By Allen H. Burdick
Two inexpensive jack-mounted products from Benchmark Media Systems can be assembled into a very powerful test set we call The Audio MicroScope™. The Audio MicroScope™ is a tool to be used in setting up analog audio systems. Because of its extremely high gain capability, over 85 dB, it is particularly useful in hunting down signal interface problems, such as hum and buzz from RF interference, magnetic induction, and ground loops.
The first device in the test set is the MP-3, a single channel microphone preamplifier. The MP-3 is mounted on an XLR type jack and has a balanced input, variable gain from 26 to 65 dB, and a balanced output. It also has the ability to feed phantom power (with an external source of +48 V required) to a microphone. This can be useful even in this test set, particularly when testing microphone input cables. The performance of the MP-3 is excellent, with a 1 dB noise figure, a 2 kHz THD at A=40 dB of 0.005% and a bandwidth of greater than 200 kHz.
The second device used is the HPA-2. The HPA-2 is a high quality stereo headphone amplifier that has approximately 300 mA of very clean output capability per channel and is mounted on a 1/4" TRS jack. The HPA-2 is a improved headphone amplifier over the original HPA-1). Although the HPA-2 is a stereo device, in this application we will be using it as a dual channel mono amp. That is, both inputs to the HPA-2 will be connected to one half of the balanced output of the second product, the MP-3 microphone preamplifier.
Figure 1. MP-3 to HPA-2 Interconnect.
Mount an MP-3, an HPA-2, in a project box along with a DPDT switch for power, and four 9 V batteries or a PS-1. Be sure to include a switch for the +48 V if it is used. The Audio Microscope™ will allow you to "look" at your system and find minute problems.
To avoid damaging your hearing, since there is over 85 dB of gain available, you must be careful to have all normal audio fully removed from the path under test. Start with low gain and move cautiously upward as you begin the cleanup of the system.
Start at the beginning of the audio chain and listen to the microphone cable that feeds your console! Back terminate the wire, that is, replace the microphone with a 150 ohm carbon film or preferably a metal film resistor. Turn on phantom power, if available. Then turn up the gain and listen while you or someone else flexes the cable. If the cable is high quality, you will hear relatively little noise, if not, be prepared for a surprise. Next listen to the output of the first piece of equipment. It should be clean and free from hum. Move to the output of the next piece of equipment. Is the interconnect between the first two pieces of equipment free from mains-related hum and buzz? If not, work with the wiring and ground system, until they are. This, of course, assumes that the equipment by itself is clean, not always a safe assumption. Continue through the chain until the output of the final stage is just what you wanted.
You will find that the "Audio Microscope™" is one of the most valuable tools in your kit. In addition to using it with clip leads for direct connections, when used with a telephone pickup coil, Radio Shack Part # 44-533 it is most useful for doing a Magnetic Field survey of your equipment space. This piece of test equipment should be used hand in hand with the "A Clean Audio Installation Guide™" by Allen Burdick, a Benchmark Media Systems application note.
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
If you look at the back of any Benchmark product, you will find balanced XLR analog-audio connectors. As a convenience, we also provide unbalanced RCA connectors on many of our products. In all cases, the balanced interfaces will provide better performance.
We build our unbalanced interfaces to the same high standards as our balanced interfaces, but the laws of physics dictate that the balanced interfaces will provide better noise performance.
This application note explains the advantages of balanced interfaces.
Benchmark has introduced a new analog-to-analog volume control circuit that features a 256-step relay-controlled attenuator and a 16-step relay-controlled boost amplifier. The volume control has a +15 dB to -122 dB range in 0.5 dB steps and is a key component in the HPA4 Headphone / Line Amplifier.
Our goal was to produce an analog-to-analog volume control with the highest achievable transparency. We wanted to be able to place this volume control in front of our AHB2 power amplifier or in front of our THX-888 headphone amplifier board without diminishing the performance of either device. Our volume control would need to have lower distortion and lower noise than either of these amplifiers. Given the extraordinary performance of these THX-AAA amplifiers, this would not be an easy task!
This application note discusses the engineering decisions that went into the development of this new analog volume control circuit. The end result is a fully buffered volume control with a signal-to-noise ratio that exceeds 135 dB. THD measures better than the -125 dB (0.00006%) limits of our test equipment.