The pin 1 problem

[cyrano]

I was a bit surprised to find this:

http://pin1problem.com/

It's a site dedicated to the pin 1 problem, with a number of links to articels explaining the problem.

Maybe we should all bookmark this?

 

[JohnRoberts]

This is relatively old news at this point, but still shows up in old legacy gear designed before this was common knowledge, so worth being aware of.

JR

 

[PRR]

Noise Susceptibility in Analog and Digital Signal Processing Systems, Neil A. Muncy
JAES 3 (6) (June 1995)
https://www.jhbrandt.net/wp-content/uploads/2018/10/Noise_Susceptibility_in_Analog_and_Digital_Signal_Processing_Systems.pdf

 

[scott2000]

Thanks PRR!

Thought I almost saw a window for some audiophile cable excitement....

I've had luck finding some issues using the test cable on the Rane site in the past as well...

 

[Bo Deadly]

Summary:

Just inside an XLR connector, pin 1 must be connected to chassis ground through the shortest possible length of wire (preferably only a few cm). The length of that wire determines the RF cutoff frequency that can be radiated into the enclosure.

Chassis ground and power ground must be connected together at one point. A good place is right next to the filter ground of the power supply.

There are XLR female connectors that have a metal spike that stabs the metal front panel and directly connects to pin 1. They're great for mic pres where a long cable with nothing on the end except for a metal can is sort of like an antenna.

Inside a mic body you can solder RC filter components directly to the XLR terminals (assuming the circuit calls for it).

 

[JohnRoberts]

pin one hummer mentioned in AES paper is a simple way to vet gear/designs.

JR

 

[abbey road d enfer]

Chassis ground and power ground must be connected together at one point. A good place is right next to the filter ground of the power supply.

As with every rule of thumb, it must be put in perspective. When optimizing a product, it is often found that moving this connection is beneficial. Some of the best designs I know use a low impedance connection between audio ground and chassis, like 10 ohms in parallels with 0.1uF, and connect the audio ground at the regulators reference point.
As to the direct connection of pin 1 to chassis, it can be the source of unwanted currents in the cable shield. Because of that, some designers purposedly insert an RC. In terms of EMI/RFI the connection between pin1 and chassis is essential at RF only. Typically, a 0.1uF between pin1 and shell is as good as direct contact.

 

[Bo Deadly]

As to the direct connection of pin 1 to chassis, it can be the source of unwanted currents in the cable shield. Because of that, some designers purposedly insert an RC. In terms of EMI/RFI the connection between pin1 and chassis is essential at RF only. Typically, a 0.1uF between pin1 and shell is as good as direct contact.

Ok. Intersting. But what if phantom power is used where "ground" is through pin 1? Is any of this still desired / applicable?

 

[abbey road d enfer]

Ok. Intersting. But what if phantom power is used where "ground" is through pin 1? Is any of this still desired / applicable?

Since phantom current is usually quite low, the additional series resistor has no effect. Indeed, it is quite important to check there is no significant parasitic voltage developped there.
Ideally, the phantom supply should be completely separate from the other rails, with its regulator reference point grounded at the chassis near the input XLR.
It is not always easy, particularly when phantom is derived from the same xfmr as the other rails via a voltage doubler or tripler.
As usual, each case must be analysed and evaluated on its own. Beware of receipes, they may work in one case, not in another.

 

[Speedskater]

pin one hummer mentioned in AES paper is a simple way to vet gear/designs.
JR

Yes the John  Windt 'Hummer Tester' is a tool that everyone should have. Cost is under $20 for a DIY unit.  Bill Whitlock has a short description in a few of his papers, but the 1995 John  Windt paper has a full three pages.

also in (you have to log in):
"Build the “Hummer” to Find “Pin 1 Problems” in Equipment"
https://www.jensen-transformers.com/wp-content/uploads/2014/08/as032.pdf