Topic: Line-in vs mic-in confusion

[yates7592]

Dumb question here:

I understand that by going mics > battery box > mic-in and adding gain beyond the unity setting of your recorder that you are using the recorder's preamps to add the required gain. If I go mics > bb > line-in, am I right in thinking the recorder preamps are bypassed? That is what I always thought. If so, and I add gain over unity on the recorder level dial again, where is this extra gain coming from if not from the recorder's preamps???

Not important, but just bothering me for some reason.

[runonce]

In general, yes...but there are so many variants in circuit design - its hard to really claim "the recorder preamps are bypassed"

I think a lot of guys use Line In for more headroom - but yes you will have to add more gain from the recorder.

Does your BB add gain?

[it-goes-to-eleven]

Most likely, you cannot "bypass" the preamp in any modern recorder.

Both inputs on the recorder typically end up going into a single chip integrated solution that provides gain in the analog and digital domains.  There may be some attenuation or impedance differences between the two inputs.  And there may be some front end circuitry that gets switched in and out depending on the mode (I think that is more likely on devices like the 7xx).

Also, the common notion of "unity" is often flawed.  The levels may sorta match at "unity", but internally the device may be adding gain and then removing it.  A better point of adjustment is probably what ever sounds best with your combination of gear.

It would be well worth doing a comp on the m10, where gain from the m10 vs. your preamp are compared.

[nassau73]

Back in Part 2 of the PCM-M10 thread, Guysonic gave a very detailed review of the machine and results of his testing which I found extremely helpful.

Quoting a section of his review:
=======
"M10 seems best choice over 09HR especially for using MIC input modes as M10 shows little (1 spike @~ 10K Hz) or virtually no high frequency digital noise.

Suggest battery box powered mics to ONLY use MIC input jack as M10 has overload resistant MIC input ability to +2 Dbu.

And if using external preamplifiers having active gain, use only the LINE input jack OVERLOAD RESISTANT TO OVER +24Dbu.
========
I've only had my M10 for about 6 months so I'm still experimenting using my mics and battery box with Line In and Mic In recordings. I'm finding that for quieter shows - Mic In has been working for me (for example - Keb' Mo solo in a concert hall). For loud shows or small clubs when I'm right near the stacks Line-In gives me the best results.

As it-goes-to-eleven says, best thing is to compare the different possibilities and see what works best.

What makes it hard is that the M10 works so well under all different environments. With the old mini-disk machines, I could try something and basically say, "that works" or "that doesn't work" and be done with it.

On the other hand, comparing results with the M10 for me has been kinda like getting a test at the eye doctor where they ask, "which is better - number one or number two? Or are they pretty much the same?" :-)

[yates7592]

OK thanks. So it seems that you do not (cannot) bypass a recorder's preamps by going line-in with a battery box, and the line-in/mic-in distinction is just a sensitivity thing where the mic-in expects a quiter signal and the line-in a louder signal. Makes sense now.

[DSatz]

> So it seems that you do not (cannot) bypass a recorder's preamps by going line-in with a battery box, and the line-in/mic-in distinction is just a sensitivity thing where the mic-in expects a quiter signal and the line-in a louder signal. Makes sense now.

Well ... no, you generally can't bypass the preamp stage altogether. But when you set the switch for line input, the circuit is reconfigured (the negative feedback is substantially increased) so that the same circuit now operates with considerably lower gain--say, maybe 40 dB lower than if you set it for mike level. And that means that (a) any noise present at the circuit's inputs will be amplified correspondingly less (by that stage of the circuit, anyway), plus (b) the overload limit of the circuit will be correspondingly higher.

Those can be very useful things, IF you are feeding high enough signal levels into the circuit to take advantage of them. (You did notice the big "if" there, I hope?)

But there are many variables here, both in the "gain structure" (design) of various recording devices, and in the signal levels that various microphones put out at various distances in various rooms from various sound pressure levels. So the optimal settings always come down to particular cases and situations.

--best regards

[yates7592]

Those can be very useful things, IF you are feeding high enough signal levels into the circuit to take advantage of them. (You did notice the big "if" there, I hope?)

Yes, I got that "if" loud and clear! Many thanks for the lucid explanation.

[stevetoney]

For what it's worth, according to something I recall reading awhile ago, Sound Devices tech support was asked by a TS.com member and they confirmed that even the Sound Devices 7xx series of recorders have an architechture that does not bypass the preamps when the switch is moved between mic-in and line-in. 

[it-goes-to-eleven]

For what it's worth, according to something I recall reading awhile ago, Sound Devices tech support was asked by a TS.com member and they confirmed that even the Sound Devices 7xx series of recorders have an architechture that does not bypass the preamps when the switch is moved between mic-in and line-in.

Yep.  I asked them that many years ago.  But it is worth repeating that flipping the switch between Line and Mic mode does change the input impedance.

[George2]

Really? I asked Sound Devices 2 times, one year apart each time.... the question.
""Does Line input bypass the mic preamp on the SD702""?
answer both times was 'yes, bypasses preamp'

[stevetoney]

Really? I asked Sound Devices 2 times, one year apart each time.... the question.
""Does Line input bypass the mic preamp on the SD702""?
answer both times was 'yes, bypasses preamp'

Is it possible that's true on a 702, but not on a 744?

This would be my preference, but I tend to think the other answer is correct for the 744 based on the note on bottom of Page 12 of the 744t manual which is provided in italics.

Phantom power is available for both mic- and line-level inputs.  Using line level inputs with microphones is useful for high SPL environments such as concert recording.  Make certain  to turn off phantom power with line level output devices susceptible to damage from DC.

To me, this note implies that the pres are in the signal path regardless of whether the switch is in the mic- or line-in position.

So for 744 owners, the implication here is that if you want to remove the SD preamps completely from your signal chain, you'd want to use Channels 3 and 4 with an external preamp.

[Gutbucket]

> So it seems that you do not (cannot) bypass a recorder's preamps by going line-in with a battery box, and the line-in/mic-in distinction is just a sensitivity thing where the mic-in expects a quiter signal and the line-in a louder signal. Makes sense now.

Well ... no, you generally can't bypass the preamp stage altogether.[snip]

At first glance, I mis-parsed the reply above before realizing my error in misreading the ellipsis. So, in a well intentioned effort to clarify the good DSatz's intent (hope I'm not out of bounds), read that as a response leaning towards the affirmative- 
(my paraphrasing)
"Well, that pretty much the situation... you generally can't bypass.." 
Basically a "Well, yes."

-rather than as a refutation of yates's summary-
"Well no.."

[/semantics]

The take away for practical purposes is that the most important factor determining which input to use is usually the output signal level of the source feeding of the recorder input.  In simple terms, for low level inputs use the microphone-input, for higher level sources use the line-input.

Another practical issue is if mic power is necessary and available or can be switched for the input as required.

As noted by others, the input gain stage circuitry is pretty much the same although the configuration values of it change, which may alter its performance. So other than mic powering and gross sensitivity/overload-point changes, the more subtle question is the noise and distortion performance differences between the recorder’s input modes (may be two different physical input jacks or a mic/line switch of some sort), and playing that variability off the performance and output gain adjustment provided an outboard gain stage such as a preamp or attenuators on the mic if something like that is being used.  That is going to vary by machine and best practice is usually determined by measurement, or experimentation, or following the established practices of others around here using the same gear, etc.

I have noticed that the specifications of the small recorders I've used indicate slight differences of input impedance for the microphone-input verses the line-input, but usually not enough to be significant IMO.  But those who have a better handle on the electrical side of things are welcome to correct me on that if I'm missing something there. [edit, see discussion of that below]

[Gutbucket]

Thanks Jon,

I based my off-the-cuff statement on the significance of input impedance differences above on the small pocket recorders I've owned which have both line-in and mic-in jacks and for which I know the input impedances.  Those are the original Edirol R-09 and the Tascam DR2d.  Granted, although I use both inputs on these machines, I've never been concerned much about their input impedance because they seemed reasonably close and I'm  always using a buffer stage (preamp or SBD output) driving either input of the recorders instead of a battery box.

Okay, let's see.. I looked up the specs on a few of the popular recorders used around here partly to fact check myself, but also to ask your opinion on how significant the difference would be for someone that was using a battery box.   

Tascam Dr2d  Mic input:  25k ohm; Line input: 22k ohm
Original R-09   Mic input:  20k ohm; Line input: 17k ohm
(not a big variance)

I also looked up a couple other similar recorders which are popular here. 
These have a more significant difference between input impedances, especially the M10.
 
R-09HR           Mic input: 30k ohm; Line input: 15k ohm
Sony M10       Mic input:  3.9k ohm; Line input: 22k ohm

[it-goes-to-eleven]

From their manuals:

Sound Devices 722 mic: 7.5K   line: 20K
Grace v3: 3K

[DSatz]

tonedeaf wrote:

> To me, this note [saying that phantom powering is available even in line-in mode] implies that the pres are in the signal path regardless of whether the switch is in the mic- or line-in position.

Knowing that one switch controls the phantom powering doesn't tell you that the other switch doesn't bypass the mike input stage! For example, consider that having the separate switch for phantom powering frees up switch contacts, which then better allows the mike/line switch to bypass the first stage IF that's what the designers of the preamp really wanted to do. Otherwise, the single switch would be hopelessly overburdened and its reliability compromised.

However, the "bypassing" approach has significant drawbacks and I'd be rather surprised if that's what was done, instead of multiplying the negative feedback to reduce the gain of the stage.

--best regards