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PostPosted: 26 Nov 2010, 21:18 
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Joined: 27 Oct 2010, 20:27
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So this will be my first large scale amp project (built a bunch of headphone amps before this), it's the S5 Electronics k12g kit in a solid aluminum chassis. Here's my grounding scheme, please critique:

- PCB isolated from chassis
- RCA inputs and speaker outs isolated from chassis with isolation washers
- AC transformer and output transformers mounted on top of the chassis (this should mean the transformer casings are electrically connected to the chassis)
- safety ground from IEC power socket wired to one of the mounting bolts of the AC transfromer inside the chassis (this should ground the chassis and all three transformers)
- signal ground from the PCB wired to the same bolt as above, going from the PCB where the kit originally specified one of the output transformer casings should be grounded to

This should ensure all three transformers and the chassis are grounded AND avoid ground loops by having the safety and signal ground connected at only one point, correct?


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PostPosted: 27 Nov 2010, 00:52 
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DisasterArea wrote:
So this will be my first large scale amp project (built a bunch of headphone amps before this), it's the S5 Electronics k12g kit in a solid aluminum chassis. Here's my grounding scheme, please critique:

- PCB isolated from chassis
- RCA inputs and speaker outs isolated from chassis with isolation washers
- AC transformer and output transformers mounted on top of the chassis (this should mean the transformer casings are electrically connected to the chassis)
- safety ground from IEC power socket wired to one of the mounting bolts of the AC transfromer inside the chassis (this should ground the chassis and all three transformers)
- signal ground from the PCB wired to the same bolt as above, going from the PCB where the kit originally specified one of the output transformer casings should be grounded to

This should ensure all three transformers and the chassis are grounded AND avoid ground loops by having the safety and signal ground connected at only one point, correct?

I would take the ground from the IEC directly to a chassis on its own bolt an nothing else with it, close to the location of the receptacle. Just a little added insurance to prevent accidental loss of case/safety.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 28 Nov 2010, 11:22 
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OK, so should I still connect the IEC and signal ground near the same point, just closer to where the IEC plug is mounted?


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PostPosted: 29 Nov 2010, 12:55 
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Honestly, I wouldn't sweat it. You're using a metal chassis which becomes a single point ground reference. I would keep the PCB ground close to the PCB. I would not isolate the RCA inputs either. A "ground-loop" situation should not be a concern in this configuration. If you were using a non-metal chassis, I would be more concerned and would make every effort to keep all grounds to a central connection, such as a buss wire or buss plate. I didn't address these in my previous post simply because there was nothing wrong with your intended scheme, per say. My main concern was that a power ground really needs to have its own connection to a chassis for simple safety purposes. Keeping it close to the power inlet is just added insurance against inadvertent disconnection. Just make sure your grounds are all electrically strong. Use ring terminals on those wires, prefereably soldered on to the wire. Use tooth/star washers between the ring terminal and the chassis. If you stack your grounds (something I personally don't care for) be sure to use a tooth/star washer between each terminal/wire. Your transformer cases will inherit the chassis ground, but can be improved by using tooth/star washers on those mounting bolts as well. I would prefer not to mount ground wires to the bolts used for mounting other hardware, such as transformers. All grounding points need to be clean. A little sanding at those points is even better. The cleaner the suface, the better the electrical bond, the better your ground will be.

Now go build something! :up:
Remember to have fun! After all, that's what all this stuff is about, right? Each build should be a learning experience. Don't be afraid to try something new.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 29 Nov 2010, 13:15 
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Thanks for the input! Just wondering why you wouldn't isolate the RCA inputs, this seems to be the opposite of what other reading has suggested. That would connect signal ground and chassis ground at the RCA plugs, would that mean I wouldn't need or shouldn't have a connection elsewhere between the PCB and the chassis?


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PostPosted: 29 Nov 2010, 23:45 
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I had to go and dig up the schematic. Fortunately, someone was kind enough to post it. I also found where you had inquired to Matt (suncalc) regarding grounds. Personally, in my SS amp builds, I never had a problem with ground loops in a metal chassis. I tend to go by experience rather than theory once I've established a "what works" basis. Since Matt has already indicated you should keep chassis and signal grounds separate and to have 1 single connection for signal to chassis, then that is what you should follow. Transformer frames are not part of signal grounds. I respect Matt's knowledge. So, for the purpose of this thread, isolate your RCA jacks, make one connection from your PCB to chassis (keep it short). The chassis will take care of the transformer frames.

viewtopic.php?f=9&t=1546&start=70
DisasterArea wrote:
Similar question: I'm mounting my transformers on the top of an aluminum chassis, should I then only ground ONE of the transformers to the PCB and make that my single ground connection from chassis to pcb to avoid ground loops... this should ground all three transformers if they're all mounted on the aluminum chassis, correct?

suncalc wrote:
A good way to think of transformer end bells and frames is as a chassis to protect the circuit portions of the transformer. They shield the transformer in the exact same way that a metal chassis shields the internal parts of the amp. As such, transformer end bells and frames should be grounded to the chassis ground. The circuit grounds should all be tied to a signal ground. Then the chassis ground and the signal ground should be tied together at one and only one point. This will prevent chassis ground loops.

__
Sorry if I have confused you. I will attempt to clarify based on what Matt has already stated;
1.) Tie the IEC ground to a point close to the receptacle - this is both a safety ground and chassis ground. Terminate the wire using solder-on ring terminal.
2.) Mount the transformers directly to the chassis, making sure to provide good clean surfaces where the bell and chassis meet. Attach no wires to the mounting screws.
3.) Isolate from chassis all signal connections
4.) Attach from the PCB board, 1 (one) single signal ground wire directly to the chassis. Keep the wire short. Terminate the wire using solder-on ring terminal.
5.) Use tooth/star washers at ALL grounding points to the chassis (these will insure a strong electrical bond);
- tooth/star washer at ground from IEC, at ground terminal from PCB, and at least one per transformer. On the wires, the washer should go between the ring terminal and the chassis. On the transformers, either between the transformer frame and chassis or on the underside of the chassis and topside of the transformer mount tab.

OR

You could follow my lead and throw caution in to the wind and don't be so concerned about ground loops. I will tell you, that one of the worst sounding amps I've ever built, I payed too much attention to avoiding ground loops. It also was built in a plastic case. No more non-metal cases for me. The most recent build was a bridged LM380 amp. I threw the ground loop theory out the door. The ONLY item that was not chassis grounded was the speaker connection. In this particular design, the speaker connection could have no ground reference what so ever. The power supply, amp circuit were chassis grounded, the input signal was grounded at both ends and the potentiometer cases were grounded to the chassis and the chassis was then "earthed". That's the cleanest amp I've ever built! No hum what so ever. Just clean audio, maybe too clean for guitar.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 30 Nov 2010, 07:56 
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Joined: 27 Oct 2010, 20:27
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Thanks again for the help, that makes things clearer. Maybe it's overkill to think about it so much, but I've always been of the "measure thrice, cut once" school of thought :) I hadn't thought of using star washers, I'll be picking some up today.


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PostPosted: 30 Nov 2010, 13:00 
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Location: South East US - Tennessee
Have fun! Don't be afraid to "wonder" from the beaten path. You'll never learn if you don't try. You're building something that you will either end up enjoying for a long time, or something that will end up teaching you something, after all, experience is something you get AFTER you really need it. ;)

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 14 May 2017, 00:45 
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Joined: 14 May 2017, 00:29
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I'm a newbie and I have a question about wiring a shielded wire to the potentiometer and to the Grid of the driver tube and input. Where have I to ground the braided wire to? In order to avoid looping. Anyone can help?


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PostPosted: 15 May 2017, 19:37 
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Joined: 06 Apr 2009, 10:08
Posts: 1419
Location: US Pacific Northwest
Can you show what are the shield leads?

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