Re: 48 v phantom power to 9 v circuit


"Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message news:4364D98F.4D9E29E2@xxxxxxxxxxxxxx 


Scott Dorsey wrote:

Pooh Bear <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:
>Scott Dorsey wrote:
>
>> Richard Crowley <rcrowley@xxxxxxxxx> wrote:
>> >"Pooh Bear" wrote ...
>> >> It's perfectly possible to 'ground lift' with a phantom powered >> >> DI.
>> >> I.e.
>> >> break the common connection between source and destination.
>> >
>> >You'll have to demonstrate your theory with a real schematic
>> >(or equivalent). I can't see how it is possible to run something
>> >on phantom power without access to the ground node. Assuming
>> >that putting a transformer on the DI box *input* (the >> >high-impedance
>> >side) is even more impractical.
>>
>> The guys at Radial actually manage to do with with the J48.... >> they
>> have a 555 timer in there that generates a 75 KC square wave that
>> goes into a little switching transformer. So they get an isolated
>> +/- 12V output that they regulate and use to power the front end. >> I
>> think that is really ingenious as hell. But as far as I know, it >> is
>> the only way to isolate the grounds with a phantom powered DI box, >> and
>> Radial is the only company doing it.
>
>Absolute nonsense Scott.
>
>Stop and think where the ground actually needs to be 'lifted' !

The ground can be lifted _anywhere_. But the front end has to be active,
which means even if you have a transformer inside there, you still have to
get power to the active front end somehow. 

Of course. Currrent sources from Pins 2&3 and returns via Pin 1

If you want to break the ground at some point where the front end can still
get phantom power, that would mean breaking the ground _before_ the front
end. Which either means sticking a transformer there and spoiling the whole
idea of an active follower with a super-high-Z input, or it means a
differential front end. 

Why would it mean spoiling the idea ?

A DI is by definition ( once ground lifted ) a differential device anyway !

I simply don't get what your problem is ! 

And you likely don't have a real-world solution for
your hi-Z transformer scheme, either.

OTOH, the in the medical biz, they like using optical
copling. It was even popular for video back in the days
before even cheap $60 TV receivers had video inputs. 

.

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