bypassing built in pres on interfaces...

[Smitty]

i'm still looking for a decent multichannel firewire interface (under $1000) that has decent, usable pres but will allow me to bypass them when i upgrade to better external pres later. i know there are plenty of HQ converters out there without onboard pres, but i need onboard ones for the time being until i can afford to upgrade. judging from the amount of questions i've seen on this particular topic, this seems like a pretty common question. what is uncommon, however, is a definitive answer.

i've done some extensive searching on this, and waded through a lot of people's (best intentioned) assumptions about whether or not this might be possible, and how to do it. I know it differs by unit:

it seems you can bypass the preamps on the MOTU 896/896HD because they say so on their website, (and because Joel said so and I believe him ), although apparently Matt from Black Lion Audio has been quoted as saying that it does not. (Link may require registration for forum viewing). I would like to know if this same option is available for the 8pre, but i kind of doubt it is.

i was confused about the response i got from Focusrite tech support about this issue on the Saffire Pro 26 i/o:
"All analogue inputs will be pre-amped by the Saffire. I would suggest plugging into the line inputs. The frequency responce is flat from 20Hz up to 150kHz so they should not colour any signal put into the line input."

and later, after i asked for clarification...

"The analogue inputs all go through the same stage, but with different input impedance and gain ranges.

Line 10kΩ -10dB to +36dB
Mic 2K4Ω +13dB to + 60dB
Instrument 1MΩ +13dB to +60dB"

It's easy to do on the Presonus Firepod/Studio as detailed here, but you have to choose between accepting "coloration" (channels 3-8 line inputs bypass the preamps, but not the gain stage entirely), or only having 2 inserts to use this way (the two inserts on the back bypass the entire gain stage).

so besides getting all of the above information in one place to perhaps save someone else the trouble of having to find it like i just did, i have some questions:

what, technically speaking, is a line input? i thought it just made reference to the level of the signal being input, but it seems like in this particular discussion it's being used interchangably with "input that bypasses the preamps." can i just look for a unit that has line inputs and call it a day?

for you folks out there using a firewire interface (w/onboard pres) and outboard pres, what is the setup that works for you?

how much would running a good preamp through a decent converter's gain stage skew the quality of it?

sorry for the great american novel here... i feel like every time i figure out what will work best for me i find another issue that makes me rethink everything. any insights or information is greatly appreciated (as always).

[cyantologist]

well it's not an interface, you might want to check out the mackie onyx 800r. it's an 8 channel rackmount preamp, but if you buy a db25 cable you can add outboard pres and just hit the "line" button on the front to bypass the onboard pres. then it's got adat out so you can buy the cheapest interface you can find that's got adat optical ins (in my case, the motu 8pre) i'm still kindof a n00b but i really like this setup and i believe the converters on the mackie are pretty decent and i like that i now have the ability to blow money on new preamps whenever i want javascript:emoticon(':D')

[skinsincyn]

RME's Fireface 800 has everything you're asking for, but it breaks the $1000 barrier.

it has four decent on-board preamps on the front, AND 8 line inputs on the back. The line inputs don't have any gain adjustments. What you put in goes right to the converters.

I have the outputs from my lunchbox going right into those. Plus, the RME let's you adjust the reference level (-10, +4, Hi GAIN) of the converters. I set them to "hi gain", and then I can drive my API pres and not clip the converters. AWESOME!!!

[trumpetgunk]

This may come across sounding a little stupid, but bear with me for a sec. The line input specs you listed go from -10dB to +36dB. If you set them to 0dB, wouldn't they be providing no gain, thus not adding much to your sound?? I think that's what the Focusrite tech was trying to say.

I would also second the RME option if you could find one used or save up a little more money.

Of course, I could be missing something big. Wouldn't be the first time...

[inverseroom]

well it's not an interface, you might want to check out the mackie onyx 800r. it's an 8 channel rackmount preamp, but if you buy a db25 cable you can add outboard pres and just hit the "line" button on the front to bypass the onboard pres. then it's got adat out so you can buy the cheapest interface you can find that's got adat optical ins (in my case, the motu 8pre) i'm still kindof a n00b but i really like this setup and i believe the converters on the mackie are pretty decent and i like that i now have the ability to blow money on new preamps whenever i want javascript:emoticon(':D')

Nice to know! I should add though that, in my view, there is nothing wrong with NOT bypassing the Onyx pres. I usually use mine for some nice clean gain after my more flavorful pres (Hamptone). I think they are really good!

[Smitty]

This may come across sounding a little stupid, but bear with me for a sec. The line input specs you listed go from -10dB to +36dB. If you set them to 0dB, wouldn't they be providing no gain, thus not adding much to your sound?? I think that's what the Focusrite tech was trying to say.

I would also second the RME option if you could find one used or save up a little more money.

Of course, I could be missing something big. Wouldn't be the first time...

well, i think you're correct... that's what the focusrite tech was trying to get across... but i guess that's part of my question:

would running a signal from a quality preamp through the gain stage of a lesser quality one (even without it adding gain) affect the sound? i tend to think it would, but i'm consistently wrong about all kinds of stuff, so who knows?

i'm looking hard at the Fireface 800 now. i love that you can double-plug the front and back inputs and route them as seperate tracks into my DAW. i'm pretty sure i could find one used for under or slightly over a grand.

part of me wants to get it just because i know it's quality and will just work and do everything i need it to and sound good... and the other part of me is hesitant to spend that much (that same part of me has a girlfriend that wants to go on vacation soon and will not understand why a small piece of electronic equipment worth approximately 1/2 of my car is preventing us from doing so).

time to save, then.

[red cross]

Try and find a used Metric Halo 2882. The "mic/line" gain stage is clean as a whistle. Convertors sound pretty damn good too.

[Andy Peters]

what, technically speaking, is a line input? i thought it just made reference to the level of the signal being input, but it seems like in this particular discussion it's being used interchangably with "input that bypasses the preamps." can i just look for a unit that has line inputs and call it a day?

You can put "input types" into three broad categories: mic, line and instrument.

Mic signals vary greatly (a D112 in a kick drum has a lot more level than a flute a foot from a C414) so mic preamps offer wide gain ranges. And since the mic preamp is the first gain stage in the chain, very low noise is a requirement. Because pretty much every microphone has a balanced output, the mic input has a balanced input. The input impedance is set as low as practical, since input Z times gain contributes to the preamp noise figure, but not too low to load down the mic. Common mic impedance is 150 ohms or thereabouts and the 2k load presented by common preamps easily bridges the mic. NB that some mics do want lower load impedances.

A line level input is presumed to be already at its nominal or proper level, so line inputs may dispense with an input trim altogether, or offer one that varies over a small range, like -10 dB to +10 dB. Input loading is set to a moderate impedance, often 10k ohms, which is an insignificant load to most drivers (and it lets you drive more than one input without degradation). Since there's usually no gain, the moderate impedance doesn't significantly add noise.

An instrument input presents a very high input impedance, often 1Mohm or more, so as not to load down the moderate output impedance of instrument pickups. While the high impedance does contribute to noise, you usually don't need a whole lot of gain and the pickup itself is noisy.

Now you often see "mic/line" inputs. Basically, these share the preamp circuit, but when in line mode a pad is inserted before the preamp (to prevent overload and to allow the gain control to have some reasonable range) and the input impedance is set higher. (Many designs that have a mic in jack and a line in jack for the same circuit prevent the phantom power from going to the line source.)

So the question you're probably asking is, "should I bypass the combined mic preamp/line channel and just use a standard line input so I don't get the 'coloration' of the mic input?" Answer? Well, choose which op-amps you prefer but I think that there really isn't any audible difference between the two circuits on these sorts of interfaces, especially when dealing with line-level signals.

FWIW, the mic preamp on the original MBox (I haven't opened up an MBox 2 yet but I assume it's similar) is a standard sort of three op-amp diff-amp with gain circuit. You have two unity-gain buffers that use very-low-noise op-amps followed by an op-amp in the usual diff-amp configuration. The signal inputs to the non-inverting inputs of each of the unity-gain buffers, and 2k pulldown resistors at those inputs set the input impedance.

-a

[Velcrocore]

I have two Echo Layla 24's (second gen)

They each need their own PCI card and don't have any mic pres, but you can pick them up cheap on ebay, they sound great, and you'll have 16 input channels. Then its just a matter of finding an old mixing desk with channel outs. The set up isn't portable, but it's cheap and easily upgradeable.

I also have a Firepod at home. 8 PreSonus mic pres, with no way around 'em. Great for portability though.

[ashcat_lt]

If the preamp on the converter is in fact flat from 20-150K, and has a relatively low %thd, then it is a quality preamp. It will be transparent and clean, and will essentially do exactly what a preamp should, which is adjust the overall level of the signal to something more suitable for the next stage.

This used to be a good thing, back in the days of tape. You got all the warmth and coloration you could ever need from the tape itself. Sometimes even if you didn't want it.

Then digital recording became popular and accessible thanks mostly to the convience of non-destructive editing in the digital domain. Very quickly people started to complain that their recordings were too clean, sterile, "digital sounding."

So they started looking for ways to replace the failures of the tape in reproduction of frequency and dynamic content. Colored or warm preamps, especially those with tubes, presented themselves as a quick and effective way to accomplish this.

Warmth and color in a preamp are actually ways of describing how it fails as a preamp. It is imparting a certain frequency response and a non-linear gain envelope. The input gain is often adjusted way up so as to push the circuitry even further out of its linear range, which causes the introduction of certain harmonics which were not present in the source material.

If you're looking to use a preamp for this warmth and color (there are a number of terms used today which are essentially synonomous), you can't really do much harm following it with a perfectly flat and clean preamp like the one on the converter. At worst, you've inserted a non-essential circuit into the chain. This will add a small (probably not audible) amount of noise, but assuming it's as clean and flat as they say it is, it shouldn't do anything else.

[Smitty]

thank you, ashcat... that's a very precise and well-worded answer to a question that i hadn't yet heard a good answer for.

so as long as you can keep from clipping the converters with too much gain (that is possible, i assume?) you'll be okay?

[snatchman]

Well, I wouldn't actually consider this as an interface, but I would like to sorta share what I 've experienced, since this is talking about converters and pre-amps!... ..I recently got a HD24-XR. I have been using a Presonus LT that had been modded by Matt at BLA. First, I tried running the LT into the XR via Lightpipe. The modded pres on the LT sound very clean and usable going into the XR. The DA converters on the XR is very smooth sounding BTW. Then I by-passed the LT pres by going into the inserts. I came straight out of an OSA mic pre into the LT insert, just using its converters. The mod Matt did to the converters really shine here! The OSA sounds big and 3-dimensional ( even tho the on board pre mods are clean, but not as big soundings as the OSA, which is to be expected).

Then I went straight into the XR from the OSA via analog input. The AD converters on the XR sound very good also! Warm, smooth. and clean. The LT's modded converters sounds faster, more detailed. I can definately see myself using the LT's converter along with the OSA preamps or other outboard pres of the same calibre! Great job on the mod Matt! Thanks everyone for listening. BTW, the XR seems to be a " big" improvement over my ...Adats XTs.... ...

[ashcat_lt]

thank you, ashcat... that's a very precise and well-worded answer to a question that i hadn't yet heard a good answer for.

so as long as you can keep from clipping the converters with too much gain (that is possible, i assume?) you'll be okay?

Well, I was talking generalities. Others may be able to provide proof that an individual manufacturers pre's are less "near perfect" than others. You might not want to buy that one unless you can bypass the pres.

It's kind of up to you how "good enough" you want it, and whether or not you could improve your mixing experience more dramatically by investing in other aspects of your studio.