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Old 18th April 2019, 10:31 AM   #11
nirupambhowmick is offline nirupambhowmick  India
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i mean...
Tone control > 2uf ?> 4k7 resistor > 100k volume > 2uf?> Final stage of the preamp.

Last edited by nirupambhowmick; 18th April 2019 at 10:42 AM.
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Old 18th April 2019, 10:43 AM   #12
mchambin is offline mchambin  France
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Quote:
Originally Posted by nirupambhowmick View Post
i dont know about coupling time constant thing. Do you thing 2uf would be adequate? Help me.
A coupling cap is in serie with some resistor R. The time constant is R x C. The corner frequency is 1 / ( 2 x Pi x R x C ), that means, lower frequencies below do not go through while higher frequencies do go through. It is not sharp, this goes gradually. At the corner frequency, signal goes through with a 3 dB attenuation which is 71% amplitude. Much lower 0%, much higher 100%.
This is explained in EE books at "First order system". Read real books, not audio oriented litterature.
What is the "some resistor R" ? : That is the sum of the source impedance ( what feeds the cap ) and the load impedance ( what loads the cap ).
So R x C depends of where you insert the capacitor.
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Old 18th April 2019, 02:19 PM   #13
nirupambhowmick is offline nirupambhowmick  India
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Quote:
Originally Posted by mchambin View Post
A coupling cap is in serie with some resistor R. The time constant is R x C. The corner frequency is 1 / ( 2 x Pi x R x C ), that means, lower frequencies below do not go through while higher frequencies do go through. It is not sharp, this goes gradually. At the corner frequency, signal goes through with a 3 dB attenuation which is 71% amplitude. Much lower 0%, much higher 100%.
This is explained in EE books at "First order system". Read real books, not audio oriented litterature.
What is the "some resistor R" ? : That is the sum of the source impedance ( what feeds the cap ) and the load impedance ( what loads the cap ).
So R x C depends of where you insert the capacitor.
I am not a student of electronics, it is just my hobby & I love it. I do not know which book to read. If possible then please mention some. Off course about basic audio.
Ok now please help me out. I do not have 6.8uf cap, but I have some good quality 1uf caps.
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Old 18th April 2019, 02:23 PM   #14
nirupambhowmick is offline nirupambhowmick  India
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How about 3uf after the volume pot?
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Old 25th April 2019, 12:17 PM   #15
nirupambhowmick is offline nirupambhowmick  India
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i think this time i'll not be wrong
At the input of non-inverting amp(first stage) i'll add 1uf mkt cap with a low pass filter(1k & 680pf). To avoid high frequency oscillation in every op-amp, across feedback resistor(between inverting input & output) i am going to add a small 15-22pf ceramic cap. Then at the input of the last stage i'll add 3.3uf mkt capacitor(input impedance of the inverting amp is 15k). The corner frequency will be then below 4hz. My lm3886 amp has input capacitor of 2.2uf with 22k Zin. In my view this is good enough. Please correct me if i'm wrong. i welcome further suggestion.
Btw thnks to all
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Old 25th April 2019, 10:45 PM   #16
Mark Tillotson is offline Mark Tillotson
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I think you can have a half-way house by adding a bias resistor to ground to the opamp input, so that pot crackling noise is much smaller. For a 10k pot a 100k resistor to ground will limit crackle to perhaps 20mV(*), rather than risking the opamp hitting the rail as the pot wiper moves over a scratchy zone.


(*) 200nA into 100k


With the tiny bias currents of JFET or CMOS opamps the slew rate of any crackling is far less - its limited by the capacitance of the input and wiper wiring.
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Old 26th April 2019, 07:47 AM   #17
nirupambhowmick is offline nirupambhowmick  India
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Quote:
Originally Posted by Mark Tillotson View Post
I think you can have a half-way house by adding a bias resistor to ground to the opamp input, so that pot crackling noise is much smaller. For a 10k pot a 100k resistor to ground will limit crackle to perhaps 20mV(*), rather than risking the opamp hitting the rail as the pot wiper moves over a scratchy zone.


(*) 200nA into 100k


With the tiny bias currents of JFET or CMOS opamps the slew rate of any crackling is far less - its limited by the capacitance of the input and wiper wiring.
Point noted Sir But one forum member have suggested to add a capacitor between volume & output gain stage. i already placed a order of 3.3uf caps & tl072, let's see.
To avoid op-amp hitting supply rail i have seen a technique, something like 1meg resistor between output & inverting input(for tone stage only).

Last edited by nirupambhowmick; 26th April 2019 at 07:50 AM.
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Old 5th May 2019, 04:35 PM   #18
nirupambhowmick is offline nirupambhowmick  India
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I have got tl072. Fake op-amps sounds terrible.
Please verify.
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