Help me understand inputs, preamps and what I need...?

[walterg74]

Hi folks,

I want to start doing some recording to my computer, and for starting out I want to buy one of the entry level Focusrite Scarlett interfaces. Specifically, either the Solo or the 2i2, but I’m a little confused on the technicalities of it...

I know the 2i2 has the separate headphone monitoring volume control, and that the 2 inputs on it are hybrid xlr/1/4” whereas the solo doesn’t have the former feature and it has 2 separate xlr and 1/4” connector.

I also know the 2i2 has 2 preamps vs 1 for the solo, but this is where my noob confusion starts...

I see they call them “mic” preamps:


- Is this because microphones require a preamp and say an instrument would not? Or is it just a name that stuck by convention?

- If in answer to the above both need one, does that mean in the Solo both inputs *share* the preamp? (Since I have seen you can record from both inputs simultaneously)

- Would the separate preamps in the 2i2 provide better quality? Control? Both?

- If one uses the solo, and wanted to record from say 2 guitars, or a guitar and a bass to name some examples, would there be any issues with connecting one of them using a female 1/4” to male XLR adapter?

Thanks!!

 

[gecko zzed]

- Is this because microphones require a preamp and say an instrument would not? Or is it just a name that stuck by convention?

It's because a microphone produces a very low level output and needs a pre-amp to bring it up to a usable level. Instruments generate a higher level output, and require a different treatment at the interface input.

- If in answer to the above both need one, does that mean in the Solo both inputs *share* the preamp? (Since I have seen you can record from both inputs simultaneously)

No. They share the same output circuitry as they enter the analog to digital conversion part of the interface, but they have different preamp stages.

- Would the separate preamps in the 2i2 provide better quality? Control? Both?!

The 2i2 is likely to use the same circuitry but with added inputs. Quality would be the same.

- If one uses the solo, and wanted to record from say 2 guitars, or a guitar and a bass to name some examples, would there be any issues with connecting one of them using a female 1/4” to male XLR adapter?

Yes. The solo has one mic input and one instrument,that's it. You may not be able to get an undistorted sound from an instrument going via XLR adapter into the mic input because of the difference in the internal gain. You would also need to makle absolutely sure the phantom power was turned off. It's possible for this to be shorted out with an jack to XLR adapter.

 

[TalismanRich]

Since you are just getting into recording, I would recommend you read a few of the "Sticky" threads here in the Newbie section. They cover a lot of the topics, and though they are a bit dated perhaps, the concepts are covered. Its like driving a car... a 55 Chevy isn't the same as a 2020 Chevy but most of the basic skills involved driving them are the same.

Recording Interfaces Thread

As for your specific questions about a mic preamp, yes... Microphones put out such low signal voltage that you must amplify it to make it usable. That is where the mic preamp comes in. Typically a mic is measured in millivolts (thousandths of a volt) whereas line level signals like a mixer, tape recorder output, stereo tuner etc puts out a signal in volts (from a few tenths to 2 volts).

Most entry level interfaces have mic preamps to boost the millivolt signal, and line levels inputs for things like keyboards, drum machines, synths, etc. Within a given range of products, like the Focusrite Scarlett series, there should be minimal quality difference. Its more of a functionality difference.

While you can use a solo with a single mic, its a limitation that I would avoid because SOMEWHERE down the line, you'll want to record in stereo, or maybe with two people or an acoustic guitar and singer with two mics. My personal lower limit is 4 inputs. When I bought an interface, I got 8 channels. That way if I want to test a half dozen mics to choose the best for my acoustic guitar, I can throw them all up and hit record.

 

[walterg74]

Thanks folks, several things cleared up

I will go read the interfaces thread and take a look at the rest of the stickies.

So 2 follow-up questions then...

- If the requirements for boosting the signal are different, does that mean that for instruments the preamo is bypassed or is it still used I some way/stage, and it is in fact shared? (Maybe at a different stage? ) or is it a completely different part of the circuitry where these 2 signal types “meet”..? (Any diagram out there somewhere that might make it easier to see?)

- To ask bluntly, understood for microphones, but say specifically to a guitar... What would this interface provide/do, that something simple like say for example the 1/4 inch to USB cable for “Rocksmith”, does, where it also shows up as an interface in DAWs..? (Just in terms of the audio processing, etc, disregarding the monitor, etc features)

 

[TalismanRich]

It depends on the design of the actual interface circuit. Since majority of the basic interfaces using a combo jack for mic and line inputs, they can either add the line level signal downstream of the mic preamp, or simply add a load resistor to drop the signal before feeding it to the preamp.

Here's a quick explanation of the signal levels. Understanding Signal Levels.

Typical guitar pickups will be around 100-300mv or so for a normal passive pickup. Active pickups can be higher.

I've never used the 1/4" to USB cable for Rocksmith, but it has to have circuitry to convert the signal to digital form, like a USB mic. I'm not a fan of that approach, as you are limiting yourself since you can't gang several USB devices as inputs.

Basically, you give up a lot in terms of functionality and quality for cheapness.

 

[walterg74]

It depends on the design of the actual interface circuit. Since majority of the basic interfaces using a combo jack for mic and line inputs, they can either add the line level signal downstream of the mic preamp, or simply add a load resistor to drop the signal before feeding it to the preamp.

Here's a quick explanation of the signal levels.

Typical guitar pickups will be around 100-300mv or so for a normal passive pickup. Active pickups can be higher.

I've never used the 1/4" to USB cable for Rocksmith, but it has to have circuitry to convert the signal to digital form, like a USB mic. I'm not a fan of that approach, as you are limiting yourself since you can't gang several USB devices as inputs.

Basically, you give up a lot in terms of functionality and quality for cheapness.

Ok, so regardless of the method used, the interface will know what you plugged in and route accordingly, right?

In any case, would it be correct to say that xlr is just the connector itself? Because for example, I have a bass amplifier that has xlr output in the back, and unless that comes straight from the input(?) I’m pretty sure I would blow something up if I connected that to the scarlet xlr input?

In short, would it be correct to say the for the *Solo* one should only connect an xlr microphone to the xlr input, and an instrument to the 1/4” input, and not get creative?

Regarding the usb cables, I understand what you said about the quality, etc. but not quite sure I understand what you meant about using several? Each one should show up as a different input, and while I haven’t tried it yet (I will since I have 2 anyway) for example in the game itself you can hookup a guitar and bass, or two guitars, etc. and both play at the same time.

 

[bouldersoundguy]

On most affordable gear where I've seen the schematic, the line input is just padded down and fed to the mic preamp.

The best way to connect an instrument or line signal to a mic preamp (when there's no High Z or Guitar input) is with a DI. If the instrument is a piezoelectric pickup on a guitar with no onboard preamp, use an active DI. Even if you have a hi-z/guitar input, having a DI or two can be really useful.

Separate USB devices have their own clocks (since the converters takes samples of the analog signal at a steady rate, the sample rate). Unless those samples are all made to happen in lock step with each other by synchronizing their clocks there will be errors. This can be overcome using "aggregate device" or similar features, which means the signal of all but one have to be resampled to match the one that's considered the master. That's fine for a game but it does mean a small loss of quality, which isn't really the best way to start a recording. So it's better to consolidate all the inputs into one device with one clock. Also, having the output converters on the same clock allows for direct monitoring etc.

 

[TalismanRich]

Ok, so regardless of the method used, the interface will know what you plugged in and route accordingly, right?

That depends on the interface and what you are plugging in. On the Solo, the XLR is for microphone inputs. Any line level inputs should go in the 1/4" Line/Instrument channel with a button to choose the level. My Tascam interface is the same way, with the mic preamps ONLY for mic level signals. There are Line/Instrument inputs on the front with a switch for choosing the level. On the back are line inputs with options for either -10 or +4 dBu levels.

In any case, would it be correct to say that xlr is just the connector itself? Because for example, I have a bass amplifier that has xlr output in the back, and unless that comes straight from the input(?) I’m pretty sure I would blow something up if I connected that to the scarlet xlr input?

In short, would it be correct to say the for the *Solo* one should only connect an xlr microphone to the xlr input, and an instrument to the 1/4” input, and not get creative?

Maybe.... Most all mic inputs are XLR. Its possible to have balanced XLR line inputs, balanced TRS 1/4" jacks or unbalance TS 1/4" jacks. As for using the bass amp, you need to consult the manual to see what voltage level is specified. You don't want to pump 1.3 volts in the a circuit designed for 20 thousandths of a volt.

Regarding the usb cables, I understand what you said about the quality, etc. but not quite sure I understand what you meant about using several? Each one should show up as a different input, and while I haven’t tried it yet (I will since I have 2 anyway) for example in the game itself you can hookup a guitar and bass, or two guitars, etc. and both play at the same time.

Windows doesn't normally allow you to specify more than one USB input device. That means that you can't plug in a 4 port USB hub and plug 4 USB mics in, fire up your DAW and have 4 channel recording. Its an issue with the operating system. Instead get an proper interface if you plan to use multiple inputs at one time.

 

[walterg74]

Windows doesn't normally allow you to specify more than one USB input device. That means that you can't plug in a 4 port USB hub and plug 4 USB mics in, fire up your DAW and have 4 channel recording. Its an issue with the operating system. Instead get an proper interface if you plan to use multiple inputs at one time.

Hmm do you mean for audio recording purposes? Because "input device" is a very generic term, which most definitely works as my many game controllers attest to.

If for audio, I will be testing it in a little while and probably be proven wrong



P.S. Regarding the interfaces yes, I meant on something like the 2i2 that has the hybrid connectors, and it routing correctly based on the inserted plug. It's clear to me now that on the "1-type" like the solo you should connect the right thing to the right input.

 

[TalismanRich]

Yes, I was talking about USB audio devices. It will do multiple HDs, mice, and other devices.

As for the 2i2, you still have a switch for the 1/4" jack to choose line or instrument. Per the user manual:

Note the Scarlett 2i2 has no “Mic/line” switch– the Focusrite preamplifier stage is automatically configured for a microphone when you plug an XLR into the input, and for a line or instrument when you connect a jack plug. Set the LINE/INST switch next to the socket to INST if you are connecting a musical instrument via an ordinary 2-pole (TS)guitar jack, or to LINE if you are connecting a line level source such as the balanced output of a stage piano via a 3-pole (TRS) jack.

 

[rob aylestone]

These products are well designed for home recordists and really do a brilliant job. Mixers of all kinds tend to be built in modules or blocks. So the smallest level signals get boosted in level my the first stage of the preamp, then the next stage allows the intermediate level audio from things like guitars 'merge' with the path, and then a little more boost is applied so that the next stage where we're talking about just under and just over a Volt can integrate properly. That's the end of the preamp part of the interface. Next, the audio hits the analogue to digital converter stage - where it gets converted into something the computer can process.

all these stages are built with matched 'quality'. No point having a brilliant first stage and a mediocre second one. In practice they also design them to work with idiots - as in us! The mic preamp will be cleverly designed just in case we have a jack to XLR lead laying around and we plug our active bass into it and squirt far too much signal into the microVolt expecting stage. Red lights come on - your speakers do their best and it sounds grim, but often, turning the gain down works - they've been built to cope with this. Plugging a CD player into the instrument jack is too much, but they cope. Clearly, the best audio comes from getting these levels matched as much as you can.

For what it's worth - inputs in pairs are always the best option - two mics, or two guitars, or stereo from a CD etc - having one mic and one guitar for instance is quite restrictive. Loads of people like the 2i2 because it sounds fine, does the job and is well designed.

 

[ecc83]

I am always wary of these very low cost, single mic input interfaces. Not only are they very limiting, as Rob said, one day you WILL want to use two mics for stereo or jam with a mate but I also suspect that costs are cut in other areas. Noise and headroom levels might be compromised and available gain restricted. They re maybe ok if all you want to do is put pods on YT but for even serious/fun music recording, go two mic channels. Yes, there are much more expensive and better specified AIs with just one mic input but they generally provide ADAT expansion.

I was not the greatest fan of the 2i2, I thought there was better value and as good AIs around but in the last two years or so, all the mnfctrs have upped their game and in the sub $200 market there is really nothing in it. (IF I were pressed I would go a wee bit upmarket and say "NI KA6 or the MOTU M4" but out of the 200 buck area.)

I will NOT have 'The Mixer' demonized! For sure, not what the newb wants nor doe he/she want a pre amp but once he has an AI like the 2i2 and has put some time in with that and a DAW the situation might arise to record 3 or 4 sources or say a drum kit? A modest 4 mic input mixer can feed the AI and deliver a 'panned stereo mix'. Mixers can also be useful for talkback, zero latency monitoring, gash extra headphone amp, general purpose signal booster, door stop.

Dave.

 

[hautbois16]

An electric guitar also needs a preamp to bring its signal up to a usable level unless the guitar is one of these newer "breeds" I have seen lately billed as acoustic/electric guitars. You might happen to know that for many years, there have been "pickups" which could be added to a standard acoustic guitar, usually by mounting the pickup across the hole by some kine of bracket or perhaps other means of screwing the pickup to the body of the guitar, then drilling a hole somewhere to accept the jack for connecting the cord to the amp. As I understand it, these "pickups" are pretty much microphones. By contrast, the acoustic?electric guitar of which I now speak has a pickup mounted, as in the case of the one I own, under the saddle. That pickup is connected to a preamp powered by a nine-volt battery mounted inside the guitar. My unit is an Epiphone having a volume control, bass, mid, and treble controls along with a phase-reversing switch that serves to reduce the likelihood of feedback when the guitar is close to the amp. Remember that the pickup is a microphone rather than a magnetic pickup. Epiphone apparently protects the battery by a trick long known by makers of guitar pedals: One terminal of the 9-volt battery goes to the ring terminal on the jack. If no cord is inserted in the jack, that terminal "floats" without being connected to the electronics. When you connect a standard two-conductor plug into the jack, the sleeve of the plug shorts that floating terminal to ground because it touches both the ring and sleeve contacts, thus turning on the electronics of the pedal or, in this case, the preamp inside the guitar. The instructions for such equipment will caution the user to remove the cord when the instrument is not in use, thus saving the battery.
The point here is that the output of these acoustic/electric guitars is probably at or very close to line-level output. It would likely need to be connected to a line input rather than a mike input. Connecting such a guitar to a mike preamp might overdrive the input, causing horrible distortion. But then, maybe that's what some guitar players might want for some of the new music coming out of late.

 

[rob aylestone]

The thing to remember is that the important feature of the preamp design is level matching. If you design an excellent performing microphone preamp, then it's probably a sensible feature to leave that circuit as perfect as possible. If you have the next common level up from a mic, a passive guitar pickup, then you LOWER it's level and stuff it into the kit level circuit. You need to cope with the balanced/unbalanced change but that's pretty straightforward. Rather like a DI box bit with electronic rather than transformer coupled conversion. Active pickups, then keyboard level, then .775V line level, then 1.4V line level. Most people find little issue using well designed DI boxes to get the best S/N ratio. Other preamp designs (usually the more expensive) inject the signals at points in the preamp where levels match - which removes the need to reduce level and then boost it again - which with poor equipment can be where noise gets generated. With all my noise generating kit I like to set my own 'maximums'. My Presonus interface conveniently means the line on the gain pots must NOT be more than the line being at the 3 o'clock position. More than that, and hiss creeps in, lower than that - it's quiet. My older Tascam that I rather liked has a little less available gain. The Lexicon Omega I still occasionally use for live recording is quite noise free to near maximum - but it took me a while to realise it's not a lower noise preamp, it just has less gain. I need the Spinal Tap elusive 11 on the 1-10 fader for distance or quiet capture.