Metering 70 Volt Lines
A 70 Volt Meter Attenuator for Sound Reinforcement
By Allen H. Burdick
Introduction
It is often desirable to monitor the levels on distributed audio PA speaker lines, known as 70 volt lines. It's simple with the right tools. This app-note shows how.
Many times a sound reinforcement system that uses the industry accepted 70 volt interface system needs to be metered. Benchmark SPM-220 and SPM-320 meter systems as well as the RPM-1, VU-1 combination can easily do the job and allow the operator to see the audio levels both in VU (average) levels as well as in PPM (peak) mode. In other words by using the peak metering capability of the Benchmark meter systems, an operator can, at any place on the system, not just the amplifier room, see whether or not his amplifier hit clip.
The following is an attenuator that will do the trick.
Fig. 1.0 Meter System Attenuator
Also in Audio Application Notes
How Loud is the Distortion from Your Power Amplifier?
Would you put a Washing Machine in your Listening Room?
If the answer is no, you may be surprised to discover that the distortion produced by your power amplifier may be louder than the noise produced by a major appliance.
Don't believe me? Take a look at Stereophile's test reports:
We selected 7 power amplifiers from Stereophile's top list of recommended amplifiers.
We took Stereophile's "THD+N vs. Power" plots for each, and replotted the data in a format that shows the loudness of the THD+N at the listening position.
The results are shocking!
Amplifier THD+N is louder than expected!
The distortion from your amplifier may be louder than a washing machine on the spin cycle, or it may be totally silent. How does yours perform? The answer is hidden in Stereophile's THD+N plots.
This application note reveals the hidden truth:
"The Distortion from your Power Amplifier may be Louder than a Washing Machine!"
I know, it sounds crazy, but this is what the measurements show!
Interpolator Overload Distortion
Most digital playback devices include digital interpolators. These interpolators increase the sample rate of the incoming audio to improve the performance of the playback system. Interpolators are essential in oversampled sigma-delta D/A converters, and in sample rate converters. In general, interpolators have vastly improved the performance of audio D/A converters by eliminating the need for analog brick wall filters. Nevertheless, digital interpolators have brick wall digital filters that can produce unique distortion signatures when they are overloaded.
10% Distortion
An interpolator that performs wonderfully when tested with standard test tones, may overload severely when playing the inter-sample musical peaks that are captured on a typical CD. In our tests, we observed THD+N levels exceeding 10% while interpolator overloads were occurring. The highest levels were produced by devices that included ASRC sample rate converters.
Audiophile Snake Oil
The Audiophile Wild West
Audiophiles live in the wild west. $495 will buy an "audiophile fuse" to replace the $1 generic fuse that came in your audio amplifier. $10,000 will buy a set of "audiophile speaker cables" to replace the $20 wires you purchased at the local hardware store. We are told that these $10,000 cables can be improved if we add a set of $300 "cable elevators" to dampen vibrations. You didn't even know that you needed elevators! And let's not forget to budget at least $200 for each of the "isolation platforms" we will need under our electronic components. Furthermore, it seems that any so-called "audiophile power cord" that costs less than $100, does not belong in a high-end system. And, if cost is no object, there are premium versions of each that can be purchased by the most discerning customers. A top-of-the line power cord could run $5000. One magazine claims that "the majority of listeners were able to hear the difference between a $5 power cable and a $5,000 power cord". Can you hear the difference? If not, are you really an audiophile?