AUDIOCLINIC (Nov. 1994)

Recording Bells Successfully

Q. I have had little success in making clean recordings of certain bells and gongs that accompany our monastic choir. I have used high-quality cassette decks. When I used Dolby B or C NR, the recordings were even worse than without noise reduction. I tried a Hi-Fi VCR and still could not make acceptable recordings.

I was successful with a DAT recorder and would like to get a second digital recorder so that I can edit. I am hesitant to do this be cause, given all the problems I have had with other formats, I'm not convinced that the MiniDisc or the DCC system will be usable. What should I do?

-Richard Young, Mt. Shasta, Cal.

A. I have had no experience with recording bells on either DCC or MD, so I cannot comment as to how well they will work for you. I have successfully recorded bells on open-reel tape, VHS Hi Fi, cassette, and even phonograph discs.

Bells are very rich in high-frequency harmonics, not to mention the transient produced as the clapper strikes the bell.

Even without asking, I am sure that most of your troubles result from your recording at too high a signal level. Never mind that the recording level meters don't indicate overmodulation. When you consider the huge amount of treble boost required when making cassette recordings, what appears to be a good recording level will surely saturate the tape when recording bells. I had problems recording bells on a VHS Hi Fi machine at first; reducing the recording level cured the problems.

You may have also closely miked the bells. The mike preamplifier, or perhaps the mike itself, may have been overdriven.

(Yes, this is possible with capacitor mikes!) I have found that, in most circum stances, a good sonic balance between the bells and voices and other instruments will be achieved when the level of the bells is well below that of other instruments or voices. In terms of absolute sound level, bells are perceived as quite loud even when they are really not.

Although a DAT recorder will work well you will still overload it if you push you levels. Because these machines work so well, use a second DAT machine for your edits if your budget allows it.

If your recordings are to be sold, keep in mind that the vast majority of buyers will likely not own DAT machines. Thus, you must come to terms with copying onto analog cassettes. By keeping the signal level produced by the bells relatively low, you will be able to make usable cassettes, even when noise reduction is used.

Apportioning Surround Amps

Q. I use two stereo power amplifiers in my surround setup, one to drive the satellite speakers while the other drives the center-channel speaker and subwoofer.

Would I be better off using one amplifier in its bridged mode to feed more power to the subwoofer, or should I use that stereo amp to drive a pair of subwoofers, one per channel? Of course, these subwoofers won' have the power that can be supplied to one subwoofer if I use the bridged mode. What would you suggest that I do?

-Wes Sterling, Omaha, Nebr.

A. I would definitely opt for the two subwoofers. Depending on your amp, you could get more power with the bridged amp feeding one subwoofer, but I think that the acoustical efficiency and better bass support from the dual subwoofers would offset this. (Either way, of course, you'll need an additional power amp for your center channel.) If you have a sufficient power reserve, you could keep the amplifiers wired as you now have them but drive two subwoofers from the appropriate amplifier, assuming that their combined impedance would not be too low for the amp. This would improve acoustical efficiency and increase some amplifiers' power output. If the two subwoofers' paralleled impedance was too low, you could connect them in series, but you would gain no power and would lose some damping. The bass will be less con trolled as damping is reduced.

Trouble in Treble City

Q. I have three tape decks. I've tried to set proper azimuth on all of them, and have checked their frequency response and found it excellent. Nevertheless, tapes made on one machine either lack highs or are too bright when played on the other decks. What can I do about it?

-Dia-Chan Tran, Aurora, Ill.

A. Did you check azimuth with a calibrated test tape? It would enable you to adjust azimuth and set up the play back frequency response in accordance with the standards used in modern cassette recorders. You may need two such tapes, one to match Type I tape EQ to the response of ferric oxide tapes and the other to do the same for chrome (Type II) and metal (Type IV) tapes.

Even if you aligned your three decks with a misaligned tape, they should then be aligned (or, rather, misaligned) alike, so alignment would not be the cause of your problem. Let us assume for this piece that your test tape is flawless. If your machines employ separate play and record heads, use the tape to align the playback heads, and then align the record heads to get maxi mum high-frequency output through the playback heads as you record. The next step is to adjust the record equalization of each machine for the flattest response with the brand and type of tape you plan to use.

Now see if recordings made on each of your decks will play back properly on the other decks. Make the first tests using no noise reduction. When there are few or no problems of high-frequency compatibility among your decks, make similar tests with each deck's noise-reduction circuitry turned on. If you achieved compatibility without noise reduction but continue to have problems with it, you must look into the calibration of your decks' NR systems.

This will require the aid of your decks' ser vice manuals.

When you have adjusted each machine, try the compatibility tests again. All should be well.

If you have a problem or question about audio, write to Mr. Joseph Giovanelli at AUDIO Magazine, 1633 Broadway, New York, N.Y. 10019. All letters are answered. In the event that your letter is chosen by Mr. Giovanelli to appear in Audioclinic, please indicate if your name and/or address should be withheld. Please enclose a stamped, self-addressed envelope.

Tape Deck Oscillation

Q. My cassette deck operates normally in play mode. However, when I press the record button, the VU meter's left- and right channel needles swing over to the extreme right and oscillate in that area. The sound system (connected to the deck) emits a repeating, rapid-fire sound that resembles an operating machine gun. What is causing this problem, and how can I fix it?

-Alex F. Soave, Plainview, N.Y.

A. The problem may be in your deck or in the way you have it hooked up to your system. If your system connections and switch settings are feeding the tape deck's output signal back to its input, you'll get exactly these results. Otherwise, several circuit defects in your deck can cause the oscillation you described. The most common one is defective bypass capacitors, especially (but not exclusively) those capacitors only active in the record circuitry. (If you plan to attempt your own repairs, get a service manual for your machine to help you identify circuit components and their purposes.) My first suggestion is to obtain a large capacitor, perhaps 2,000 µF, with a 50 V or higher rating. Place the machine in "record." Jump this capacitor across the connections to various bypass capacitors; if the oscillation ceases, you've just bypassed the defective capacitor.

Other things can also account for this oscillation. Be sure that all the screws that hold the circuit board in place are tight; sometimes these mounting screws also ground some components. Where such a ground is missing, the signal may still find its way to ground, but through a high-impedance path; this can lead to common coupling and a tendency toward oscillation.

Dirty switch contacts, poorly soldered connections, a cracked circuit foil, etc. can also cause oscillation.

Making an Omni Indoor FM Antenna

Q. I understand that the "T-type" antennas supplied with most receivers have an uneven dipole pickup pattern. Would it be possible to obtain more of an omnidirectional pattern by arranging two of these antennas so that they meet in the middle and are perpendicular to each other? Will this increase multipath interference or cause other drawbacks?

-Steve Hirsch, Windsor Locks, Conn.

Yes, more omnidirectional reception can be obtained by arranging two folded dipole antennas as you have de scribed. But although it's a simple matter to mount one such antenna on a wall, mounting the crossed dipoles is another matter. If you were to mount them on a wall in such a way that one dipole ran horizontally and the other vertically, you would only add multipath problems and not obtain quite the omnidirectional results you would like.

You will have to mount your two dipoles on a wooden frame and find a way to suspend it in the clear.

The antennas will combine into one feed line. This arrangement will cause a slight mismatch of impedance between the antenna and the tuner, but it should be of no consequence.

If space is a problem, you might try mounting one of these antennas vertically.

Because FM stations are both vertically and horizontally polarized, this mounting arrangement should permit the antenna to operate with a more or less omnidirectional pickup pattern.

However, if multipath is a problem, an omnidirectional antenna will just make it worse. The best way to deal with multipath is to use as directional an antenna as possible, aiming it at the clearest signal of the many bouncing around in the air. The signal you want may not be the strongest signal; sometimes the direct signal from a transmitter will be accompanied by many multipath reflections from the same direction, while a single, clean reflection can be found by aiming the antenna more or less away from the station.

Maintaining Head Azimuth

Q. How can I keep my tape heads from going out of alignment? I have a three-head cassette deck whose record and play heads are in a single block.

-Danny Tse, San Lorenzo, Cal.

This is most often a problem in frit decks whose head assemblies move and stop rapidly. The only remedy that I know of is to put a small dot of glue, nail polish, or Loctite on the azimuth adjusting screw, to hold it in place. Be sure to use something that you can remove when necessary. Nothing is really permanent, and when your heads wear down and need re placement, applying a tiny dot of "glue" will let you turn that azimuth screw to set up the new heads.

Playing Mono Phonograph Records

Q. Now do I obtain a mono signal from a stereo turntable? I plan to record some noisy discs and understand that setting my player to mono will help in this regard.

Do I wire the phono outputs in series, with this series combination connected to the phono inputs on my receiver?

-John Comer, Cincinnati. Ohio

A. I have found that, in most instances, rewiring a turntable for mono has little audible effect on background noise even though theory says otherwise. You will, however, notice a slight decrease in tracing distortion on some highly modulated discs, and the huge output pulses caused by warped discs will be reduced. This is especially true of 78s.

(Editor's Note: Switching your preamp to mono will give you most of the noise improvement but will help less with the tracing distortion and output pulses.-I.B.)

You actually want to wire your outputs in parallel, not series, but it's simple. Plug both phono outputs into an ordinary Y-connector, and feed its third terminal to one of your phono inputs. If you wish to feed both, you need a second Y to fan out from the first.

If you'll often switch between stereo and mono, you can mount four phono jacks and a single-pole, single-throw switch in a metal project box. One pair of jacks will accept the cables from your turntable; the other will accept a cable to your preamp's phono input. Wire each pair of jacks to the opposite pair, and wire the switch's two contacts to the "hot" terminals of either jack pair, so that closing the switch ties the two channels together. This will give you monophonic playback, even with stereo discs.

You will discover that there is very little change in frequency response between the mono and stereo arrangements.

Old House Wiring

Q. I am putting a new stereo system together-just a straightforward sys tem, with no home theater use in mind. But I'm concerned that the only available listening room in my small apartment has just one electrical outlet, which accepts two two-prong, ungrounded plugs. The wiring is very old; it is covered in fabric rather than plastic.

Will this outlet suffice for my setup? Also, is there a potential hazard to stereo components when they are connected to an ungrounded electrical outlet?

-Gregory Smith, Toronto, Ont., Canada

A. Most stereo systems require more plugs than the two in your listening room. But many devices include convenience outlets that can be used to supplement the two that are available.

I'm not nearly as concerned about the lack of grounding as I am about the old wiring. Old wiring implies that the circuits are poorly segregated. This means that several other outlets will share the same fuse or circuit breaker. This, plus the fact that the wire gauge is probably lighter than would be used in modern house wiring, will lead to voltage variations, depending on how many appliances are operating at any given time. When the operating voltage falls, the maximum available power output from your amplifier will decrease, possibly with an increase in distortion; this may or may not be audible. Most other components have regulated power supplies and so are less likely to be affected.

As you know, many audio components employ three-wire line cords whose plugs have a third terminal that automatically grounds them. These grounds often cause more audio problems than they solve; they can lead to ground loops, which cause hum and noise. Common adaptors let you plug three-prong cords into two-prong outlets.

To restore grounding, place the adaptor's grounding lug or pigtail under the screw that holds the cover to the outlet. If the box was properly wired, this screw will serve to ground the equipment. This setup also lets you disconnect grounds if you have hum problems: Disconnect all but one, then re store the grounds, one at a time, skipping any components whose grounding causes hum.

If you should discover that the outlet box is not grounded, ground one piece of your equipment to the nearest radiator or water pipe. Because of the arrangement of interconnecting cables, this will ground all other equipment. Choose the preamplifier or integrated amplifier as that one piece to be grounded.

(Audio magazine, JOSEPH GIOVANELLI, Nov. 1994)

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