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Author Topic: How DI Boxes Work?  (Read 8266 times)

Dan Mortensen

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Re: How DI Boxes Work?
« Reply #30 on: September 10, 2018, 01:22:40 PM »

Dan, the irony is that you answered your own question in that first post.

You were dealing with TechoDummies®.

The funny thing was that they were definitely NOT TechnoDummies. Their academic technical qualifications are head and shoulders above mine, if not clear down to the ankles, although their PA skills are another story IMHO.

They felt differently about that last part EDIT: and reminded me of the former several times during the troubleshooting.
« Last Edit: September 10, 2018, 01:46:01 PM by Dan Mortensen »
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Peter Morris

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Re: How DI Boxes Work?
« Reply #31 on: September 18, 2018, 04:20:36 AM »

In the case of a passive DI you need to match a high impedance input such as a guitar to the low impedance input on your desk - say 20,000 ohm's to 200 ohm's so you need a transformer with a turns ratio of 100:1 hence the insertion  loss.

With an active DI the active circuit provides the impedance match. Typicall a FET or an IC using  FET input stage is used because of their high input impedance. The active design allows you to design it to have no insertion loss, in fact you could even design it to have gain.  In this case Ian designned it to have a nominal insertion loss of 15dB so that there was no need to put a pad on your desk or DI.

Being able to connect a high impedance source, and providing earth isolation when needed to the low imdedance balaced input of a mixing desk are the two main fuctions of a DI.

There is another reason why manufactures have a gain reduction or insertion loss of typically 15 dB on phantom powered DI's.

Phantom power is usually 48 volts and is typically supplied through a couple of 6K8 resistors (in accordance with IEC 61938).   This limits the power available to any device using it.  In this case it needs to drive a low impedance mic-pre with an impendence of around 2K.

The current required to drive the 2K load will cause a significant voltage drop across the 6K8 supply resistors and hence the ability of the DI to produce a large voltage swing. 

It’s not possible to output a signal from the DI at the level that some instruments may produce (eg. a keyboard) without clipping the DI because of the phantom power supply limitations.

The simple solution is just to reduce the output voltage to a mic level instead of a line level. By doing that if you hear clipping it will be in the keyboard for example not the DI … one less problem to worry about :)


https://en.wikipedia.org/wiki/Phantom_power
« Last Edit: September 18, 2018, 04:24:42 AM by Peter Morris »
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Steve M Smith

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Re: How DI Boxes Work?
« Reply #32 on: September 18, 2018, 10:01:21 AM »


It’s not possible to output a signal from the DI at the level that some instruments may produce (eg. a keyboard) without clipping the DI because of the phantom power supply limitations.

How much signal are you expecting from the instrument?  I would expect to see between 12 and 18 volts available for the active circuit inside the DI after losses in the 6K8 resistors in both the mixer and the DI.


Steve.
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Peter Morris

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Re: How DI Boxes Work?
« Reply #33 on: September 18, 2018, 11:18:07 PM »

How much signal are you expecting from the instrument?  I would expect to see between 12 and 18 volts available for the active circuit inside the DI after losses in the 6K8 resistors in both the mixer and the DI.


Steve.

What the best DI's do is use a DC to DC inverter to give perfect earth isolation.  The DC to DC inverter is not 100% and you must account for the current used to drive the electronics as well as the load they are driving.  In addition to the 6K8 resistors in the mixing desk there are also resistors in the DI as part of the supply (see below - note this design does not use a DC to DC inverter).

In terms of maximum input voltage … it should be around 10 volts p-p  (typically they are around 12 to 15 dBu)
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Steve M Smith

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Re: How DI Boxes Work?
« Reply #34 on: September 19, 2018, 03:31:48 AM »

I assume you mean the BSS 133.  That is an example of great design in my opinion.  Being able to make a dc to dc converter efficient enough to run on the 7mA (max) available via the resistors is quite an achievement.

I make my own DIs, usually with a 5M high input impedance for piezo pickups.  I don't bother with a zener diode to regulate the voltage.  I found that if I put an LED with a 3K3 series resistor across the supply, it would pull it down to around 18v, making the 24v zener superfluous.


Steve.
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Peter Morris

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Re: How DI Boxes Work?
« Reply #35 on: September 19, 2018, 09:37:01 AM »

I assume you mean the BSS 133.  That is an example of great design in my opinion.  Being able to make a dc to dc converter efficient enough to run on the 7mA (max) available via the resistors is quite an achievement.

I make my own DIs, usually with a 5M high input impedance for piezo pickups.  I don't bother with a zener diode to regulate the voltage.  I found that if I put an LED with a 3K3 series resistor across the supply, it would pull it down to around 18v, making the 24v zener superfluous.


Steve.

Yes ... the BSS and a few others.

I also posted this link a little eariler ... http://www.leonaudio.com.au/active.htm

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Steve M Smith

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Re: How DI Boxes Work?
« Reply #36 on: September 19, 2018, 10:37:52 AM »

I also posted this link a little eariler ... http://www.leonaudio.com.au/active.htm

Yes. I saw that when you posted it. That looks very good.


Steve.
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Rob Spence

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Re: How DI Boxes Work?
« Reply #37 on: September 19, 2018, 04:36:46 PM »

There is another reason why manufactures have a gain reduction or insertion loss of typically 15 dB on phantom powered DI's.

Phantom power is usually 48 volts and is typically supplied through a couple of 6K8 resistors (in accordance with IEC 61938).   This limits the power available to any device using it.  In this case it needs to drive a low impedance mic-pre with an impendence of around 2K.

The current required to drive the 2K load will cause a significant voltage drop across the 6K8 supply resistors and hence the ability of the DI to produce a large voltage swing. 

It’s not possible to output a signal from the DI at the level that some instruments may produce (eg. a keyboard) without clipping the DI because of the phantom power supply limitations.

The simple solution is just to reduce the output voltage to a mic level instead of a line level. By doing that if you hear clipping it will be in the keyboard for example not the DI … one less problem to worry about :)


https://en.wikipedia.org/wiki/Phantom_power

Peter, what 2k ohm load were you referring to?
I always thought of DIs driving an input on a mixing desk which I thought had a much higher impedance?


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John Roberts {JR}

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Re: How DI Boxes Work?
« Reply #38 on: September 19, 2018, 08:41:35 PM »

Peter, what 2k ohm load were you referring to?
I always thought of DIs driving an input on a mixing desk which I thought had a much higher impedance?


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I hate myself for posting this, but (actual) DIs are traditionally designed to feed mic preamp inputs that have 2k ohm input termination impedance.

we have discussed all this ad absurdium... but every time we go around in circles because people conflate DIs, with instrument preamps, and sundry other boxes all considered generic "DIs"

JR
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Dan Mortensen

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Re: How DI Boxes Work?
« Reply #39 on: September 20, 2018, 02:06:52 AM »


we have discussed all this ad absurdium... but every time we go around in circles because people conflate DIs, with instrument preamps, and sundry other boxes all considered generic "DIs"

As I said before, I searched before posting for this kind of DI discussion and couldn't find it in these forums, which may well be due to my poor search skills. I asked you ITT for a link to the previous discussions but got nothing. Which is not to say it isn't there, but I couldn't find it on my own.

With all due respect, it may be ad absurdum for you, but I for one am appreciating people's different viewpoints and explanations of how these things work with as much detail as they've cared to share, which has been considerable and enlightening.

Hopefully this thread can be found the next time somebody has a question about what exactly a DI box does and how and why it does it, since I couldn't find anything other than "what's the best DI?" and the like.
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Re: How DI Boxes Work?
« Reply #39 on: September 20, 2018, 02:06:52 AM »


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