Questions about preamps and DI Boxes

[Farview]

Ok, I am back with another quick question.

Let's use the Lexicon Omega as a reference:
http://www.musiciansfriend.com/product/Lexicon-Omega-Desktop-Recording-Studio?sku=245505

Manuals are here: http://www.lexiconpro.com/ProductDownloads.aspx?ProductID=6&Type=4

This has two preamps (one on each mic input). It also has 4 line inputs and one instrument input.

Now, there are "gain" knobs for both the mic inputs and all the line/instrument input(s).

This is where my questions lies. Doesn't "gain" mean the signal is being boosted? If so, how can there be a "gain" knob on the line/instrument inputs if they do not have preamps? What are they actually doing? Thanks!

There are only two MIC preamps. The rest are line amps.

A line signal may, or may not, need to be boosted. There are line preamps in there that do that. They will not provide the same amount of gain as a mic preamp. In fact, the line inputs on a 'mic' preamp will generally either bypass one of the amplification stages or will go through a 20 db pad before going through the preamp.

Not all preamps are mic preamps and not all amplifiers are 'preamps'.

You are either getting stuck on the terminology, or you think someone is trying to put one over on you.

It really isn't this complicated.

Any piece of audio equipment will:
1.Have an input that is designed with the proper impedence, level, connector, etc... to take the signal it was meant to take.

2.Then through preamps, transformers, opamps, rectifiers, etc... changes the signal into what it needs to in order to perform it's function.

3.It performs it's function.

4.Through preamps, transformers, opamps, rectifiers, etc... changes the signal into what it needs to in order to output it in the expected manor.

That's it. It's really that simple.

In the case of these all-in-one audio interfaces, the inputs are the proper impedance to accept the signals that they are meant to accept. The signal then is amped, unbalanced, attenuated, buffered, etc... whatever needs to be done to match the signal to what the converters need to recieve. After the converters convert the signal to digital, it is then converted into whatever protocol firewire needs to function. Then, once in the computer, it turns into the digital stream the DAW uses.

I'm not sure what you are getting at with all this.

If there is a gain knob, there is an amplifier somewhere. Even if there isn't a gain knob, there might be an amplifier.

 

[ahrenba]

You are either getting stuck on the terminology, or you think someone is trying to put one over on you.

LOL! That was actually genuinely funny!

Ok, I understand that in a dedicated preamp, the line inputs may bypass the "mic" preamp.

With these all-in-one devices, like the Lexicon Omega interface, it specifically says there are two "mic preamps" in the box.

So, my main concern is when you connect a guitar directly to a line input. I realize that the input is expecting a higher voltage than the guitar can give, but since you said that these inputs do have an amplifier (because it has a gain knob), why can't you plug a guitar into the line input and just use the gain knob to turn it up?

So, let's see if I can make sense of this. The only real reason for a preamp (other than tone shaping) is to get the voltage up to a level that the input is specifying? Is this why no preamp is needed when you plug into a instrument input?

I believe my logic above is correct. The only thing that mystifies me is this: the level of the signal from a guitar is so much lower than that of a line level.... Even if you were to plug into an input designed to accept this lower voltage, wouldn't the level (volume) of this signal be much lower than something that, let's say, plugs into a line input? Since these signals both get outputted to something, i.e. computer, wouldn't this affect the recording (or output) since one will be higher?

Man, I am probably angering you with all these questions. I am still a newb. I can't tell you how much I appreciate your time though.

I am done after this post....seriously

EDIT: Here is a diagram of the signal routing for that Lexicon. https://img253.imageshack.us/img253/8486/examplelj2.png
What do the little triangles and things mean near the line inputs?

 

[Farview]

So, my main concern is when you connect a guitar directly to a line input. I realize that the input is expecting a higher voltage than the guitar can give, but since you said that these inputs do have an amplifier (because it has a gain knob), why can't you plug a guitar into the line input and just use the gain knob to turn it up?

Line inputs are a different impedance than instrument inputs. The line inputs might also be balanced, your instrument is not.

So, let's see if I can make sense of this. The only real reason for a preamp (other than tone shaping) is to get the voltage up to a level that the input is specifying?

Tone shaping is not a reason for a preamp. Some mic preamps have a sound, but that is not the purpose or the point of a preamp in general.

Is this why no preamp is needed when you plug into a instrument input?

There probably is a preamp on the instrument input, just not a mic preamp.

I believe my logic above is correct. The only thing that mystifies me is this: the level of the signal from a guitar is so much lower than that of a line level.... Even if you were to plug into an input designed to accept this lower voltage, wouldn't the level (volume) of this signal be much lower than something that, let's say, plugs into a line input?

It would until the signal was amplified up to line level.

Since these signals both get outputted to something, i.e. computer, wouldn't this affect the recording (or output) since one will be higher?

No, because all of this is compensated for within the unit.

I will copy and paste this from my last post. Keep reading it until you understand it.

It really isn't this complicated.

Any piece of audio equipment will:
1.Have an input that is designed with the proper impedence, level, connector, etc... to take the signal it was meant to take.

2.Then through preamps, transformers, opamps, rectifiers, etc... changes the signal into what it needs to in order to perform it's function.

3.It performs it's function.

4.Through preamps, transformers, opamps, rectifiers, etc... changes the signal into what it needs to in order to output it in the expected manor.

That's it. It's really that simple.

 

[ahrenba]

Lol, ok. Thanks for your help.

 

[ahrenba]

Oh crap!! Found something else!!

http://www.musiciansfriend.com/product/Alesis-MultiMix-8USB-Mixer-with-USB-and-DSP?sku=630166

That mixer has 4 channels with high-gain knobs. Each channel has an XLR input and a 1/4" line input. The manual instructs the user to plug a guitar into the line level inputs with gain, boost it, and it'll work fine. It also says that line level devices can be plugged into it.

Here is what is says about these "gained" inputs:

These inputs accept line-level instruments such as keyboards and drum machines. If you find that your instrument has a weak line signal, just plug it into channels 1 – 4 and crank it up with the Gain knob.

Yet, it shows a guitar being plugged directly into the line input on the diagram.

How can this work since the impedances, voltages are different for instrument and line level? If this works, why couldn't a guitar be plugged into that line input on the Lexicon (since it too has gain).

I guess I just don't see why you can't just plug a guitar into a line input, as long as it has gain, given that it works with this mixer. Can't you just crank the gain up high enough until it's workable?

Damn, I knew I said I wouldn't post again, but it was too tempting.

 

[Farview]

They have done one of two things:

1. They have split the difference with the impedance, so it will work fine with either a line signal or a guitar signal

OR

2. They assume that since you are buying this piece of crap, you won't notice the difference cause by your pickups being loaded improperly.


My guess is that you would hear a big difference in sound between using a direct box and just plugging right into the line input. It might sound fine for your application, it might not.

A lot of this impedance matching stuff isn't the end of the world. Some people are perfectly fine with plugging guitars right into the line input on a board. If you do it and you like the sound, you're fine. If it sounds thin (or muddy) you probably have a big mismatch, and you will need a direct box.


It's kind of dangerous trying to gleen technical knowlege from advertisements. They are trying to sell something, so they will write the description so that it sounds like it's performing magic.

These are the same clowns that tell you to 'record as loud as you can without hitting 0dbfs' in their manuals, and we all know what a load of horseshit that is.

 

[ahrenba]

Ok, this'll probably be an easy question for you. It is a digression from the original questions.

Just to experiment, I tried plugging my guitar directly into the mic input and line input (both are 1/8" inputs) on my computer's soundcard. Just to see what happens.

I believe the input impedances of these are:
Mic input: 600 oHm
Line input: 46 koHm

I am assuming my guitar pickups are around 100 koHm's.

I made sure that software boosting of signal was turned off. For some reason, I got a much louder signal when plugging into the mic input than when I plugged into the line input.

This would be the opposite of what I would expect. Since the mic input has a MUCH lower impedance, wouldn't this severely limit the voltage that gets passed on, thus creating a quieter signal than the line input (with the higher impedance)?

However, this was opposite. I don't understand. Shouldn't the line level input be creating a louder signal than the mic input?

Why is this happening? What is causing it to do so?

 

[Farview]

Ok, this'll probably be an easy question for you. It is a digression from the original questions.

Just to experiment, I tried plugging my guitar directly into the mic input and line input (both are 1/8" inputs) on my computer's soundcard. Just to see what happens.

I believe the input impedances of these are:
Mic input: 600 oHm
Line input: 46 koHm

I am assuming my guitar pickups are around 100 koHm's.

I made sure that software boosting of signal was turned off. For some reason, I got a much louder signal when plugging into the mic input than when I plugged into the line input.

This would be the opposite of what I would expect. Since the mic input has a MUCH lower impedance, wouldn't this severely limit the voltage that gets passed on, thus creating a quieter signal than the line input (with the higher impedance)?

However, this was opposite. I don't understand. Shouldn't the line level input be creating a louder signal than the mic input?

Why is this happening? What is causing it to do so?

That's because The mic input is expecting a mic level signal and the line input is expecting a line level signal.

The mic input is amplified more than the line input. That amplificaton more than makes up for the loss due to the impedance mismatch. Remember, the line input is still lower impedance than an instrument input. So the pickup is (possibly) loaded in both situations.

You might not notice a tonal difference between the inputs because neither are the correct impedance and it's a POS stock soundcard.

 

[ahrenba]

Ah, ok. Cool. Thanks for the explanation. That makes sense.

So, basically all inputs (mic, instrument, and line) will usually be amplified somewhere in the circuitry, even if it doesn't say so or has an adjustable knob?

If so, is the main point of all this amplification to get the signal to line level?

If I am correct here, one would assume that the line input wouldn't need to be amplified at all since the signal is in fact (usually line level). Since it is amplified, is this to give you a little extra headroom in case the signal isn't quite line level, and in turn, it'll boost it just the amount needed?

Thanks again man!

 

[Farview]

So, basically all inputs (mic, instrument, and line) will usually be amplified somewhere in the circuitry, even if it doesn't say so or has an adjustable knob?

'amplifier' is a generic term. There are hundreds of different kinds of amplifiers that are there for hundreds of different reasons.

If so, is the main point of all this amplification to get the signal to line level?

The preamp on the mic input is. I can't speak to the rest of it because I don't have the schematic in front of me.

If I am correct here, one would assume that the line input wouldn't need to be amplified at all since the signal is in fact (usually line level). Since it is amplified, is this to give you a little extra headroom in case the signal isn't quite line level, and in turn, it'll boost it just the amount needed?

I never said that the line input was amplified. There might be an opamp in there as a buffer, but I don't know that for sure.

Like I said, 'amps' are used for many more things than just boosting signal.

 

[ahrenba]

Alright, cool. Last question.

When something has a dedicated instrument input, wouldn't that theoretically mean that the circuitry has to have a preamp (or amp) for that input?

I realize that the impedance and voltage expectations for an instrument input are geared down to what something like a guitar would output. However, this ensures a good connection, but as we know guitars (instruments) are not line level.

A good connection allows the maximum voltage transfer and minimizes frequency loss, etc., but really does nothing else, correct? By itself, a connection doesn't boost the signal, right?

If a dedicated instrument input didn't have a preamp somewhere in its circuit, what would the benefit of it versus a normal input....other than minimal frequency roll off? This is why I would assume that these instrument inputs are preampped in order to get the signal up to line level. Would it be safe to assume so?

The above summed up: If not preampped, what is the benefit of the instrument input and if I use a instrument input, do I still have to use an external preamp?

Last question ....Is it possible for me to add like 10 positive comments for your rep. You have been awesome.

 

[Farview]

When something has a dedicated instrument input, wouldn't that theoretically mean that the circuitry has to have a preamp (or amp) for that input?

It's possible. It could have just a transformer. There is more than one way to skin a cat.

I realize that the impedance and voltage expectations for an instrument input are geared down to what something like a guitar would output. However, this ensures a good connection, but as we know guitars (instruments) are not line level.

A good connection allows the maximum voltage transfer and minimizes frequency loss, etc., but really does nothing else, correct? By itself, a connection doesn't boost the signal, right?

The impedance of a circuit is a result of the circuit design. The 'connection' doesn't exist without the circuit, so your question doesn't make sense. Of there is an amplifier on the input, that amplifier would be designed to have proper impedance for the duties it was designed to perfrom.

If a dedicated instrument input didn't have a preamp somewhere in its circuit, what would the benefit of it versus a normal input....other than minimal frequency roll off?

there are other ways the system could be configured. For example, the input could connect directly to a digital converter that was calibrated for guitar level signals. You would not need a preamp because the level difference would be compensated for in the converter.

This is why I would assume that these instrument inputs are preampped in order to get the signal up to line level. Would it be safe to assume so?

It's a good bet that there is some sort of preamp on most instrument inputs, but it's not a given. And it doesn't matter.

The above summed up: If not preampped, what is the benefit of the instrument input and if I use a instrument input, do I still have to use an external preamp?

If it's an instrument input, you can rest assured that it is designed to accept an instrument level signal in a useful way.