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Kieran C

Joined: 29/05/08
Posts: 3
Loc: Derbyshire

Level Standards and D.I.Boxes
#993393 - 18/06/12 02:00 PM

Hello Sound On Sound forum people,
I have a D.I. box that has 2 In Line -20dB pads on the input. I believe that this allows me to input a line level signal into the box instead of the expected instrument level.
Can anyone explain what kind of voltage differences and what kind of difference in dB I should expect between speaker, line(+4), instrument and mic levels? Also, if I were to simply attenuate a line level signal to for example an instrument level voltage, would that make it safe to plug into an instrument level input? Or would the fact that it was originally output at line level with a low impedance cause damage to the instrument level input that I plug it into?

I hope that all makes sense.

Thanks,
Kieran

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James Perrett

Joined: 10/09/01
Posts: 9706
Loc: The wilds of Hampshire

Re: Level Standards and D.I.Boxes new

[Re: Kieran C]
#993429 - 18/06/12 03:19 PM

Attenuating a line level by around 20-30 dB to feed an instrument level input should work fine.

It is difficult to give typical levels for things like mic signals as different types of mic have different output levels and something like loud shouty vocals into a high output condenser mic could easily give levels more typical of line outputs whereas distant mic'ed quiet acoustic instruments with a dynamic mic would be much lower.

Speaker levels can also vary from a few hundred millivolts up to 100 volts or more, depending on the volume and impedance involved.

James.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

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ef37a

Joined: 29/05/06
Posts: 5669
Loc: northampton uk

Re: Level Standards and D.I.Boxes new

[Re: Kieran C]
#993430 - 18/06/12 03:23 PM

Hi Kieran,

I don't know if this is the info you want but..

The lowest likely valve power amp is 5watts? This gives a voltage output on a 4 Ohm tapping of 4.5V rms = +13dBu
On the 16R tap that would be about 9V rms or +21dBu. 8R will be something inbetween (4.5 x 1.414 in fact) These voltages are "just" within any decent line level input.

Going up to a firebreathing 100watter Vout 4R is 20Vrms (+28dBu) and 16R 40V rms (+36dBu).

But, going into a highly reactive load such as a 4x12 the peak voltages can get much scarier! 100watters spitting out 100V peak is quite normal (+42dBu, and B loud!)

Do not of course forget that if you are DI'ing at speaker levels the amplifier MUST still be loaded at its rating and if using a resistive, "silent" load that load should be rated at at least twice the amplifier power delivery.

Dave.

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Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18530
Loc: Worcestershire

Re: Level Standards and D.I.Boxes new

[Re: Kieran C]
#993448 - 18/06/12 04:44 PM

Quote Kieran C:

I have a D.I. box that has 2 In Line -20dB pads on the input. I believe that this allows me to input a line level signal into the box instead of the expected instrument level.

Correct.

Quote:

Can anyone explain what kind of voltage differences and what kind of difference in dB I should expect between speaker, line(+4), instrument and mic levels?

In very round figures, most DI boxes assume speaker feeds will typically bw around +40dBu (~75V) -- that's what you'd expect from a 100W amp. Line level is around 0dBu (~1V), but most line inputs won't clip until they get to about +22dBu (~10V). Instrument level is about -20dBu (100mV), and microphone level is about -50dBu (~3mV) -- but that can vary enormously (see below).

However, I must emphasise that these are very much 'ball-park' figures. Speaker feeds could be 10dB up and 30dB down on that notional +40dBu. Pro line level is ususally a nominal +4dBu, but semi-pro line level is -8dBu, and the headroom margin means the loudest signals will be 20dB higher.

Instruments could be as low as -35dBu or as high as +4dBu -- depending on whether they are active or not.

Microphones vary enormously, with spoken voice into an old-school ribbon producing about -70dBu, and a D112 inside a kick drum kicking out 0dBu. But a typical dynamic mic with gently sung vocals will be about -50dBu or so, and a capacitor mic is probably about 10dB hotter with the same source.

Quote:

Also, if I were to simply attenuate a line level signal to for example an instrument level voltage, would that make it safe to plug into an instrument level input?

Yes.

Quote:

Or would the fact that it was originally output at line level with a low impedance cause damage to the instrument level input that I plug it into?

No. The source impedance of pretty much all line level deevices will be very low (~100 ohms or so), and will expect to see an impedance of about 10k ohms or more. Instrument inputs generally present an imput impedance of 250k ohms or higher, so there won't be any loading issues at all.

However, going the other way, most instruments need to see a very high input impedance (>250k ohms) and won't be at all happy with a measly 10k Ohms from a line level input!

You are also much more likely to have issues with the balanced nature of most line outputs, and the unbalanced nature of most instrument inputs!

Hugh

--------------------
Technical Editor, Sound On Sound

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Nathan

Joined: 13/09/04
Posts: 1872
Loc: lincolnshire government experi...

Re: Level Standards and D.I.Boxes new

[Re: Hugh Robjohns]
#995528 - 30/06/12 04:22 PM

40dB attenuation has always done me when I've taken a speaker feed from a keyboard combo (the only time I think I've ever done this). Wouldn't want to risk it on the back of big PA amps (>1kW), but should be ok for small stuff.

>

--------------------
planet nine
lincoln, uk.

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