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Thread: The future of microphones & amplifiers

  1. #1
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    The future of microphones & amplifiers

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    I put this thread in the DIY section, because if you DIY the whole chain, you can design with your ultimate goal in mind.

    And what is that goal? Well, for most people now, that's a converter. So our goal should be to feed the converter a signal it wants. What do converters really want? Most of the IC converters use a +5V power supply, which is common for digital circuits. That means the desired input signal is not more than 5Vp-p, or about +5dBV = 0dBFS. That's right about the consumer operating level of 0VU = -10dBV.

    Most preamps are designed to have lots of headroom; another way of saying that is they are capable of a very hot output, +20dBV or more--sometimes much more. But it's clear to see that any preamp operating at +4dBu "pro" level (+2dBV) is going to need to be padded into the average converter chip.

    What's the cost of all this unnecessary headroom? Well, larger and more expensive power supplies and wasted power, mostly. And that's just the preamp!

    Again, let's look at our goal, we'll keep the math easy--let's say we need +5V into 10K ohm input impedance of the converter (many are higher). That's a whopping 0.5mA, for 2.5mW of power. That's it. That's all we really really need.

    Let's ask a very basic question (which spawned a very good thread last year on a different BBS): do we really need a preamp? The answer to that question is yes, but we need to think about what a preamp is very carefully. What we think of as a preamp is arguably not necessary at all.

    First, microphone types:

    - dynamic. low output, usually no active electronics onboard.
    - externally biased condenser. high output, but must have an amplifier onboard. needs high voltage to polarize capsule.
    - electret condenser. high output, must have an amplifier onboard, does not need high voltage.

    Hmm, so we already have amplifiers on condenser microphones. Why do we need preamps (other than to supply power to the mic amp)?

    Actually, in some cases we don't. There is nothing magically different about the amplifier in a mic versus a preamp versus the buffer amp at the front end of the converter. Actually, they can be quite similar.

    When do we definitely not need a preamp? If the mic's dynamic range is less than that of the converter, then the mic's output level can be set such that no analog adjustment of gain is ever required. Wow. That's deep, man

    Let's see an example. Let's say we have a very small-diaphragm electret condenser with a dynamic range of 120dB (that's maximum SPL less self-noise). If the mic amp is designed such that the mic's maximum level is 0dBFS, and the converter has a dynamic range of say 124dB, then the mic's self-noise will exceed the converter noise, and no analog gain adjustment would ever be required.

    It's not that simple in the real world; many mics have a dynamic range that exceeds most converters; there are issues like noise spectrum and noise summing and so forth. But it's pretty easy to show that more than say +30dB of gain should never be required.

    Why is this important? Because the trend is towards small, portable, battery powered devices, and we would like to have high-quality audio available for these devices. If we want to avoid wasting power and therefore maximize battery life, we need to consider the efficiency of the entire system.

    How efficient is the average condenser mic at driving a converter load? Let's say we are just using an external phantom supply into the converter. An average transformerless condenser uses let's say 3mA of +48V phantom power, which is 144mW, or 58 times the power of our converter load. Hmm. That's terrible! Add a preamp, say using a +/-15V supply and a dual opamp drawing 8mA idle current, and there goes another 240mW straight down the drain!

    Why is so much power wasted? Because the mic needs a high polarization voltage, and it has a single voltage supply across dropping resistors (which waste 31mW in our example). Further losses are to the amplifier transistors inside the mic, which have the job of shedding most of the rest of the unnecessary voltage that the buffer circuit doesn't need. For the losses in the preamp, there is really no excuse--we are using lots of power to boost the mic to a level far beyond what the converter needs, and which must be shed by an input pad on the converter. Smart.

    Why do we care? That 144mW per mic hurts a small battery pretty bad; the preamp losses will kill it dead. Portable devices manufacuters are smart enough to design their input stages to suit the converter though, so they don't lose that power. But they have no control over what the mics demand, and if the don't hit the wasteful P48 standard, then they are punished by the marketplace.

    A much more efficient method would be to send a mere +5V to the mic. For an electret, that's plenty, and the mic would run happily on 15mW or less. An externally biased condenser would need an onboard DC converter, which since it needs almost zero output current could still be much more efficient than a +48V phantom supply.

    Further savings come from moving from a phantom supply to a separate +5V pin, requiring a 4-pin connector. Without the supply resistors, the efficiency of the system is greatly improved. If we want to keep a 3-pin phantom configuration, we'll probably have to compromise with a +12V supply (the existing P12 standard would work OK). That will still help a lot, and it will maintain backwards compatibility with existing gear.

    OK, now we have improved the mic's efficiency greatly, all we need to do is eliminate the preamp. That's very simple: put a low-current opamp in the mic as its output driver, and a pot for a limited range of gain control right on the mic (again, +30dB should be plenty, too much for some microphones). Since the desired max output level is a mere 0dBV or so, we don't even need a superfast opamp to do the job. There are off-the-shelf opamps that will work great!

    And that, my friends, is the wave of the future! the wave of the future! the wave of the future! the wave of the future! the wave of the future! the wave of the future! the wave of the future!

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    I completely agree, I think. I am curious what sorts of problems you'd run into with the capacitance of those long XLR cable runs between the preamp and the converter stages, though.

    I think if I were designing a microphone, it would probably involve a four-pin XLR with four independent lines---two for a differential bit clock and two for a differential return data stream---all the advantages of a USB mic without the impossibility of synchronization with other mics, without the distance limitations, and without the crappy quality converter packages... maybe....
    Quote Originally Posted by Obi-Wan
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    Quote Originally Posted by dgatwood View Post
    I completely agree, I think. I am curious what sorts of problems you'd run into with the capacitance of those long XLR cable runs between the preamp and the converter stages, though.

    I think if I were designing a microphone, it would probably involve a four-pin XLR with four independent lines---two for a differential bit clock and two for a differential return data stream---all the advantages of a USB mic without the impossibility of synchronization with other mics, without the distance limitations, and without the crappy quality converter packages... maybe....
    Exactly! The format of USB on a 4-pin XLR! But again, 12V with 3-pin is backwards compatible, still much more efficient, and thus generally more palatable. So all we need is mics with onboard pres and converters with the existing P12 standard!

    And the mic source impedance need not be any different from a preamp source impedance, so the cable capacitance issue is the same.

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    Hmm?
    Interesting, not a hope in H of it happening! But interesting.

    Of course we already have "integrated" usb mics, and what an inflexible piece of ***t THEY are!

    There is of course a very good reason why mic pre's and converters have the in/out levels they do, they have to interface with all the other mixers and outboard gear in the world!

    Consumer levels? P'tah! High operating levels give better noise immunity.

    The future is already here. Put the CONVERTER in the mic, but a very GOOD one not some 16bit crap.

    And what will "they" DO with this beautiful 120dB DR? Turn it into MP3!

    You'll be saying we don't need valve guitar amps next! Now THERE'S inefficient!

    Dave.

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    Quote Originally Posted by ecc83 View Post
    Hmm?
    Interesting, not a hope in H of it happening! But interesting.

    Of course we already have "integrated" usb mics, and what an inflexible piece of ***t THEY are!

    There is of course a very good reason why mic pre's and converters have the in/out levels they do, they have to interface with all the other mixers and outboard gear in the world!
    My proposal is 100% backwards compatible, and already runs off an existing standard: P12. Most of the time I've run live sound, the mixer output isn't approaching anything like +20dBV; it's usually quite low. If you run a digital mixer, you can run all DSP noise-free and set the output at whatever level you need for your power amp.

    If you are recording to tape, yes you probably do need higher levels, but despite the dedication of the Analog boys that will continue to grow more rare.

    Consumer levels? P'tah! High operating levels give better noise immunity.
    True, but not relevant. Why? Because if the existing CMRR of a balanced line operating at mic level (say -50dBV or lower) is not sufficient noise immunity, you are doomed already. So the difference between -10dBV and +2dBV is superfluous. +32dBV would give you better noise immunity, wouldn't it? Why not rectify the mains voltage and use that?

    But let's back up--which system really offers higher operating levels? Today we send a mic signal at -30dBV to -50dBV over a 100m cable, amplify it +50dB or so, then send it maybe 3m to a converter.

    My proposal has the amplification inside the mic, so the signal going down the 100m cable is -10dBV, and the converter IC is only a few mm from the input buffer amp on the converter PCB.

    Let's talk about dynamic mics. Today, their transformer turns ratios are limited by the need for their outputs to be low impedance. That means we need a superlow input noise preamp to offer good dynamic range (see every thread regarding ribbons and preamps, for example). Redesign the transformer, and put the amp in the mic, and those problems disappear.

    The future is already here. Put the CONVERTER in the mic, but a very GOOD one not some 16bit crap.
    That is not backwards compatible, is not always necessary for applications like live sound, and still needs power anyway.

    You'll be saying we don't need valve guitar amps next! Now THERE'S inefficient!
    They are inefficient, but nobody expects to be able to run a valve amp off of 2x AA batteries for six hours. Whereas when somebody buys a portable recorder that purports to offer P48, and it can't do 10mA each for two mics (1W total draw, plus losses to the DC converter), they get angry.

    The future is portable devices--not like the Zoom stuff, I mean the iPhones of the future being used for absolutely everything--so that's where I am looking.

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    I can't wait till I start studying electrical engineering...I am so going to come back to this thread and understand it. Someday...

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