SY said:
Ok. Then if you're working with lethal voltages, use a reverse pair of high current diodes instead.
Though unless the switch, control knob, connector, whatever is also electrically tied to the chassis, the fuse won't blow even if your supply ground is connected directly to the chassis.
se
That's something I often do, but several people whom I respect have tsk-tsked me for that. Diodes usually fail short (a good thing in that situation), but I was told that this is not always the case.
It probably isn't always the case for resistors either, so I better just not build anything. You'll find me in my bedroom with a blanket pulled over my head.
It probably isn't always the case for resistors either, so I better just not build anything. You'll find me in my bedroom with a blanket pulled over my head.
SY said:It probably isn't always the case for resistors either, so I better just not build anything. You'll find me in my bedroom with a blanket pulled over my head.
Don't have to go to that extreme. Just don't build anything with !@#$% AC power supplies in them. They're the root of the evil. Without them you don't have to worry about all the noise and grounding issues that they cause and you can just go straight to the chassis.
And short of that, there's always transformers.
se
Um.... the purpose of the fuse inthe AC line is to blow whether there is an AC OR DC short. So if something in the DC side of things goes bad and draws more curent than it should , the fuse will go. One can also easily add more fuse to the DC side of things.
Neither extreme is necessary to go to...
Gabe
Gabevee said:
You missed my point, which had nothing to do with fuses. But then I probably didn't make it as clear as I could have.
Tying the AC safety ground and the audio reference grounds (which includes the power supply ground) to the equipment chassis can result in ground loops and chassis leakage currents which are the number one cause of hum problems in interconnected pieces of equipment.
To try and reduce these effects, various methods are used to try and isolate the audio reference grounds from the chassis, using resistors, resistor/capacitor combinations, diodes, etc.
My point was that if you use a straight DC supply (i.e. batteries), then that issue is moot. You can just tie your audio reference grounds straight to the chassis without having to worry about the chassis leakage currents caused by the safety ground connection of an AC supply.
se
Actually.. I apologize. I was supporting your comment or trying to. I should have also included the previous post where SY said that if one uses a cap to ground the signal common, the cap would cause a problem.
Basically, a cap from signal ground to chassis would be fine no matter what. The reason one might use it is to avoid the ground loop problems while keeping the AC and DC worlds separate.
Someone else said that to have a separate common, AKA a floating ground, was a bad design. But it is not if one wants to keep DC ground separate from signal ground... common... whatever you want to call it.
It is a practice found in many RF applications, and is NOT considered a bad design.
One application where one might want to keep signal common separate from chassis and from DC common is in some audio applications where multiple power supplies and types are used, such as having one section need a split supply versus a single ended supply, and one needing one voltage level and one using a different voltage level. Especially is this imperative when it comes to recording studio amps, where a "phantom" power supply is also used.
Hybrid applications, apps which use tubes and transistors, both ICs and descrete components as well as PNP and NP stages, as well as bipolar and MOSFET applications. Combine these separate coponents and one might find themselves needing different power supply voltages, both positive and negative, where it would not be prudent to connect all the DC commons together.
These are not bad designs, but designs of necessity in many cases. All of which need a common point for signal, but not necessarily for DC. So a cap from the commons to chassis might be needed.
Gabe
Gabevee said:
Heheh. No problem. Seems we both just zipped right past each other.
Well SY's point was that with the capacitor there, if the DC hot managed to contact the chassis, there would be no return path for the fault current and if the voltage were high enough, could present a shock hazard (for example if you contacted the chassis and say the ground barrel on one of the RCAs).
So it wouldn't necessarily be fine in all instances.
The reasoning behind using a capacitor is not so much to keep AC and DC separate, but rather to provide a high impedance at power line frequencies to reduce noise caused by interchassis leakage currents, while providing a much lower impedance at RF so that the chassis can better function as an electrostatic shield.
I mean, other than shielding, there's no particular reason to tie your audio reference ground to it in the first place.
Not sure what you're meaning here. You mean like in a split supply situatoin where you don't tie the signal reference ground to the transformer's center tap?
Mmmm. Not sure I quite see the problem there.
se
If I had a cap from the common to the chassis, and for some reason high voltage (now assuming a tube amp) B+ got to the chassis, one would merely find B+ on the chassis. Without a return path, how can it be harmful?
If the signal jack was also isolated from the chassis, and one happened to put their hand across the jack and the chassis, then yes, I can see that.
But, now, if such a thing could happen, THAT is what I would call a bad design not thought out well enough to avoid such a problem.
However, ideally, if one has a metal chassis, it needs to be isolated from the circuit and grounded to earth for safety.
Please do not think I am arguing against anyone here. All of the input is expertly given. I am just questioning so I can be on the same page as everyone else. As many say "your mileage may vary". Well our experiences vary also.
I have worked on many older circuits which do tie the chassis to neutral. Just about ALL of the "All American Five" tube radios do. many old TVs do. This is why there is a big label warning people not to open the sets and only allow authorized servicemen to do so.
All transformer tube equipment which used the old two conductor AC were not a problem since the primary generally never touched the secondary or the chassis, as was mentioned before.
We don't worry too much about it now, because just about all electronic equipment is made in plastic.
Steve, I will reply to the rest of your comments later.
Gabe
If the signal jack was also isolated from the chassis, and one happened to put their hand across the jack and the chassis, then yes, I can see that.
But, now, if such a thing could happen, THAT is what I would call a bad design not thought out well enough to avoid such a problem.
However, ideally, if one has a metal chassis, it needs to be isolated from the circuit and grounded to earth for safety.
Please do not think I am arguing against anyone here. All of the input is expertly given. I am just questioning so I can be on the same page as everyone else. As many say "your mileage may vary". Well our experiences vary also.
I have worked on many older circuits which do tie the chassis to neutral. Just about ALL of the "All American Five" tube radios do. many old TVs do. This is why there is a big label warning people not to open the sets and only allow authorized servicemen to do so.
All transformer tube equipment which used the old two conductor AC were not a problem since the primary generally never touched the secondary or the chassis, as was mentioned before.
We don't worry too much about it now, because just about all electronic equipment is made in plastic.
Steve, I will reply to the rest of your comments later.
Gabe
Gabevee said:
Ok gentlemen I am now ready to begin wiring up my Roberts mono block amps, converting them to mic pres with balanced inputs and outputs. This is is what i observe:
It appears that the present unbalanced input phone jack's ground IS connected to chassis , as is the unblanced output jack ground. There appear to be many other wires going to chassis ground as well (the original factory wiring) So given these facts should i just connect both the new input transformer "ground" or "common" along with the transformer frame to the same ground or should I completely electrically isolate the new transformer from the existing chassis and run a new ground wire over to the newly installed safety ground ? (wouldn't this introduce a ground loop?). The same question is raised concerning the output phono and the installation of my new output transformer. Please advise ASAP.
Gabe,
Can you advise in regard to my questions in my previous email too concerning my intended transformer connections, at this moment I am totally confused where to connect both the "common" and the case ground on both the input and the output trannies, do i simply hook them up to the existing unbalanced phone jack connections or do i only hook up the signal side to existing phone jack signal lug and wire-in some more elaborate grounding system. These questions must seem very absis to you all but i have had so many conflicting posts in the past that I'm totally confused when i look at the actual circuits.
Can you advise in regard to my questions in my previous email too concerning my intended transformer connections, at this moment I am totally confused where to connect both the "common" and the case ground on both the input and the output trannies, do i simply hook them up to the existing unbalanced phone jack connections or do i only hook up the signal side to existing phone jack signal lug and wire-in some more elaborate grounding system. These questions must seem very absis to you all but i have had so many conflicting posts in the past that I'm totally confused when i look at the actual circuits.
Sorry you missed the first part of my post, I/O's will be balanced after the mod., so it complicates things. If i mount both input and output transformers so they are electrically isolated from the chassis and take a dedicated ground over to the safety ground from both the primary side of the input transformer and the secondary side of the output transformer will that suffice. I will surely have to connect the secondary of the input transformer and the primary of the output transformer directly to the existing unbalanced phone jack conections for both signal and common (chassis) won't I? Do you see whrere I am getting confused about how to go about doing this.
OK.. here is what you will do:
Disconnect the inputs. Mount isolated balanced jacks. XLR are the best ones I know of (expensive). Stereo, or tip/ring/ground 1/4 inch phone jacks can be balanced and cheaper, though. Take the primary center tap and connect it to common/chassis/ground. Connect the positive lead to the positive of the jack (tip), the negative to the negative of the jack (ring), and the common wire to the shield to ground/common/chassis. For XLR it is 2, 3, and 1, respectively.
Here is a cool page that explains it better, with pictures! ;-)
http://www.dplay.com/dv/balance/balance.html
He shows two conductor balanced with shield, but there are three conductor with shield for isolated signal grounds.
The secondary of the transformer, if single ended, or unbalanced, is simple. Common to chassis and hot lead to input. Just do the same on the opposite end. Common to chassis, hot to output.
Just remember, if you want to isolate even the ground of the input, the shield must connect to the chassis.
Connecting the commons of all inputs and outputs together will keep ground loops at bay. While there are some benefits (what, I don't know) to keeping signal ground isolated, yet it is what leads to the cause of ground loops. Most equipment does not use it.
Hope this helps.
Gabe
Disconnect the inputs. Mount isolated balanced jacks. XLR are the best ones I know of (expensive). Stereo, or tip/ring/ground 1/4 inch phone jacks can be balanced and cheaper, though. Take the primary center tap and connect it to common/chassis/ground. Connect the positive lead to the positive of the jack (tip), the negative to the negative of the jack (ring), and the common wire to the shield to ground/common/chassis. For XLR it is 2, 3, and 1, respectively.
Here is a cool page that explains it better, with pictures! ;-)
http://www.dplay.com/dv/balance/balance.html
He shows two conductor balanced with shield, but there are three conductor with shield for isolated signal grounds.
The secondary of the transformer, if single ended, or unbalanced, is simple. Common to chassis and hot lead to input. Just do the same on the opposite end. Common to chassis, hot to output.
Just remember, if you want to isolate even the ground of the input, the shield must connect to the chassis.
Connecting the commons of all inputs and outputs together will keep ground loops at bay. While there are some benefits (what, I don't know) to keeping signal ground isolated, yet it is what leads to the cause of ground loops. Most equipment does not use it.
Hope this helps.
Gabe
Gabevee said:
Thanks Gabe, that makes it much more clear, I'm very familiar with XLR & TRS connections since I operate a studio, I'm just not familar with Tube amps.
One final question, on the amps as they currently exist prior to any mods, the inputs and outputs are unbalanced phone jacks. Until I actualy plug-in a 1/4 plug the signal (tip) actually goes to chassis. The action of inserting the plug actually breaks that connection, is this normal? I had asumed that the signal side (tip) would always be isolated from the chassis. Of course if i totally bypass these old jacks (disconnecting any wiring) and then install only isolated TRS jacks or XLR connectors I guess the question becomes mute.
That 1/4 inch jack, by connecting the input to chassis with no plug, is a sort of... mute. ;-) With the input open it will pick up hum and noise that one might not want.
It is the same as setting the volume control all the way down. It literally puts the input to ground/chassis/common also.
You might want to consider putting in a mute switch, a DPST (double pole single throw) switch on the single ended side in order to keep a sudden high signal condition input form causing a loud "pop". I am sure you are familiar with that. ;-)
Gabe
It is the same as setting the volume control all the way down. It literally puts the input to ground/chassis/common also.
You might want to consider putting in a mute switch, a DPST (double pole single throw) switch on the single ended side in order to keep a sudden high signal condition input form causing a loud "pop". I am sure you are familiar with that. ;-)
Gabe
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