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impsick 1st June 2007 03:03 AM

Protect line from 48v
Hi everyone.

is there a solution to protecting a line instrument from 48v phantom power.
Basically what im doing is wanting to run a line input on my mic pre. I dont like the idea that every time you have to switch off phantom power. cause of course i will forget. So without complicated rerouting im wondering if there is a solution without degrading audio signal.
would something like the standard hot plug/esd circuit work?


Minion 1st June 2007 04:24 AM

If you want to add a DI feature (Line in) on your Mic preamp I would suggest that you use a seperate Input Buffer for the DI/Line Input.....

You could use a dual opamp (one as a Buffer and one with adjustable gain) that will output to your Mic preamp after the INA217 but before the Line driver stage so you will have unballanced inputs (Like most Line and DI inputs) but have Ballanced outputs.....

You could make it so when you stick something into the 1/4in Input jack it also disconnects the Ina217 from the Line driver stage so no extra noise is introduced or if you want, it could sum the Mic preamp signal with the DI input signal and give you a Ballanced mono mix of both......

I would strongly suggest that you do it this way or in a simular way because the INA217 isn"t really designed to handle Line inputs and it also has a much reduced performance at higher Input impedances .......

My next Rack of Mic preamps based on the INA217 are going to have DI/Line inputs useing pretty much the same topology I just talked about, but I have so many projects on the go right now (One 20w Guitar amp(Opamp Input Discrete Output), One 100w Bass amp(based on 2 LM3866 in Bridged mode) and a 75w Guitar amp(That is fully discrete with a High power Darlington output stage)that I don"t know when I"ll get arround to designing it.......

Good luck

impsick 1st June 2007 06:03 AM

ok i get it. Thanks Minion.
Im using a jensen for line to mic. So this is why i ask. i was thinking relay but im not reading much good about using relays with audio signal.
How eagle? you been using yet? have fun with guitar amps, and post em when your done. i'd like to check them out.

Minion 1st June 2007 06:05 PM

Hi Impsick,

Yes I have used eagle But I find it very hard to use, Much harder than I thought....I actually haven"t really been able to do anything with it yet as it takes Practicly forever to find even the most basic component and I don"t have the time or Patience to to do this with every Part, I actually can"t find pretty much any Parts that I use regularilly......

I"m sure it is a great program for those who use it but I haven"t been able to do a single thing with it and I have spent several hours trying to tigure it out.....


impsick 2nd June 2007 12:59 AM

I know it can be a pain, but what you can do is delete all those library's you dont use. that will help out a lot. just find the parts you use and delete the rest. Dont give up on it!

impsick 2nd June 2007 02:05 AM

2 Attachment(s)
Minion check out this layout.
see how pins 1 & 4 from the jensen, go to the THAT1510 / or INA217 after the diodes. is there any potential problem here? do you think this would work?

Minion 3rd June 2007 09:48 PM

Hi Impsick , I really don"t know what you are trying to do in that PCB (I can"t read a PCB as well as a Schematic)....

You are probably better at this stuff than me anyways....


impsick 3rd June 2007 10:05 PM

no i dont think so, but thanks. ha ha
i have the line input connecting after the diodes.(coming from pins 1&4 from the jensen) those diodes and BP caps are to protect the ina217 from 48v phantom power right? so it should keep the 48v from line input too right?

Minion 3rd June 2007 10:29 PM

Yes, the 47uF caps are for blocking the 48v DC Phantom power so I would connect the Jensen between the Caps and Diodes or between the Diodes and the INA217.....

The INA217 schematic says to use Non-polar Caps if turning off the Phantom power but I have found that You can use Polar electro"s without any problem......


impsick 3rd June 2007 10:42 PM

cool. thanks.
i think in the ina217 data sheet it says to use non-polar if you plan to turn off the phantom power. i think it has something to do with distortion.?.

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