Mic preamp vs line input - what's the diff for line level?

[wearashirt]

I would like to ask, what would be the sonic difference between feeding line level signals into the mic input vs the line input.

in my case, my line signals come from my interface. Prematurely, I had pre-made 1/4" to XLR cables, to feed into the mic inputs of my analog console, with the aim of minimizing output "wear" from my audio interface, since mic inputs have less impedance and will thus pass level louder than the line input.

It has been successful so far, but I've noticed that line levels in my analog console are quieter even when the gain knob is raised up to 10db shy of the max. Though, the only legitimate way to compare noisefloor is by re-doing all my cables into 1/4 to 1/4".

In the past, did mixing engineers pass tape outputs into the mic inputs of their high-end analog mixers?

[ef37a]

I am not really sure what you are up to here friend! If you are feeding line line level (+4dBu?) into a microphone input then, even if you can get the mic pre's gain low enough to avoid overload the input impedance is much lower, ~1k5 typically, than line outs are designed to drive and could get distortion on peaks and possibly some tonal changes.

You really should give the make and model of the mixer and the interface as well.

Re your point of "avoiding wear on the AI outputs"? This is a good point if, as I assume you mean wear to the jacks? It is good practice to make 'sacrificial' cables, short stubs, XLR-XLR say to avoid constant use of the XLRs on a valuable piece of kit. Jacks are more of a problem because there is only one really reliable 'cable' jack IMO, the Neutrik lockable line jack but they's expensive if you need a lot of them. Better to makeup 'breakout boxes' with decent chassis mounted jacks. The quality and longevity of the jacks on some quite prestigious interfaces can be poorer than you would expect.

Dave.

[Hugh Robjohns]

I would like to ask, what would be the sonic difference between feeding line level signals into the mic input vs the line input.

If you can get the gain structure right -- which can be a big IF in some cases -- then very little. Lots of preamp line inputs simply pad the signal down by 30dB and route through the mic preamp. Its not ideal, but the added noise and distortion from following this route is negligible in most cases and of no practical consequence.

... since mic inputs have less impedance and will thus pass level louder than the line input.

I've no idea where that notion came from, but its nonsense. A lower input impedance actually 'loads' the output more and reduces the signal level. It also may cause the output driver to struggle on signal peaks and sustained low frequencies, resulting in distortion.

The reason mic inputs are 'louder' than line inputs is because they introduce more gain, by design.

I've noticed that line levels in my analog console are quieter even when the gain knob is raised up to 10db shy of the max.

It might help with your understanding if read up on gain structure....

[wearashirt]

I am not really sure what you are up to here friend!

You really should give the make and model of the mixer and the interface as well.

Re your point of "avoiding wear on the AI outputs"?

Hi Dave!

I'm doing the hybrid-mixing thing. My tracks are sent out through a Behringer 1820 + Focusrite 6i6, then into a 1990 Soundcraft Spirit Live.

Initially (i.e., several months ago), after figuring out that mic inputs are louder than line inputs on the console, I decided to fabricate 1/4" to XLR cables. This way, my level passes louder, without having to max the output from the AI's. I have this idea that I might "burn" them out or something.

Also wanted to pick senior engineer's brains on whether the preamps that are always talked about in vintage consoles, were the same path that tracks were sent back in for mixing (e.g., from tape machine or ADAT (?) )

[wearashirt]

If you can get the gain structure right -- which can be a big IF in some cases -- then very little. Lots of preamp line inputs simply pad the signal down by 30dB and route through the mic preamp. Its not ideal, but the added noise and distortion from following this route is negligible in most cases and of no practical consequence.

... since mic inputs have less impedance and will thus pass level louder than the line input.

I've no idea where that notion came from, but its nonsense. A lower input impedance actually 'loads' the output more and reduces the signal level. It also may cause the output driver to struggle on signal peaks and sustained low frequencies, resulting in distortion.

The reason mic inputs are 'louder' than line inputs is because they introduce more gain, by design.

I've noticed that line levels in my analog console are quieter even when the gain knob is raised up to 10db shy of the max.

It might help with your understanding if read up on gain structure....

Hi Hugh, thanks for your reply.

-I've a hunch that how you describe is exactly the way my console is designed. In the manual it says "when LINE is selected, the gain range is reduced by 20db". In this case I'll try a bit harder to tell whether there is more or less noise between line and mic inputs on this console.

Context is I'm torn on whether I should pass vocals using the mic or line input, since it goes through so much outboard compression (about max of 15-18db reduction) thus amplifying the noisefloor picked up. But something in my aural gut tells me that going through the mic input is really beefing up the vocal in a desirable way.

-In the manual, the impedance for the mic input is listed 2kohms, and line input 10kohms. Am I understanding that correctly?

[Hugh Robjohns]

In the manual, the impedance for the mic input is listed 2kohms, and line input 10kohms. Am I understanding that correctly?

Those numbers are fairly typical.

[James Perrett]

But something in my aural gut tells me that going through the mic input is really beefing up the vocal in a desirable way.

That's because it is louder - louder always sounds more impressive. If you actually match the levels properly when comparing you should hear little difference unless the output of whatever you are feeding the mixer with is struggling with the low impedance.

[ef37a]

I have found that the max line output of the 1820 is +16dBu, about 5V, at I assume 0dBfs. If you are averaging around -18dBfs (as you should) that gives you an average of 0.5 to 1.0 volts into the Spirit which should be an adequate level for good noise levels. The mixer is always going to be noisier than the AI. I have had several Behringer products and they have always had very decent noise levels, my last was the UMC-204HD which was very good IMHO.

So, drive the mixer as hard as reasonably possible so as to keep its gain and hence noise low. The exchange of line and microphone signals involves incredibly low energies. You will not 'wear out' anything.

Dave.