mshilarious
Banned
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!
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!
