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Old 11th May 2009, 03:19 PM   #11
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What would be a good value for the input cap? So that it doesn't affect the audible frequencies?

Thanks!
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Old 11th May 2009, 03:51 PM   #12
Atilla is offline Atilla  Norway
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Something like C = 1/4*pi*R where R is your input impedance is one of the suggestions I've seen.

More calculations are explained here : AC Couple Input Value?

As suggested, a bypass with a small value good quality cap might also be a good idea, especially if you're using an electrolytic
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Old 15th May 2009, 05:12 PM   #13
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Quote:
Originally posted by Puffin
I am using Ampohm Polypropylene in my Gainclone monobocks. I sourced these from a company whose name escapes me, but their details should be found on www.audiojumble.co.uk.

They normally have a stall at the audiojumbles and their prices are very good. I think mine are 3.3uf and were about a fiver each.

Much better sound than ICW polyprops (IMO)

Here is the link :-http://www.audiocap.co.uk/special-offers-56-c.asp
And we'll be at the AudioJumble on the 21st June in Letchworth - see you there!
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Old 18th May 2009, 10:17 PM   #14
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Quote:
Originally posted by rectumfrier
Borrowing the thread

What would be a good value for the input cap? So that it doesn't affect the audible frequencies?

Thanks!
It's straightly depends on of your circuit.
In my circuit I placed a good value cap (2.2uF MKP). But for that it doesn't effect audible frequencies I placed 10nF MKP in parallel.

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Old 19th May 2009, 02:11 PM   #15
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Okay thanks!
What would be a good value for the "standard" circuit.. posted on: http://www.national.com/mpf/LM/LM3886.html ?

Btw, how do I measure the input resistance? If it has anything to do with this...
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Old 19th May 2009, 02:37 PM   #16
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If you can find "film and foil" polyprop's of around 100V, you
may get somewhat better sound than MKP's which are metalized
polyprop's. I would also go with a smaller value bypass
(x10 - x100 times lower value) of something like polystyrene
or if you can afford, Teflon!
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Old 19th May 2009, 02:42 PM   #17
AndrewT is online now AndrewT  Scotland
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The input resistance of the chipamp can be considered near infinity.
The 1k0 feeding that adds almost nothing to infinity.
The 10k pot stes the input impedance/resistance @ 10k.
measure it and you will find it does not vary for different positions of the volume control.

If you add a DC blocking capacitor before the pot, then a resistance measurement will show infinity. The impedance is set by the capacitor at DC and low frequencies and gradually as the impedance of the capacitor falls with increasing frequency the 10k pot becomes the dominant impedance.
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Old 19th May 2009, 05:17 PM   #18
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Quote:
Originally posted by rectumfrier
Okay thanks!
What would be a good value for the "standard" circuit.. posted on: http://www.national.com/mpf/LM/LM3886.html
An optimal value for this circuit could be 2.2uF.
But it's advisable to raise input impedance to 47kOhm and feedback capacitor to 47uF, so you can use 1uF input coupling capacitor with much better performance than with standard circuit. Or you could use other schematics available at this forum for the best performance from that chip.
To determine required components values likewise to obtain calculated performance characteristics NSC has "Overture_Design_Guide15" which you can find at their website.
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Old 19th January 2010, 01:52 PM   #19
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What would be a good capacitor value (speaking of coupling cap) for a LM3886 with

A) 10k input resistance

B) 100k input resistance

?? Thanks
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Old 19th January 2010, 02:03 PM   #20
AndrewT is online now AndrewT  Scotland
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this answer depends on the passband of the amplifier.
There is no point in telling you to choose a high pass filter with F-3dB=1Hz or 10Hz or any other, if you don't tell us that the amplifier has a LF high pass F-3dB=26Hz.
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