A few "PIN 1" questions

[Lowfreq]

Hey all,

I've been doing a lot of reading about grounding, and the whole "what to do with pin 1" thing.
Before anyone tells me to read up on it or search, let me just say I have, and that reading and searching has helped a lot, but also has confused me a bit, so I'm trying to clarify some things more specifically to what I'm doing.

On most of my line level gear 1176's, La2a, EQ's etc, I take pin 1 directly to chassis ground at the tab on the xlr connector, and not to the circuit itself (like in the Rane doc), and that seems to work best for keeping noise, interference and computer sounds out of my equipment. (Even on my Slient-arts La2a pcb, where they say that it's not needed to do pin 1 to chassis ground, I found that it's still quieter if I do)
My limited understanding is that we should have three types of ground "audio, power, and chassis" and that chassis and audio should be kept separate from each other except for the star ground point.

But when it comes to preamps (specifically my JLMs and my APIs), I'm a bit unsure on the best option.
From looking at the JLM schematic pin 1 on input & output goes to 0v instead of chassis ground.
I would've thought that the output pin one would at least go to chassis rather than 0v. ???
http://www.jlmaudio.com/Dual%20JLM99v%20with%20Lundahl%201538.pdf

The Baby Animal pre's even connect 0v directly to the chassis through the back of the conductive dc connector. So does that mean there is no chassis ground?

http://www.jlmaudio.com/Baby%20Animal%20Mic%20Pre%20with%20JLM14%20&%20OPA2604.pdf

But when I look at all the documents from Jensen, they always ground pin 1 at the chassis. I have my API's wired up like this and they seem to work really well.
http://www.jensentransformers.com/as/as016.pdf
http://www.jensentransformers.com/datashts/110khpc.pdf

I had a good read through some of the grounding threads, and from what I can gather, with preamps, you want to connect pin 1 on input directly to the star ground so current from phantom power doesn't make it's way back to the star ground through the chassis.
So how come the Jensen docs don't do this. Is it wrong to have pin 1 on preamps only connected to chassis ground?

One exception to pin 1 on XLR's. If an XLR is used for a mic input
requiring phantom power, It should go to the star ground point.
This is the return path for the phantom circuit and carries current.
The chassis should not be used as a return path for any circuit.
Shields in XLR cables should be connected to the provided "tabs" in the
XLR cable connectors. Good high quality mic cables are wired this way.

This is all true, RonL.  I would say that on a mike preamp, pin 1 of the female XLR(s) should still be connected to the chassis, and that should be the one point where circuit ground connects to chassis ground.  This is how this stuff gets so complicated and confusing.  In my designs, which use circuitboards, there is a Chassis Ground plane on the part of the circuitboard where the XLR connectors go.  This lets me keep Pin 1 and the shell connected to Chassis Ground instead of Circuit Ground, but also gives me a copper conductor for the phantom return.

Also, the shield on an XLR cable can be soldered to both the shell tab AND pin 1.

So I guess I'm really trying to figure out why the JLM go to 0v instead of chassis ground? And why Jensen say to only chassis ground input pin 1



Ok onto my next question. Microphones this time.
I was reading the Jim Brown paper on grounding, and he showed a couple of examples of microphones with pin 1 problems. It seems the issue with connecting pin one to both the mic body and the mic circuit can create issues with RF interference.
The shorter the connection between the two the better, but he never really explained the best way for mics. He seems to go onto regular outboard when showing "the right way" of doing things.

So is it bad to connect to the mic body at pin one?, even on the little tab?

Take the Royer mod for example, in the schematic it tells you to do this. ???
So what is the best way? Do we want to take chassis ground up the shield of the mic cable and use one of the inside cables for 0v?

Some (not all) of my mics  have a little zip zip sound going on in them, even comercially made ones like my audio technicas.
I've tried them in a yamaha desk (to get away from any potential DIY wrong doings), and still get the zip zip buzz. My outboard gear sometimes picks up the zip zip when the lid is off but is quiet when all boxed up.

Anyway, that's enough questions for now............ I have more, trust me...... a lot more. (my brain hurts)
It seems the more I read and search on this, the more answers I get, but also more questions and even opposing ideas from different sources..........

I hope I've explained myself clearly. Sometimes I find it hard to get onto paper all my crazy thoughts and directions.......

 

[PRR]

> a little zip zip sound going on

Find the SOURCE of the zip-zip and kill it.

The "right" answer sometimes depends on the problem. It does seem to make sense to take Pin 1 to chassis every time. And then to use a dedicated floating Phantom supply returned only AT the mike connectors. But if your cellphone or microwave is spewing garbage all over the chassis, that may be "worse". While a constant 60Hz hum comes with modern living, a "zip-zip" is something odd and probably kill-able.

 

[Lowfreq]

Ok, I'm going zip zip hunting tomorrow.........

Any other thoughts on my crazy questions??

 

[Lowfreq]

I have located my zip zip..... and killed it!! ;D

It was our wireless phone in the lounge, but the base, not the handset....

I tried plugging in a different wireless phone that we haven't been using (just coz the wife liked the look of the other one better) and it works well, with no zip zips making their way into my room.

Ok, now that's sorted I'm still keen to hear peoples thoughts on grounding and pin 1

 

[3nity]

It was our wireless phone in the lounge, but the base, not the handset....

Quasi audio related...case closed??
My 1290s pin 1 are just connected to 0V and its quiet than to case.
Any toughts?

 

[Lowfreq]

Is that the EZ1290? I've been reading through the assembly guide as I've got a couple of these to assemble soon. I noticed that input pin one is tied to 0v, and that in the project the only time anything goes to chassis ground is via a 10 ohm resistor. The JLM power supply also has this 10 ohm difference going on in the power supply from what I can see.

I've heard Neves are fussy about their grounding. The guide even suggests not grounding the output pin 1 at all to avoid ground loops.

 

[abbey road d enfer]

In a perfect world where pin 1 would carry only electrostatic screens, it is clear that connecting pin 1 to chassis the shortest way is good practice.
You still may have to deal with strong earth current created by induction in the mains cabling. That's why I always install an RC circuit (100R/0.1uF) between pin 1 of the inputs and chassis.
I have never seen a case of pin 1 problem when a mic is connected directly to a mixer or preamp (unless the mic's body touches a metallic stucture); problems arise when there is a mic patchbay with commoned grounds.

The imperfect world syndrome is when connections are unbalanced.
It is impossible to do a quick review of the techniques and constraints of grounding.
You must consider that every ground connection is in fact a resistor and analyse the circuit.

In some ways, connecting pin 1 to audio ground makes more sense than connecting it to chassis, because you don't have to deal with all the garbage that's carried by the mains earth. The problem with this is that it's a receipe for catching every radio interference in the vicinity.
Thirty years ago, when there were no portable phones, no WiFi, disconnecting audio completely (except for a 10k/0.1 RC) from the chassis and connecting pin 1 to it was ok (except for broadcast equipment operating in RF environment).
Today, because of the need for RFI protection, there is no choice other than pin 1 to chassis (although it may be done through a capacitor of suitable value).
As a consequence the earth loops created by the conjunction of audio cable screens and mains earth are to be dealt with. Standard balanced connections are good at it, but unbalanced ones are difficult. Some manufacturers have devised smart circuits that actively compensate hum loops.
Remember that, in order to break a hum loop, it is not necessary that both the input and output are balanced.
In case of hybrid connection (one balanced-one unbalanced) use a balanced cable and strap cold and shield only at the unbalanced side.