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Project: Separate Power and Preamp into two separate chassis - help please
Project: Separate Power and Preamp into two separate chassis - help please
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Old 14th June 2018, 11:30 AM   #1
Heifetz1000 is offline Heifetz1000
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Default Project: Separate Power and Preamp into two separate chassis - help please

Hello,
Your comments and advice would be most welcome.
I have a tube preamplifier with the power supply (rectifier tube) inside - the unit is noisy. It has a 50Hz hum and I have tried everything I know to solve the ground loop. A picture of the preamp is attached.
I have decided to separate the power side and the signal side into different chassis' in the hope that this will help.
I will not touch the circuitry or their connecting wires except replace rca wiring and redo the grounding. I'll also replace the 'umbilical' as it will need to be longer. This project is really a separation and rearrangement of the components. The PDF plan is attached. I have not indicated wiring that connects the components as I will keep them intact.
I'm not very knowledgeable but I feel confident that I can do this.
I have a couple of quesitons:
1. Should the PSU chassis be steel or aluminium?
2. Should internal grounding wires be the same wires as the signal wires?
3. I am looking at Jupiter 23awg
4. Which kind of wire should I purchase to use as the umbilcal wire?
Any comments or suggestions would be most helpful.
Gratefully, Bob
Attached Images
File Type: jpg Ravel AN Preamp.jpg (975.2 KB, 119 views)
Attached Files
File Type: pdf Ravel PSU and Preamp Component Layout.pdf (172.4 KB, 36 views)

Last edited by Heifetz1000; 14th June 2018 at 11:33 AM.
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Old 14th June 2018, 02:25 PM   #2
Miniwatt is offline Miniwatt  Netherlands
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Are you sure this will solve the problem? You call it a ground loop problem, removing the power supply probably won't solve this.
Is this a commercial unit? If so it probably didn't hum when it was new.
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Old 14th June 2018, 03:01 PM   #3
kodabmx is offline kodabmx  Canada
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Try rerouting that grey power cable. It's not twisted and it's running right across the preamp section which might be inducing the hum.
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Old 14th June 2018, 09:50 PM   #4
Heifetz1000 is offline Heifetz1000
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Quote:
Originally Posted by Miniwatt View Post
Are you sure this will solve the problem? You call it a ground loop problem, removing the power supply probably won't solve this.
Is this a commercial unit? If so it probably didn't hum when it was new.
Hi Miniwatt,
The unit is not a commercial unit. I bought it for very low dollars and it had the hum. I thought to myself "no problem, I'll be able to solve that". Well, its been a lot more difficult than I thought I.ve tried every combination possible. I've also tried a Ground Zero.
I think there may be induced hum between components. Also, I thought grounding would be easier with two separate chassis - am I correct?
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Old 14th June 2018, 09:53 PM   #5
Heifetz1000 is offline Heifetz1000
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Quote:
Originally Posted by kodabmx View Post
Try rerouting that grey power cable. It's not twisted and it's running right across the preamp section which might be inducing the hum.
I just tried twisting some wires and the grey wire has just moved to that position accidently - so the hum is not due to that.
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Old 14th June 2018, 09:56 PM   #6
Heifetz1000 is offline Heifetz1000
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Quote:
Originally Posted by kodabmx View Post
Try rerouting that grey power cable. It's not twisted and it's running right across the preamp section which might be inducing the hum.
Oh, I think you mean the grey cable at the bottom of the picture. Right!
Its connected to the switch on the front panel and continues around the front and other side to the terminal for the transformer. It is sheilded though.
Ok, I'll have a closer look. Thank you,
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Old 14th June 2018, 09:58 PM   #7
scottjoplin is offline scottjoplin  Wales
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Hi Bob. The layout seems very poor, input sockets very close to the transformer and the mains power routed all round the chassis and very close to the valve, I'd be tempted to remedy these first. Ground loops are not necessarily easier to deal with using separate chassis, you may even make them larger. It looks like the signal wires are twisted left and right channels together and ground wires run separately, this is not correct, the signal flow and return (gnd) should be twisted together
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Last edited by scottjoplin; 14th June 2018 at 10:05 PM.
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Old 14th June 2018, 09:59 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Heifetz1000 View Post
I just tried twisting some wires and the grey wire has just moved
to that position accidently - so the hum is not due to that.
It's also possible there are either defective capacitors that need to be replaced,
or design errors that must be corrected. Separating the circuits into different
enclosures might do little or nothing about the hum. Can you get a schematic
from the previous owner?
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Old 15th June 2018, 04:53 AM   #9
stocktrader200 is offline stocktrader200  Canada
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try disconnecting an RCA input and move it away from the power transformer.
if it is the transformer, the hum will be 60hz. if it is the rectifier / filter assembly the hum will be 120 Hz.
Easy to see on a scope
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