AUDIOCLINIC (Jun. 1992)

Four Speakers on One Amp

Q. When specs for a power amplifier say "two sets of speaker terminals," does this mean the amplifier has four channels? And when four speakers are connected at once, does the power to each speaker stay the same or is it cut in half?

-Kevin Blackwell, Topeka, Kans.

A. All that the statement means is that the amplifier can power more than one loudspeaker per channel, unless the literature specifically says the amplifier in question is a four-channel unit.

Many stereo amplifiers have double speaker terminals, usually with switches that let you select either pair and, often, both at once. One pair of speakers may be in the main listening room, and a second pair located elsewhere.

When an amplifier channel drives two speakers at once, the load it sees is lower than either speaker's impedance. Two 8-ohm speakers become a 4-ohm load, which is usually no problem; two 4-ohm speakers become a 2 ohm load, which many amplifiers can not handle safely. Don't try to drive such a load unless the manufacturer states specifically that the amp can handle it.

If both speakers have the same impedance, each will receive half the available power; if their impedances differ, the one with the lower impedance will receive more power than the other speaker. However, when both speakers are connected, the available power will usually go up, because most solid-state amplifiers deliver more power into lower impedances.

Some amps deliver twice as much power into 4 ohms as into 8-ohm loads, some deliver about 50% more, and others deliver about the same into both impedances. So when you hook two pairs of speakers up at once (assuming their impedances are equal), each speaker may get just as much power as a single speaker would, or half as much, or something in between.

DAT or VCR

Q. I have been contemplating buying a DAT recorder but am wondering if an 8-mm video deck with PCM is comparable in performance to a DAT. I have read that the 8-mm format uses a 32-kHz sampling rate, which is the same as a DAT running in the 4-hour mode. Some 8-mm decks allow audio only recording, with recording times ranging from 8 to 24 hours on a single cassette.

What is your opinion about using a VHS Hi-Fi deck for audio recording in the 2- or 6-hour mode?

-John F. Beasley, Augusta, Mich.

A. When samples are taken at 48 kHz, we can expect the resultant frequency response to be flat to 20 kHz.

When the sampling rate is reduced to 32 kHz, frequency response is not likely to be flat very much above 12 kHz, with highs absolutely limited to 15 kHz.

How bad is this 32-kHz sampling rate? Most of our TV signals are sent via satellite at a 32-kHz sampling rate. Surprisingly good sound is possible.

The DAT system employs a 16-bit "word." The 8-mm system uses an eight-bit data "word," which should limit the S/N ratio to 48 dB. However, my 8-mm machines do better than that, because the system uses compression to improve its S/N.

From a theoretical standpoint, there fore, we can readily see that the DAT machine has some clear advantages.

One such advantage of the DAT system over 8-mm is that the protection against lost data is better.

Another advantage of DAT machines over any VCR that I know of-8 mm or VHS-lies in the manner by which the "Pause" mode is handled. In any video system it is absolutely necessary to maintain sync; there cannot be a blank part of the tape on which no sync is written. The VCRs that I know of maintain sync by moving the tape backwards a couple of frames (or about 1/15 S), so that there will be sync pulses to lock onto when you start re cording again. Thus, we have to anticipate this by waiting a short time after the last desired data has been recorded before pressing the "Pause" button. Otherwise the last note of music or the last word of a speech recording may be clipped off.

The DAT system is better thought out. When "Pause" is pressed, recording is immediately muted, but the tape continues forward for a frame or two.

Then it moves backward to the exact point it occupied when the button was pressed. Thus, you don't lose data and are ready to begin recording at just the right place.

Cassettes used for 8-mm machines are less expensive than those for DAT machines. The 8-mm tapes can, with some systems, hold more than can be accommodated on a DAT, making the price of the 8-mm tapes even more attractive.

The VHS Hi-Fi format works well, with S/N ratios of 80 dB and better and with frequency response flat to 20 kHz. Tapes are often less expensive for VHS than for DAT or 8-mm, and they can hold up to 6 hours of recording. There are tapes designed for 8 hours of re-cording, but I do not recommend these for VHS Hi-Fi use.

The error protection with VHS tapes is not good. Any tape edge damage will result in mistracking, which causes a fluttering background. At times you can compensate for this by adjusting the deck's tracking controls.

Long-Distance AM

Regarding Juan Ferry's question concerning the reception of distant, AM broadcast signals (April 1991), my recent experience might be helpful.

For many years I have relied on an old "boat anchor," the Hammarlund HQ129X receiver-reasonably sensitive, quite selective, and also useful for holding things in place in the San Francisco Bay area, where nature often tends to move the S-meter needle as much as a strong local signal does.

I found a unique device selling for less than $50, with which virtually any AM receiver that incorporates a built-in ferrite antenna can outperform the HQ129X on the broadcast band, even if that venerable receiver were hooked up to hundreds of feet of wire. It's called a "Select-A-Tenna." It requires no power and is about 11 inches in diameter and 2 inches thick. No wire connections to the receiver are needed; signal transfer is by induction.

Operation is simple. After tuning the receiver to the desired frequency, the knob on the Select-A-Tenna dial scale is set to about the same frequency.

Without having time yet to make measurements, I would believe the gain to be 25 to 30 dB. Rotating either the Select-A-Tenna, the receiver, or both provides excellent rejection of interfering signals where differences in transmitter direction exist. It can not only reduce adjacent-channel interference but identical-frequency "QRM" as well.

Mr. Ferry might also want to consider the Sangean ATS-803A receiver, which provides dual-conservation AM coverage from 150 kHz to 29.99 MHz, as well as the FM band. This sells for about $200.

Both products are available from the C. Crane Company in Fortuna, Cal.

(800-522-8863).

Bob Crane and his crew provide something sadly missing in many retail operations today-at any price-service!

-Lee Neidow, Kentwood, Mich.

A. The C. Crane Company offers a modification for the Select-A-Tenna so that direct wire connections can be made to receivers having external AM antenna terminals. Mr. Crane tells me that this hookup should be used for receivers whose AM antennas are not accessible or cannot readily be rotated-in other cases, the inductively coupled version will do a better job.

Definitions, Please!

Q. I have a few unrelated questions for which I would like clarification: What is a power isolation strip? What is an intermittent tweeter? What are imaging and soundstage?

-Jimi Phillips, Jr., Detroit, Mich.

A. A power isolation strip is a plug strip (a box containing a row of a.c. outlets, hooked to a single power cord) with extra protective circuitry. Its circuits may be designed to keep sudden line-voltage surges from getting through to the equipment plugged into the outlets, to filter out noise, or both.

An intermittent condition is a problem that comes and goes; an intermit tent tweeter would be one that failed some of the time.

Imaging refers to the ability of the equipment and recording to let listeners hear precisely where the instruments and performers were during the recording session, just as they would have heard had they been there. The soundstage is the space where the performers were; it is to the ear what a theater stage is to the eye.

(Source: Audio magazine, JOSEPH GIOVANELLI, Jun. 1992)

= = = =