Input Capacitor

[otherside]

Hi All,

I was trying to find a good capacitor and available for DC blocking/input.

Has anyone ever tried this one:

http://onecall.farnell.com/ampohm-w...-0-5-300/audio-capacitor-10uf-300v/dp/1636701

It's quite pricy and i was wondering about two things:

1. Is it worth it?
2. With such huge size you got a long signal path, isn't that going to affect the signal more than a small MKT?

 

[Atilla]

This is a paper in oil cap, they tend to be rather big. It's also rated for 300V, which is, well, an overkill.

I think there are smaller, cheaper and better caps you could put in your signal path.

 

[otherside]

Atilla thanks for the reply, which brings me to another question

What voltage rating should this caps have (input signal)? I've seen threads in here where people are using such high voltages, up to 300V or even more and never quite understood the reason. The actual voltage across the cap will be almost zero so what's the point of high voltage rating?

Also has anyone tried instead of these polyester, polypropylene, paper etc. caps to use multiple smt ceramic ones in parallel? And if so is this approach any good?

Cheers

 

[Mr Evil]

Exotic capacitors are a waste in my opinion. If you want something nice, try polypropylene. They are much cheaper and much smaller. They are still large, but you can actually fit them into circuits rather than trying to fit the circuit around the capacitor.

 

[Stream]

try polypropylene.

It's not polypropylene, it is polyester... Polypropylene is much bigger, more expensive and "sounds better"

otherside , I can recommend You polypropylene caps made by Icel.

 

[Mr Evil]

Oops, yes those are polyester. Polyester has a higher dielectric absorbtion, but you won't hear that. Anyway, let's try that again: Polypropylene.

 

[Atilla]

I'd say go for a decent brand polyprop. People would put anything in their signal path, but I think I'm deaf and sane enough not to find enormous difference, unless they do it *wrong*.

A few searches through those forums will give you a bunch of nice and sane suggestions about what to use. You might also be interested in checking out Tangent's article on input caps here - http://tangentsoft.net/audio/input-cap.html

 

[otherside]

Thanks for your suggestions guys.

I've certainly heard the difference between an electrolytic and polyester, now i've never tried polypropylene before so i don't know how different would sound.

I'll try out a few for sure. But to get back to the original question about this huge paper in oil cap, has anyone tried it? not the specific ones but any with such dielectric?

I have to say i'm tempted, and since i'm planning a couple of new designs i don't have any space concerns at the moment. I'm just curious to know if forking over 70£ for them will make me "eat my hat"

Cheers

 

[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

 

[Atilla]

I don't know, 70 for one input capacitor sounds a little bit of a scary price. I can't guarantee any hat-eating, unless it's related to the fact that you just purchased a gigantic cap for a ludicrous price, that does nothing special in the application it's being put in.

I do know I've seen people report on similar caps, paper-in-oil, beeswax, and say good things about them, but all in all they were a little ... smaller.

I'll try beeswax, just for fun, some day

Oh and the link above has some caps with 1% tolerances. I'd be more interested in getting 2(or any number) of low-tolerance caps with good ratings, than putting 2 obscure 10% to 15% tolerance caps in the different channels. A worst-case scenario could end up with quite mismatched channels, after all.

 

[rectumfrier]

Borrowing the thread

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

Thanks!

 

[Atilla]

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 : http://www.diyaudio.com/forums/showthread.php?s=&threadid=136770

As suggested, a bypass with a small value good quality cap might also be a good idea, especially if you're using an electrolytic

 

[MCampbell]

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!

 

[AndriyOL]

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.

Regards.

 

[rectumfrier]

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...

 

[coloradosound]

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!

 

[AndrewT]

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.

 

[AndriyOL]

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.

 

[rectumfrier]

What would be a good capacitor value (speaking of coupling cap) for a LM3886 with

A) 10k input resistance

B) 100k input resistance

?? Thanks

 

[AndrewT]

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.