Capacitors – Why They Are Important

Capacitors – I am always on about capacitors and how important they are. I have literally thousands of the damn things in my parts inventory and I like to replace them, especially in certain types of circuits – but why?

To answer this we perhaps need to look first at what they do and how they are made. Capacitors store charge, like little reservoirs. They also block the flow of direct current and therefore DC voltages. Some are made of aluminium foil and are polarized, some of metalised plastic, some of ceramic materials or tantalum.

The Best Cap is No Cap…

A good rule of thumb is that any capacitor in the signal path is best avoided. Having said that, there are some you would rather have in the signal path than others, and sometimes you have to have them there, for example to block DC from being passed on to the next stage in a circuit.

Often, manufacturers will use cheap aluminium electrolytic capacitors in this role, but polarized capacitors can never be symmetrical in their operation, because they are polarized. This means they introduce distortion into AC signals – the signals we want to be able to hear.

A selection of capacitors. The larger can-types in the top row are electrlytic capacitors and have huge capacitance for their size, but poor high frequency performance. The first four capacitors on the bottom row are also electrolytics of smaller capacitance. The final six capacitors on the bottom are film and ceramic types which have much better HF performance, but much smaller capacitance per unit volume.
Signal Path

Signal path capacitors are best when they are metalised plastic or polyester, oil filled or other non-polarized types, but these are more expensive and are often omitted for this reason. I often change out the aluminium electrolytic signal path capacitors for types that sound far better for my customers. They cost more, hundreds of times more sometimes, hence the reason they are often not used as original equipment from the factory.

These silver nica capacitors have excellent high frequency performance and are often used in RF applications like radios and old tube tv sets, as well as premium audio gear and laboratory equipment

Because capacitors store charge they are used in power supplies to smooth out the ripple in the rectified AC and remove the noise. Here we need high capacitance for a given size and aluminium electrolytic types are perfect because of their very high capacitance to volume ratio.

The problem is again one of cost though. Manufacturers need to save money so they often use economy parts to boost margins, but in turn this reduces reliability and longevity. Cheap parts are rated for shorter working-life and lower operating temperatures.

They also have poorer electrical characteristics such as higher equivalent series resistance or ESR. In effect this wastes energy and reduces the performance of equipment that needs high power, low impedance supplies, like amplifiers.

Paper in oil capacitors are great for using in the signal path to warm things up a little.

What we can do here is use better quality parts and replace old capacitors that wear out. Aluminium electrolytic capacitors do actually wear out. Their performance gradually degrades over time, resulting in increased ESR, increased leakage and decreased C.

This reduced performance over time is due to the gradual increase in internal resistance inside the capacitor, usually due to a gradual deterioration of electrolyte. This is all bad, but all easily fixed. Good electrolytic power supply caps can cost $10 – $100 EACH depending on size. You can see that in a DAC that has to sell for say $1500, you can’t possible have several hundred dollars worth of capacitors in the power supply when the WHOLE DAC has to cost no more than that price to manufacture.

Herein lies the great conundrum – designers would of course rather use the best parts. If they did, the DAC that should sell for $1500 would have to sell for $5000, and this is not an option in almost every case. Instead, the manufacturer’s marketing and production droobs will work out how to save money and this will mean trimming each part down to the most basic spec that will get the job done.


Finally we need to consider decoupling, which is where small value solid capacitors are often used to reduce high frequency noise. Again, better capacitors sound and perform better. I often use military parts bought at surplus, in large quantities. These military caps are fantastic because they are not made for the ‘audiophile’ market, and so there is no marketing bullshit involved.

What you DO get with mil-spec parts is parts that are the best performers, and have the best lifespan. Remember – these parts are made to operate under military conditions 0f extreme heat, vibration, humidity. Knowing this, you can probably see why these parts are best and why I use them where I can.

To summarise, capacitors have a huge impact on the sonic performance of audio gear and the measured technical performance as well. Almost any piece of equipment – even brand new – will benefit from some careful consideration of the capacitors installed and alternative replacements.

Thanks for reading, leave a comment and let me know what you think!