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Author Topic: Extended test leads on Kelvin Bridge  (Read 2001 times)

John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #30 on: April 29, 2018, 09:50:15 am »

IIRC this was also used to improve damping factor on long cable runs. Pretty simple concept since the sense lines just become part of the negative feedback loop in the power amplifier. In theory you can make a long run of 18 gauge behave like really heavy wire, except for voltage drop and reduced headroom (of course).
To continue down this music veer, when designing power amplifiers being able to publish a crazy high damping factor is consider a meritorious specification, while in the real world an amplifier with DF of thousands will not realize much benefit with even a short length of speaker wire attached. ::) When chasing high amplifier DF numbers even the short wires inside the amplifier going to the output terminals matter. 

Last century while product manager for power amps at Peavey, I was given a power amp with near infinite damping factor due to a clever feedback arrangement (Jack Sondermeyer RIP got a patent for it). In fact he could tweak some terms and create a power amp with negative output impedance so it could compensate for the external speaker wires too. We even momentarily considered adding a control to the power amps allowing you to adjust compensation for wire length (resistance).

But nah... we quickly realized this would just create more confusion with Peavey customers than provide any practical benefit, but it was nice to publish DF with extra zeros for the customers who believed it was important.  8)

FWIW amplifiers using sense lines was useful but impractical for live sound reinforcement where systems were set up and torn down repeatedly. An open sense line would lead to bad behavior from the amps. In design for mass markets we have to ASSume customers will do the worst, not the best.

JR
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #31 on: April 29, 2018, 12:41:06 pm »

Back to Kelvin Bridge theory. Here's a really good explanation of how it works: https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/ and as you can see, you can insert a lot of resistance in the test leads without adding to the resistance being measured. Because there's two different loops, one low-z path for current and one high-z path for voltage, the lead lengths are automatically cancelled out of the measurement. Here's a picture of the basic setup, the most import part being the Kelvin Clamps which have each half of the clamp isolated from each other.
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Mike Sokol
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #32 on: April 29, 2018, 02:07:01 pm »

FYI: Here's exactly what I used to use back in the 80's, and which I still have on my shelf of vintage (old) test gear. https://www.ebay.com/itm/ESI-Portametric-PVB-300-Potentitometric-Voltmeter-Bridge/272452748026?hash=item3f6f7302fa:g:GewAAOSwHMJYLVuD
It should still work if I make a replacement holder to substitute a battery stack for the original mercury battery, which of course is no longer available.

I was originally going to take this on my gig, and it would have worked. But it is a bit cumbersome to null-out and explain to other engineers, while the Greenlee CLM-1000 meter is really quick and easy to use.
 
« Last Edit: April 30, 2018, 10:21:59 am by Mike Sokol »
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