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To Screen or not to screen Vbe multiplier leads in HK1400
To Screen or not to screen Vbe multiplier leads in HK1400
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Old 13th August 2014, 06:10 PM   #1
db0360 is offline db0360
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Join Date: Aug 2010
Default To Screen or not to screen Vbe multiplier leads in HK1400

Hi, I'm hoping for a fast response please.

I'm upgrading/modifying my Harman Kardon HK1400 Line amp and need a little advice.

The split bobbin mains transformer was very easy to split into two matched winding as the centre tap was brought out to tags on the transformer itself. With this in mind, I've provided split supplies and bringing the 0V (ground) connection to the power star point on the main amp board as four wires from 4 individual 25A bridges. (2 left + 2Right channel). This is to provide a far improved power supply to the power amp board.

Whilst evaluating the amp for it's sound quality which is very fast, detailed but a touch grainy in the very highest treble and upper mid-range, I have determined that because the VBE multipliers (a transistor and 3 resistors on a small PCB) are wired to the main driver stage via approximately 4 inches of el cheapo FIG8 wire and are susceptible to induced signals from elsewere. I figure that replacing this fig8 wire with some screened 2 core signal cable and taking the screen to the signal star ground point, I can protect these lines from induced eddy currents and RF.

Can anyone offer a measured opinion on this course of action please with any thoughts on whether this might affect electrical stability (not thermal as I can't see that being affected)

Many thanks

DaveB
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Old 13th August 2014, 08:34 PM   #2
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
Location: Coffs Harbour, NSW
The Vbe transistor will already be located in the noisy environment of the power amplifier heatsink. You could speculate on what could escape the large cap that usually shunts AC across the Vbe multiplier but unless there is evidence that any significant problem actually exists, there's little point making guesses what it could be and what you think you should do about it. Eddy currents BTW, are induced in the transformer iron. Copper braid won't stop them.

I think an oscilloscope and sine wave input signal is the way to observe and measure but first you need a control measurement to establish what (if any) ambient noise is there when the amplifier is not emitting EMR. (i.e. no signal).
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Old 13th August 2014, 09:17 PM   #3
db0360 is offline db0360
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Join Date: Aug 2010
Thanks Ian, I take your point re: eddy currents..........wrong phrasiology indeed.
I guess, what I'm trying to establish is whether, in the absence of measurement, shielding these flying leads could in any way be detrimental. The bypass capacitor only seeks to shunt AC differential mode signal across the Vbe multiplier circuit. It can not, in any way serve to suppress any induced pickup from stray fields.
By virtue of the Bias circuit and it's bypass cap, the bases of the driver transistors form an imaginary centre point in the signal path and can serve to drive the output stage with any stray signal pickup.....at least, that's my interpretation.
Shielding the umbilical leads would serve to ground any stray fields and prevent pickup..........in my mind that is!?
In this amp, a great deal of attention has been paid to grounding techniques but it seems to have been at the expense of a more logical and technically correct layout. It's a well regarded amp but has a slightly fizzy, sibilant top end which seems to be tamed somewhat when Wifi sources and cordless phones are powered down. Of course, I can't ask my neighbours to shut down their kit to give my amp a good life so I'm trying to minimise/remove this problem by engineering it out.

Dave
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