Phantom power blocker for synth outputs

[k brown]

Being strictly a location engineer with no studio experience, why aren't alll studios wired with XLRs for mic in, and TRS for line in?

 

[Whoops]

Being strictly a location engineer with no sudio experience, why aren't alll studios wired with XLRs for mic in, and TRS for line in?

Studios are not wired with XLRs or TRS. Studios are all wired to a Patchbay.

The most common patchbay used is with Bantam connectors. (there's others)
Any input or any output will have the same connector, which is very practical an convenient.
But someone that is not experienced might connect for example a Compressor output into a Mic Input that has the phantom power On.





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As for professional hardware units/equipment, the units itself
Mic In and Mic Outs are normally XLR
Balanced Line In and Balanced Line Out can be XLR or Jack TRS, depends on the manufacturer choice.

XLR has a locking mechanism so it's normally preferred in Professional situations to Jack TRS

 

[k brown]

OP stated: "straight to the console (through an XLR patch panel)."; not a patchbay.

Hence my question why would anyone build a patch panel that used XLR for mic and line, rather than TRS for line, to easily avoid this phantom poweer issue?

 

[Brian Roth]

In a few studio builds, I specified that the mic lines from the wall panels in the tracking space(s) terminate in a panel in the control room which had male XLRs for the incoming signals. The same panel also had female XLRs for connection to the mic preamp inputs. Short male-female XLR cables did the patching.

The reasoning was that the bantam patchbays used for the line level patching weren't really reliable for the mic levels. For instruments. we used DI's.

Bri

 

[ruffrecords]

Being strictly a location engineer with no studio experience, why aren't alll studios wired with XLRs for mic in, and TRS for line in?

Because a lot of pro outboard is line level and uses XLRs. You idea will certainly work but you would need a lot of XLR to TRS adaptors and sooner or later someone is going to plug the XLR end of one into an input with phantom power on it.

Cheers

Ian

 

[Cqwet Dbdfte]

OP, are you sure your synth does not have capacitor coupled outputs?
I see no reason to DC couple a low level output, unless you want 0.001Hz bass response.
Adding output coupling caps shouldn't be too hard, cut a wire or a trace put in a leaded cap or an SMT cap, if some bonehead forgot to include it.
Mic inputs are special, they are not "line" level.

 

[Rob Flinn]

OP, are you sure your synth does not have capacitor coupled outputs?
I see no reason to DC couple a low level output, unless you want 0.001Hz bass response.
Adding output coupling caps shouldn't be too hard, cut a wire or a trace put in a leaded cap or an SMT cap, if some bonehead forgot to include it.
Mic inputs are special, they are not "line" level.

Even if the outputs are capacitor coupled, one would need to be sure that the caps are rated for 63v, which isn't gauranteed on much of the gear I have looked at over the years....

 

[bmaughan]

OP, are you sure your synth does not have capacitor coupled outputs?
I see no reason to DC couple a low level output, unless you want 0.001Hz bass response.
Adding output coupling caps shouldn't be too hard, cut a wire or a trace put in a leaded cap or an SMT cap, if some bonehead forgot to include it.
Mic inputs are special, they are not "line" level.

Even if the oututs are capacitor coupled, one would need to be sure that the caps are rated for 63v, which isn't gauranteed on much of the gear I have looked at over the years....

I’ve not checked. Two that I’m most immediately worried about are a Juno 106 and an ensoniq esq-1 if any one knows about the output sections on these.
If they have capacitor coupled ouputs with caps rated at or above 63v they should be safe correct?

What do you mean by “I see no reason to DC couple a low level output, unless you want 0.001Hz bass response.” ?

In a few studio builds, I specified that the mic lines from the wall panels in the tracking space(s) terminate in a panel in the control room which had male XLRs for the incoming signals. The same panel also had female XLRs for connection to the mic preamp inputs. Short male-female XLR cables did the patching.

The reasoning was that the bantam patchbays used for the line level patching weren't really reliable for the mic levels. For instruments. we used DI's.

Bri

This is exactly how I’ve always done it as well.
This setup is my personal one and I have basically zero budget haha.. I just had a few tt patchbays laying around so I made due.

 

[Cqwet Dbdfte]

0.001Hz bass response:
What is the lowest frequency of the output you intend to generate?
Is 20Hz low enough? What is the load impedance of the input?

https://www.digikey.com/en/resource...sion-calculator-low-pass-and-high-pass-filter
Use a "big" enough cap to pass the lowest freq of interest, given the load impedance and size constraints. Use at least the voltage of the phantom, 50V or 63V.
With a 6.8k load and a 10uF cap the -3dB roll-off is at 2.3Hz.
Maybe UVP1J220MPD ? 22uF 63V Bi-polar

 

[Somkereki]

I just checked the M1516 schematic. It has a pair of switchable mic inputs which are fed phantom power. There is a +4dBu input position on the mic gain switch which bypasses the mic pre but for some unknown reason it does not disable the phantom power. Even in the +4 position, the input impedance is only 1K3 which could be problematic for some synth output. I think you definitely need some form of protection such as that proposed by Bill.

Cheers

Ian

I agree! Unfortunately, I have also come across many cases where there was phantom power at the input of the line. Otherwise, it is worth turning it off, even for dynamic microphones, because it can produce significant dielectric noise in the cables (when they are moved or stepped on). (I used to be a sound engineer for rock bands.)

 

[Somkereki]

The output is probably an (electrolytic) capacitor in most cases, but the +48 volts is higher than the output circuit voltage, so it will be reverse polarized and the output amplifier stages may fail. A long time ago, I put a 10:1 matching transformer on the output of my friend's synthesizer, so the 47 kOhm unbalanced output became a 470 Ohm balanced output. The sound has also become much better. Of course, this is the most expensive, but the best sounding method.

 

[Newmarket]

There is a problem with this design. The pair of 47K resistors provide a return path from the two electrolytics back to the 0V of the phantom supply. This means that when the phantom is switched on, since the voltage across the capacitors cannot change instantaneously, the two 47K resistors have 48V phantom across them until they charge up the capacitors. The time constant of 47K and 100uF is 4.7 seconds. In theory, this in itself is not sufficient to do any damage as long as the two voltages remain the same because any difference between them will result in a current flowing in the microphone. This means you really need to plug the blocker into the preamp with phantom on and then plug the mic into the blocker. Only in this way will the caps have had time to charge up. Bottom line is this may not work when re-plugging.

At least one phantom blocker manufacturer is aware of this limitation:

https://www.tritonaudio.com/product/phantom-blocker/

but good old Hugh Robjohns seems to have missed it:

https://www.soundonsound.com/sound-advice/q-there-easy-way-block-phantom-power-cable

Cheers

Ian

Use a bipolar capacitor or bipolar configuration of capacitors such that DC polarity is not an issue. Or 'active' solution with series capacitors having their junction tied to a fixed bias voltage that is not derived from P48.

 

[ruffrecords]

Use a bipolar capacitor or bipolar configuration of capacitors such that DC polarity is not an issue. Or 'active' solution with series capacitors having their junction tied to a fixed bias voltage that is not derived from P48.

Bipolar caps are definitely a good idea. Bottom line is that no matter what you do, any change in the phantom power dc level will be transmitted through the capacitors. The only other thing I can think of to mitigate this is to deliberately load the 48V with say a 1K resistor to pin 1 on each of the hot and cold legs. This would at least limit any sudden changes to about 6 volts.

Cheers

Ian

 

[soapfoot]

What nitwit outfit sold pramps where phantom was always on?

Rupert Neve, for one, on the very best-sounding consoles he ever designed

 

[k brown]

Rupert Neve, for one, on the very best-sounding consoles he ever designed

Why on earth would he have done that? To what possible advantage?

A really rough way for people to find out they had a ribbon mic with faulty wiring! But hot-plugging also NOT recommended for most condenser mics either (?!).

 

[Newmarket]

Bipolar caps are definitely a good idea. Bottom line is that no matter what you do, any change in the phantom power dc level will be transmitted through the capacitors. The only other thing I can think of to mitigate this is to deliberately load the 48V with say a 1K resistor to pin 1 on each of the hot and cold legs. This would at least limit any sudden changes to about 6 volts.

Cheers

Ian

Yes. And low tolerance capacitors are a lot harder to achieve than resistors. It's difficult to make this simple and absolutely foolproof. Solution points towards delayed switch/relay (similar to Poweramp output solutions) but it seems a lot of bother and some £s in context. Rather than manual sequencing as previously outlined.
The simplest things...
For context my dayjob problem tomorrow likely involves diagnosing/"root cause" an arc event at somewhere north of 35kV

 

[soapfoot]

Why on earth would he have done that? To what possible advantage?

A really rough way for people to find out they had a ribbon mic with faulty wiring! But hot-plugging also NOT recommended for most condenser mics either (?!).

I can’t speak to motive; only to the facts

Phantom was global on 80-series desks; to switch it off for any channel required removing the module (which switched it off for ALL channels)

With moving coil mics and condensers, this is pretty much inconsequential

It only becomes consequential for ribbons if patching around normals on a trs, tt or longframe bay

If you always connect the ribbon via an xlr panel to a normaled or pre-patched input, it shouldn’t be much of an issue (at least in theory)

Worth remembering that ribbon use was probably at its lowest ebb in the late 1970s-1980s

In the modern world (post-ribbon renaissance) I agree it’s not optimal

 

[zzzzz]

Phantom was also always on for 1970s Auditronics consoles, but they used a pair of 20 kOhm resistors to pin 2 and pin 3 instead of the "standard" 6k8~ish.

Their "condenser mic power supply" was also an optional add-on board to their console PSU, with the earliest versions having a totally separate little mains transformer versus coming off the main PT secondary winding with everything else.

Sort of off topic. Sorry!

 

[Brian Roth]

The MCI JH-416 and 428 desks I "grew up with" back in the 1970's also were all-or-nothing with the phantom supplies. I don't recall if the MCI 500 desks had individual phantom switches per channel, but the 600 desks did have the switches.

Bri

 

[Whoops]

Why on earth would he have done that? To what possible advantage?

I don't see any advantage at all.

Phantom Power was still a new thing in late 60s early 70s,
so the standard was still starting to be implemented.

Not all Neve consoles of that period were installed with a Phantom Power supply, you had to ask for it when you ordered,
and yes if the Phantom supply was installed it was on in all channels.

But some studios did modify the 48V supply on those Neve consoles so they could turn On and Off the 48v individually in each channel.
Oceanway did that, there's also a studio in my city with a Neve 80xx console that has individual switches on the back to turn 48v on and off, so that was a modification also.