Input capacitors; (in what way) does size matters?

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hansibull

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#1
Hi!
I'm looking for a great input capacitor to replace my stock WIMA 1uF metal film caps. can someone explain the difference from choosing a small input cap vs a large one?

At Ebay most of the input audio capacitors are between 1-10uF. What to choose? Why choose it?
 
#5
Put another way... a series capacitance with a parallel resistance (which is what your input circuit is) form a first-order high-pass filter (because the capacitor blocks the lower frequencies down to DC). JS forgot a two - the formula is F = 1/2*pi*R*C. At that frequency, the response is down -3dB, and drops 6dB per octave. He is right that moving too far from the designed value creates a possibility for problems.
 
#6
What about increasing voltage ratings? I have experienced cleaner sound when I replaced e.g. 63v caps in speaker crossovers with 100v or 250v types. The sound opens up , less congested, more dynamic... and this is when the amplifier is putting out much less than 50v on to the speaker... would this relationship hold true for input caps as well?
 
#7
hansibull - what speakers do you have? if they are stand-mounts then 3.3uf or 4.7uf should be fine. If they are big floor standers then try 10uF. Do you know the value of the associated parallel resistor?

I wondered what the correct input cap for my system was and found the lowest freq response of my speakers (60hz) then used an online calculator to find the appropriate cap value.

Is it correct that two bipolar caps in series are very good for input? I believe you have to polarise them slightly with a 9v battery.
 
#9
The input capacitor is just one of many Low Frequency roll-offs built into your music reproduction system.

If there were four (filters) and all were set to the same frequency, then the output would be 12 dB down at the roll-off frequency.
Changing the input capacitor alone would not significantly change the roll off frequency, due to the effect of the other unchanged roll-offs.

I recommend a different way to build and interface a Power Amplifier.
The stages before the amp should have a pass band that is at least one octave wider at both the top end and the bottom end than the amp.
The internals in the amp should have a wider passband than the passive filter built into the input of the amp.
Now you can "tune" the passive input filters, both the RF filter and the DC blocker, to investigate the effect of moving the DC blocker value around.
This ONLY works if the input filter defines the pass band of the system.
Other bottlenecks must be investigated and if necessary eliminated before you can reach useful conclusions on the value of the DC blocker.
 
#11
Yes, physically small caps can pick up less noise.

Raising the working voltage of the cap will not have the same effect as it may have in a crossover, as a coupling cap has much less signal voltage across it. However, in some cases a coupling cap may have significant DC voltage across it too. Avoid slightly non-linear dielectrics like polyester (mylar) if the cap dominates the LF rolloff.

To decide whether a cap 'upgrade' will have any effect you have to reverse-engineer the circuit and determine exactly what the cap is doing. Or just make the swap and believe that it must be better because you spent time and money on it.
 
#12
hansibull said:
Hi!
I'm looking for a great input capacitor to replace my stock WIMA 1uF metal film caps.
can someone explain the difference from choosing a small input cap vs a large one?

At Ebay most of the input audio capacitors are between 1-10uF. What to choose? Why choose it?
Click to expand...

If they're Wima Metal FILM/FOIL caps (Wima FKP), you already had optimum type
and great sounding caps. Almost all MKP caps are worse. So why change them?

This is the Seas Mimir:

An externally hosted image should be here but it was not working when we last tested it.


Pardon me, but I cannot see any deep bass response at all. 😉

If you'd lower the -3 dB point of your amp from 10 Hz to 5 Hz, do you really think
it would be audible? Certainly it will not. It won't even make any difference, if it
would drop from 20 Hz to 10 Hz.

So again, why change anything?
 
#13
I know that the Mimir kit doesn't have a deep bass response. That's why I'm building a 2.1 amplifier. I'll use three separate amplifier boards, one for each speaker (Where I'm using the WIMA caps). The subwoofer (SEAS L26ROY) is driven by three LM3886 in parallel. I found out that this might me the best method to give a lot of power to a 4 Ohm speaker. But if the WIMA 1uF is enough (and gives great sound) I shouldn't be changing them 🙂

EDIT:
I found a datasheet for what looks like is my WIMA caps. the datasheet says that my cap (WIMA MKS4 1.0uF/100) is a PET capacitor
DATASHEET
 
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#17
Increased voltage ratings

Increasing the voltage rating of a film capacitor can be accomplished a few different ways. The simplest one is by adding additional dielectric between the electrodes.

The other (more complicated to explain) is switching the electrode system from a discrete foil to a metalized electrode system.

Now how that effects what you are experiencing??

K-amps said:
What about increasing voltage ratings? I have experienced cleaner sound when I replaced e.g. 63v caps in speaker crossovers with 100v or 250v types. The sound opens up , less congested, more dynamic... and this is when the amplifier is putting out much less than 50v on to the speaker... would this relationship hold true for input caps as well?
Click to expand...
 
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