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Seperate Power Supply Chassis and Grounding
Seperate Power Supply Chassis and Grounding
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Old 19th December 2013, 02:33 AM   #1
lordglum is offline lordglum  United States
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Default Seperate Power Supply Chassis and Grounding

Hello,
So I just finished rebuilding an amp that I built about 5 years ago. Its a Time bandit 2A3 more or less based on the 'simple' design. I wanted to use it as a headphone amp but the AC hum was pretty bad.

So I decided to move the power supply, rectifier tube, chokes into a separate chassis. I run 2 x 2.5 filament supply, 6.3V supply, 2x 330V and ground through the umbilical cord.

After some troubleshooting I finally got it working again tonight (right channel 4 pin tub socket pins were not contacting the 2a3).

My only issue is after adjusting the hum pots I still have hum in my grados.

The method I used to ground everything was follows:

I used bare copper wire in the power supply chassis and ground my IEC ground and every other ground to this buss bar. I then took a lead from the buss bar and put that into the "amp" section. From there I grounded everything to another bar in the amp section and then tied this to the "amp" chassis near the rca jacks.

My question is there a flaw in the logic I'm not seeing? Anybody have past experience on how to knock down a slight AC hum? On my Hawthornes its darn near silent, but in my ears not yet perfect. I'm gonna play with the grounding a bit this weekend I was looking for some ideas to try.

Maybe shielded cables for the filament supplies?
Maybe I have some induced noise on the ground from running the AC with it from chassis to chassis?

Thanks for the help.
Jeremy

I have pics but I need to snag them off my phone. The schematic is attached.

PS sorry for the wall of text.
Attached Images
File Type: jpg 2a3_3.jpg (146.5 KB, 794 views)
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Old 19th December 2013, 03:41 AM   #2
Michael Bean is offline Michael Bean  United States
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Instead of using a ground "buss bar", use a star ground to establish a single "0 volt" common connecting point. A buss bar, no matter how large, will still develop voltage drops along its length when current flows.

Mike
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Old 19th December 2013, 05:06 AM   #3
Richard Ellis is offline Richard Ellis  Argentina
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My 12B4 under construction will be using the aluminum chassis plate at 5mm for a ground plane. To wit, the incoming green ground will be centrally located & grounds for signal will be on it.

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Old 19th December 2013, 06:11 AM   #4
Conrad Hoffman is offline Conrad Hoffman  United States
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Do you have anything with a transformer within about a foot of the cartridge? It may be unusually good at picking up magnetic fields, so shielding and fooling with grounds won't help. My preamp has a nice toroid, but if it's in the rack under the turntable, I get hum. Distance is the easy cure.
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Old 19th December 2013, 06:38 AM   #5
rhone is offline rhone  Australia
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If I understand correctly there is flaw. If you regard your two chassis as being one amp then you only want the circuit to be connected to chassis ground at ONE point. So the circuit ground doesn't get connected to the chassis in the amp section. Pass the circuit ground through the umbilical to the buss in the PS section, where you have your chassis ground.
You MUST also have a separate "chassis ground/earth" wire in your umbilical that connects ROBUSTLY to each chassis (to create one chassis) and then the PS chassis is connected to the IEC earth. Now both chassis have a safety earth and the circuit has one ground point.
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Old 19th December 2013, 06:46 AM   #6
Vinylsavor is offline Vinylsavor  Germany
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Hi,

try to isolate the hum source check this:

VinylSavor: Trouble Shooting Hum

Looking at the schematic, I would see two possible sources:

not enough B+ smoothing
AC filaments might be an issue with head phones

Thomas
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Old 19th December 2013, 01:00 PM   #7
lordglum is offline lordglum  United States
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Thanks for the reply's A bit more than I expected.

I have read many articles in favor of a star grounds as those that are opposed. Right now the amp is not set up for star ground.

Thanks for the hum troubleshooting link started to look through it this morning.

I have a couple general questions...

Shorting the inputs to see if hum goes away...I assume short them to ground?

It sounds like I need to ground the PS chassis on the top plate as well correct?

The 2.5V filaments center tap are not grounded. I am not sure why not?

Thanks
Jeremy
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Old 19th December 2013, 05:14 PM   #8
mark02131 is offline mark02131  United States
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Quote:
Originally Posted by lordglum View Post
The 2.5V filaments center tap are not grounded. I am not sure why not?
I believe the center tap from the filaments of a DHT has signal in it. If you were not using hum pots you could use that connection to your cathode resister/cap. Whats the hum look like on a scope, 60hz?

I was looking at your power supply, Check to make sure the HT is completly smooth, also your hum may be coming from the AC on the heaters at ether or both stages. If you have a bench supply you could try some DC on the 6SL7.
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Old 19th December 2013, 05:51 PM   #9
Vinylsavor is offline Vinylsavor  Germany
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Hi!

Quote:
Originally Posted by lordglum View Post
Shorting the inputs to see if hum goes away...I assume short them to ground?
Yes

Quote:
Originally Posted by lordglum View Post
The 2.5V filaments center tap are not grounded. I am not sure why not?
Grounding them would short out the cathode resistor which sets the op point of the output tube. Don't ground them

Thomas
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Old 19th December 2013, 09:23 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by lordglum
The method I used to ground everything was follows:

I used bare copper wire in the power supply chassis and ground my IEC ground and every other ground to this buss bar. I then took a lead from the buss bar and put that into the "amp" section. From there I grounded everything to another bar in the amp section and then tied this to the "amp" chassis near the rca jacks.

My question is there a flaw in the logic I'm not seeing?
A bus ground can work fine if everything connects in the right order. You haven't told us what order you used so we can't comment on it. All you have said is that you have two busses (one in PSU, one in amp) and they are connected together.

You may have mixed up safety ground and signal ground. The IEC ground must go the chassis, not the bus. There must be a chassis ground connection between the two boxes in addition to the signal ground. Then somewhere you have one and only one connection between chassis and signal ground.
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