car amplifier. When you upgrade, get a receiver with pre-ins and pre-outs for maximum flexibility in adding outboard decoders and power amplifiers.
Q I am wondering how I can shield my loudspeakers. I want to do this without buying new ones.
A It is difficult to answer your question directly without knowing your specific speaker system, but here are a few options available. The traditional way that speaker manufacturers use is the bucking magnet technique. Simply put, a second magnet is placed behind the speaker magnet to counter its magnetic field (reversed polarity, so that the same pole of the bucking magnet is against the same pole of the speaker magnet). Obviously, it would have to be held in place; otherwise, it would jump away from the repelling force of like poles. It's the speaker driver's magnetic field that causes the video distortion seen on your TV when an unshielded speaker
comes close to the monitor. Performing this modification to an existing
speaker will undoubtedly change the T/S (Thiele/Small) parameters of the driver, and may possibly change the overall sound of the speaker system. If you use this procedure, make sure you have access to measuring equipment to see if the driver specs change appreciably. Alternatively, a company called Orca sells a product called "Silent Shield". This product was initially designed to be used for particular Focal mid drivers and tweeters. They can be adapted to fit other tweeters and smaller mid drivers up to 5 1/2". The "Silent Shield" has little or no effect on the driver's T/S specs. Another way to shield a loudspeaker system is to line the speaker cabinet with sheets of iron or steel (1/8" thick). This blocks the magnetic field from escaping through the cabinet. The T/S parameters are not changed in this case, since the iron is far enough away from the speaker magnets. You could also put a steel cup around the driver magnet. When attempting to shield your speakers, make sure that you can reverse whatever modification you perform, in the event it doesn't work.
Q I have compiled this system based on reviews from Secrets of Home Theater, Home Theater Magazine, and Home Theater Buyer's Guide. I would appreciate ANY comments on it!
Sony KP53V45 RPTV
Martantz SR880 AV Reciever w/ AC-3
B&W DM603 Front Speakers
B&W DM602 Rear Speakers
B&W CC6 Center Channel
Sunfire True Subwoofer
Sony DVPS7000 DVD Player
Mitsubishi HSU760 SVHS VCR
Right now it costs $9,200 at MSRP, but will probably cost $8000 in the end. The only limiting factors are keeping it under $10,000, keeping the front speakers relatively small, and having a 53" RPTV. That seems limiting, but it just gets rid of the $20,000 projectors, speakers, preamps, processors,
and amps. Give me some suggestions on speakers, subwoofers, and televisions.
A Looks like a good system to me! The only suggestions I have are considering a TV with component video inputs. The Sony S7000 DVD player has the outputs, and it would be a shame to waste the excellent image quality that results from using component video. Also, keep in mind that you would not be able to copy DVDs onto your SVHS recorder. Copy protection on the DVDs prevents this.
Q I recently upgraded my Yamaha 2090 receiver with a Sony EP9ES AC-3 decoder. Since the 2090 has only 5 discrete inputs, I connected the
sub-out from the Sony to one sub input and the sub-out from the receiver to
the other input on the sub. (note - this method of wiring was suggested by
you in another question's answer)
I've found this connection to cause a few problems. Switching the
receiver to 5 channel discrete input does not relinquish gain control to the
decoder. That way, I control volume from both units. The manual recommends
setting the volume to -20 dB and using the receiver's volume control. All
good except that the receiver cannot control the sub-out from the decoder.
So, I shut it off for CDs or Pro-Logic, letting the receiver provide bass
output. Problem? AC-3 sources have LFE that I really want and do not want to
downmix. What can I do about the two volume levels and the LFE situation?
Just looking for the best hook-up and control situation. I sure wish there
was a way to change the setting on the Yamaha unit so the sub-out shuts off.
Do you have similar problems when using this connection method?
A When using Dolby Pro Logic, there will be no output from your AC-3 decoder to the subwoofer, so you can leave that connection in at the time. When using AC-3, the LFE output from the Sony will drive the subwoofer with its signal. The subwoofer output from the receiver will send low frequency signals that are in the decoded AC-3 sound running through the receiver to the subwoofer as well, but they just add to the bass in the subwoofer. You could disconnect the receiver input to the subwoofer when using AC-3 if you like, but we have never found the double connection to cause a sonic problem. The only difficulty is controlling the volume of the sub with AC-3 since the receiver is not routing the LFE output through its volume controls. We simply adjust the sound level of the AC-3 from the receiver then adjust the subwoofer volume with the volume control on the back of the subwoofer. A little inconvenient, but it works. I hope the next generation of receivers with a set of 5.1 inputs have the .1 jack too for sending LFE signals through the volume control with the rest of the signals. Right now it is only 5 jacks, not 6.
Q Is a pre-amp truely necessary?
A A preamplifier has several purposes. One is to a provide volume control for all channels (that includes stereo and 5.1 channel digital surround). A second is to allow switching among several source inputs (CD, AM/FM, TV, VCR, DSS, DVD, LD). Lastly, the preamplifier maintains a constant impedance at the output regardless of volume (ideally), so that it is only the volume that changes, not the tonality. I assume you are asking the question because of the volume controls that are present in some CD players. These are usually just inexpensive (about $2) ICs that can degrade the sound quality.
Q I'm in a real jam - should I go for an A/V receiver with S-Video, slightly higher in price, or go with one that has only regular A/V jacks for less? I have a DVD player, DSS, VCR, etc... The problem - the DVD and DSS for the best picture quality say to use S-Video. Now my TV has only one S-Video input jack, so an option is get an A/V receiver. Am I right or not?
A Both DVD and DSS reside in the component domain. Therefore their Chroma
(C) and Luma (Y) signals are separated (Y/C or S-Video) from the start. When
you use the composite cable, you are forcing the DVD or DSS to join the
signals together and then asking the TV to separate them apart. This will
not be a clean process, and a lot of the artifacts seen in TVs today is because
of the poor separation.
It is best to keep the signal of those two sources in the S-Video
domain. Some lower costing receivers might actually damage the signal
of those sources because the bandwidth of the S-Video switching in the
receiver might not be enough. By bandwidth I mean the size of the pipe that
the information must flow through.
Of the two sources, DVD is the one you want to keep in the S-Video
domain the most. You can look for an external video switcher or
manually switch the two inputs. You should base your purchasing the
receiver on the overall sound quality.
Q What would be the reason for using a Dolby Digital rear channel delay other than 0 (except what is needed for time alignment)?
A The Dolby Digital (DD) delay is supposed to be 15 ms less than what you have
Pro Logic set at. For example, if you have used the standard Dolby Surround
(DS) formula that comes in most manuals and set your DS delay to 25 ms, then
your DD delay would be set to 10 ms. If you have used the time alignment
feature in your (Meridian) processor, then you leave the DS delay at 15 ms (Default)
and DD delay at 0 ms. (Default)
Q I am looking for a reasonably priced A/D converter to handle a single stereo audio input with a Toslink and/or coax digital output. I would like to be able to ditch my current analog Lexicon CP-1+ Dolby Pro Logic
processor and only use my Sony SDP-EP9ES digital processor for all
Pro Logic and Dolby Digital processing. All my source devices have
digital outs (DSS, LD, DVD) except my S-VHS VCR, hence the A/D requirement
and the need to keep the Lexicon for now.
I am also using a Sony SBV-3000 matrix switcher for all my video and
analog audio switching. A processor loop out of the switcher connects to
the Lexicon analog processor and loops through the Sony digital processor
to the amps. All the digital audio outputs are connected directly to the
Sony processor.
A Your Sony digital processor should have a built-in A/D converter that
should be more than adequate for your S-VHS machine. I am currently
using the Meridian 562V (A/V switcher) that has a built-in A/D converter, and
I use it only for S-VHS because the other sources are all straight digital
inputs.
Q I've got a question about the sound output of DVD players. We all know that every DVD player has a digital AC-3 out as well as
conventional Left and Right analog RCAs. My question is about the content of those RCAs. According to Dolby, any DVD player is able to downmix the Dolby Digital stream to a Left and Right Pro Logic Matrix, a straight stereo or even mono signal. Would it not be a true statement then, that such DVD players have an onboard AC-3 decoder of sorts? Is it in fact true that those of us not ready (financially) to replace our hard earned (and not bad sounding) Pro Logic equipment will still be able
to hear DVD soundtracks in surround (not just straight stereo)? I'm asking all this because it is obviously important to me but has yet to be dealt with in any meaningful degree by ads, salespeople, or magazines. Is this downmixing capability part of all DVD players or
is it something I need to inquire about with every model?
A The RCA outputs can have both AC-3 downmixed to two channel or a PCM (Pulse Code Modulated) track. Some DVDs will have a full fledged PCM track just like a CD or LD,
even though the PCM data take up space that could have been used for video. All DVD players have a
built-in AC-3 decoder, though not all will give you 6 analog outputs like an
outboard decoder. (One or two DVD players have the 6 analog outputs on the back of the player.) They have the decoder built-in to convert the AC-3 to two
channels, that will "act" like normal stereo PCM sound (except that it is compressed). This
will work with your Pro Logic decoder to give you surround sound.
� Copyright 1997
Secrets of Home Theater & High Fidelity
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