AUDIOCLINIC (Nov. 1989)

Adding a Single Speaker

Q. My receiver will accept two pairs of speakers. I'd like to use an 8-ohm stereo pair in one room and a single 8 ohm mono speaker in another. Can I run the single speaker from one channel, push the balance control to that channel, and press the mono button? Can I leave the second channel of the "mono" room unterminated?

- Marc Richman; Washington, D.C.

A. Yes, you can use a single speaker, and you have two options: One option assumes you will have all three speakers running at once, and the other assumes you'll just have the mono speaker running.

First, let's consider the case of all three speakers operating at one time. Set your mode switch to mono and your balance control to center, just as you normally have it for stereo. Thus, the listening room serviced by two speakers will get the benefit of both speakers and the added "feeling" which two speakers provide--even when you are listening monophonically. There is no problem about terminating the unused channel; it is effectively loaded by one stereo speaker.

If you are mainly interested in having the single speaker operating, connect it to one channel and move the balance control to favor that channel. Again, you don't need to be concerned about terminating the unused channel because the balance control probably kills all signal to that channel. If not, I suggest you terminate it with an 8-ohm resistor of suitable wattage.

Even here, one speaker will be "live" in the stereo room. If the balance control does not completely kill the unneeded channel, some sound will also be produced by the other speaker in this room. If you want silence in the stereo room, add a switch which will open the "hot" leads to both speakers in the stereo location.

Removing Vocals

Q. I am looking for a device or at least a way to remove or muffle the lead vocalist or instrumentalist on recorded music. What can I do to accomplish this? I am a musician and need to use these stripped tracks for practice.

-Tom Tuttle, San Antonio, Tex.

A. Although I have heard of a device which supposedly strips off the vocals from recordings, I don't know anything about it. Assuming that you have the necessary equipment, you probably can do this yourself. If the program source is stereo but the lead is placed on just one channel, all you need to do is copy the recording with just the channel which does not contain the music you want stripped. Of course, you also lose anything else which happened to be on that channel.

If the music source is monophonic, there's nothing you can do. If the recording is stereo and the material to be removed is centered equally on both channels, you have a fighting chance of accomplishing your goal. The final recording will, however, be mono.

What you require is a mixer with at least two inputs which can be assigned to just one output. You also need a device that inverts the phase of one channel of the music source so that its output is 180° out of phase with the input signal. Although such an inverter could be made from scratch, chances are that among the equipment you have on hand, there is either a cassette recorder, equalizer, or some other unit which just happens to invert phase.

To make this system work, connect the left channel of the music source directly into one mixer input. Connect the right channel of the program source into the input of the device which will invert phase. The output of that device is fed into the second input on the mixer.

Next, send the music to the mixer. Set the two mixer pots to equal knob settings, and adjust the master gain to a suitable level. Start the music. If you hit it just right, the lead vocal will be reduced or removed. If not, move the mixer control associated with the phase-inverted signal up or down. If this makes no difference, advance the volume control on the device which inverts phase. If this device is a cassette recorder, be sure to set its selector to monitor the input. Reset the mixer control to its original position and adjust the volume control on the phase inverting device up or down until you hear the least amount of signal from the lead which you are attempting to strip. If the lead increases in volume instead of decreasing, chances are that the device you are using to invert phase is not doing so. Try a different piece of equipment.

Once you get things straight, you'll notice a distinct decrease in the volume of the lead you are removing. You may never remove it completely because some of this sound will be found in the overall reverb, which will have random phase and hence cannot be cancelled.

Should the sound be "thin" in bass, you may be able to use an equalizer to boost the bass of one channel. Loss of bass is common because bass is often centered along with the lead.

If you only expect to strip sounds from phonograph records, skip the phase-inverting device and reverse the leads of one channel of your phono cartridge. Use the mixer as above.

I have been asked to do this trick many times during my years in the recording field. I hate to do it because the audible results are poor in most cases and can never be stereophonic. When possible, purchase specially prepared background tracks.

The backgrounds for many popular religious songs are often released with no vocals. These tracks are recorded stereophonically and are often of very high musical and sonic quality. There is sometimes an "alternate" provided, with a vocal which you can use for tutorial purposes.

Digital Remastering Of Old Recordings

Q. I've heard a lot lately about old recordings being re-mastered digitally for better fidelity. I am wondering what is done, aside from transferring the original performance from analog to digital.

-Joe Mazza, Long Beach, Cal.

A. Much of the digital remastering is done to permit the performances to be recorded onto CD. Beyond this, there's no telling what happens to any given recording between the moment it is removed from the archives until the time it finds a new home.

Perhaps the safest answer I can give you is that what happens to a performance depends on what the producer thinks should happen to it. For instance, I have recently come upon some really nice jazz trio performances which were much better sonically as analog recordings than they are after transfer to the CD format.

Sadly, I must therefore say that re mastering is not necessarily the salvation of old recordings. There's nothing at all magical about digital recording.

Any transfer is only as good as its engineers and producers allow it to be.

Assuming that the reissue is really better than the original, the engineer may bring this about in a number of ways. He may add bass, for the original master may have been deliberately left bass-shy to fit the groove limitations on a long LP side. Or the producer may add treble if he believes that the studio session would likely have been brighter than the final master tape showed. This can happen because of the limitations imposed by tracing distortion of LPs or by treble saturation on either cassette or open reel tapes. The producer and/or engineer may also kill treble via a filter or by rolling off all treble to reduce background hiss. (I'll accept hiss, so long as the music remains intact.) Many older recordings were made in very "dead" environments. Thus, the producer may elect to add some reverberation to simulate a real concert hall, or he may like the sound and just leave it as is. I have heard many recordings ruined by overuse of reverb!

Connecting a TV Set to a Stereo System

Q. I recently purchased a stereo TV receiver, assuming that it had high-level stereo output jacks which could be used with my component audio system. Instead, it only has two tiny, tinny speakers about 14 inches apart-obviously unsuitable for stereo! I am considering adding stereo jacks to the unit by tapping into the wires connecting the set to its speakers. I assume that the audio circuitry is all on printed circuit boards, making it difficult to tap directly into them. Do I need to match impedances somehow and, if so, how? Should I feed the signal directly into the high-level tuner or tape inputs? Will I need any additional resistors in the line?

-Randy Webb, Sweet Valley, Pa.

A. I surely thought I'd received the last of letters like this once stereo TV sets came along. Like you, I figured they'd all contain line-level output terminals so the set could be properly connected to component or even "packaged" systems. Oh well.... You must be careful when tapping signal from the speaker outputs. It is essential that the "low" side of the speaker be at true ground potential. I have seen many sets which were not designed that way. If your set is not usable as it is, connect an isolation transformer between the speaker outputs and the inputs of your audio system. This will prevent damage to either the TV or the audio system. You should be able to wire your cables so they will plug into the high-level inputs of your sound system.

When you use the loudspeaker terminals, the volume control setting on the TV set will affect the amount of audio which actually gets to your main system. If you set that control too low, you'll need to advance the main system's gain control too much and will risk picking up too much background noise from the electronics in the TV set.

Turning it too high may lead to distortion and may also make it hard to control the volume of the main system. Experiment to find a good setting, and mark it for future reference. You will probably also want to provide a method of turning off the internal loudspeakers when they are not needed.

In some circuits, it is possible to obtain signal by connecting your cable across the volume control terminals. Again, you can't count on the designers of TV sets to use standard audio volume control circuitry. Further, with modern ICs, volume is often controlled by negative feedback. Assuming that you can readily connect to the volume control and that it is wired in a conventional manner, you may still run into problems. The volume control could be wired ahead of high-frequency de-emphasis. I think this is less likely with a stereo TV than with older sets; nevertheless, you may need to supply your own de-emphasis.

You may also find the impedance to be fairly high at this point in the circuit. If so, you may be all right if you use short, low-capacitance cable between the set and the audio system.

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(Source: Audio magazine, Nov. 1989, JOSEPH GIOVANELLI)

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