Audioclinic (May 1979)

Meeting Test Standards

Q. I recently purchased electronic test equipment to test and repair my audio equipment. When I wish to test something according to the test standards like IHF A-202-1978, Section 2.5.2, which states: "Each output terminal whose primary function is to deliver signal voltage to a subsequent device shall be terminated with a load consisting of a 10,000 ohm, 5-percent resistor in parallel with a 1,000-pF, 5-percent capacitor." I constantly come across references to this arrangement when reading electronics literature, but I never get a thorough explanation of why this is done. Please clarify this for me.

- Michael Bartholomew; Hellertown, Penna.

A. As for connecting the output of a preamplifier, tuner, or whatever, to a standardized 10-kilohm load shunted by a capacitor, this is done to simulate general use conditions. It is assumed that most input devices have around 10 kilohms impedance and that the capacitance of the interconnecting cable is around 1000 pF. This standardized output load is not intended as a means of matching impedances, but to make a device under test appear to "see" a typical input circuit.

If your main object is to repair your own equipment or, perhaps, that of your friends, you need not be involved with rigorous details of this kind. Problems with equipment are generally gross ones calling for logical judgments rather than pure test setups.

If, on the other hand, you are in the business of evaluating equipment, then you should follow all the test standards precisely, so you cannot be accused of subverting them by not adhering to the letter of the law. Using the standard procedures in this manner assures that all tests can be repeated by any other person doing similar evaluation work.

Impedance & Amplifier Output

Q. In the "good-old days" of audio, tube amplifier specs stated one output power for the nominal impedances of 4, 8, and 16 ohms which was usually found on the back panel. The implication was that the output power would not vary with the load impedance.

Transistor amplifiers state their power at specific loads, in effect the power varies considerably with the load.

Why?

-Frank A. Bertolotti, Lucca, Italy; and Kevin Rowley, Rockford, Mich.

A. The output terminals on most tube amplifiers were composed of common 4-, 8-, and 16-ohm connections which were connected to appropriate taps on an output transformer.

When loads were connected to their respective output taps, power output would be substantially the same in all cases.

Solid-state amps are, generally, equipped with just two output terminals marked "common" and "hot." No matter what the impedance is, these are the only terminals to be used.

The output impedance of an amplifier is very low, therefore the lower the impedance of the load connected to the amp, the more closely it matches the amplifier's impedance, and the more power that can be drawn. A 4-ohm load is the minimum for most amplifiers.

Signal Boosters

Q. I am considering the purchase of a moving-coil phono cartridge but I am confused by the fact that some re quire step-up devices while others do not. With regard to those that require step-up devices, what is the difference between a step-up transformer and a pre-preamplifier?

- S. Campanile; Pleasant Hills, Cal.

A. Some moving-coil cartridges pro duce so little signal that they cannot drive a preamplifier to anything like full output. Under such conditions, background noise would be higher than it should be, and to overcome this problem, means must be provided to bring this signal up to a usable value.

Because the impedance of such cartridges is low, a step-up transformer is one way of obtaining the necessary voltage to drive the preamp. If the step-up is really high, this means a lot of turns in the transformer secondary and this capacitance will limit the highest audio frequencies which, in turn, can reduce transient response.

These losses are often subtle and may not be detected. Most transformers made for this purpose are well enough designed that this is not a problem.

It is also possible to increase the amount of signal through use of additional electronics to amplify the tiny voltage produced by the moving-coil cartridge. This added amplifier is placed between the cartridge and the conventional preamp, and is called a pre-preamplifier.

Channel Dropouts

Q. Sometimes when playing a record, one channel will be silent. If I lift the tonearm and wiggle the cartridge a bit, the sound will reappear when the stylus contacts the record.

What is wrong?

- O.B. Madenwald; Hines, Or.

A. It is important that good electrical contact be made between the wiring contacts on the cartridge and those on the tonearm. Clean them with a solution like WD-40, and if the channel loss or intermittent hum still occurs, then the contacts may be either bent or distorted. If you can, bend them to make sure they fit snugly enough to insure good electrical contact.

It is also possible that the small wires within the tonearm are broken and defective by having been moved back and forth so often. This could also cause intermittent operation and hum. This means that you will have to replace those wires.

Added Output

Q. I am considering the purchase of a moving-coil phono cartridge, but I am confused by the fact that some require step-up devices while others do not. Will a moving-coil cartridge that doesn't require a pre-preamplifier sound audibly better if one is used?

-S. Campanile; Pleasant Hills, Cal.

A. If a moving-coil cartridge doesn't need a step-up device and you elect to use one, then this would cause the cartridge to produce too much signal, leading to overload of and distortion in the preamplifier.

(Source: Audio magazine, May 1979; Joseph Giovanelli )

= = = =