signal earthing/ground scheme

[PeteC]

Just a couple of questions as I near end of my dual channel REDD build.

1) Jensen mic input schem suggests earthing the mic input to chassis as close as possible to the input.

I am using Neutrik combo xlr/jacks which are isolated - so had planned to make a star earth somewhere close to TX and the main filter cap and run everything back to that.
Also I will have safety earth point next to IEC inlet.

Whats a recommended approach ?
I don't see a clean way of earthing the mic input on the front panel without another screw hole which I want to avoid.

2) My Sowter input TXs have flying leads that I planned to twist , which will go the the phase/z/pad switching. 
How do I screen the input wiring - or will twisted pairs be OK ?

Also whats the usual approach for dealing with the unused leads on an input TX ( CT , 34.5% tap ) as I noticed how I vcoil them up or move them affects the noise floor ( when I hagd the amp testing on the breadboard )  ?

Rgds
PeteC

 

[dmp]

1) Jensen mic input schem suggests earthing the mic input to chassis as close as possible to the input.

I'm not sure what schematic you are talking about. Typically, they show only the shielding connections of the transformer going to chassis ground. These connections are not the mic transformer input.
For instance in  JT-16-A in Single Stage Low-Noise IC Mic Preamp (16KB PDF) on their site, the black and white wires are grounding the case and core of the transformer.

The input XLR p2/p3 should go to the input of the transformer. XLR p1 should have a good connection to chassis ground.
Twisted pairs are very good when going to a transformer, since the transformer has excellent CMRR.
Shielding can help if there is interference or crosstalk between channels though.I wouldn't expect it would be needed.

 

[CJ]

easy rule for grounding is keep your electronic grounds from running thru the chassis as much as possible,

a star ground will do this,

this way, all your pin 1 currents and xfmr shield currents and induced EMF's will not use the chassis as a mixer,

 

[PeteC]

thanks chaps,

this jensen note ......

see last comment in notes section

http://www.jensen-transformers.com/as/as016.pdf

thats what I had seen suggesting connect pin 1 to chassis.

cheers
Pete
\

 

[CJ]

yes, pin 1 should go to the chassis near the jack,

however, i bought a bunch of Blonder Tongue 1RU boxes to use for projects, these are hi freq switch gear jobs that cable companies buy by the zillions, anyway, they had 2 feet of wire running from the XLR jacks on the back along the perimeter of the chassis to the front of the chassis where there were other XLR connectors installed, joining the pin 1 grounds together, so another rule is to experiment and use whatever works,

a little black magic with grounding, but not nearly as complex as, say, antenna theory,

there are no EE classes on antennas because it is all figured out by experiment,

bunch of weird Phd's sitting around bending coat hangers,

 

[PeteC]

CJ,

my plan was to use a star ground for the whole preamp, and therefore not to ground pin 1's at the chassis - instead running them back to the star point, near main filter cap.

If I ground the pin 1's at the chassis wont I get a ground loop ?

That's what I didn't really understand about the Jensen approach .
Does the Jensen approach assume that the pin 1 chassis connection becomes the single ground point ?

Rgds
Pete

 

[CJ]

pin 1 has the nasty stuff on it that you want to keep out of your audio,

so you want to ground pin 1 away from the star ground,

 

[PeteC]

OK,
thanks CJ
i think I get it now - I am too used to unbalanced connections and bringing earth to single point.

I guess I can earth the pin 1 on the neutrik combo jacks ( which are isolated rear mounting ) using a star washer on the inside of the panel mounting hole.

cheers
Pete

 

[CJ]

i never liked pin 1 being ground either,

came into studio electronics from the guitar amp world, everything unbalanced, so same story as you.

i always though that pin 1 should be hot, pin 2 cold, and pin 3 ground,

but some jackwagon had to mess it all up for the rest of us logical thinkers,

you know what i'm sayin? cuz you know i ani't playin,  ;D

 

[okgb]

It's not my idea , but I've had good success taking a piece of copper house wire [ 14 ? fits right into the xlr  ]
and making a bus to connect the i/o's pin one's and bringing all grounds to there ,  preamp board & psu , if individual
preamp boards closest to their respective jacks , It's how Dan Kennedy suggested wiring his preamps .
kind of a star bus bar .
then I make some kind of jumper to link chasis ground or not  [ that's the part I'm not firm on  ]

 

[PeteC]

Yeah, that's kind of what I had in mind originally - and what I have done on previous tube hi-fi amps I have built.
I will try a few different schemes and see what gives me lowest noise factor .

Seems like there's no hard and fast rules - so much dependent on the specific build layout etc I guess.

Rgds
Pete

 

[ruffrecords]

Here is a link to a document on grounding that I recently wrote for the EZTubeMixer builders but it applies equally to mic preamps:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf

Cheers

Ian

 

[PeteC]

Thanks Ian,
thats really helpful.  I will follow these guidelines.

PS - Would you mind if I send you a couple of pics as I progress with the finish of my dual P2P REDD47 so that you can pick me up on anything that looks iffy from a grounding point of view ?

cheers
Pete

 

[ruffrecords]

Thanks Ian,
thats really helpful.  I will follow these guidelines.

PS - Would you mind if I send you a couple of pics as I progress with the finish of my dual P2P REDD47 so that you can pick me up on anything that looks iffy from a grounding point of view ?

cheers
Pete

That would be fine.

Cheers

Ian

 

[alexc]

Thanks Ian - great read to add to my sheaf of 'Ian Thompson-Bell Tube Designs' collection.

Keeping it 'Ruff 

 

[PeteC]

Hi Ian/UK Chaps ....

any particular ( twin ) screened cable brand you recommend for hooking up screened connections ?

Is maplins miniature ( 6mm ) twin screened stuff good enough, or should I look for a particular cable ?
http://www.maplin.co.uk/twin-individually-screened-audio-cable-134

Cheers
Pete

 

[ruffrecords]

Hi Ian/UK Chaps ....

any particular ( twin ) screened cable brand you recommend for hooking up screened connections ?

Is maplins miniature ( 6mm ) twin screened stuff good enough, or should I look for a particular cable ?
http://www.maplin.co.uk/twin-individually-screened-audio-cable-134

Cheers
Pete

I use Van Damme 'Claasic Install Cable'. Good tough cable, easy to strip and it comes in a variety of colours. Even Maplin do it:

http://www.maplin.co.uk/van-damme-pro-grade-classic-xke-1-pair-install-cable-623211

Or from here:

http://www.distributedsoundandvideoltd.co.uk/store/van-damme-pro-grade-classic-xke-1-pair-install-cable-black.html


Cheers

Ian

 

[PeteC]

Many thanks Ian,
I have a convenient Maplin store so will go with that.

Newbie alert........I read through your earthing document Ian, which was really helpful, but I have some questions about earthing the added Phantom power in my dual REDD47 build.  I need Phantom for a couple of the mics I have.
Should I have Phantom earth at the star point ?

Rgds
PeteC

 

[ruffrecords]

Many thanks Ian,
I have a convenient Maplin store so will go with that.

Newbie alert........I read through your earthing document Ian, which was really helpful, but I have some questions about earthing the added Phantom power in my dual REDD47 build.  I need Phantom for a couple of the mics I have.
Should I have Phantom earth at the star point ?

Rgds
PeteC

If the phantom power comes from an isolated winding on a mains transformer then the answer is no. The phantom power 0V should be wired directly to the mic input XLRs pins 1. If your phantom power already has a 0V in common with the HT 0V, because it is derived from a winding used for something else as well then it will already be connected to the start point via the HT 0V connection. A lot depends on your power supply schematic.

But the rule is; try to ensure your phantom power is isolated from all other supplies (by using its own winding) and connect its 0V to mic input XLR pins 1. 

If you want to post your PSU schematic I can tell you which it is.

Cheers

Ian

 

[PeteC]

Hi Ian,

I am using your Phantom circuit together with the jensen standard mic input scheme , i.e. with the 220Uf cap and a 220R resistor at the switch.
http://www.jensen-transformers.com/as/as016.pdf

I have a dedicated toroid supplying the PSU board which has linked 2x25v windings for 50v into the board.

I am not deriving my phantom power from the main TX secondary.

all my XLR pin 1's are now connected straight to chassis.  I didn't realise at first but the insulated Neutrik combo jacks have a really neat way of doing this via a couple of sharp pointed ground connectors which dig into the faceplate when you tighten the socket in place.

Cheers
Pete