Using a microphone preamp with a DAW

S

sdwest

New member
Hi folks.

I've been using audio interfaces with built-in preamps, but wanted to try a really high-end microphone preamp. The problem is, most do NOT include an A/D converter. The question is: how do I get this pristine microphone signal into my DAW? It seems to me that running a high quality preamp into an existing audio interface with a built-in preamp limits the signal quality to that of the built-in preamp. If I run the external microphone preamp signal into the "line in" input of the audio interface, will the signal go directly to the A/D and bypass the built-in preamp? Am I required to buy a really high quality A/D converter only, with nothing else between the external preamp and the DAW input? How do you "pros" do it?
Any recommendations are appreciated!

Wade
 
arcaxis said:
You didn't mention what interface you have, but if you state the manufacturer and what model it is someone may know if the pres are bypassed. Even if they aren't bypassed, an external preamp might be worthwhile if it's one that can "color" the sound a bit. I use external preamps for both a Tascam digital recorder I have as well as a Tascam interface and of different preamps I have each has a subtle difference to the sound it imparts to a mic.
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Thanks, Mark. The interface is a Focusrite ISA One Digital. I also have a Focusrite Saffire 24 DSP and a Focusrite Saffire Pro 40. I generally use the ISA One, but recently obtained a high-end mic pre that I want to try out in comparison. I take your point about "coloring" the vocal sound, but this is just to be able to technically compare preamps.

Thanks for responding!
Wade
 
sdwest said:
Hi folks.

I've been using audio interfaces with built-in preamps, but wanted to try a really high-end microphone preamp. The problem is, most do NOT include an A/D converter. The question is: how do I get this pristine microphone signal into my DAW? It seems to me that running a high quality preamp into an existing audio interface with a built-in preamp limits the signal quality to that of the built-in preamp. If I run the external microphone preamp signal into the "line in" input of the audio interface, will the signal go directly to the A/D and bypass the built-in preamp? Am I required to buy a really high quality A/D converter only, with nothing else between the external preamp and the DAW input? How do you "pros" do it?
Any recommendations are appreciated!

Wade
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You've identified an important issue with high-end preamps (and, indeed, any high-end gear). It will deliver high-end results only to the degree that the rest of the system will allow. Going through a line-in on the interface should bypass the interface's pres. However, even doing this may not give you the results you would otherwise expect if the rest of the process is not also sufficiently high-end-ish. For example, poor mike placement (something that costs nothing to get right) could wipe out any notional benefits of the pre-amp.
 
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Ah but there are "line inputs" and Line Inputs!
Those combined with mic inputs on combo XLRs are just the mic pres with a 20dB or so pad, clearly not so good in the "pure access to the converter" stakes!

So, use the dedicated TRS jack line ins? Yes, pretty much fine err, except they probably use the same IC as the mic amp and simply change the NFB around it. If you believe that ICs affect the sound quality (other than straight added, measurable noise and distortion) .
 
Ah, Auto Save did not save all of last night's waffle, maybe just as well!

The bottom line is that you cannot get very "close" to the input of the A to D processor for technical reasons so there is always going to be a "bog standard" chip* in the signal path from your high end pre amp. Probably the closest you can get is an AI with an insert? Better still would be a pre amp with a built in (but good!) converter. One such was the Audient Mico and you might be able to find one on The Bay. The Mico was always super value, maybe TOO super and it was deemed unprofitable by the Bean Counters?

*Such as the often maligned NE5532/4? Actually a superb chip despite being 30+years old. The LM4562 has a better spec' in almost all departments but I doubt very much if they could be heard apart. In fact, used as a unity gain follower the humble 741 is quite "invisible"!

Dave.
 
This is very interesting. I'm hearing that the A/D box preamps remain active when there is a line level signal coming in?

Is this a technical limitation? Or are there A/D boxes that avoid this? What would I look for in the specs of my A/D box to see if this is happening?
 
Robus said:
This is very interesting. I'm hearing that the A/D box preamps remain active when there is a line level signal coming in?

Is this a technical limitation? Or are there A/D boxes that avoid this? What would I look for in the specs of my A/D box to see if this is happening?
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If an AI has dedicated line ONLY inputs they will not be mic pre amps (tho' they will probably use the same chips as I said!) .

Yes, there is a technical reason why you cannot "get at" the inputs to the ADC. You would blow up the fronts ends! The inputs of an ADC can only handle up to about 6volts peak to peak, about +8dBu and any 1/2 decent mic pre can put out two or three times that voltage and at enough current to blow the protection devices inside the ADC chip.

So, bottom line, there is ALWAYS going to be at least one analogue IC twixt you and the converter. In practice there will be more since the line input needs to be capable of handling +22dBu, preff' more and thus there will Be a balanced input chip followed by AN ATTENUATOR!

If all this bothers you (it shouldn't) start looking for a Mico!

Dave.
 
I have often wondered if switching the switch or plugging into the XLR "microphone" input actually increases gain in that stage, or does it just attenuate less? Which way is better really depends on all of the details of the circuit, but if the gain stage is the same, then usually the path with less attenuation will be better for noise purposes.
 
ashcat_lt said:
I have often wondered if switching the switch or plugging into the XLR "microphone" input actually increases gain in that stage, or does it just attenuate less? Which way is better really depends on all of the details of the circuit, but if the gain stage is the same, then usually the path with less attenuation will be better for noise purposes.
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I have not seen any actual schematics Ash' (will varder) but I would think that the TRS circuit in a "combo" XLR goes down a different route which includes an attenuator? The actual gain of the mic amp circuit would stay the same. This is of course bad engineering, attenuating a signal by 20-30dB then boosting it back up again but tis the way it is done in 99% of AIs and quite a few mixers, some quite famous ones.

In practice of course, so long as the mic pre is low noise the net result is likely to be very acceptable. In any case, for the purposes of THIS discussion the "posh pre" would determine the overall noise to a very large degree.

In other situations the line sources from such as synths, guitar amps, pedals etc are likely not uber low noise anyway.

Compromise! Most of us have to live with it and never forget, in terms of noise and distortion, the cheapest, bog S interface is still WAAAAAY better than tape!

Dave.
 
"So, bottom line, there is ALWAYS going to be at least one analogue IC twixt you and the converter"


Dave, that's good information, just what I was looking for. I suspect that to do this right, the cost could be almost twice what a good preamp would cost. OR, I can just live with what I already have! Thanks all, for responding!

Wade
 
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