> "pro" line level is +4dBm. So would you want a mic preamp to be able to amplify any mic signal to this level?? Then why do some manufacturers say that their pres can put out +20dBm??
How do you measure the "+4dBm"?
The reference is the zero mark on a VU meter that reads zero with a steady +4dBm tone. On speech/music, when hovering around this 0 mark, on the slow VU meter, the peaks will be 10dB to 20dB higher: +14dBm to +24dBm.
When using an averaging meter such as the VU, on speech/music, the actual peak level is statistically "random". If you could hold speech/music to "0VU", you would get a 10dB peak about every second, a 15dB peak every few hours, a 20dB peak every few days.
In broadcast audio with soft-overload amplifiers, 10dB headroom to rated output was considered acceptable, and you sometimes see amps rated +18dBm working +8dBm nominal (though they didn't usually push that hard). Music recording, especially after the invention of the Peak light, tended to use similar +18dBm or +22dBm amps but at nominal level of +4dBm, for 14dB or more headroom. Some engineers and systems allow 20dB headroom. Numbers of 16dB and 18dB are widely accepted.
Also remember that engineers can't watch the meters all the time, and musicians are unpredictable. They may be cruising along at -5VU and twang a +5VU chord. If that chord really has a peak 16dB over the slow VU meter reading, that is a +21dBm peak. If passed through a +18dBm amplifier, it will be clipped 3dB.
So headroom is good. But headroom is expensive: bigger output stage. But a funny thing is: the price of a small amp is roughly constant, the price does not soar much until you get over about 100 milliWatts. Which is +20dBm. So for good performance at low price (with costly amplifiers), you use +18dBm in internal stages, and just a couple +28dBm amps for output stages that may have to drive long lines or a loaded recorder-bus.
But in transformerless outputs, there may be a rise in price beyond 40V supply, or 30V with large margin of error. A +28dBm output on 600 ohms is 20 volts, or 56 volts peak to peak, demanding 60 or 80 volt output transistors, which are not as nice (price and performance) as the selection below 40V. And the old long-lines and heavily loaded recorder buses are not common in modern small studio practice. So we see a lot of outputs that are only good for +20dBm single-ended. Sometimes they double-up the output as a bridge, call it Balanced, and get +26dBm or a bit more. These are ample for any small studio, though obviously not as powerful as the push-pull 6V6 8-watt outputs that real consoles used to use.
Yes, you want at least 7 Volts RMS, 20 volts peak to peak, in any box with any pretense of being "Pro". And in rare but important situations, it should manage a 600 ohm load without strain or much sag.
I think most small studios would work just as well, even cleaner, using the old TASCAM semo-pro levels. 0VU is 0.3V, peak to peak never exceeds 5.6V, loads are 10K or higher are rarely paralleled more than 4 wide (2.5K total load). But interfacing between TASCAM ins/outs and the strange things that pass for "Professional" these days is tough.
