slow ramp 48v phantom

I decided to follow Oliver's advice and use a slow ramp phantom
with my old v72 & v78 mic amps.I built it with the 2k2 feed to the regulator and it slow rises to 47v on my cheapo meter.

This was the circuit I found that Oliver had posted in another forum.

I have a ckt for this that a friend designed for me, but I have not yet proto-ed it up just yet. It is slow ramp up and down. Pretty straightforward and the ramp slope can be changed for up and down seperately with one resistor each.

I'll work on getting this posted too...!

That would be nice.I'd like to try that out.

I guess I should leave my mic hooked up for a few minutes after the phantom pwr is turned off, then the mic would drain the caps . right?

Output will sag badly if the mike draws more than (55V-48V)/2KΩ= 3mA, and many mikes draw that much and 10ma should be allowed.

Putting a monster cap on the regulator's Vref pin should give a very slow rise. It will also blow-out the regulator in certain abnormal but not impossible situations. The datasheet and app-notes show where to put some diodes to protect against this.

thanks PRR, Of the mics I have the largest draw is 3mA , so I'm
barely OK for now. A better design will have to be built in the long run.

another question- Is it bad practice to daisy chain the 48v to the various
mic ch ?

[quote author="PRR"]Output will sag badly if the mike draws more than (55V-48V)/2KΩ= 3mA, and many mikes draw that much and 10ma should be allowed. [/quote]
in v.reg app-notes, there's often a circuit given which is slow turn-on characteristic and makes use of a transistor-cap arrangement. maybe try that?

Putting a monster cap on the regulator's Vref pin should give a very slow rise.

Click to expand...

you mean, in parallel with the zener in the above circuit?
will that also give a slow decrease after turn-off? because that's as much important as rise on turn-on.

what about the 1000u cap on the o/p leg? will that make a noticeable time constant on it's own for decrease?

This has come up before...

Why doesn't anyone ever want to use the LM317HV set to 48V and apply the slow turn-on app in the data sheet?

[quote author="Flatpicker"]use the LM317HV[/quote]
Just a small note. You don't nescesarily have to use the HV version - in many cases the standard 317 will be fine.

Best regards,

Mikkel C. Simonsen

I use 317T for 48v phantom. Haven´t got any problems doing so. (yet)

flatpicker,
that's just what i meant by v.reg app-notes... =)
don't have it at hand now, but will that slow turn-on app from the data sheet also provide slow turn-down?

When I built my first mic preamps I used TL783 for phantom power but now, for long enough time I use the regular LM317. No problems.
Even f_o_c_u_s_r_i_t_e use the 317 in the I_S_A 430.
The TL783 is absurdly expensive compared to the LM317.

chrissugar

[quote author="volki"]flatpicker,
that's just what i meant by v.reg app-notes...[/quote]I know - just curious why folks continue searching for alternatives and if there was something wrong with doing it this way...

[quote author="volki"]...but will that slow turn-on app from the data sheet also provide slow turn-down?[/quote]During turn-off the filter caps on the regulated side will discharge through the load resistance - whatever time constant that ends up being. Looks like the only difference here is that the 25uF cap will discharge through the diode along with the filter caps (which are not shown) and lengthen the time constant a little.

[quote author="chrissugar"]Even f_o_c_u_s_r_i_t_e use the 317 in the I_S_A 430...[/quote]That?s strange. DIY might be fine, but I?d be afraid to do that on a commercial unit just to save 2 or 3 bucks! Makes you wonder if f_o_c_u_s_r_i_t_e knows something we don?t ? like that it?s the same chip as the ?HV and National's just charging more for the 'HV?

[quote author="Flatpicker"]That?s strange. DIY might be fine, but I?d be afraid to do that on a commercial unit just to save 2 or 3 bucks! Makes you wonder if f_o_c_u_s_r_i_t_e knows something we don?t ? like that it?s the same chip as the ?HV and National's just charging more for the 'HV?[/quote]
The HV versionis only needed if the difference between the input and output is above 37V. It doesn't matter that the output voltage is above 37V.

Best regards,

Mikkel C. Simonsen

[quote author="mcs"]The HV versionis only needed if the difference between the input and output is above 37V. It doesn't matter that the output voltage is above 37V.[/quote]Right, but I took it that they were dropping the entire 48V across the 317. Is that not the case? :?

It's input-output differential. You have to ensure that the input voltage is never more than so-many volts above the output voltage. The 317 is in fact a 1.5 regulator that you "float" on a reference voltage to get your desired output.

Talked to Kevin from K and K audio at the show. He has a circuit for a ramped phantom that won't limit current draw. uses a transitor in the ref leg of the reg with a rc network. will get the circuit from Tommypiper tomorrow.
cj

[quote author="cjenrick"]uses a transitor in the ref leg of the reg with a rc network.[/quote] Sounds pretty close to the one I have.

[quote author="Flatpicker"]Right, but I took it that they were dropping the entire 48V across the 317. Is that not the case? :?[/quote]Wait... Now I see - I was thinking that the voltage between ground and the output had to be less than 37V for some stupid reason. :roll:
Oh well, at least I don't have to buy the expensive HV regulators any more.

> I don't have to buy the expensive HV regulators any more.

You may have mysterious failures. If you have a capacitor (or a short) on the output, there is 60-70V across the regulator. It is only rated 35V-40V, and over-voltage kills very quick, and can't be self-protected the way over-current and over-power can be.

I don't see a thing wrong with fairly fast Phantom. But if you insist on slow-decay phantom, you are going to need a BIG capacitor somewhere. Couple hundred uFd per mike-jack.