Countryhick* and Bo Hansen DI, an alternative phantom quest?

[smallbutfine]

Hi everybody,
i took a deep look into the di schems on this board and elsewhere...as i got two high quality transformers (LL1538) today from a very good friend who wants the best possible di boxes built from them (which i can use whenever i want)...and i wish to do the best on this one.

I would be very nice to have a possibility to power it by phantom...if not automatically , then switched. Nowadays nearly all desks have phantom power, so the battery power should only be used in emergency cases.

I first took a very, very deep look into Bo Hansens DI. Then i heard of the countryhick and read the specs and such, i really like the speaker level option and heard it is maybe a bit more open in the highs...

The original countryhick di claims for a 9V battery, but 18V with two batteries are a better choice for better headroom, i think.

As i already know, the original country* does phantom powering with a second xformer to bring the 48V down with an extra dc-dc converter ckt containing an extra transistor 2N5550 and a quite high parts count overall...

So my idea is using the countryhick circuit with some replacements and a new phantom power ckt:

1) substitute the fet 2N5459 with BF245A as i already have them (european equivalent?)
2) substitute 2N4401 with BC547C (in fact i use B types for audio uses with hfe compatible to B and C spec for audio as these overlap....would BC550C do a better job?)

and

3) paralleling a 24 V Zener switchable to the batteries.
4) soldering 2x 6k81 Ohm resistors between XLR pin 2 and pin 3 and batteries + pole

for getting phantom powered option...

Am i right with this?
I have a strange feeling it cannot be so simple...
(Sorry i'm really a very autodidactive guy, but willing to learn a lot...so correct me if i'm wrong)

At this moment (phantom power limited to 24V) there's no earth lift option available anymore.
For this case i want to include a safety mechanism at pin 1. I looked into Rod Elliots Active DI at ESP. He did the trick with 100n cap paralleled with 10R there, I saw something similar at Bo's box with 47n and 47Ohm at pin 1.

Next question: what is the deeper meaning if the 1N4004 diodes at Bo Hansens circuit? Do they improve quality of the phantom power stability? Are they optional of essential?

Man, i have to learn alot more, i know.

Anyone willing to help on my questions? I'm quite confused if i'm on the right way and analog electronics to me is still like a new terrain...

Kind regards

Martin

 

[BYacey]

I haven't looked at the specs for the transistor numbers you are considering, but this isn't what I would call a critical application, just maintaining low noise is the biggest factor. If the FET biases at about half the DC supply voltage you are off and running. The ground lift schemes you cite from the other schematics don't really achieve true galvanic isolation. The only true way to do this is with a transformer coupled DC to DC converter.

 

[smallbutfine]

The ground lift schemes you cite from the other schematics don't really achieve true galvanic isolation. The only true way to do this is with a transformer coupled DC to DC converter.

Yes, you are definately right about that, Bill.
I think that's the reason the original goes the hard way (ok, you probably expected this in a near $300 DI... :grin: ).
I thought Bo's Box has a very good reputation, as well as Rod's projects, and their ideas have been prooved in praxis over the years.
So this little teardrop about loosing the full isolation is probably worth having an easily DIY-able phantom power option ...


Am i right about the way to get ~24V DC supply for the circuit from phantom power (here i'm really unsure...)?

EDITED...to be kind...for obvious reasons


Kind regards

Martin

 

[BYacey]

HS = High speed switching. The converter frequency is probably around 100KHz. And yes, a resistor off each leg (pin 2 and 3) tied to a zener and filtered will give you the necessary voltage drop to run the DI.