Semiconductor crosspoint switch array? Unbalancing/balancing circuitry? Nein, danke.
connectors in a signal path
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moamps
Well-known member
The first patchbays I installed in the late 80's were Switchcraft. Microphone lines were fully normalized, line signals were half normalized. After about 10 years of use in the microphone parts of the patchbays occasional interruptions and noise began to occur, usually where the phantom power was turned on. I explained it to myself that metal-oxide diodes started to form on the contacts, because by switching the phantom voltage on and off several times, the problem would temporarily disappear.
Critical parts on patchbays are normalling contacts quality and open frame structure prone to dust etc.. That's why I later made the microphone lines isolated, and I would use patch cables to achieve the desired paths. And of course, if it was possible, I used XLR based patchbays instead of TT versions for microphones. Later I also used Neutrik and Audio Accessories patchbays with about the same results.
TRS patchbays are basically very bad (I tried Neutrik, Presonus?, Behringer, etc.) and they should only be used in isolated mode, not as half or full normalized and of course only for line signals.
I also installed a couple of Ghielmetti patchbays, but I wasn't a fan of them, they were unacceptably overpriced at the time.
scott2000
Well-known member
Sort of like a souped up Kramer?
Yes, or Extron, which latter coined the term "Matrix Switcher" for it. These products used to be called Audio Routing Switchers. Well-known manufacturers were Leitch, Hedco, Grass Valley, Central Dynamics, Sierra video (later bought by Kramer), Sigma Electronics, Utah Scientific, etc.. I enclose an ad example. They were typically used in a broadcast studio environment or in control rooms of large auditoriums and theaters. With the advent of digital technology, they have almost disappeared.
In the audio systems for event and conference rooms, we no longer have analogue technology. It seems to me that analog technology only survives in the recording studio and with musicians.
In the audio systems for event and conference rooms, we no longer have analogue technology. It seems to me that analog technology only survives in the recording studio and with musicians.
The main issue with microphone jack patchbays is the short that happens each time a cord is inserted or disconnected, particularly when phantom power is applied.
Although most console/preamp/mic manufacturers have implemented the necessary protection circuitry, there are still many pieces of equipment that don't have them.
The consequences can range from straight non-function to progressive loss of performance.
XLR, relay or Ghielmetti patchbays are immune to this problem.
Although most console/preamp/mic manufacturers have implemented the necessary protection circuitry, there are still many pieces of equipment that don't have them.
The consequences can range from straight non-function to progressive loss of performance.
XLR, relay or Ghielmetti patchbays are immune to this problem.
soapfoot
Well-known member
Also, the non-simultaneous connection can be dangerous for ribbon microphones in the case of older consoles that have P48 always on all channels
According to IEC 61938:2018-10, the short-circuit current must be 7mA. If the 6800 Ohm resistors from the 48 VDC supply do not tolerate this current but burn, the circuit does not conform to the standard. The cause of the burning is that 1/4 watt resistors are used in cheap devices. The short circuit current is 1/3 watt though, so a decent circuit should have at least 1/2 watt resistors.
The issue is not with resistors burning.
Transformerless mic preamps typically have input capacitors that charge at about 48V. When an input pin is shorted to ground, it produces a negative peak that can destroy the junction of the input devices.
For the same reason, Schoeps-style microphones have PNP output transistors that can be damaged if their output is shorted.
Do you have any evidence for this statement? I know that's been said time and time again, but I've never found any hard evidence for it.
Brian Roth
Well-known member
I've in and out of pro studios since early/mid 70's (studio with a desk and a 8/16 recorder). Phantom power was new, especially to me. An older mentor told me to keep phantom off when patching. He gave me a demo with a Shure 57 while he was intentionally doing patching screw ups; I could clearly HEAR the element making clicking noises.
Shrug.....with phantom...I always prefer to err on the side of caution.
Bri
Last edited:
It is important to analyze the issue.
Regarding microphone connection, the problem occurs during phase 2 of connection, where the tip of the jack connects with a phantom-powered point and the ring is still grounded.
Depending on the type of preamp the current can be significant.
If steady-state only was considered, the power dissipation would be limited by the 6.8k phantom geed resistors, to a very small level. 7mA into 200 ohms results in less than 10 microwatt, which is without any danger.
However the pulse of 48V may result in an instant peak power of a few watts for a duration of 20-30 milliseconds, which is enough to produce a loud click.
It is usually harmless to transformer output mics and dynamic mics, but can result in large displacement of the diaphragm in ribbon mics, which distends the ribbon and results in loss of level and frequency response.
Transformerless mics can also suffer. The most pernicious way is when the output transistors go into Zener mode, which often degrades them in a subtle (and cumulative) way, so the
re's no obvious sign of problems.
Although there are more cases when patching hot mics results in no damage than cases otherwise, it is still good practice to avoid doing it.
Regarding microphone connection, the problem occurs during phase 2 of connection, where the tip of the jack connects with a phantom-powered point and the ring is still grounded.
Depending on the type of preamp the current can be significant.
If steady-state only was considered, the power dissipation would be limited by the 6.8k phantom geed resistors, to a very small level. 7mA into 200 ohms results in less than 10 microwatt, which is without any danger.
However the pulse of 48V may result in an instant peak power of a few watts for a duration of 20-30 milliseconds, which is enough to produce a loud click.
It is usually harmless to transformer output mics and dynamic mics, but can result in large displacement of the diaphragm in ribbon mics, which distends the ribbon and results in loss of level and frequency response.
Transformerless mics can also suffer. The most pernicious way is when the output transistors go into Zener mode, which often degrades them in a subtle (and cumulative) way, so the
re's no obvious sign of problems.
Although there are more cases when patching hot mics results in no damage than cases otherwise, it is still good practice to avoid doing it.
soapfoot
Well-known member
Yes, exactly.
To be clear—it’s hot-patching, with P48 live, using trs, tt, or longframe connectors, that poses the hazard.
It’s not the phantom itself… it’s the momentary non-simultaneous connection that causes the risk (as described by abbey above)
As others mentioned, in some cases you can actually hear it.
This has been overblown over the years into a “phantom kills ribbons!” myth. Strictly speaking, that isn’t true except in this one narrow circumstance—but that circumstance is very much real and well-documented
To be clear—it’s hot-patching, with P48 live, using trs, tt, or longframe connectors, that poses the hazard.
It’s not the phantom itself… it’s the momentary non-simultaneous connection that causes the risk (as described by abbey above)
As others mentioned, in some cases you can actually hear it.
This has been overblown over the years into a “phantom kills ribbons!” myth. Strictly speaking, that isn’t true except in this one narrow circumstance—but that circumstance is very much real and well-documented
Thanks for taking the trouble to explain the problem. Very enlightening.
At TI I found the schematic of a microphone amplifier front end, which nicely shows what measures should be taken to avoid damaging the first amplifier stage. (Scroll down to the last circuit diagram and its explanation.)
https://e2e.ti.com/support/amplifie...forum/665925/ina163-regarding-circuit-diagram
or
Yes, it's the typical recommanded protection for this type of circuit.
However, THAT's app note DN140 recommends adding 10 ohm resistors. Also google "The 48 Volt Phantom Menace Returns".
I have to admit that I underestimated the problem.
I was able to download the papers you mentioned about "The 48 Volt Phantom Menace Returns", they can be found in the AES E-Library
AES E-Library.
The "That Design Note 140" can be found here:
https://www.thatcorp.com/datashts/dn140.pdf
Thanks for the valuable hints.
I was able to download the papers you mentioned about "The 48 Volt Phantom Menace Returns", they can be found in the AES E-Library
AES E-Library.
The "That Design Note 140" can be found here:
https://www.thatcorp.com/datashts/dn140.pdf
Thanks for the valuable hints.
ThatCorp also has a copy on their web site in the published papers section:
Phantom Menace Returns paper by Bortoni and Kirkwood
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