Looking to DIY a DI box -10dBv to +4dBu - diyAudio
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Old 21st August 2012, 07:43 AM   #1
NEO Dan is offline NEO Dan  United States
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Default Looking to DIY a DI box -10dBv to +4dBu

Well not exactly a DI as it will not be provided any source of phantom power...

In a nutshell I want a single channel line stage buffer with switch selectable gains 6,12,@18dB.

Normal input would be -10dBv via cinch, but an XLR in would be nice too.

Seeing as XLR input would be +4dBu and I want an +18db gain option and some headroom left I'll need roughly 48V p-p - I think?

I'm not familiar with what's out there so I'm asking here

What say you?
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Old 21st August 2012, 10:37 AM   #2
AndrewT is offline AndrewT  Scotland
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Could explain what a DI box is and does?
And what is different in your application?

+22dBu is 9.75Vac, or about 28Vpp

You can get that from a rail to rail opamp on +-16Vdc. But you'd do far better avoiding all the rail to rail opamps and use +-18Vdc to +-24Vdc. A discrete circuit capable of driving a 1k load would need a current capability of ~18mA.
Add on a further 10mA for capacitive cable loading and you are still in the region that can be supplied by an opamp.
A composite opamp + buffer as described by Jung?
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Old 21st August 2012, 11:01 AM   #3
NEO Dan is offline NEO Dan  United States
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Hi Andrew,
DI stands for direct injection, commonly used as a preamp between an instrument like a guitar and the mixer. Some DI boxed actually get power via 48v phantom power provided from the mixer that the DI box is connected to.

In my application none of that applies, I just need a single channel of quality preamp conversion to balanced interface.

Actually After I posted this I went on to find exactly the type of device I am looking for, except for the $200 usd cost and a couple extra features I was not looking for it looks good. And it has +-18V capability...

SC-1mk3 Mic Preamp

Anyone heard of these guys? Any opinions?
Thanks
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Old 21st August 2012, 11:18 AM   #4
AndrewT is offline AndrewT  Scotland
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This is a mic pre-amp with a maximum gain of +72dB

You are looking for a line driving pre-amp with gains (switchable) of +6dB to +18dB

If you want unbal to balanced, there are chips specifically made for this duty.
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Old 21st August 2012, 12:09 PM   #5
NEO Dan is offline NEO Dan  United States
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I know what it is

Although it has up to 72dB gain it is still only +-18V capable... Kind of a moot point IMO.
The switchable gain settings and gain trim is nice as is phase inversion... If it were half the cost I'd accept it's short comings and ignore that it was ever intended to be a mic pre

What about the conversion chip??? Sounds interesting.
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Old 21st August 2012, 10:32 PM   #6
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Andrew we did this one year ago!

DI Box question
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Old 22nd August 2012, 03:03 AM   #7
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Remember that in this context, the -10dBV and +4dBu are 0VU levels and *not* peak outputs. You'll need to allow for at least 14dB and preferably 20dB for peaks, plus a little headroom. (And this has nothing to do with a DI box.)

All good fortune,
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Old 22nd August 2012, 09:06 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Speedskater View Post
Andrew we did this one year ago!

DI Box question
That was a year ago. What happened last week?
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Old 22nd August 2012, 09:09 AM   #9
NEO Dan is offline NEO Dan  United States
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Focus - Andrew U can do it
About the chip???
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Old 22nd August 2012, 08:02 PM   #10
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Quote:
Originally Posted by AndrewT View Post
That was a year ago. What happened last week?
The Olympics, don't you remember?
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