slow rise phantom power schem

On the old forum some of us were having a discussion about phantom power vs vintage mic transformers...

It started after Cjenrick quoted Oliver from Tab-Funkenwerk about how turning phantom on and off slowly kills vintage mic transformers, specially those with very hight turns ratio.

So, Does anyone would be able to draw something to solve this problem. As I have said, I´ve bought LOTS of vintage iron for future projects and I like the sound of many vintage transformers, like beyers, UTC, peerless, etc...

Someone mentioned a very simple passive RC circuit for this purpose, but I don´t remenber who.

Thanks again!

I think Gus or Adrian had something.
A simple fet woth an rc network on the gate.
I am wondering about ramp down. That would be tougher, because you have to catch the spike at the x-former primary, on the other side of the 6.8 k resistors, if you are using any.
cj

wheres Fabio?

Hey, instead of a bunch of circuit trouble, why not just some zeners across the pri or sec?
Are they fast enough?
I mean, talk about an easy fix!
Gotta get the storage scope out.
cj

Back to back Zeners will work great!
The analog devies SSM2017 / 2019 datasheet should discuss this.
As a solid state mic pre IC gets hit hard when the phantom
is unplugged and plugged it keeps the junctions of the input transistors in the
IC from shorting.
They are using 1N750 5.1 400mW zeners. since it is a step up transformer
that will be too low. I would use 12-24 volt versions.
Take two diodes lay them together with the cathode (black band)
side by side. Twist the cathode of on to the cathode of the other.
Solder this connection. Then connect the anodes of each diode across
the transformer secondary. Ensure that when the unit is being driven with
the maxinium input level that the diodes still are not turing on and clipping the signal. Leave several volts above this max level befroe the diodes
clip the signal.
since the diose are "back to back" the capacitance will be low
in the under 50pf range.

CJ try this in the lab with the scope!

Are you sure you need zeners? I though that's what the criss-crossed 914s on the input of op amps like the JH990 were for?

Hey Adrianh, sorry for being a dumb, but could you draw the schem you´ve just described? The zeners on the transformer secondary...

...

The Zeners are not so good for transformer circuits, since they reduce/eliminate the galvanic isolation that you get from a transformer. If there's a static potential difference (which is a great reason to use transformers for their isolation) then the Zeners will start to conduct and your isolation is gone. They are for transformerless circuits, where the lack of isolation renders the base-emitter junction vulnerable to surges in common-mode potential.

Keith

Here?s a link to the LM317HV data sheet. On page 6 there?s a circuit described as a ?Slow Turn-On 15V Regulator?. I haven?t tried it, but playing with R3 and C1 values should give different time delays. Obviously it?ll work using any output voltage you set R1 and R2 for.

http://cache.national.com/ds/LM/LM117HV.pdf

Would that need a different shunt transistor? Looks like the 2N2905 is only good for 40 Vce.


OldHouseScott

You can probably do it with a TL783C as well..

My take on phantompower;

It's a lame idea. Really. I prefer a 48V PSU close to microphone. And one for each microphone. Granted, I'm not recording with +50 condensers at the time. I don't like the phantom PSU in my pres.

I see it's giving a lot of folks headaches. Voltage triplers and what not... really bad DC to DC converters.. :evil: .. I DO think the quality of the phantompower makes a sonic impact. There's no need to fuss about preamp PSUs if the supply for the microphone is lousy. The internal amp in the mic is often more affected by bad PSUs. It's lower signal level and there's no room for decent filtering. Their CMRR isn't as good as a other gain stages either..

Also, I do not need a slow risetime. I've never fried a mic or trannie. If you don't like the <POP!>, hit the mute button

sismofyt, the 48v problem obviously only applies to certain types of transformers, namely some vintage types from the pre-phantom-power era (e.g. the siemens/telefunken/tab v7x series). but there it's definitely something to keep in mind. if i may quote oliver/tab-funkenwerk:

"Theoretically this shouldn't pose a problem for a none center taped transformer, but in practice there is always the capacitive property; primary to core or the Faraday shield, then also the primary windings to secondary windings etc. These will become charged if 48V DC is applied and the transformer (standard tube input x-former and tube mic outputs have ratios from 1 to 10 and even higher) acts like a ignition coil creating approx. 1 to 2KV!! on the transformers secondary. Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!! and this is enough for a millisecond energy burst that is powerful enough to burn microscopic holes into the insulation.

It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insolated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.

If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitve property of the transformercan charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years. "


a friend of mine has two v72 in a profession rack with p48 etc., i believe that supply is ramped. i'll post details when i get a chance to look at it.

I have more than 30 old Telefunken/TAB/Siemens modules and a bunch of other gear with old, and perhaps fragile, trannies. Never had a problem.

I've seen that quote from Ollie, and he knows a LOT more than me, but in practice I've not encountered any problems. Same thing with phantompower and ribbonmics.

The FMR Audio RNP that I have has the ramping phantom power. Don't have any schematics though. I didn't know why it did that, but this sheds some light on why they did it.

Flatpicker,

doing it with the 317 means one for every single phantom power switch... excessive and silly expensive.

As for not having to worry about the aspects of preamp quality if the phantom PSU isn't of good design... anyone else ever had this problem? -Sounds a little like superstition to me... If the preamp has even halfway decent CMRR, a 50Hz ripple at 1V is inaudible on a Neumann U87 plugged in... try it.

Noise on the phantom power feed should be inherently self-rejecting. having said that, my own designed ('bi-phase' multiplier) phantom supply will drive two mics and happily continue to provide 48V for a couple of seconds after power is removed... With that much reserve, there can be no question of it being a weak supply... and it costs under $2 to add to the ±18V supply.

Back on-topic, if you take the output of the +48V 'on' switch through a 100Ω resistor, then shunt it to ground with a 100µF electrolytic to ground, then connect that junction to the two 6.81K resistors, you'll have slugged the on/off. if the mic is a ribbin ir similar, the 100µF cap will take a LONG time to discharge, so you might like to add a 10K resistance in parallel or something. Don;t make this resistance too small, or it will start to tug down your 48V, forming a potential divider with the 100Ω resistor.

Keith

...

You're correct, no cap on the reg leg. Some massive filtering, RC, after the TL783C is also nice. Be sure to put a diode across the Vreg though. Ripple is very low & stability high.

Anyway, SSLtech, I think we debated this on techtalk as well. In my experience, the PSU, any PSU, do have an effect on the sound. Sometimes not a lot, but often quite a bit. YMMV and all that jazz..

As far as the U87 I dunno. I don't use that mic. Doesn't it have a DC to DC converter? If the amp is powered by that one, 5V ripple may not do a lot of harm. I know my Oktavas do care a lot about the phantom PSU quality though..

[quote author="SSLtech"]...doing it with the 317 means one for every single phantom power switch... excessive and silly expensive...[/quote]Yep, you have to use it on every channel? (did I leave that part out? :wink: )

It's certainly excessive, but I wouldn?t hardly call it ?silly expensive? for 2-4 channels. It?s only about $5-$6 extra a pop, and you?re going to spend a little extra to use the ?passive? method anyway. Maybe just "expensive".

Like I said, I haven?t tied it yet, but if it works as advertised, I?ll probably use it on my pres.

[quote author="OldHouseScott"]Would that need a different shunt transistor? Looks like the 2N2905 is only good for 40 Vce...[/quote]The version "A" (2N2905A) is 60Vce. Looks like it's the only version Digi-Key sells.

BTW, that's a LM317HV - NOT a LM317. The 'HV is the higher voltage version. I edited my original post so no one would blow their LM317s.

I don't remember just how HV the LM317HV is.. isn't it just 57V?

You could also do it with an emitter follower and a big cap from ground to base. Cheaper than the Vreg and more reliable too..