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Old 26th October 2005, 05:22 PM   #1
Sheldon is offline Sheldon  United States
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Default Choke induced Hum

I had a choke that I ordered by mistake, so I thought I'd play with it in a cap input supply. When I modeled it in PSUD, it showed a big reduction in B+ ripple with only a loss of about 10V. So I plugged it in per the following schematics. When I did the various measurements, the voltages all checked out as predicted, but my output hum went from less than 1mv to about 4mv. At first I thought it was the clip leads strung around, so I just removed the choke and extra cap and completed the circuit with the leads. Hum dropped back to <1mv, even moving the leads around. Could the choke be influencing the power transformer?

Edit: Thinking about it, I didn't ground the choke chassis. Could this be a problem?

Amp:
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Old 26th October 2005, 05:24 PM   #2
Sheldon is offline Sheldon  United States
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Default Cap only PS

Original supply:
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Old 26th October 2005, 05:25 PM   #3
Sheldon is offline Sheldon  United States
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Default With Choke

Choke and input cap:

Note that the only change other than cap and choke, is one less dropping resistor for the driver supply, to keep driver voltage the same. Taking this out did not affect the hum.
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Old 26th October 2005, 08:38 PM   #4
anatech is offline anatech  Canada
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Hi Sheldon,
Was the supply loaded? Below a critical current the choke effect stops. At such low levels of ripple the core should be grounded and you may have to play with the choke orientation to prevent coupling with the power transformer.

-Chris
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Old 27th October 2005, 02:28 AM   #5
Sheldon is offline Sheldon  United States
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Hi Cris,

Yes, the supply was loaded. I just power up the amp same as for the non-choke supply. No signal connection, but all systems go. I don't think orientation was the problem, as it was outside the amp and at least 10 cm away from the output transformers. Also, moving the choke around had no effect. Is it loading my ground in some way?

Sheldon
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Old 27th October 2005, 09:21 AM   #6
Colt45 is offline Colt45  Serbia
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The B+ tap is on the wrong side of the choke.
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Old 27th October 2005, 09:53 AM   #7
billr is offline billr  New Zealand
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hi

did you check the orientation of the choke?

these things have a magnetic field around them, and unsurprisingly, it can couple with the output transformer. Try having the magentic cores at 90 degrees to each other. this is normal practice when building amps.

Also, earth the case of the choke, or the laminations, this too keeps hum down.

Hope that this helps.

kind regards

bill
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Old 27th October 2005, 10:34 AM   #8
kmtang is offline kmtang  Canada
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According to your schematic, it is not a Choke Input LC filter. It is just a C-L-C PI filter. The 350V B+ should be tapped after the choke.

If you don't have hum problem with the original circuit, the hum probably caused by the magnetic flux generated from the choke and coupled to the OPT.


Johnny
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Old 27th October 2005, 11:02 AM   #9
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I noticed that the original power supply has a 100uF input capacitor. The second power supply has a 47uF input capacitor.
That would explain the higher hum level. I would move the B+ connection to the output side of the choke.

Also. 100uF is too much capacitance. Most tubes are rated for 40uF to 60uF maximum. 100uF will shorten the life of your rectifier tube.
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Old 27th October 2005, 12:54 PM   #10
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Sheldon.

What is the max rating for inductor current? Isn't it saturated while loaded?

Pawel
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