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Driving capacitive loads
Driving capacitive loads
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Old 9th March 2013, 11:13 AM   #1
ginetto61 is offline ginetto61  Italy
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Default Driving capacitive loads

Hi !

I have a question about preamps "driving ability"
I see that usually they are measured using pure resistive loads, even very low loads like 600 ohm, but never with capacitive loads
Looking at the specs of some amps I see that the input capacitance can be up to almost 1000 pF
and moreover there is a cable in between with its own capacitance
Driving inputs of this value could not effect the performance of the preamps ?
is it any power amp completely within the driving ability of any preamp ?
Thank you very much indeed for any kind advice
Kind regards,

Last edited by ginetto61; 9th March 2013 at 01:17 PM.
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Old 9th March 2013, 12:30 PM   #2
willamp is offline willamp  United States
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Interesting questions. Looking at specs for a few amps I don't see input capacitance listed except on phono stages. A quick ltspice sim does seem to show that input capacitance is at least as signifcant as source impedance which is the thing I usually concern myself with. Hopefully someone more knowledgable will chime in.

I'd like to know how to set up a test circuit to measure input capacitance of the equipment I have built.

First screen shot below is a source with 1300 ohm output impedence driving load of 100k and 100 to 2000pf.

Second screen shot is a source varying from 300 to 2000 ohm driving a load of 100k and 1000pf.
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File Type: jpg source_imp.jpg (264.0 KB, 103 views)
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Old 9th March 2013, 03:15 PM   #3
jcx is offline jcx  United States
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few small signal amplifying devices in common amplifier input circuits will have more that a few 10s of pF effective input C

some amps will add a few 100s of pF to gnd as RF filter

cable can range from 10(rare, foamed dielectric)-50? pF per ft so it can be the largest C load

some feedback circuits can become unstable when loaded with C on the output - often a isolating series Z is recomended

usually this has to be > ~10 Ohms, its not too strange to see up to 100 Ohms in series with line level audio source/preamp outputs

some CD players used muting circuits that added up to kOhm series output R - less common today than earlier when noise at "digital black" level was a competive number in ads
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Old 18th March 2013, 10:40 AM   #4
ginetto61 is offline ginetto61  Italy
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Hello !
please excuse me for another question
Could a tube line stage have problems when asked to drive a capacitive load like 500-1000 pF ?
Which could be the sonic drawbacks ?
I ask this because we made a comparison between a CJ PV10 and a Bryston preamp
The Bryston had a much stronger low end and even the top end seemed cleaner.
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