line level

[TKC]

*first post*

 

Hey all, I was experimenting with different inputs on a board the other day and tried using a headphone out

from various amps into a line level input, the results were very varied(volume). I am now curious as to

what exactly is a line level input and how it differs from a mic level or headphone out level.

 

..although maybe this should be in Elec & Power?

 

anyway any guidance?

[paulears]

def not elec & power - here is fine.

 

In essence, and this is simplified quite a bit, there is a large difference in levels.

Microphone level is millivolts - in reality, tiny amounts. Inputs designed for microphones need bags of gain to boost this level to something mixers and other equipment can deal with.

Line level tends to be centred around a couple of fixed level - domestic and pro equipment do use slightly different ones, just below or just above a volt. If it's a consumer style piece of kit, or a pro model that has phono type uput sockets, then a full scale reading on the meters should give you .775 of a volt. As few manufacturers seem to set their maximum levels that accurately, it's often a bit more.

 

Loudspeakers and Headphones

The idea here is not just a voltage issue, it's a current one too.

You need to have enough umph to MOVE a loudspeaker cone. A small one in the case of a walkman, a big one in the case of an 18" sub. Voltage here depends on the equipment. The walkman with batteries can't produce more than the maximum voltage of the battery supply. A mega PA may have 80-100V available at considerable current.

 

Line level inputs often have a bit of gain too - handy for things like guitars, which don't produce as much output as a CD player, but more than a mic - sort of half way house.

 

Some mixers have mic inputs that can handle mic level AND line level. Some have separate inputs often on jacks for line, and XLR for mic.

 

Problems

 

Plugging a CD line level out into a mic input on lower level spec kit produces distortion - the input, even with gain turned right down is overloading.

 

Plugging a mic into a line level input (ignoring the balance/unbalanced issue) produces hiss, because the gain has to be cranked right up.

 

Plugging a guitar into a real line input may be a bit noisy because of the extra gain required.

 

Plugging a guitar into a mic input might be a little distorted, but on most kit is just about ok.

 

Plugging a walkman headphone out into a line input normally works

 

Plugging a pa loudspeaker out into a line input is usually EXPENSIVE TO FIX

 

Plugging the output of a guitar stomp box into a mic input is usually a little too much.

 

Plugging a mic into a guitar amp, sort of works - a bit, but is usually very hissy and always sounds rotten!

 

There are huge simplifications in this text, but the basics will hold till you try it for yourself

[StevieR]

As a related-point on this issue...

 

The problem presented by different signal levels is one where a DI (Direct Injection) Box can assist. One of the functions provided by (most) DI boxes is signal attenuation. The unit will allow you to connect most inputs to a mic input on a mixing desk and you use the gain attenuation switch to select how much you would like to drop the signal level. Most boxes normally have three settings (0db, -20db and -40db). So, for example, if you are connecting an instrument with a very low signal level such as an acoustic guitar then a setting of 0db would be fine. However, if you want to connect a keyboard whose output is at 'Line Level' then a setting of -20db or even -40db will be more appropriate.

 

Steve

[Carey D]

Hey all, I was experimenting with different inputs on a board the other day and tried using a headphone out from various amps into a line level input, the results were very varied(volume). I am now curious as to what exactly is a line level input and how it differs from a mic level or headphone out level.

 

I'll add another angle to Paulear's helpful post...

 

MIC level - A few millivolts - Very small voltage created by air waves vibrating the tiny diagphram of a mic capsule. Converts sound into electrical voltage. Varies according to how loud the source is, for example a loud rock singer close to an SM58, or quiet distant pickup of a float mic in a stage production.

 

INSTRUMENT level - Tens of millivolts - Similar to mic level, a small voltage produced by guitar and other instrument pickups. These sources are often high impedance which means that there are other considerations apart from level. A DI box may be used to convert these to low impedance mic level to feed a console channel.

 

LINE level - Around a volt - The 'normal operating level' of mixing consoles and signal processing equipment such as compressors and cross overs, as well as sources such as CD players and recorders. This derives from the power supply voltages used to run such circuits. In pro gear, a dual rail supply such as +/-15V is typically used. Here, a 0dBu (0.775V) or +4dBu (+1.23V) standard allows enough headroom before clipping and keeps the signal clear of the electronics residual noise floor (hiss). In consumer gear, a lower voltage or single rail power supply means a lower voltage should be used to maintain reasonable headroom. This is where the -10dBV (310mV, equivalent to -8dBu) standard comes from. Because of these different line level standards you may find +4/-10 switches on some gear. The console meters help the operator set gains and faders so that the signals average around the optimum line level.

 

SPEAKER level - Tens of volts - A much higher voltage needed to energise a speaker cone to move large amounts of air. The amplifier has high voltage power rails so that its line level input can be boosted to the level required to achieve the desired speaker volume without clipping. Speaker level varies according to power output, for example a few volts driving a home hifi system, or around 70V driving a high powered PA speaker. A headphone output is an example of low speaker level almost at line level. In fact, the headphone output can sometimes be used as a line level source.

 

With MIC, INSTRUMENT and LINE signals, maximum voltage transfer is what matters. For this reason, output impedance is low and input impedance high so that the source voltage is not attenuated by the destination. Very little current flows. With SPEAKER signals, power matters, so low speaker and cable impedance are used to ensure maximum power transfer through high current flow.

 

A DI box may have several inputs marked INSTRUMENT, LINE or SPEAKER. These provide the necessary attenuation to reduce those inputs to mic level to feed the console. In this case they can also convert an unbalanced high impedance signal to balanced low impedance able to drive a long cable run with minimum interference pickup... but that's another coffee session! :-)

 

Hope that makes sense,

Regards,

Carey

[TKC]

but that's another coffee session! :-)

 

thanks to all of you

 

but one more question for this coffee session, I think I read somewhere phantom power for use with a condensor mic etc, is +48V and normal dynamic mic uses millivolts, I've used a dynamic mic with phantom power on but didn't notice any adverse effects, is there some piece of circuitry that stops the +48V when using a dynamic mic?

something to do with the third pin in a balanced connection?

[Andrew C]

is there some piece of circuitry that stops the +48V when using a dynamic mic?

something to do with the third pin in a balanced connection?

Yes & No, in that order. There are capacitors on the input stages of the mixer to keep the phantom where it belongs. The 3 pins on a balanced connector are 1 - Screen, 2 - Signal Hot & 3 - Signal Cold. This has the effect that any interference that gets through the screen is induced equally into the hot & cold cables, but the wanted signal is only on the hot. In the mixer the hot & cold are summed cancelling out the noise but leaving the signal intact.

[Lamplighter]

TKC

 

The positive of the 48V phantom power is fed equally to the active signal lines of the microphone connection (XLR pins 1+2) and the negative side is connected to the cable screen (XLR pin 1 ). The element of the dynamic microphone is connected only between XLR pins 2 and 3 which are at the same potential ( +48V) so no current flows through the microphone coil. The +48 V is connected via resistors so that the signal between pins 2 and 3 is not bypassed. Simplified explanation!

 

Brian

 

PS must learn to type with 2 fingers

[ojc123]

While we're on phantom power, I know about not plugging in anything with the volume up and the phantom power on (my first post on this forum was to get new drivers which had blown because of this). What happens to cause the annoying and expensive thump?

[bruce]

In the mixer the hot & cold are summed cancelling out the noise but leaving the signal intact.

 

[pedantic]

difference (ie subtracted), not summed. So the +48V on both legs get cancelled. And any common noise on both legs also gets cancelled.

[/pedantic]

 

 

What happens to cause the annoying and expensive thump?

 

My guess is that it's to do with one "leg" connecting just before the other when you put the plug in....

[paulears]

I think I read somewhere phantom power for use with a condensor mic etc, is +48V and normal dynamic mic uses millivolts,

 

I thought I'd just tag this because, although it probably doesn't matter that much, you don't want to get the wrong things in your head.

 

A condensor mic has an on board preamp and this is what needs the 48 volts, fed, as has been explained down the mic cable.

 

It produces millivolts in terms of output, so as far as the mixer is concerned, the mics appear quite similar in terms of level. Dynamic mics don't need power to operate.

[Carey D]

While we're on phantom power, I know about not plugging in anything with the volume up and the phantom power on (my first post on this forum was to get new drivers which had blown because of this). What happens to cause the annoying and expensive thump?

 

When you select phantom power, the +48V DC gets switched to the + and - inputs of the channel preamp so it can feed the microphone via the XLR cable. The preamp input is protected from the DC voltage by a 'blocking' capacitor on each input. However, the transition from 0V to +48V when switching phantom on behaves like an AC voltage for that moment. It 'jumps' across the capacitor causing a brief high voltage spike. Because it is so big compared with the millivolts of typical mic signal being amplified this spike results in a very big thump. The same applies when the +48V is switched off.

 

A similar situation can arise when simply plugging a microphone into a preamp with phantom power already switched on. The +48V drops to a lower voltage according to the current drawn by the mic and attenuation of the series resistors routing the voltage from the console phantom supply to the XLR pins. That drop behaves as a brief AC spike that hops across the capacitors and causes a thump.

 

Hence the advice to mute the channel while switching phantom power on or off, or plugging mics into inputs already powered.

Hope that makes sense.

Regards,

Carey

[ojc123]

Hope that makes sense.

 

It does. Thank you.