Audioclinic (Audio magazine, Jan. 1968)

by Joseph Giovanelli

If you have a problem or question on audio, write to Mr. Joseph Giovanelli at AUDIO, 134 North Thirteenth Street, Philadelphia, Pa. 19107, USA. All letters are answered. Please enclose a stamped, self-addressed envelope.

Hum from a phonograph

Q. I would appreciate your advice.

My equipment consists of a record changer, a receiver, two tape decks, and two speakers.

Frequently, when playing the phonograph, I hear a loud hum from one or both speakers. At first I thought that the hum might be caused by loose jacks.

When I wiggled the jacks which lead into the turntable, the hum temporarily disappeared, but would soon return.

I also noted that touching the jacks at the other end caused the hum to momentarily disappear.

I have tightened the ground connection from preamplifier to turntable, but to no avail.

What could cause this problem? Would it have anything to do with a ground?

-Roy W. Hoffmann, N. Bergen, N. J.

A. I would say that "grounds" are the most likely cause of your hum trouble. If you are using commercially made, molded cable, it is possible that the shield has come loose from the shell of the connector where it cannot be seen. Replace the entire cable or at the very least, replace the defective connector.

The typical connector used in home music systems employs an outer shell, or skirt, as a ground connection. It is designed to have a broad contact surface to ensure a good ground connection. However, it often happens that the shell expands, leading to poor contact with the shell of the female connector mounted on the equipment chassis. This poor contact can result in hum. Squeeze the shell of the male connector slightly with pliers until it fits snugly when re-inserted. This remedy should improve the situation.

It is also possible that one of the ground lugs in your phonograph cartridge has become loose. Remove the lug. Squeeze it lightly with pliers so that it fits over the cartridge clip more snugly.

In addition, hum can result if a ground connection made to the input connector inside your receiver is improperly soldered.

The same holds for the grounds inside your record changer.

Harpsichord effects from a piano

Q. Can you suggest some sort of equalization circuit to simulate the plucking of a guitar or the sound of a harpsichord, using the input from a piano?

-Joseph C. Cavella, Hollywood, California

A. To achieve the effects which you are seeking, I think that you do not need an equalizer as much as you need some form of a square-wave generator.

The harpsichord is rich in harmonic content. Such harmonics can be produced by square-wave oscillators. Your problem is further complicated because you want to obtain your input from something other than pre-tuned oscillators which are always running or at least ready to run at a definite frequency. I believe you will have far less trouble if you change your approach from the use of a standard piano to the approach taken by electronic organ designers. If you continue to use the piano idea, you will then have to use some kind of limiting to drive a multiviibrator whose frequency will be determined by the frequency generated by the piano. However, this would mean that only one tone at a time could be generated. Chords would represent a real problem. Further, I am not at all sure you could generate the transient wavefronts needed in order to simulate the harpsichord. Therefore, I once again suggest that you follow electronic organ practice for the generation of percussive sounds.

You will need to construct only those parts of the electronic organ circuit which would fit your particular requirements.

Circuits of the type under discussion can be found in texts dealing with the design philosophy of electronic musical instruments. I suggest that you consult such works because the subject is too complex to be dealt with here.

If you have some reason why it might be necessary for you to use your present piano keyboard, you possibly do exactly that if you are able to locate an Organo inside or outside "setup." What these units are is a bracket with rods protruding from it. Each rod, when the bracket is mounted properly, extends to touch a particular piano key.

When the key is depressed, spring loading behind the rod enables it to move and actuate a switch.

The outside "setup" mounts above the keyboard and attaches to the piano by an easily-removable clamping arrangement, designed to fit most any style of piano.

Where the system is to be permanently installed, the bracket is different, and is called the "inside setup." It fits inside most pianos. The exact operation escapes me, but electrically it does the same thing as the "outside setup" does.

This equipment is no longer manufactured, but it was intended for the Organo, a small organ for limited-space applications, activated from a piano keyboard. I have described it here with the thought that you might come upon one by some good luck.

If space is no problem, you might just as well use a standard organ keyboard.

I do not know what all of this would cost. If the price rises high enough, it just might pay you to investigate the possibility of constructing a harpsichord kit. That will present you with a challenge of another kind.

(Audio magazine, Jan. 1968)

Also see:

What's New in Audio

AUDIO EQUIPMENT PROFILES (Jan. 1968)

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