Line level versus mic level improvement

[John Teel]

I'm considering getting a preamp mixer that offers line level outputs (like a Beachtek DXA-10) instead of mic level outputs like on my Beachtek DXA-8. I'm trying to understand what type of noise improvement I can expect from line level inputs over mic level inputs.

When I plug an empty mini-plug (no mic on cable) into the mic input with the switch set to mic I hear ALOT more noise than I do if I switch it to line level. Is this difference in noise representative of the improvement in noise I can expect if I go with a line level input instead of my mic level input?

Thanks,
John

[David Ennis]

No. You'll see improvement, but not of that magnitude. The unterminated plug, especially if there is a cable attached is an antenna for electrical noise, so that a weak signal gets applied to the input. All that "test" is doing is confirming that more amplification is applied when the cam's input is set to mic level.

What really matters is the signal to noise ratio of all amplifiers in the signal chain. All amplifiers introduce random noise due to the physics of their components. This includes the preamps in condensor microphones, preamps in mixers and camcorders, and output amplifiers. Preamps are the most vulnerable because they are beginning with a small signal. Any noise generated by the components on the low signal side of the amplifier gets amplified with the signal. Amplifiers with better designs and higher quality components introduce less noise and yield higher signal to noise ratios.

An external preamp is an advantage if it is of highter quality than the cam's preamp, which is often the case. So if you use a good external preamp with a line level output and switch the cam to "line in" you are effectively substituting the better preamp.

There is an additional benefit in preamping a signal up to line level for transmission over a long cable, and that is that electrical noise induced in the cable will be amplified less by the cam, as you saw in your experiment.

[Mike Rehmus]

I seem to remember an ex-member of this forum who reported that the pre-amp is in the signal path no matter which you select, line or mic. I think that is why the BBC mod was created.

Place a shorted mini-plug in the input and then test for noise.

Truth is that all of these cameras are noisy compared to the pro cameras. Still, if you are operating in a noisy environment or there is a music bed present, you won't really notice the noise. What they don't do very well is capture voice-over. A MD recorder or a pro camera (or anything better) will give better results in a quiet studio.

[John Teel]

I guess the validity of my test depends on how much of the noise I heard was coming from RF pickup and how much is physical noise from the amplifiers.

After talking to both Greg Winters and Harry @ Beachtek it's my understanding that when you switch from mic to line level the same preamp is used but the closed-loop gain of the feedback circuit is reduced. By using a line level preamp I'm replacing this noisy internal gain with lower noise gain from my preamp. By have the BBC mod done this whole preamp circuit is bypassed and the signal is fed right into the ADC. As far as I'm concerned there's not much use in having an audio system that is as noisy as the VX2100's. I'm getting a Beachtek DXA10 with line level outputs which I'll feed into the line level mic input and if that doesn't give me the low noise floor I need then I'll have Greg's mod done so that I can move all of the gain to external amps.

John

[David Ennis]

Originally Posted by Mike Rehmus

I seem to remember an ex-member of this forum who reported that the pre-amp is in the signal path no matter which you select, line or mic. I think that is why the BBC mod was created.

Nah, that doesn't make sense. You're probably remembering some reference to other parts of the signal path that the two settings have in common.

[Mike Rehmus]

Originally Posted by Fred Retread

Nah, that doesn't make sense. You're probably remembering some reference to other parts of the signal path that the two settings have in common.

It is a common technique in electronics and John has just reported that the pre-amp is always in the circuit. That Sony changes the gain rather than attenuating the signal is a minor difference.

[David Ennis]

I guess it's semantics. In usage, "amplifier" can refer to the whole circuit that gives the function of amplification, or it can refer to the active component in that scheme. But either way, if the gain of the amplifying chip is cancelled, the thermal noise locally generated on the input side isn't amplified as the line level signal passes through, so I'd stand by my earlier statement that "you are efffectively substituting the better preamp."