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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Passive pre amp - 50k or 100k pot?
Passive pre amp - 50k or 100k pot?
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Old 21st March 2020, 02:20 AM   #1
Asterix2k10 is offline Asterix2k10  United States
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Default Passive pre amp - 50k or 100k pot?

I built a passive pre amp with a 100k ohm pot - should I have used a 50k instead?
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Old 21st March 2020, 02:24 AM   #2
rayma is offline rayma  United States
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For solid state sources, 10k is best.
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Old 21st March 2020, 02:25 AM   #3
Michael Bean is offline Michael Bean  United States
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How long is a piece of string?
That question can't be answered without knowing the output impedance of your source(s) and the input impedance of your amp.

Mike
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Old 21st March 2020, 02:33 AM   #4
Asterix2k10 is offline Asterix2k10  United States
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Quote:
Originally Posted by Michael Bean View Post
How long is a piece of string?
That question can't be answered without knowing the output impedance of your source(s) and the input impedance of your amp.

Mike
The output impedance of the source is 32 ohms and the input impedance of the amp is 100k ohms according to the manuals.

Last edited by Asterix2k10; 21st March 2020 at 02:45 AM.
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Old 21st March 2020, 02:39 AM   #5
JMFahey is offline JMFahey  Argentina
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Then you can easilybdrive a 10k pot, even a 5k one.

That eases a lot connecting cable requirements.
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Old 21st March 2020, 03:42 AM   #6
sgrossklass is offline sgrossklass  Germany
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Strictly speaking, the output impedance still won't tell you much about the output's actual driving abilities. That said, any source brave enough to have only 32 ohms of output impedance (sounds more like a nominal load impedance to me, actually), would be likely to have a decent amount of oomph, easily driving 10 kOhms and possibly even <1 kOhms still.
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Old 21st March 2020, 01:39 PM   #7
Ken Newton is offline Ken Newton  United States
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An possible alternative (which you may not wish to do), is to mount the volume pot. INSIDE your power amp, effectively turning it in to an integrated amp., then the concern over passive volume control pot. resistance value mostly goes away. The key is whether or not the pot. forms an RC low-pass filter with the interconnect cable capacitance (commonly, approximately 50pF per foot), producing an audible high response roll-off. The higher the pot. resistance, the worse the roll-off if it is mounted before the interconnect cable. When mounted after the interconnect, such a roll-off is not produced. Which enables the pot. resistance to be chosen so as to provoke the lowest line driving circuit distortion. However, the higher the pot. resistance the more it will produce measured thermal noise, which still may not be audible as an increase.
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Old 23rd March 2020, 01:18 PM   #8
Mark Tillotson is offline Mark Tillotson
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100k is usually capable of discernable hiss at the tweeters, as that's about 70ÁV RMS at the output of a typical power amp. Typically you'd only hear this fairly close to a speaker.

100k pot has its max effective noise resistance of 25k, when set to 50k+50k position, and that noise gets amplified about 25 times in a typical power amp.

I reckon if noise is an issue (all depends if you are irritated by the hiss close to the speakers), change to 10k.
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Old 23rd March 2020, 01:30 PM   #9
sgrossklass is offline sgrossklass  Germany
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As an aside, power amps may exhibit worsened distortion performance if driven from high-impedance sources. They are often using rather hot bipolar inputs with plenty of input bias current and associated nonlinearity.
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Old 24th March 2020, 03:02 AM   #10
rsavas is offline rsavas  Canada
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On the bjt IPS example, say with a unipolar input and not complementary with some degree of bias current cancelation, is the base bias current not isolated by the input coupling cap? So this ips bias current would be determined by a resistor that is connected to the ips base and usually connected to the ground reference returned through the emitter? The base bias current would not be determined by the source or the pot in this case?
This is how a jfet IPS has an advantage over a bjt type.

Last edited by rsavas; 24th March 2020 at 03:06 AM.
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