Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Quickie consensus on input grid shield
Quickie consensus on input grid shield
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th September 2019, 05:08 PM   #1
Windcrest77 is offline Windcrest77  United States
diyAudio Member
 
Join Date: Feb 2019
Location: Lockport, IL USA
Default Quickie consensus on input grid shield

Generally do you prefer to ground a shielded cable connecting the grid to a jack or volume pot at the grid end or at the Jack/pot end? On my breadboard it seems better to ground the grid side and leave the pot/Jack floating. But one can make the theoretical argument of keeping the shield intact all the way across your external interconnect right up to the grid.
  Reply With Quote
Old 8th September 2019, 05:25 PM   #2
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
GoatGuy's Avatar
 
Join Date: Dec 2012
Location: SF Bay Area
Just to be “âhnal”, why not both ends? I know the signal levels of the input of an amplifier are modest. And for “preamp output” on the order of volts.

So, really “volts” are much greater than the millivolts associated with magnetic cartridge pickups for records. They clearly need shielding to protect against sub-microvolt AC induction, or pickup of errant AM radio signals, and so forth.

But amp-inputs with 0.775 V sensitivity (i.e. 'dBu', which is to say, 1 milliwatt of power into a 600 Ω load! …
P = E²/R …
PR = E² …
E = √(PR) …
E = √( 0.001 W × 600 Ω ) …
E = 0.775 VRMS
… bet many people don't know that!!!)

Anyway, good shields are good to have. Grounding them at either end to a good star or big-fat-copper ground bus ought to be perfectly acceptable. Especially for nominal amplifier input levels. Not magnetic pickup levels.

GoatGuy ✓
__________________
At a certain point, burnished unicorn horn only goes so far. When the BUH magically fails, its best to rely on maths. JustSaying™
  Reply With Quote
Old 8th September 2019, 06:32 PM   #3
lcsaszar is offline lcsaszar  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
Ideally the pot end GND should be connected to some common ground. I prefer that is the cable shield, connected at the grid end to the real signal ground (that goes to the PSU star ground, all isolated from the chassis).
  Reply With Quote
Old 9th September 2019, 07:13 AM   #4
6A3sUMMER is online now 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
For my low wattage power amps, I do the following:

The RCA input jack ground 1, goes to the 'zero' end of the volume control 2, and to the bottom of the input tubes self bias resistor 3, and bypass cap 4. Those 4 ground points connect together before connecting them with a separate wire that goes to the amplifier's central ground point. Those first 4 points make a purposeful short local ground loop.
I do not need to use a shielded wires there, I keep this part of the circuitry short and away from the other amplifier wiring.

I also make another short purposeful local ground loop:
The center tap of the B+ secondary 1, negative of the first filter cap 2, and the second filter cap 3. Only after that local loop do I connect another wire from there to the amplifier's central ground point.

Of course I also have to pay attention to the spacing and orientation of the output transformer, power transformer, and B+ filter choke.
I use aluminum chassis (steel has the problem of coupling those electro-magnetic parts).

Then there is the requirement to watch where you allow the other wiring to be placed.

Those practices give my amplifier's output hum to be equal to, or less than 100uV.
  Reply With Quote
Old 9th September 2019, 07:48 AM   #5
mondogenerator is online now mondogenerator  England
diyAudio Member
 
mondogenerator's Avatar
 
Join Date: Sep 2009
Location: GB
Tbh I always connect screen at one end only.

Saves having ground loops to worry about, when connecting to other hifi components, that may have a route to return that 'loop'
__________________
Testing everyone's patience since 1978.

Last edited by mondogenerator; 9th September 2019 at 07:52 AM.
  Reply With Quote
Old 9th September 2019, 02:17 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
For coax cable the shield is the ground so the question disappears, although it can reappear in the form 'can I get away with a small ground loop'.
  Reply With Quote
Old 9th September 2019, 03:31 PM   #7
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
GoatGuy's Avatar
 
Join Date: Dec 2012
Location: SF Bay Area
Alert / Apology… While it is rather amazing what “the mind will come up with at 3 AM”, this very early I woke up in a sweat: my opinion of “ground at both ends” is wrong. It creates a kind of low impedance ground loop which can and will pick up everything from low RF all the way up to UHF.

Now, the “Faraday Shield” effect keeps the oscillating electric fields on the outside of the shield for the most part. So the “ground loop problem” isn't in reality a very large one.

But the best advice is just to attach shield/braid at either end, and let it do its work. There might be a subtle advantage in grounding nearest to the wiper of the front-end attenuator pot. But if so, it is subtle.

Again, my apologies for misrepresenting the ground-connection advice. Tho' he didn't claim it, this discussion and my incorrect advice was corrected by none other than DF96 a few years back. See? Even at 3 AM, I remember!!!

GoatGuy ✓
__________________
At a certain point, burnished unicorn horn only goes so far. When the BUH magically fails, its best to rely on maths. JustSaying™
  Reply With Quote
Old 9th September 2019, 06:42 PM   #8
6A3sUMMER is online now 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
There is a measurement of coax cables, it is called Reverse Transfer Impedance.

The simplified explanation:
If I remember correctly, the Reverse Transfer Impedance measurement determines how much signal will be induced into the center conductor, when an external field impinges on the coax.
That field is non contacting, and is away from the body of the coax.

"Grounds are Commonly Misunderstood" - Me
  Reply With Quote
Old 9th September 2019, 07:29 PM   #9
mondogenerator is online now mondogenerator  England
diyAudio Member
 
mondogenerator's Avatar
 
Join Date: Sep 2009
Location: GB
Quote:
Originally Posted by GoatGuy View Post
Alert / Apology… While it is rather amazing what “the mind will come up with at 3 AM”, this very early I woke up in a sweat: my opinion of “ground at both ends” is wrong. It creates a kind of low impedance ground loop

[b]
That's what I was trying to say, without sounding smug (when I'm often very wrong)



In my amateur experience of 2 amps, it almost always creates a ground loop, if we are building stereo amplifiers.

This could just be because I'm stupid though.

If the L and R ground are isolated throughout then it's moot
__________________
Testing everyone's patience since 1978.

Last edited by mondogenerator; 9th September 2019 at 07:35 PM.
  Reply With Quote
Old 9th September 2019, 07:51 PM   #10
Windcrest77 is offline Windcrest77  United States
diyAudio Member
 
Join Date: Feb 2019
Location: Lockport, IL USA
I always like jacks at the back and volume at the front of the chassis so interconnects can be hidden. Seeing coax and power cords and spealers entering anywhere other than the rear is weird looking to me. So a small diameter coax is necessary. Breaking whatever ground is coming in on the interconnect, I believe is good, who knows where that cable is coming from or if it has any ac potential different than my amps ground.. So I run the jack ground to the star and the volume ground to the same star which is established as close to the grid as I can get it. It seems to break any hum loops caused by the source device. Basically I stop the interconnects ground at the front door and hand it all over to grounds I can control. Does this sound like a good approach for a wide variety of unknown sources?
  Reply With Quote

Reply


Quickie consensus on input grid shieldHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Power Amp input shield ground rperry Solid State 3 12th December 2018 03:43 PM
Screen grid input instead of control grid tim845 Tubes / Valves 10 28th June 2018 01:06 PM
"PEAK INPUT SIGNAL GRID-TO-GRID" question. howe0168 Tubes / Valves 9 26th January 2014 03:31 PM
grid stopper and grid choke on pentode input YouAgain Tubes / Valves 3 12th February 2012 09:32 PM
Connection of input shield? EE Solid State 2 17th November 2007 09:35 AM


New To Site? Need Help?

All times are GMT. The time now is 08:34 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki