AUDIOCLINIC (June 1997)

Music Blending with DJ Mixers

Q. I am assembling a portable DJ-style sound system to play dance music. Is there a mixer that can help blend tracks without clashing the beats? I need a constant beat as songs change so that the dancers can maintain their rhythm. Also, are there CD players available whose speed can be adjusted the way a phonograph turntable's speed can be? And are there dual-transport CD players similar to dual-well cassette decks?

-John McKinson; Queens Village, N.Y.

A. The mixer has nothing to do with whether beats can blend properly as one song ends and the next one starts; all the mixer does is let you fade between songs or overlap them. The DJ-style mixers I've seen have at least one cross-fader to blend songs from separate program sources. Producing a smooth transition between tracks depends mainly on your skill and the music you select, not on your equipment: Even when you have a variable-speed player, you need skill to cue your music so that the two machines are playing "in beat" before you fade from one player to the next.

I don't know what CD players have the features you need, though I believe they do exist. To find them and the mixers you require, try musical-instrument or DJ-equipment dealers, not hi-fi stores.

Who Needs D/A Converters?

Q. What is a digital-to-analog converter used for? When is it necessary to use one in a system?

-Derek S. Collins, New Port Richey, Fla.

A. In recording, analog signals from microphones or other sources are converted to digital pulses. These digital audio signals need to be converted back to analog form before we can run them through pre amps, amps, and, finally, speakers or headphones in order to hear them. Components that play CDs, DATs, MiniDiscs, or DCC tapes, therefore, normally have built-in digital-to-analog (D/A) converters. Many of these components also have digital outputs for connecting external D/A converters in the pursuit of higher sound quality. Since many audiophiles use external converters, high-end CD players are often available as transports without internal D/A sections.

When 8 + 8 = 8--and When It Doesn't

Q. If I connect an 8-ohm loudspeaker to each channel of my stereo amp, is that amp loaded by 16 ohms?

-Sal Puzzanghera, Medford, Mass.

A. Each of your amp's channels is effectively a second amplifier, so connecting a speaker to one channel has no effect on the other channel's load.

If you were to connect two 8-ohm loudspeakers to one channel, then the load "seen" by the amplifier would no longer be 8 ohms. If you wired the speakers in parallel (both wires of each speaker connected to the amp), their combined impedance would be 4 ohms. If you wired them in series (one wire of each speaker going to the amp and the remaining wire connecting the speakers to each other), the combined impedance would be 16 ohms.

Lowering Speaker Impedance

Q. Can I improve my system's sound if I lower my speakers' impedance from 8 to 4 ohms? And if it would help, how can I do it?

-Dwayne Pellegrin; Houma, La.

A. Some amplifiers deliver more power into 4-ohm than 8-ohm speakers; however, changing your speakers' impedance might well lower their sensitivity, wiping out some or all of that power gain. There would be no other advantages. What's more, altering your drivers and crossovers for 4-ohm use is wildly impractical. I don't recommend it.

Using Two Speakers as a Center Channel

Q. My home theater system's center channel amp requires an 8-ohm loudspeaker. I do not have one, but I do have two 4-ohm speakers. Is it safe and appropriate to connect them in series to produce the 8 ohms that my amplifier calls for?

-Peter Nguyen; San Jose, Cal.

A. Assuming that you have two identical 4-ohm speakers that are magnetically shielded, you have nothing to lose by trying. (Placing an unshielded speaker near a TV screen will likely distort and discolor the picture, and some of these effects may persist even after you take the speakers away.) I think that the overall effect will be best if you stack the two speakers, one above the other. This will give you broad horizontal coverage while slightly narrowing the vertical coverage, both of which are desirable in a center channel. Be careful to wire the speakers so that they are in phase with one another and with the main speakers; otherwise, you'll lose bass and have imaging problems.

If the two speakers are not identical, wiring them in series may cause their impedance curves to interact adversely, producing audible aberrations in frequency response. Moreover, odds are that your 4-ohm speakers were not designed to match the sound of your main speakers. Together, these factors may make sounds change character as they move across the sound field; depending on the degree, it can be quite distracting. And I hope your home theater system lets you adjust the center channel's relative level, as your two-speaker array is unlikely to match your other speakers' sensitivity.

Replacing Old Filter Capacitors

Q. My old Philco radio from the 1930s picks up stations quite well, but it has a loud, almost unbearable, hum that comes on slowly as the radio warms up. The volume control setting makes little difference to the hum level, though by setting the volume rather high I can improve signal-to-noise ratio enough to make listening tolerable. Is this the nature of the beast?

-David M. Hudson, Dallas, Tex.

A. The hum you hear is not the nature of the beast but is probably caused by aging electrolytic filter capacitors. You'll find these capacitors in your radio's power supply section, which will likely include a power transformer and should include a rectifier tube, such as a type 80. The filter capacitor will be a can mounted on the chassis, probably a two-section unit. It might be a dual 8-microfarad capacitor rated at 450 volts DC. You'll need to replace that capacitor. Any replacements must have the same value (you'll find that microfarad, then abbreviated "mfd," is now abbreviated as "µF") and an equal or higher voltage rating. I doubt you'll find a capacitor that can physically mount to the chassis as the present unit does. However, because capacitors are smaller today, you may be able to find two tubular caps that you can easily place under the chassis. To install the replacement capacitors, detach the wires from the old can but leave it in place to maintain the radio's original appearance.

Most old electrolytic cans were polarized, using the can as the negative terminal. Usually the can was grounded to the chassis, but in many cases it was insulated from the chassis, with a ground lead going elsewhere. Positive terminals were (and are) usually marked with red dots or plus signs. Make sure your replacement capacitors' positive and negative leads connect to the wires that led to the equivalent leads on the old capacitor.

I'd also suggest replacing any electrolytic capacitors in the audio stages. That may give you a bit more bass and overall volume.

Foam Surrounds

Q. I recently helped my mother rehab an old pair of speakers. The foam surrounding the woofer cones had turned to dust, and we replaced it with foam from a kit ordered from one of Audio's advertisers. The project was successful, and the speakers sound quite good again. But now I wonder about the foam surrounds on today's speakers. Have the manufacturers improved this material at all, or should I expect to see the new foam also deteriorate over time? If the material hasn't been improved, why do manufacturers continue to use it? Can anything be done to slow the deterioration? What other materials can be used in its place?

-Daniel Pratt, King of Prussia, Pa.

A. According to Ken Kantor of Now Hear This, "Foam is typically more compliant than rubber and easier to glue to paper cones (but harder to attach to polypropylene). Foam also ‘rolls' differently under dynamic conditions, so any effects on response that are related to edge-termination effects will be different. Most modern foam is treated against the ultraviolet light and microbial agents that used to decimate the old stuff, and the life expectancy for premium foam is as high as for other parts of the speaker. Tropical climates don't help, however.

Summing Bass Without Simmering Amps

Q. In the March '95 "Audioclinic," you stated that most subwoofers add the bass from two channels together to feed one driver with one voice coil. Please explain how this is done, inasmuch as some amplifiers can't tolerate having their right and left outputs connected to each other.

-Allen Schramm; Tehachapi, Cal.

A. You're right about the amplifiers, but mixed-bass subwoofers don't connect the two channels directly to each other.

Usually each amplifier channel feeds a stereo crossover whose two bass outputs are then summed together. Without the isolation provided by the crossover and summing circuits, the amplifier's output stages could be damaged.

Amplifier Bypass Tweaks

Q. I'd like to tweak my integrated amplifier to improve its sound. For one thing, I could bypass the U-shaped jumpers that bridge the preamp-out/main amp-in jacks. Is this a job I can do myself on the circuit board, or is it better left to a technician? To further improve sound quality, I want to bypass the fuse in the loudspeaker output. Would this be a simple modification?

-August Timmermans; Bangkok, Thailand

A. If you're handy with a soldering iron, it should be a simple job to get inside the amplifier's chassis and solder a short jumper wire between the hot (center) connector of each channel's preamp-out RCA jack and its power-amp input jack.

Each wire will replace the friction link of the U-shaped jumper with a permanent soldered connection. I do not recommend you attempt to do this at the circuit board; that job is best done by a technician.

Whichever method you choose, I question whether you will be able to detect any difference in sound quality from bypassing a mere few inches of wire.

If your output fuse-holders are accessible, you can bypass the fuses by connecting a wire between the two terminals of each fuse-holder. But don't do it unless you've first made sure that these fuses are in series with the speaker output connectors and not in your amplifier's positive and negative power-supply rails. Fuses in these rails have no effect whatever on your amplifier's sound but do protect the loudspeakers from damage if an output transistor shorts out. (Such a short would cause excessive current to flow, which undoubtedly would burn out the loudspeaker's voice coil if the fuses were not in place.)

Hi-Fi VCR to Mono Dubbing

Q. When dubbing from a Hi-Fi VCR to a mono VCR, is a Hi-Fi track created or is the resulting copy mono? What is the best way to dub, Hi-Fi to mono or mono to Hi-Fi?

-Mona Williams, Houston, Tex.

A. A conventional mono VCR is incapable of playing a VHS Hi-Fi track or recording one. A mono VCR will play back or record only the linear mono track on the edge of the videotape, which is read by a tape head similar to the head in an audio cassette deck. If you dub from a Hi-Fi machine to a mono VCR, you'll get a mono recording, the quality of which will be determined by the audio response of your mono VCR's tape head and electronics. Typically, the mono edge track yields adequate but certainly not high-fidelity sound.

There are two pairs of audio inputs and outputs, for the left and right channels, on a Hi-Fi VCR; there is just one audio input and one audio output on a mono VCR. If you have a choice, dub from the mono machine to the Hi-Fi VCR by connecting the single audio output of the mono recorder to the single lead of a Y connector. The remaining split connectors of the Y are hooked into the left and right audio inputs of the Hi-Fi machine. The resulting copy will still be mono, but the losses in sound quality won't be as dramatic as they would be when dubbing between two mono machines or from a Hi-Fi machine back to a mono machine. Ideally, you should dub between two Hi-Fi VCRs--even with a mono signal--to best preserve the fidelity of the audio signal.

(Audio magazine, JOSEPH GIOVANELLI, June 1997)

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