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

Help in choosing a potentiometer as a "Passive preamplifier"
Help in choosing a potentiometer as a "Passive preamplifier"
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Old 29th August 2017, 01:40 PM   #91
Mixi is offline Mixi  Europe
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so scottjoplin says that it will be fine and andrewt says that the current pre amp configuration together with a 10k pot will lead to problems with the bass. did i get that about right?

seems there's never just one correct answer to a question like that

btw: i talked to the guy who designed my phono stage and he said the output impendancenis 10 Ohms. He suggested using a 100k pot or a buffer amp.

i'm a bit confused

Last edited by Mixi; 29th August 2017 at 01:42 PM.
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Old 29th August 2017, 01:52 PM   #92
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Mixi View Post
btw: i talked to the guy who designed my phono stage and he said the output impendancenis 10 Ohms. He suggested using a 100k pot or a buffer amp.
The lower the output impedance the greater the load it can drive, so that doesn't make sense
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Old 29th August 2017, 02:00 PM   #93
DF96 is offline DF96  England
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1uF driving 10k gives an LF rolloff at 16Hz. You may or may not find that acceptable; this partly a matter of taste, and partly a matter of which speakers you have and where they roll off. If the 1uF is an electrolytic then you really need 10uF instead, so the cap has very little signal voltage across it at the bass end; alternatively, use a 100k pot instead.

10 ohms output impedance can easily drive a 10k pot. 10 ohms plus 1uF can only drive 10k if you are happy to have some bass rolloff.
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Old 29th August 2017, 05:47 PM   #94
Mixi is offline Mixi  Europe
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thanks for the explanation!

my speakers are rated 35 - 20.000 Hz, so I guess a rolloff at 16Hz wouldn't really matter, would it?

aren't frequencies below 20 Hz inaudible anyways?
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Old 29th August 2017, 06:18 PM   #95
scottjoplin is offline scottjoplin  Wales
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It's a good idea to roll off the low frequency to avoid excessive excursion of speakers, particularly if a turntable is your source. Yes, 20Hz is about the limit of audibility
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Old 1st September 2017, 03:37 PM   #96
Mixi is offline Mixi  Europe
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thanks for all your advice.

i talked some more to the designer of the phono stage and figured out that a 20k pot would probably be ideal. i'll keep the phono stage as it is and in combination with a 20k pot the rolloff should be at 6 or 7 hz.

i seem to remember that somewhere on this forum i read about the relationship between S/N ratio and pot values. unfortunately i can't find the post or article anymore. can somebody verify my assumption that a lower pot is desirable because the lower resistance will "kill" (for a lack of a better word) less of the signal and thus less amplification is needed after the pot. With less amplification whatever noise appears in the amp will also be less amplified giving a better S/N ratio. Did I get it right so far?

Would that mean that - all other things equal - a lower resistance pot will play louder (because it "kills" less signal) in any given setting than a higher resistance pot?
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Old 1st September 2017, 04:02 PM   #97
DF96 is offline DF96  England
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No. All resistors generate thermal noise, no matter how 'good' they are. The thermal noise voltage is proportional to the square root of the resistance, so higher values generate more noise. The signal attenuation, on the other hand, just depends on resistance ratios (given a suitable pot value) not absolute values.
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Old 1st September 2017, 04:32 PM   #98
stocktrader200 is offline stocktrader200  Canada
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blue velvet 50k pot will do the trick.
I'm looking at the 10k or 20k one, however I have a preamp before the pot
Less amplification after the pot will reduce noise levels
It won't be any louder but noise levels will be less. Lower resistance will also make it flatter response when driving cables or stages with larger miller input capacitance.

Last edited by stocktrader200; 1st September 2017 at 04:36 PM.
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Old 2nd September 2017, 08:43 AM   #99
Mixi is offline Mixi  Europe
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cool, thanks for the explanation!
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Old 2nd September 2017, 09:20 AM   #100
scottjoplin is offline scottjoplin  Wales
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Where to have gain and attenuation is a balancing act. This is worth reading Setting Sound System Level Controls
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