Phantom power blocker for synth outputs

[ruffrecords]

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

In those days, phantom was rarely required. It was a bit of an afterthought. Phantom power is a very poor design concept and I suspect Rupert knew it. In any case most condenser mics were tube based and came with their own power supply. Dynamics were often used for snare and guitar cabs. Ribbons were rarely used.

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 (?!).

If some fool wires a mic wrongly you can hardly blame phantom power for that. The real lesson is not that phantom is bad, but that you should take more care wiring your ribbons.

Cheers

Ian

 

[Silas]

What I would personally do in that situation, and the cheapest way (for Free) to be protected would be to dedicate some channels of the mixer for Line Inputs and in those channels I would remove (desolder) one leg of the 390r resistor after the 48V switch.
I would desolder one leg of it from the PCB and put heatshrink tube around it so that it could be reversed back to stock easily in the future in case needed.
This way you don't need any DI boxes, you don't need to spend money on DI boxes, you are protected from phantom power because it's not reaching the input anylonger, and don't forget that besides the Line inputs you can still use those channels for Dynamic microphones in case you need to use the Mic Pres

I not saying this is the only solution, or the best of all solution, other people might have much better ideas,
I'm just saying that this would be what I would do in a pinch in the situation you described


View attachment 137747





Many Output sections and output IC's have phantom power protection,
but not all have, and we never know which ones are protected and the ones that aren't...
So the best thing to do is to never send Phantom power into a Line level output.
People that are not well versed on Bantam Patchbays do that mistake easilly all the time.

Could it also be possible to move the legs of the 6.8ks to M1 or M2 (pre switch) so that it only affects the input you’d like to use with microphones, if that is the case that one is being connected to synths continuously? Or is that still too risky
Would there be a risk of spikes when switching between inputs still…
S

 

[Whoops]

Could it also be possible to move the legs of the 6.8ks to M1 or M2 (pre switch) so that it only affects the input you’d like to use with microphones, if that is the case that one is being connected to synths continuously?

If I'm seeing it correctly yes, you could move the 2x 6k8 resistors to Pin 2 and Pin 3 of M1, before the M1/M2 switch.
That way you would have Phantom Power only in Input M1 and not in Input M2.
You could use then input M1 for Microphones and input M2 for your synths

But you still need to check something, please read below

Or is that still too risky
Would there be a risk of spikes when switching between inputs still…

As far as I can see, and please guys correct me if I'm wrong,
to do this you need to check if the Switch that changes between M1 and M2 is a Make before Brake switch (shorting) or a Break Before Make switch (non-shorting).
If it's a BBM switch then I don't think you have any risk and problem is solved.
But if it's a MBB (shorting )switch then if phantom is turned on when you move the switch from input M1 to input M2 there will be for a tiny fraction of time 48V on input M2 (during the travel) until the switch reaches the final position and then there's no 48V.

I don't know if this tiny fraction of time would be enough to cause any danger, but I think you should check what type of switch is installed there just to be sure.

Some reading about BBM and MBB switches:

https://e2e.ti.com/support/switches...tches-multiplexers-and-how-are-they-different

https://e2e.ti.com/support/switches...tches-multiplexers-and-how-are-they-different

 

[Whoops]

Ribbons were rarely used.

Hi mate,
you made me think about this, I though that Ribbon microphones were used a lot during that time,
seen many old photos in classic recording studios using the RCA 44 and 77, STC/Coles and Beyer M160 and M130.
I see know I was not defining correctly the different eras, even in the same decade, as those photos are from "vintage" studios but from very time frames.
The time frame you're talking about I think is late 60s until Mid 70s.

So while doing some research on this I learned a lot, thanks for spicing up my curiosity.
RCA 44 and 77 were used mostly in 30s, 40s and 50s and it seems they lost popularity to Condenser microphones that were introduced by Neumann,
the U47 (1949) then the U67 (1960) and the U87 (1967).
By 1976 RCA was out of business.

The STC Coles I've seen in the Beatles recordings photos used on Overheads are probably from their records from 63 to 66 (or 67),
maybe in the late 60s and 70s they were not that popular anylonger.
Anyway production of the 4038 never stopped from the 50s to the present. But maybe there's a reason production went from STC to Coles in the Mid 70s.
But thinking about it I don't think I've seen photos of recording sessions from the 70s with the 4038 being used, maybe I was distracted.

The exception I remember of Ribbons being used in the 70s in very successful records was in Led Zeppelin IV record (1971) where Beyerdynamic M160 were used and actually some of the technics used became classics like the sound of "When the levee breaks".
But I guess this use was an exception and not the most popular mic choice of the time.

So like you said, late 60s and 70s (and 80s and probably 90s), Ribbon mics were Rarely used,
and it seems they fell out of favor to condenser microphones between the mid 50s to mid 60s being rarely used from the late 60s on,
until the Digital age of recording came.

There's a lot of reasons for that,
but what I think are the main reasons are these ones:

"Ribbons are known for a very musical, natural sound. But, in the 1960s, ribbons fell out of favor — not because of the microphone itself, but due to changes in the way voices and music were recorded.

Ribbon microphones have an inherent, high-frequency roll off that is similar to the way people hear. Between the 1930s and 1990s, the majority of people were recording to tape machines through consoles which both had a high-frequency roll off. Those tapes were then put on vinyl which also had a high-frequency roll off.

Then those vinyls were played on radio, which also had a high-frequency roll off. By the time the original source track hit the home listener, much of the high-frequency content was totally lost.

Until the late 1950s, condenser microphones could not compare to the ribbon’s frequency response. But condensers steadily improved and swept the recording industry. They became the “go to” mic for many recording studios. Because of their high-tuned system, frequencies in condensers in the top-end were exaggerated and hyped.

Sometimes these condensers were so hyped that monitoring vocals directly through the console would sound harsh. But by the time the condenser was recorded to tape and then transferred to vinyl and then played back on radio, the top end was greatly rolled off and sounded smooth.

The difference between a condenser and a ribbon at the end of the chain was pretty drastic with an RCA 44BX sounding muddy next to a condenser sounding crystal clear. By the 1970s, ribbon mics had passed out of style and RCA’s ribbon mic division was closed down."

https://www.thebroadcastbridge.com/...crophones-make-a-full-circle-in-audio-history


This last part of my past I already knew and found, but I write it anyway to complete the post,
In the Digital Age of recording the High End frequencies are not lost in the recording process, or on each playback, or in mixing, mastering or in transfer to a CD or Streaming (final consumer format), so condenser microphones might sound too harsh in some situations.
The Ribbon microphones comeback made sense because now, although they're darker than condensers the limited high end they have will not get lost, and can make when needed digital recordings sound warmer or less harsh for some people (myself included, but I like all mics and having all the tools)

Sorry for hijacking the thread with this post,
This subject is interesting to me because this is my passion and my job, but also because I'm a tutor of Recording for 7 years, and I always start my lectures with the History of Recording.

Thanks Ian

 

[Whoops]

If some fool wires a mic wrongly you can hardly blame phantom power for that. The real lesson is not that phantom is bad, but that you should take more care wiring your ribbons.

Other potential dangers that can happen in a recording studio with Phantom Power and Ribbon mics is hot pluging a TRS Bantam patch cable between Mic Pre input and Mic Tie Lines.
And also an XLR cable that failed and some of the wires inside the connector broke and are shorting to other pins.

But most of the times even those situations are not guaranteed to break the Ribbon.

Anyway, I think equipment should always have switches for turning Phantom On/Off in each input individually,
and engineers should know very well the safe procedures to use phantom power.

 

[RoadrunnerOZ]

Just a few points to ponder - assuming no bad wiring:
Ribbon mic transformers are not grounded one side or the other on the secondary so no path to ground for phantom current
One side of the winding secondary goes to Hot, the other goes to Cold at the mic’s output - Hot is at +48V, Cold is at +48V with phantom engaged and require a path to ground for phantom to have any spike effect back into the ribbon - there is no potential difference of 48V between the Hot and Cold.
Hot plugging any mic that does not use phantom like ribbon or dynamic should only cause a spike to the input preamp of the console which should be muted if plugging in a mic anyway.
A condenser mic will send a spike to the destination input preamp whether you have a phantom switch or not as the mic preamp starts up - with no phantom switch you get a spike on plug in, with a phantom switch you get a spike on turn-on of the switch - mute the channel input in either scenario. Hot plugging a condenser mic should be no different to using a phantom switch for the mic. Should not cause any problem for the mic either.
Balanced output from a synth without DC coupled outputs shouldn’t have problems if there’s no ground reference but not something you want to test out, not ideal - it’s when you have unbalanced outs from a synth connecting to a phantom destination you can get problems.
That means either a DI, hard disconnecting the phantom or a phantom switch.

 

[Whoops]

Hot plugging any mic that does not use phantom like ribbon or dynamic should only cause a spike to the input preamp of the console which should be muted if plugging in a mic anyway.

In this video you can clearly see that the ribbon moves when the mic is hot plugged with a TRS:




Now, even when hot plugging the ribbon seems to withstand the spike, so I guess most ribbon mics will take the abuse
What do you guys think?

 

[RoadrunnerOZ]

The above video instance uses a TRS which on plug insertion the tip of the plug will engage the ring terminal of the socket, the ring of the plug will engage the shield of the socket thus providing a path to ground through the ribbon transformer. This does not happen with XLR plugs/sockets.
This is why in the studio any patchbay connections should be done BEFORE connecting the ribbon mic cable to the wall plate. In the studio I work in at the moment I wired all wall plates to the patchbay coming up on XLR male sockets, then when plugging in a ribbon mic (or any mic) I made XLRF to bantam adapter leads - the bantam is inserted first, then the XLR end of the bantam to XLRF lead into the male XLRM socket on the patch panel.
If changing to another channel on the desk the XLRF plug is removed from the patch panel, bantam patch change is done, then the XLRF re-inserted.
We have several ribbon mics in the studio - 2 Coles 4033, 2 Coles 4038, a Royer clone with custom ribbon and transformer (and a couple of others I forget the makes/models now). No pulse of the ribbon when plugging in using the XLR end. This will occur only if patching while mic is plugged into the XLR and then inserting the bantam TRS end which we do not do.
Ribbons will handle a lot of ribbon movement and are pretty robust unless you drop one!

Edit: this is all connected to a Neve console with permanent phantom, no switches, on the mic inputs.

 

[bmaughan]

Could it also be possible to move the legs of the 6.8ks to M1 or M2 (pre switch) so that it only affects the input you’d like to use with microphones, if that is the case that one is being connected to synths continuously? Or is that still too risky
Would there be a risk of spikes when switching between inputs still…
S

If I'm seeing it correctly yes, you could move the 2x 6k8 resistors to Pin 2 and Pin 3 of M1, before the M1/M2 switch.
That way you would have Phantom Power only in Input M1 and not in Input M2.
You could use then input M1 for Microphones and input M2 for your synths

But you still need to check something, please read below



As far as I can see, and please guys correct me if I'm wrong,
to do this you need to check if the Switch that changes between M1 and M2 is a Make before Brake switch (shorting) or a Break Before Make switch (non-shorting).
If it's a BBM switch then I don't think you have any risk and problem is solved.
But if it's a MBB (shorting )switch then if phantom is turned on when you move the switch from input M1 to input M2 there will be for a tiny fraction of time 48V on input M2 (during the travel) until the switch reaches the final position and then there's no 48V.

I don't know if this tiny fraction of time would be enough to cause any danger, but I think you should check what type of switch is installed there just to be sure.

Some reading about BBM and MBB switches:

https://e2e.ti.com/support/switches...tches-multiplexers-and-how-are-they-different

https://e2e.ti.com/support/switches...tches-multiplexers-and-how-are-they-different

This is awesome!! I’ll check the switches. It may take me a bit.. half of my room has to come out for me to get access and I’m in the middle of a project haha

 

[ruffrecords]

Anyway, I think equipment should always have switches for turning Phantom On/Off in each input individually,
and engineers should know very well the safe procedures to use phantom power.

This is exactly what I do in my own mixer designs but sometimes even that is not enough. Earlier this year I delivered a 4 channel mixer to a customer in Pasadena. All four channels have phantom power switches. However, I then designed a 4 channel passive mic mixer, very much like the one used by the Beatles at Abbey road for Ringo's drums. The customer wanted it for the same purpose and he was going to use only dynamic and ribbon mics (including a 4038). Now that I have sent him a working prototype he is concerned that he might damage his 4038 if someone accidentally switches on the phanom power. So now I am designing a MK2 version which includes phantom isolation. So you see, even with a phantom switch per channel it can still cause problems.

Cheers

Ian

 

[ruffrecords]

Sorry for hijacking the thread with this post,
This subject is interesting to me because this is my passion and my job, but also because I'm a tutor of Recording for 7 years, and I always start my lectures with the History of Recording.

Thanks Ian

No problem. I would add a couple of additional points. The US and Europe developed differently. Neumann condenser mics were available in Germany from 1928. At that time, until the advent of vinyl, both radio and records had an upper bandwidth of about 7KHz. With the introduction of FM radio (1940) and vinyl records (1948) the required bandwidth increased to 15KHz (FM) and 20KHz (vinyl). If you look at the the frequency response of a U47 (released in 1947) for example you will see it is only specified up to 15KHz. So I think extended high frequency response from condensers was available much earlier than you suggest and I would dispute the claim that "Until the late 1950s, condenser microphones could not compare to the ribbon’s frequency response"

Cheers

Ian

 

[Whoops]

No problem. I would add a couple of additional points. The US and Europe developed differently. Neumann condenser mics were available in Germany from 1928. At that time, until the advent of vinyl, both radio and records had an upper bandwidth of about 7KHz. With the introduction of FM radio (1940) and vinyl records (1948) the required bandwidth increased to 15KHz (FM) and 20KHz (vinyl). If you look at the the frequency response of a U47 (released in 1947) for example you will see it is only specified up to 15KHz. So I think extended high frequency response from condensers was available much earlier than you suggest and I would dispute the claim that "Until the late 1950s, condenser microphones could not compare to the ribbon’s frequency response"

Cheers

Ian

Thank you so much my friend.

 

[Whoops]

The above video instance uses a TRS which on plug insertion the tip of the plug will engage the ring terminal of the socket, the ring of the plug will engage the shield of the socket thus providing a path to ground through the ribbon transformer. This does not happen with XLR plugs/sockets.

And that's exactly what I explained in post #45

"Other potential dangers that can happen in a recording studio with Phantom Power and Ribbon mics is hot pluging a TRS Bantam patch cable between Mic Pre input and Mic Tie Lines.
And also an XLR cable that failed and some of the wires inside the connector broke and are shorting to other pins."

90% of the professional studios I worked in (and worked in some major ones) had Bantam TRS patchbays with a row for the Tie Lines coming from the Live Room (or rooms) and the row under it was the Mic Pres inputs (or console Mic Inputs). Although the most common usage is to have those 2 rows Normalled, sometimes you want to send a specific tie line to a different mic pre input, and for that you need to insert a TRS bantam patch cable so the "potential danger" is hot plugging it

This is why in the studio any patchbay connections should be done BEFORE connecting the ribbon mic cable to the wall plate.

For sure that's the procedure, and most of us know the procedures to use phantom power, but the discussion we're having is not about the correct procedure, what the OP "bmaughan" is trying to aviod is the potential danger when someone forgets to leave 48v on, or when someone makes a mistake, or when an XLR cable fails. These are just a few examples that can happen in studio sessions.

From my experience in many years recording in professional studios and doing very long sessions, you can be so exhausted that at some point is easy to make a mistake. Also a lot of sessions are made in an hurry, budgets are smaller now so the band/artist can only book a few days but they still want to record the full record in those days, so many sessions are so rushed and you have to work so fast that mistakes can easily happen also, even if you know all the procedures.
We can't also forget studio interns or assistants that still don' have the same experience as the main engineer as they are still learning, and it's normal and understandable that they make mistakes.

So in my opinion and experience, any Engineer should know very well the Phantom Power procedures but if the studio is setup in a way that is more mistake proof the better.

My 2 cents

 

[emrr]

In this video you can clearly see that the ribbon moves when the mic is hot plugged with a TRS:




Now, even when hot plugging the ribbon seems to withstand the spike, so I guess most ribbon mics will take the abuse
What do you guys think?

Shinybox had a great video (now removed) showing the effects of various misapplied phantom states to a ribbon. One state would immediately break the ribbon.

There are vintage ribbons with grounded secondary center taps that would give a symmetrical path to ground for phantom. The resistive symmetry of the winding would dictate any level of disturbance to the ribbon for an otherwise perfectly hot plugged situation.

There's always the newb assistant/musician/client who 'wants to help' and inadvertently does something stupid with a mic and an input.

I did walk by a Coles 4038 in a session recently that was making acoustic noise on it's own - phantom applied from such a case as above. Turned the phantom off, mic seemed fine, but it was alarming to hear a mic producing an acoustic output, I didn't expect survival.

I did have a nearly new M160 damaged by a client replugging it to a different XLR input with active phantom. Can't at all say why, but it indeed needed a repair.

There is also the 'Phantom Menace' and 'Phantom Menace Part 2' papers to consider. The right unfortunate phantom hot patch short can dump several amps of current with potential for damage.

For the OP I'd propose a Switchcraft barrel with a pair of inline NP caps and a pair of 10K resistors from pins 2/3 to pin 1 on the synth sidee, leave them attached to the synth output, assuming they're balanced outputs. They probably aren't, so some variation of that may be the simplest solution if the mixer is hard to get into and modify.

 

[Somkereki]

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.

In addition, for the sake of perfect sound, it is worth connecting a minimum 100 nF Mylar or polypropylene capacitor in parallel with the biolar electrolytic capacitors in order to eliminate the high-frequency phase shift. I recently repaired a friend's Quad 66 preamplifier, all coupling capacitors were 50uF / 25 V bipolar. I measured the phase shift at 20 kHz at 48 degrees. (With a Neutrik A2-D Audio Test & Service System instrument.) In addition, when testing with a square signal, I saw "endless ringing" on the oscilloscope, at the top of the signal. I installed 680 nF polypropylene capacitors everywhere and reduced it to 14 degrees. The ringing has stopped. Synthesizer sounds are full of harmonics, phase shifting can result in a bad sound.

 

[Somkereki]

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

I haven't heard a better-sounding mixing desk either, but is this a newer model? Unfortunately, the newer models of most companies are designed for "cheaper" production. For example, from the Sennheiser MKH 416 and MKH 435 condenser microphones, the old 12 Volt phantom power (T12) has a much better sound than the 48 Volt MKH 416 models with the new SMD electronics. Movie sound engineer friends showed me and the difference is amazing! (In favor of the old one) Now I'm glad that I bought quite a few from the theaters when they bought new ones.

 

[Somkereki]

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

35 KV? You are brave! I worked in a plasma physics research laboratory with 10 KV capacitor banks, but I had to be so careful because a mistake would have been fatal. When one of my colleagues dropped the wrench right onto the capacitor bank, it exploded like a hand grenade. That's when we built an automatic discharge device for it, because one forgetfulness can mean death. Back to the electrolytic capacitors, the worst part in terms of sound. But there is nothing better! For this reason, I use Mylar or even more polypropylene capacitors in the signal path in all demanding audio circuits. If this is not possible due to the size of the capacity, I try to connect the largest possible inductance-free polypropylene or Mylar capacitors in parallel with the electrolytic capacitors. You can really hear the difference! (I am an electrical engineer and sound engineer, already in 1981, during the tour of the most popular Hungarian rock band, my colleague brought an article from the IEEE magazine about the sound of various capacitors. Since then, it has been proven by measuring and listening! He lived next to the Technical University, he visited its library regular. But it was good to be so young! However, our sound engineer also had to pack the equipment into the truck. True, we got paid extra for it. In two days I got paid as much as working for a company as an electrical engineer.

 

[RoadrunnerOZ]

When I started out servicing colour TV at Philips the 110° picture tubes on the K9 sets had anode voltage of 25KV, the black and whites were around 16KV, we had a lot of very high voltage capacitors in our parts store that some techs would charge up and put back into the bins for a joke - and also had HT bench supplies with the required voltages to drive a picture tube - you learned real quick to be careful!

 

[JohnRoberts]

Back last century when I was still designing mixers, or at least mapping out feature sets for them, I recall one mixer design (RQ3014) where I offered the option to switch phantom power on/off independently on only a handful of "special" channels, while the rest of the mixer's phantom was switched on/off with a single switch. This feature allowed you to power just one channel, or power all but just one channel, as needed.

JR

 

[Cqwet Dbdfte]

I haven't heard a better-sounding mixing desk either, but is this a newer model? Unfortunately, the newer models of most companies are designed for "cheaper" production. For example, from the Sennheiser MKH 416 and MKH 435 condenser microphones, the old 12 Volt phantom power (T12) has a much better sound than the 48 Volt MKH 416 models with the new SMD electronics. Movie sound engineer friends showed me and the difference is amazing! (In favor of the old one) Now I'm glad that I bought quite a few from the theaters when they bought new ones.

Not sure exactly what part of the earlier non-smd product had worse sound.
Cheap ceramic caps have a pretty severe voltage coefficient, and NPO caps have limited values, so if a coupling cap, better to use a film cap, or even electrolytic.
Most SMT resistors are thick film, metal film are many times more expensive and not very common.
Active devices, I don't think SMD suffers any disadvantage.