Hi List Mates;
For those who may not be aware regarding the
copper strap around the transformer
(as Alan N1AL says) it acts as a shorted turn to external "H" fields
make sure that the copper strap goes around the OUTSIDE of the windings
as well as the core and NOT thru the window of the core where the
windings go otherwise you will introduce a dead short circuit to the
magnetic flux in the core which will short out the primary & blow the
mains fuse(s).
Another approach is to make up a magnetic shield of about 4 or 6 sheets
of 0.020" (0.5mm) thick transformer core lamination steel; each sheet
cut to about 6" (152.4 mm) square - or- the same size as the side panel
of the linear am side panel tape them together and fit them inside the
amp adjacent to the core of the power transformer- in particurlar if the
gap between the two halves of the "C" cores is facing toward the side of
the linear amp.
The sheets of transformer core steel will shunt the stray (leaking)
magnetic field emanating from the two faces of the "C) cores. Will work
the same for the junctions of the "E's & I's of a conventional E&I built
transformer (although there is more stray magnetic field from
transformers of E&I construction.
Lowest stray magnetic leakage is from transformers using toroidal core
construction.
Hope this information is of use.
Eric VK3AX. K34250.
On 2/4/2012 7:51 AM, Alan Bloom wrote:
> One trick I have used in the past is to put a copper strap around the
> transformer. It acts as a shorted turn for the radiated fields,
> reducing their amplitude. You could try that both on the power
> transformer and on the audio transformer in the K3. Of course, the
> strap needs to be soldered so it makes a continuous loop.
>
> Alan N1AL
>
>
> On Fri, 2012-02-03 at 12:21 -0800, k8...@mho.com wrote:
>> This is a "common" problem when a amplifer is placed next to the left
>> side of the K3 (especially if the transformer in the amp is on the
>> rigt side - like the Alpha amps).
>>
>> The problem is caused because the K3 line in has a isolation
>> transformer conveniently located on the left side. Place a power
>> transformer next to it, and all hell breaks loose (something to do
>> with the magnetic lines of force interacting).
>>
>> It took me a few hours of contemplating and experimenting to figure
>> this out. If someone has come up with a easy "fix", please let me
>> know. I tried buying a steel plate (alum will definitely not work) at
>> Home Depot and placed that between the K3 and amp; it did nothing to
>> fix the problem (or even help). One sure fire fix is to turn off the
>> amp!
>>
>> Dick K8ZTT
>>
>>
>>
>>
>> --- k6...@me.com wrote:
>>
>> From: Lew Phelps K6LMP<k6...@me.com>
>> To: "Elecraft@mailman.qth.net List"<elecraft@mailman.qth.net>
>> Subject: Re: [Elecraft] K3 Hum on Audio...
>> Date: Thu, 02 Feb 2012 10:24:07 -0800
>>
>> Thank you, Tony!
>>
>> I had developed a maddening audio hum suddenly, and couldn't find the cause.
>> The curious thing was that it was WORSE when no mike was plugged into the
>> jack on the back panel.
>>
>> Based on your email, I checked the menu settings. Sure enough, MIC + LINE:
>> ON. Switched to OFF and the hum totally disappeared. You don't need to
>> adjust the LIN volume; just cut it off entirely.
>>
>> Lew K6LMP
>>
>>
>> On Jan 14, 2012, at 9:02 AM, N2TK, Tony wrote:
>>
>>> Make sure the LIN IN is not turned up. Early on I had a problem with hum in
>>> my audio, especially if an amp was close to the K3. For some reason my LIN
>>> IN was at max. Turned it down and hum went away.
>>> 73,
>>> N2TK, Tony
>>>
>>> -----Original Message-----
>>> From: elecraft-boun...@mailman.qth.net
>>> [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Phil Hystad
>>> Sent: Friday, January 13, 2012 12:54 PM
>>> To: j...@audiosystemsgroup.com
>>> Cc: elecraft@mailman.qth.net
>>> Subject: Re: [Elecraft] K3 Hum on Audio...
>>>
>>> Jim,
>>>
>>> Well, I would call it mostly a buzz. Also, the only near magnetic field of
>>> the KPA500 power transformer is off, I mean powered off and unplugged, so
>>> that does not seem to be the problem.
>>>
>>> I just ran another test with everything disconnected, including ground and
>>> coax. With the K3 in TEST mode, key-down on the mic still produces the hum.
>>> I double checked the grounds. I did this because if I were to touch
>>> anything metal on the k3, the metal part of the PL259 or the ground strap,
>>> the hum is damped quite a bit to almost insignificant. It seems that if my
>>> rig were already well grounded this would not happen so I am curious if this
>>> is something I would normally expect.
>>>
>>> I will experiment with the TXEQ but this is a real puzzle I would like to
>>> solve, not merely erase it.
>>>
>>> 73, phil
>>>
>>>
>>> On Jan 13, 2012, at 9:37 AM, Jim Brown wrote:
>>>
>>>> On 1/13/2012 9:20 AM, Phil Hystad wrote:
>>>>> Unfortunately, it is in both mics but not as strong in the Yamaha CM500
>>> mic as in the MH2 but it is present.
>>>> Clarification question. Is it HUM (pure 60 Hz), or BUZZ (mostly
>>>> harmonics of 60 Hz)? If it's HUM, I would suspect magnetic field
>>>> coupling into the audio, either from a big power transformer (like the
>>>> one in a power amp or a big linear power supply) or from a AC power
>>>> wiring fault called a double-bonded neutral.
>>>>
>>>> The K3 has unshielded audio transformers at all the audio inputs and
>>>> outputs, and an unshielded transformer is a sitting duck for magnetic
>>>> fields.
>>>>
>>>> The good news is that the K3 has excellent audio equalization (TXEQ)
>>>> that allows us to remove that 60 Hz hum by filtering. The lower audio
>>>> frequencies in the human voice make NO useful contribution to speech
>>>> intelligibility, but they do waste transmit power. So it is ALWAYS a
>>>> good thing to set the TXEQ for maximum cut of the lowest two bands,
>>>> and at least some cut of the third band. This is true with virtually
>>>> ALL mics and ALL voices. AND it will reduce that hum enough that you
>>>> may no longer hear it.
>>>>
>>>> There are several solutions to magnetic field coupling. 1) Rotate the
>>>> noise source or the victim circuit to put the fields at right angles
>>>> to the victim. 2) Move the noise source further from the victim. 3)
>>>> If the hum field is produced by that AC power wiring error, fix the
>>>> error to eliminate the field.
>>>>
>>>> 73, Jim K9YC
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