Mixing desk inputs - help, I'm confused!

[Walker]

I'm experimenting running my bass directly into the desk, via a DI box.

At the desk, should I go into the 'XLR input' or the 'Line input'? Or does the DI box convert the line level to mic level so the XLR input can be used?

The attached pic from my Yammy manual has confused me as it has a DI box into the line level input.

My DI is a VT Bass Deluxe (from the manual)...

[i]XLR Out Pad Switch: -20dB pad to match the output to equipment with different input level requirements. In the line level (up) position, the output is 0dB. In the instrument level (down) position, the output is -20dB.[/i]

We have a uke, mandolin, banjo and accoustic guitars, all going through DI boxes. Would it be the same for them?

Thanks very much,

Chris

Edited March 11, 2013 by Walker

[BigRedX]

Use an XLR cable from the DI box to the XLR input on the desk. Ideally you don't need to pad unless you are clipping the input of the desk with the channel gain on minimum.

[Walker]

Excellent, thank you.

[JoshOmachine]

And yes it is the same for all of the other instruments. As a rule DI boxes should always go straight to a mic input as their job is to take a line level unbalanced signal and transform it into a mic level balanced signal.

[rmorris]

[quote name='JoshOmachine' timestamp='1364157070' post='2022703']
And yes it is the same for all of the other instruments. As a rule DI boxes should always go straight to a mic input as their job is to take a line level unbalanced signal and transform it into a mic level balanced signal.
[/quote]

But in this case the DI has already provided a 'Line' (or thereabouts) level so it would likely be best to take the '0dB' line level output to a line input.
The details really depend on the internal circuitry of the mixer but it's essentially all about gain mapping. Attenuating a signal then boosting it again results in a worse signal / noise ratio so ideally you only want to attenuate at the last stage you can so you are cutting all the noise as well as the signal.
It gets a bit more complicated depending on the available headroom etc but that's the gist.

Where it's essentially a passive transformer DI then that will want to go to a mic input. Essentially the transformer trades a higher impedancce / higher level signal for a lower impedance / lower signal level and balances it into the bargain. But where you have active electronics you can get a 'line' level output that is low impedance and balanced.

Strictly 0dB is a relative level but I guess they mean 0dBu or similar which defines an actual signal level. 'Standard' proffesional line level is nominal +4dBu witha maximum level of around 21dBu or greater.
The '-20dB' pad makes the level generally suitable for amplifier inputs / fx pedals etc ( I have a Sansamp BDDI with similar output level options)

[Walker]

RMORRIS - this is incredibly helpful and coincidentally was supported by the book I was reading last night (the section was all about input signals and having non-contradictory EQ and gain structures across your gear). I'm going to take some time to carefully experiment with the equipment.

Thank you.

[JoshOmachine]

[quote name='rmorris' timestamp='1364247951' post='2023915']


But in this case the DI has already provided a 'Line' (or thereabouts) level so it would likely be best to take the '0dB' line level output to a line input.
The details really depend on the internal circuitry of the mixer but it's essentially all about gain mapping. Attenuating a signal then boosting it again results in a worse signal / noise ratio so ideally you only want to attenuate at the last stage you can so you are cutting all the noise as well as the signal.
It gets a bit more complicated depending on the available headroom etc but that's the gist.

Where it's essentially a passive transformer DI then that will want to go to a mic input. Essentially the transformer trades a higher impedancce / higher level signal for a lower impedance / lower signal level and balances it into the bargain. But where you have active electronics you can get a 'line' level output that is low impedance and balanced.

Strictly 0dB is a relative level but I guess they mean 0dBu or similar which defines an actual signal level. 'Standard' proffesional line level is nominal +4dBu witha maximum level of around 21dBu or greater.
The '-20dB' pad makes the level generally suitable for amplifier inputs / fx pedals etc ( I have a Sansamp BDDI with similar output level options)
[/quote]

Why would the DI be giving a line level?
Also traditionally you should be attenuating at the source so that the mixing desk 'sees' a level that is as close to +4db (u/v/m). That way it can be processed by the desk/effects/outboard without the need for any further gain structures until the point it is sent to the sound reinforcement (loud speakers/amps etc) for the reasons you gave. Obviously gain structures do appear but the idea is to eliminate as many as possible.

dB(u/v/m) are all relative levels as they are all relative to ground hence partly the need for that 3rd ground wire. The reason the figure 20dB often used for attenuating pads is that it is 4 times less the perceived volume of the signal and that it is roughly half the voltage of the signal, which is the important bit for the desk. The only 'defined' dB rating commonly used is dBspl or decibels over sound pressure levels which is the calibrated measure used to denote a loud speakers sound reinforcement capabilities. Or for licensing venues/events an agreed volume at a certain distance.

I wrote a paper on DI box design while at uni that I may have to dig out now. Unfortunately it was research into digitally controlled analogue gain structuring but the principles are the same.

[rmorris]

[quote name='JoshOmachine' timestamp='1364810273' post='2031025']
Why would the DI be giving a line level?
Also traditionally you should be attenuating at the source so that the mixing desk 'sees' a level that is as close to +4db (u/v/m). That way it can be processed by the desk/effects/outboard without the need for any further gain structures until the point it is sent to the sound reinforcement (loud speakers/amps etc) for the reasons you gave. Obviously gain structures do appear but the idea is to eliminate as many as possible.
[/quote]

Some level clarifications needed i think. Out of necessity these are generalisations but : A mic level is relatively tiny. The level from a passive bass / guitar is a fair bit higher than a mic signal but a fair bit less than professional audio line level (well established at +4dBu). Now if you have active gain available in the DI box then why would you choose to attenuate that to mic level just so you have to boost it back up with the mic preamp in the desk with the inevitable added noise.
So you'd probably choose to either put it out at unity gain and lift it to line level in the desk input gain stage or boost it to 'proper line level' at +4dBu so that the desk can take it at unity gain.
In general a you'd choose to get it to the 'correct' level as early as possible in the signal chain ie the DI box although it might depend on the detailed design of the DI and mixer input stage.
As for 'attenuating at the source...+4dB' - that implies that you have a signal larger than +4dBu to start with ??? That's only likely to be the case if you are taking a speaker feed into the DI.

[quote]
dB(u/v/m) are all relative levels as they are all relative to ground hence partly the need for that 3rd ground wire. The reason the figure 20dB often used for attenuating pads is that it is 4 times less the perceived volume of the signal and that it is roughly half the voltage of the signal, which is the important bit for the desk. The only 'defined' dB rating commonly used is dBspl or decibels over sound pressure levels which is the calibrated measure used to denote a loud speakers sound reinforcement capabilities. Or for licensing venues/events an agreed volume at a certain distance.
[/quote]

No. A dB figure is a relative figure. But dBu and dBv (and for that matter dBV) figures define an absolute voltage level. 0dBu is defined as 0.775Vrms ( giving +4dBu = 1.23Vrms). and dBu levels are used everywhere when quoting analogue levels in audio equipment , especially pro audio mixing desks etc.
dBm is actually a power level but is generally not applicable in modern audio systems although sometimes still seen quoted.
All this has absolutely nothing to do with the '3rd ground wire'. It's simply a level measurement and is valid for all wiring configurations - balance / unbalanced / floating etc...
20dB isn't 'half the voltage' . That would be 6dB ( dB = 20log(V1/V2).

[rmorris]

btw - before somebody points it out - yes some mixing desk line stages do attenuate a line signal before feeding it to the same differential amp as the mic input. But that is something of a design compromise - it's cheaper of course.

[Walker]

Thank you for your help with this everyone.