Input cap selection

[Deltat]

For an input cap what material is usually the best? Also is it usually better to go with a small capacitance/large resistance or large capacitance/small resistance combination for the input filter?

Im looking at possibly using the Nichicon ES Muse series caps for my subwoofer amplifier. Suggestions?

Kurt

[ostie01]

HI, my rule is the bigger is the better, at least 1000uF per amp so for 20 amp this mean you'll need at least 2000uF of capacitance. Nichicon is a good choice.

[Deltat]

Oops should have been more specific, this is for the input to my preamp. So the signal coming from the subout is like 100mVrms thinking probably very little current aswell.

[lndm]

Are you referring to a cap in the signal path (apart from power supply)? If so, a film cap sounds good (if possible). The resistance to use depends on the circuit.

[AndrewT]

Hi,
the input DC blocking capacitor forms an RC high pass filter with the following resistor to ground.

eg. amp with 100kohms input impedance using a 1uF blocker will have a time constant= 100k*1/1000000=0.1seconds=100mS
the high pass filter frequency is 1/2/Pi/RC=1.59Hz.

But there is a problem that is often overlooked.
The source probably has a DC blocker as well. If the source has a 470nF cap at it's output then the effective cap value to put into the RC calculation becomes .47*1/(.47+1)=.32uF and the frequency has changed to 5Hz. No problem when the input impedance is high (100k) like the example I have used.

Try some numbers using lower input impedance say 22k or even 10k. Now the DC blocker on the input to the amp needs to be 4u7F to 10uF. Now repeat the correction for the source cap and sure enough the high pass filter has jumped above 10Hz and can even, in extreme cases of mismatch, approach 50Hz.

Do the calcs and be informed.

[Deltat]

^
So basically your are saying to keep the impedance high and the inductance low so that a possible series capacitance on the output of the source wont have a large effect on the cutoff frequency of my hp filter. Makes sense, I cant believe i never thought about that.

When designing low pass filters, say the one on the output of my prog. switch cap filter IC that i use to block the high frequency switching noise, should i use a film cap there?

Another question. All these audio grade electrolytic caps that Nichicon and im sure other mfgs make, those are for what? Power supply decoupling?

[AndrewT]

Hi,

So basically your are saying to keep the impedance high

no! Do the sums and make an informed decision.

A low input impedance has some attractive advantages that, otherwise, are almost impossible to achieve. Each design decision or modification should be based on knowledge not guesswork nor opting to follow the fashionable trend.

I believe that film caps perform better than electrolytics in every location. But the cost and size effectively removes the film choice from many duties. Similarly the very compact nature of ceramic makes them very suitable for bypassing if implemented correctly. No other option, in my opinion, can approach their performance in this area.

[Deltat]

I know how the math works, if i didnt, well i wasted a lot of money in college. When it comes to audio appliciations there appears to be a lot of subjective elements that cant be solved by calculation.

Since this is a subwoofer amplifier, I CAN NOT afford any output capacitance to have a great affect on my input filter's cutoff frequency. My statement that i should keep the imedenace high was not a statement that i ment to sound like im generalizing for ALL filter designs, merely just this particular case.

I dont know if my source has any output capacitance, its a cheap Sony reciever, and im sure the documentation for it is crap. Would it be wrong for me to design with the assumption that it does?

Any mfgs of film caps should i look into? Ive also seen from some searching that polyproplyne(sp?) seems to be the favored material.

[lndm]

If I understand correctly, AndrewT was suggesting that your source may have been AC coupled, and naturally that cap would limit your loading options.

Might I suggest you open your Sony, and tailor the output to your needs. In my opinion there is no sense in running the signal through unhelpful or unnecessary circuitry when we have the skills to 'marry' the two components in the best way.

Polypropylene is a good material generally speaking. You might try this page.

http://www.humblehomemadehifi.com/Cap.html

[Deltat]

Originally posted by lndm
If I understand correctly, AndrewT was suggesting that your source may have been AC coupled, and naturally that cap would limit your loading options.

Might I suggest you open your Sony, and tailor the output to your needs. In my opinion there is no sense in running the signal through unhelpful or unnecessary circuitry when we have the skills to 'marry' the two components in the best way.

Polypropylene is a good material generally speaking. You might try this page.

http://www.humblehomemadehifi.com/Cap.html

I think i would rather make this more universal.