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Old 12th May 2010, 05:30 AM   #1
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Default Driving Long Cables, how much current?

Hi,

I am intending to build an Aikido type line stage, i need to drive 10meters of screaned cable to remotely located poweramplifers, with an input impedance of 10K ohm. I am guessing the cable capacitance is approx 200pf per meter.

I am intending to use a 6DJ8 as output tube with 10 mA of standing current. This will give me an output impedance of approx 200ohms.

Will the output impedance be low enough, and standing current adequate to drive a 10K/2000pf Load?

Would a White cathode follower be more suitable for this load?

Thanks.
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Old 12th May 2010, 05:42 AM   #2
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You tell me: Fo = 1/(2*pi*R*C).

As for current, you did not specify required drive voltage, so it is impossible to say. I can tell you this: at 20kHz, the cable capacitance will have a reactance of 7.9kohms.

Tim
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Old 12th May 2010, 05:54 AM   #3
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Hi,

From the power amp spec, input voltage requirement is quoted as:

0dBm(775mV) (+3dBm peak) over 10K for 100W Output.

Thanks
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Old 12th May 2010, 06:00 AM   #4
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775mV RMS + 3dB peak = 1.55V peak, so a peak current of 10mA will drive 1nF at frequencies up to about 1MHz.

...Ya, it'll drive it...

Tim
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Old 12th May 2010, 06:23 AM   #5
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Thanks Tim, Maths is not my strong point.

Cheers, much appreciated
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Old 12th May 2010, 07:32 AM   #6
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It doesn't matter if you have 1mA or 100mA going thru your tube. It's the RC values that'll set the respons either way. The 10mA is going thru your circuit, and not thru the load. When that much of the current starts running thru the load, the preamp is overloaded.
At 20kHz the load impedance (10kohms in parallel with the 2nF capacitance) is 10kohms in parallel with 4kohms which is 2860ohms.
200ohms will drive it fine. You probably wont hear the slight loss of highfreqs, besides that long cables may pick up unwanted HFs, and a little filtering may be a good thing.
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