Noise from an Unterminated Phono Stage
Q. When I set my receiver to "Pho no" and turn the volume up, with no cartridge connected, there is a lot of noise. Is this a reflection of the receiver's phono S/N ratio?
-Richard Vin yard, Jacksonville, Fla.
A. Electronic devices produce more noise when their inputs are not connected or "terminated" than when they are terminated. Thus, it is not surprising that you hear a considerable amount of background noise in the situation you describe. This noise does not represent the amount of noise which will be produced by the phono graph stage when a cartridge is connected. The signal-to-noise ratio should always be measured with the appropriate cartridge connected.
Speaker Phasing In an Ambience System
Q. I have an ambience system consisting of a set of main-channel speakers and a set of rear speakers driven by a separate amplifier and reverberation system. One of my pairs of loud speakers is, however, not marked as to phase. How do I obtain correct phase between the front and rear speakers?
-Name withheld
A. Because the reverberant sound is likely to bear an irregular phase relationship to the main program, phase coherence between front and rear speakers should be of little importance. However, if the speakers used for the front or main channels are not marked as to phase, the phase relation between that pair could be incorrect, leading to very poor bass response.
The simplest cure for this is to experiment, using a monophonic pro gram source applied to both speakers equally. By alternately reversing the leads of one of the speakers, you can determine by ear when that monophonic signal has the most bass. This indicates correct phase.
Let's assume now that the rear speakers are the pair which are un marked as to phase. Again, you can experiment with the phase relationship as described above. In this instance, however, I suggest that you use the actual reverberation signal as the source, and do not make that signal monophonic.
VHS/Beta HI-FI And Digital Recording
Q. My friend claims that hi-fi sound on video Hi-Fi (VHS or Beta) is "digital." Is this true?
- Brian Grifman, New York, N.Y.
A. The audio process employed in VHS and Beta VCRs is not digital. It is definitely analog. Stating it simply, the audio is produced by placing two FM subcarriers within the video band.
These subcarriers are modulated by the audio signal. During playback the signals are demodulated and fed to the audio output of the machine.
However, digital audio can be re corded on a VCR using a "black box" processor such as Sony's PCM-F1. All of the processors with which I am familiar are external units which make the VCR totally dedicated to digital audio so that no video is possible.
All 8-mm VCRs have FM (sometimes called AFM) audio tracks. Many of the newer ones also have PCM sound tracks which can accompany the picture, and some of these models can lay down PCM audio in place of the video tracks, for a digital sound-only capacity of up to 24 hours per cassette. However, this PCM system's parameters (32-kHz sampling rate and 8-bit samples with analog companding, rather than CD's 44.1-kHz rate and 16 bit samples) limit its frequency response and dynamic range.
Comparing S/N Ratios
Q. There is a mixer/preamplifier selling for $130. In terms of its phono stage S/N ratio, how would this unit compare to a receiver, costing many times more, which boasts an 88-dB S/N in its phono stage?
-Richard Vinyard, Jacksonville, Fla.
A. To put it succinctly, you can't compare the units. This does not mean to say that the mixer can't actually have a better S/N ratio than the more expensive receiver. Remember that a receiver contains a great deal more circuitry and functions than a mixer/ preamplifier does. For this reason alone, it makes sense that the receiver costs much more than $130. There is no way of knowing how much attention was given to the design of the receiver's phono circuit, compared to that given to the mixer's phono stage.
Let me give a rough example. I own a battery-operated mixer which cost around $40 when new. It is good enough, despite its conservative S/N spec of 56 dB, for me to easily use it with ribbon microphones. In fact, I measured it as having close to 80 dB S/N. So you just never know.
Ventilation and R.f.i.
Q. Will ventilating the cover of a pre amplifier by inserting a fine-mesh steel screen increase its susceptibility to r.f.i.?
-Tom Unger, Gardena, Cal.
A. Because no screen is 100% efficient in its ability to shield a piece of equipment from radio-frequency interference, I believe there would be some increased susceptibility to such interference by using a screen rather than a solid enclosure. This is especially true when using a steel screen rather than copper.
If you are not bothered by such interference now, chances are that ventilating the equipment will not create problems unless you are in close proximity to radio noise. But I can't help wondering why it is necessary to ventilate a preamplifier at all. Such equipment generates only a small amount of heat.
Fuse Failure
I would like to share my experience with the reader whose monophonic amplifier has been blowing its power fuse ever since the amp was incorrectly connected to an organ ("Audio clinic," September 1985).
I think Mr. Lapenna may be replacing a "slow-blow" fuse with a "fast-blow" fuse. All "brute-force" monophonic amplifiers have large power sup plies that draw much more than idling current when first turned on. This surge takes place when filter capacitors are charging to their normal operating volt ages. Because this surge lasts only a fraction of a second, it can blow a "fast" fuse but will not blow a "slow" one. I had the same problem as Mr. Lapenna's, and blew two fuses before I understood the reason why.
-Mark Schlegel, Lincoln, Nebr.
VU Meters Across Loudspeaker Lines
Q. How do I connect VU meters to my speaker lines?
-F. Keenan, Lima, Ohio
A. I assume your purpose is to know when your amplifier is delivering some given power level. You can buy meters designed for this purpose (such as Radio Shack's Cat. No. 42-2107, $24.95), and perhaps you might prefer to VU meters cannot be used in this way without external circuits to drop the signal voltages from the amplifier to the much lower voltages the meters are designed to read.
Figure 1 shows such a circuit, but requires some explanation. The 5.6 kilohm resistor across the meter terminals damps the meter movement to give it the characteristics of a typical VU meter. The 15-kilohm resistor and the potentiometer, in series with the amplifier's "hot" terminal and the meter, drop the signal voltage down to the proper level. By using one fixed and one variable resistor, we get a circuit which is easy to adjust and calibrate yet still gives the meter some protection against overload if the variable resistor should be set to zero. Note in Fig. 1 that the potentiometer is wired as a rheostat, with its wiper contact wired to one end terminal.
We now need to use Ohm's Law to calculate the value of the pot and to calibrate our circuit. First, we must calculate the voltage across the amplifier for the signal level which we wish to have read "0 VU" on our meter. The formula for this is V = \Px Z where V stands for voltage, P for power, and Z for the impedance of the speaker. For example, an amplifier delivering 20 watts into an 8-ohm speaker would develop 12.7 V (N 20 x 8 = \ 160 = 12.7).
A VU meter should read "0" for a signal of 0.707 V, so we need a circuit which will drop a 12.7-V signal to 0.707 V, a ratio of about 18:1. (Since we're using a variable resistance which we can calibrate later, ballpark figures are close enough.) If the ratio between the meter's impedance and that of the whole meter circuit is 18:1, we'll get our desired voltage drop. The internal impedance of a VU meter, as I recall, is about 5.6 kilohms; the 5.6-kilohm damping resistor across its terminals effectively cuts the impedance at This point in half, to about 3 kilohms. So the total meter circuit should have an impedance of 18 x 3 kilohms, or 54 kilohms. Subtracting the resistance of the damped meter and the 15-kilohm fixed resistor, we get 36 kilohms (54- 15- 3). To get a safety margin, use the next higher value commonly available, either 50 or 100 kilohms.
To calibrate the circuit, set the potentiometer to its maximum resistance, feed your amplifier--a test signal from an oscillator, test CD, or test record, and gradually raise the input level until an a.c. voltmeter across the amplifier terminals registers 20 V. Then adjust the pot until the meter reading climbs to "0 VU." Some cautions are in order: Use a middle frequency, such as 400 Hz, rather than a very low or high one, which might damage your woofers or tweeters. Feed the tone in short bursts-it won't serve any purpose to wreck your speakers or destroy your amplifier's output stage if you have miscalculated somehow. And be aware hat amplifiers have headroom, which allows them to deliver more than their rated power for brief bursts; if you've calculated your circuit so that your meter will read zero when your amplifier reaches is full rated output, be prepared to have the meter "pin" from time to time, and possibly even suffer damage, if you drive the amp beyond the meter's highest calibration point.
Fig. 1--Circuit for measuring signal levels in speaker lines with a VU meter.
(The formula for calculating the value of the pot, and calibration instructions,
are given in the text.)
Vertical Antenna Polarization
Q. What is "vertical polarization" of an FM signal, which supposedly improves automobile FM reception? Does it degrade regular reception in any way?
-Name withheld
A. Generally, automobile radio antennas are vertically mounted. FM transmitting antennas used to be arranged to transmit horizontally, meaning that these signals would be best received by using an antenna mounted horizontally. To overcome the problems of receiving FM signals in auto mobiles, stations now use a combination of the two antenna types so that any receiving antenna can be used. There is no signal degradation.
(Audio magazine, Oct. 1986, JOSEPH GIOVANELLI)
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