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Old 26th July 2012, 03:00 PM   #1911
disco is offline disco  Netherlands
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- Was an power amp connected?
- Did you take measurements at the output socket of the pre or at the input socket of the power amp?
- What happens when you feed the pre-amplifier 2Vpp?
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Old 26th July 2012, 03:08 PM   #1912
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I've been looking at 2" aluminium tube to put around the 26s, 1.5mm wall thickness. The 26 is 45mm diameter, both ST and globe, so may help. Haven't bought any yet.

Wondering if the hum comes from the tubes or the 156C chokes

Andy
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Old 26th July 2012, 03:16 PM   #1913
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Many years ago I was building a 56 pre and had problems with hum.
An aluminum beer can was cut down,
placed around the 56 and grounded with a clip lead.
No more hum. So that works.

Last edited by slowmotion; 26th July 2012 at 03:36 PM.
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Old 27th July 2012, 03:09 AM   #1914
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Quote:
Originally Posted by disco View Post
- Was an power amp connected?
- Did you take measurements at the output socket of the pre or at the input socket of the power amp?
- What happens when you feed the pre-amplifier 2Vpp?
No power amp was connected.
Yes, I took the measurement from the output socket with the signal generator injects 10v @1k to the preamp.
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Old 27th July 2012, 03:12 AM   #1915
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Quote:
Originally Posted by andyjevans View Post
I've been looking at 2" aluminium tube to put around the 26s, 1.5mm wall thickness. The 26 is 45mm diameter, both ST and globe, so may help. Haven't bought any yet.

Wondering if the hum comes from the tubes or the 156C chokes

Andy
I cut them out of the legs of the coffee table which people trashed at the yard.
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Old 27th July 2012, 03:30 AM   #1916
iko is offline iko  Canada
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Quote:
Originally Posted by andyjevans View Post
I've been looking at 2" aluminium tube to put around the 26s, 1.5mm wall thickness. The 26 is 45mm diameter, both ST and globe, so may help. Haven't bought any yet.

Wondering if the hum comes from the tubes or the 156C chokes

Andy
When I built my 26 preamp first time I've documented (earlier in the thread) that the 156C chokes pick up hum like crazy. No problem whatsoever with gyrator/ccs. In your case I don't think it is the 26s that pick up the hum, but the chokes.
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Old 27th July 2012, 06:14 AM   #1917
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If it's the chokes picking up hum, then, how do you screen them? I've never tried, any more than putting them inside the chassis.

What extra measures can I take to screen chokes? Steel, alu, cages? Never tried mu-metal.

andy
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Old 27th July 2012, 06:49 AM   #1918
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Hi Andy,

- orientation: if you are lucky, rotating the choke will help, since the electromagnetic fields are oriented.

other methods to try:

- all power transformers located on a remote chassis.
- wiring between power transformer secondary and rectifier - and from rectifier to caps must be kept to an absolute minimum length. This is especially true with filament supplies, where the peak currents in the rectifier are 3A or more. The bandwidth of the rectifier pulses is very high (may be hundreds of MHz) and long wiring "transmits" these pulses to sensitive nodes of your audio circuit.

If you have a remote power chassis, and wire the rectifiers short, and locate it 1.5m away or further, and there are no large filter capacitors on your signal chassis, AND all filaments/heaters are well filtered or regulated dc, the hum-generating sources should be very small.

Screening with thin Alu or steel sheet will work if electrostatic fields are the problem (presence of voltages nearby that swing rapidly from one value to another (eg anode signals). This is much less effective against magnetic fields (proximity of rapidly changing current, eg power supply capacitor/rectifier node). In case of magnetic field, distance (from the generating source) is your only practical friend.
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Old 27th July 2012, 06:52 AM   #1919
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Hi!

Quote:
Originally Posted by albertli View Post
It shows the higher the frequencies, the more it drops. Is there anything I could do to improve this ??
It's probably the winding capacitance of those chokes which impacts the high frequencies.

Best regards

Thomas
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Old 27th July 2012, 06:54 AM   #1920
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Hi!

Quote:
Originally Posted by andyjevans View Post
What extra measures can I take to screen chokes? Steel, alu, cages? Never tried mu-metal.
Mu metal is very effective in screening magnetic fields. Rather expensive though

Thomas
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