I've been pondering a somewhat different mike-amp.
There is absolutely nothing new in it. Yet I have never seen this exact plan before. (Citation of prior art welcome.) It has shortcomings, and the way I have fleshed it out it would be a poor commercial product. But it may be easy to build, be useful in many situations, look good, and generally be "an original design", as much as can be in this well-explored field.
Thoughts in my head:
First look at noise. All feedback amps can be represented in this form:
To change gain, you can change either feedback resistor. But for a number of reasons, changing Rshunt is most common in mike preamps.
Many mike-amps use 50K feedback resistance (Rseries), though for differential they may do it as two 22K resistors.
Mike-amp gain usually varies from 60dB down to 20dB, or even 0dB (unity gain) for hot mikes on loud sources. The diagram shows the other feedback resistor's value for various gains.
At 60dB gain, the amplifier sees 200 ohms of mike resistance noise at one input, and 50 ohms of feedback resistor noise at the other input. (Really 50 in parallel with 50K, but that's essentually 50 ohms.) The mike sees a total noise resistance of 250 ohms, of which 200 ohms is "good" (microphone signal) and 50 ohms is "bad" (feedback network dead resistance). Noise figure is 1dB, very good. If the mike maker claims mike self-noise equal to 20 dB SPL, we will observe noise equal to 21 dB SPL, which is about as good as we can get.
At 40dB gain, we have 700 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 5dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 25 dB SPL, which is not so good, though in many situations it isn't a problem. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 40dB gain for sounds of 108 dB SPL. In pauses, the noise floor is 25 dB SPL. The total dynamic range is 83dB, acceptable only because acoustic background noise may be around 25 dB SPL in many studios. (But I'm booked for a 108dBSPL gig where the noise drops below 15 dB SPL after rush-hour.)
At 20dB gain, we have 5,200 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 14dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 34 dB SPL, which is pretty poor. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 20dB gain for sounds of 128 dB SPL. In pauses, the noise floor is 34 dB SPL. The total dynamic range is 94dB, barely 16-bit quality.
We would like to reduce feedback network impedance. But if we reduce Rseries, it loads the amplifier output, and increases the strain on the amplifier. Maximum output falls (or the output must be beefed-up), distortion rises.
Next: how many stages? A single-stage BJT amp "can" make 60dB of voltage gain, but not much more. When you also consider input and output impedances, it doesn't work. A 3-stage BJT amp can have a voltage gain of a million, but three stages is hard to apply feedback to, and simpler is probably better for sound. Let's start with a 2-stage design, though we may end up with one "stage" being a 2-device compound to get enough current gain to meet our impedance needs.
The most obvious 2-stage amp is sketched on the left:
Even you RCA B3C tube-amp fans will recognize this layout. Feedback from output plate to input cathode. The pins have new names on BJTs, but it has the same drawback. The 2nd stage plate resistor pulls up, but the feedback resistor loads it the other way. In low-gain tube amps the feedback resistor may equal the output load, doubling the amplifier's work and stress.
The amp on the right uses complementary devices to get the load resistor going toward the feedback network. In fact it works well if the load resistor IS the feedback resistor. And that gives the lowest feedback network inpedance, lowest noise for any gain setting, highest first-stage gain when needed.
I have omitted DC Bias, coupling caps, and differential operation. All these present problems, but it turns out that the solutions fit together elegantly.
---- enough for now. More later.....
There is absolutely nothing new in it. Yet I have never seen this exact plan before. (Citation of prior art welcome.) It has shortcomings, and the way I have fleshed it out it would be a poor commercial product. But it may be easy to build, be useful in many situations, look good, and generally be "an original design", as much as can be in this well-explored field.
Thoughts in my head:
- Transformerless. I like transformers, but good ones are expensive. In effect you buy performance, and that isn't really DIY. (Unless you do considerable set-up to wind-your-own and learn all the little secrets, which is maybe too DIY.)
- Low-Noise, of course. And while many designs give low noise at maximum gain, often the input noise level rises at lower gains. And with today's hotter mikes and sources, we often work at lower gain, but with today's 16-bit and 24-bit recorders, we often hope for huge signal to noise ratio even on loud sources.
- "Professional" interfaces: differential input, differential 600-ohm output, +4dBm nominal with ample headroom. I still argue that "semi-Pro" interfacing is more appropriate for small studios and cleaner sound, but there is a historical legacy that is hard to fight.
- All modern mike preamps have to have a +48V supply for Phantom. It seems silly to also have various other voltages like +/-15V. You can build a fine preamp with just +48V. The preamp will take more power than just the Phantom needs, but it is easier to beef-up the one supply than to build several supplies. Single-supply tends to force coupling capacitors, but Phantom already forces the use of caps. (Or transformers, but see above; or very clever common-mode design.)
- S-i-m-p-l-e-! Don't make the audio go through a lot of tricky-stuff. Don't design something you can't easily build or debug or tweak.
- Wide bandwidth and low distortion, but not obsessively so. A few-dB droop can be fixed in the mix. If the signal path is simple, the distortion will be inoffensive.
- Feedback, because I am used to that type of design, and because it greatly simplifies setting of gain and balance.
First look at noise. All feedback amps can be represented in this form:
To change gain, you can change either feedback resistor. But for a number of reasons, changing Rshunt is most common in mike preamps.
Many mike-amps use 50K feedback resistance (Rseries), though for differential they may do it as two 22K resistors.
Mike-amp gain usually varies from 60dB down to 20dB, or even 0dB (unity gain) for hot mikes on loud sources. The diagram shows the other feedback resistor's value for various gains.
At 60dB gain, the amplifier sees 200 ohms of mike resistance noise at one input, and 50 ohms of feedback resistor noise at the other input. (Really 50 in parallel with 50K, but that's essentually 50 ohms.) The mike sees a total noise resistance of 250 ohms, of which 200 ohms is "good" (microphone signal) and 50 ohms is "bad" (feedback network dead resistance). Noise figure is 1dB, very good. If the mike maker claims mike self-noise equal to 20 dB SPL, we will observe noise equal to 21 dB SPL, which is about as good as we can get.
At 40dB gain, we have 700 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 5dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 25 dB SPL, which is not so good, though in many situations it isn't a problem. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 40dB gain for sounds of 108 dB SPL. In pauses, the noise floor is 25 dB SPL. The total dynamic range is 83dB, acceptable only because acoustic background noise may be around 25 dB SPL in many studios. (But I'm booked for a 108dBSPL gig where the noise drops below 15 dB SPL after rush-hour.)
At 20dB gain, we have 5,200 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 14dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 34 dB SPL, which is pretty poor. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 20dB gain for sounds of 128 dB SPL. In pauses, the noise floor is 34 dB SPL. The total dynamic range is 94dB, barely 16-bit quality.
We would like to reduce feedback network impedance. But if we reduce Rseries, it loads the amplifier output, and increases the strain on the amplifier. Maximum output falls (or the output must be beefed-up), distortion rises.
Next: how many stages? A single-stage BJT amp "can" make 60dB of voltage gain, but not much more. When you also consider input and output impedances, it doesn't work. A 3-stage BJT amp can have a voltage gain of a million, but three stages is hard to apply feedback to, and simpler is probably better for sound. Let's start with a 2-stage design, though we may end up with one "stage" being a 2-device compound to get enough current gain to meet our impedance needs.
The most obvious 2-stage amp is sketched on the left:
Even you RCA B3C tube-amp fans will recognize this layout. Feedback from output plate to input cathode. The pins have new names on BJTs, but it has the same drawback. The 2nd stage plate resistor pulls up, but the feedback resistor loads it the other way. In low-gain tube amps the feedback resistor may equal the output load, doubling the amplifier's work and stress.
The amp on the right uses complementary devices to get the load resistor going toward the feedback network. In fact it works well if the load resistor IS the feedback resistor. And that gives the lowest feedback network inpedance, lowest noise for any gain setting, highest first-stage gain when needed.
I have omitted DC Bias, coupling caps, and differential operation. All these present problems, but it turns out that the solutions fit together elegantly.
---- enough for now. More later.....
)))
