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  • Measuring Mic-Preamp Noise

    Measuring Mic-Preamp Noise

    By Allen H. Burdick

    To perform valid noise level measurements, two specific conditions must be established:

    1. A 150 Ohm source impedance
    2. A 20 kHz measurement bandwidth

    The microphone preamplifier must have the proper source impedance at its input. If it does not, the preamp will amplify the noise of its internal bias resistors, resulting in much higher noise voltage. Typically, the easiest way to terminate an input on the Benchmark MP-4 is to engage the 20 dB attenuator, since that presents a 150 ohm source to the input stage. When measuring other preamplifier systems that do not have an internal attenuator, a 150 ohm metal film resistor connected between pins 2 and 3 inside an XLR type connector shell, such as the Neutrik NC3MX or Switchcraft A3M, will do the job.

    The use of an carbon film resistor is usually ok, but the carbon composition resistor should never be used due to a phenomena known as "excess noise" found in these resistors.  This noise is created by electrical currents having to jump over small but real physical spacing between the carbon particles which are held apart by the resistors binder material.  These days, the discrete carbon comp resistor is almost unavailable.  However, the thick film chip resistor and thick film resistor SIP or DIP packs are in fact carbon composition resistors and should be avoided.

    The second condition that must be met is the limitation of the measurement bandwidth to 20 kHz. Manufacturers never specify, nor do we really want to measure, the noise outside the 20 kHz audio bandwidth. Be sure the test instrument that you are using has an internal filter for that purpose. Audio Precision markets a 20 kHz "brick wall" filter for their Systems 1 and 2.

    The output noise of the microphone preamp will be the noise of the source resistance, -130.82 dBu for 150 ohms, plus the gain of the preamp, plus the noise figure of the preamp in dB. The expected noise of a preamp operating at 50 dB of gain would then be -130.82 dBu, plus 50 dB, plus the noise figure of the preamp, which is < 1 dB for the Benchmark MP-4, or a total of approximately -80 dBu.  Using bipolar 15 volt power the MP-4 has an output clip point of +27 dBu thus yielding a preamp dynamic range of 107 dB.

    The only way to get lower noise and therefore greater dynamic range (with a given power supply voltage source) is to operate both the mic and the preamp at much lower (i.e. cryogenic) temperatures. Unfortunately, at this point in the history of man, this is not possible.

    Also in Audio Application Notes

    Closeup of Plasma Tweeter

    Making Sound with Plasma - Hill Plasmatronics Tweeter

    by Benchmark Media Systems June 06, 2023 2 min read

    At the 2023 AXPONA show in Chicago, I had the opportunity to see and hear the Hill Plasmatronics tweeter. I also had the great pleasure of meeting Dr. Alan Hill, the physicist who invented this unique device.

    The plasma driver has no moving parts and no diaphragm. Sound is emitted directly from the thermal expansion and contraction of an electrically sustained plasma. The plasma is generated within a stream of helium gas. In the demonstration, there was a large helium tank on the floor with a sufficient supply for several hours of listening.

    Hill Plasmatronics Tweeter Demonstration - AXPONA 2023
    While a tank of helium, tubing, high voltage power supplies, and the smell of smoke may not be appropriate for every living room, this was absolutely the best thing I experienced at the show!
    - John Siau
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    Benchmark AHB2 Power Amplifier

    ChatGPT Reviews the AHB2 and Includes Listening Impressions!

    by John Siau June 02, 2023 2 min read

    What happens when you ask ChatGPT to write a review?

    I asked ChatGPT to review the Benchmark AHB2, and I was shocked at the results!

    It turns out that ChatGPT wrote a review by gleaning information from the many publicly available reviews of the AHB2. I recognized phases and sequences of words that had appeared in prior reviews, and yet the text was new, original, and well written. The ChatGPT review seemed to summarize the collective impressions of the reviewers without directly quoting any particular reviewer.

    It should be possible to ask ChatGPT to write a review on any well-known product and the result will be an AI-generated review that reflects the collective opinions of many human reviewers.

    These AI-generated reviews may be more useful than traditional search results!

    If I am not mistaken, ChatGPT seems to have golden ears!

    Here is my dialog with ChatGPT:

    John Siau PFP

    John Siau:

    Write a review of the Benchmark AHB2 complete with listening impressions.

    chatGPT logo


    Title: Unleashing Sonic Purity: A Review of the Benchmark AHB2 Amplifier

    Introduction: The Benchmark AHB2 amplifier has gained a reputation for its exceptional audio performance and unwavering dedication to sonic purity. In this review, we delve into the heart of this powerhouse amplifier, exploring its features, design, and most importantly, the listening impressions it evokes...

    Read Full Post
    ChatGPT Logo

    ChatGPT and John Siau Discuss Star-Quad Cables

    by Benchmark Media Systems May 30, 2023 7 min read

    Today, Benchmark's John Siau and OpenAI's ChatGPT had a conversation about star-quad cables. It turns out that ChatGPT got quite a few things wrong, but learned quickly when these errors were pointed out by John Siau.

    ChatGPT is quick to recognize mistakes, and quick to provide detailed corrections. In each of the first few corrections, additional errors were made. As each of these were pointed out, ChatGPT began to provide accurate information.

    Here is my conversation with ChatGPT: 

    John Siau PFP

    John Siau:

    How do star-quad cables reject interference?

    ChatGPT logo


    Star-quad cables are designed to provide improved rejection of ...

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