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Rick Miller 1st August 2010 10:31 PM

First filter cap - Big or Small?
Just wondering what you have found to sound the best in your power supplies. Should the first filter cap after the diode bridge be large or small for the best sound? This can apply to both solid state and tube equipment. Some people like a large cap after the bridge and some do not, what do you recommend?

Speedskater 2nd August 2010 12:02 AM

If we are talking about a typical power supply with no series resistors or chokes, then large caps. near the bridge and small caps near the audio circuit. It's not about best sound (directly) it's about good circuit design.

Sch3mat1c 2nd August 2010 01:42 AM

Neither. The best size.

Typically 2000uF at 12V, 1A, 120Hz, I think for 10% ripple. C = 2.88F * IDC / (VDC * F), where IDC is maximum DC load current in amperes, VDC is the desired DC voltage, and F is the ripple frequency (twice line frequency for full wave rectifiers).

If this is too much ripple, add additional filtering (pref. CLC), regulate (pref. switching reg at high power), or just build a better circuit. Anything with low PSRR isn't worth listening to anyway.

Big capacitors worsen power factor considerably. This invites ground loop induced hum (from the huge peak currents) and reduces power available from a given transformer or line circuit. With the above value, power factor is typically around 0.5, depending on source impedance. A 100VA transformer will only deliver 50V 1A (= 50W) within ratings, much less if a huge cap is used. A 2kW amplifier cannot operate continuously from a 120V, 15A circuit because the input will be 4kVA! This is an excellent reason to supply high-power amps from a PFC supply.


DF96 2nd August 2010 11:01 AM

Small cap gives low voltage, high ripple. Big cap gives large peak currents, which get into audio and overheat cap and transformer. As Sch3mat1c says, pick the right one for your design. Don't follow fashion or your favourite guru. All electronic design is a compromise.

Rick Miller 2nd August 2010 08:39 PM

In the past I have picked the smallest uf value that would keep the voltage across it up to near the calculated value of X 1.414, or peak charge. I have seen some designers use a small uf value for the first cap in order to keep down RFI and simulate a vacuum tube rectifier. The larger the first cap the longer the current charge pulse gets in time. There is not always a direct correlation between measured performance and the best sound, so that’s why I was asking for feedback on what sounded the best. It seems to me that the greater the value of the first filter cap the stronger and better defined the bass gets in sound.

DF96 2nd August 2010 09:23 PM

No, a large cap means the charge pulse get shorter in time but larger in amplitude. It might help reduce droop, which in some circuits might help bass. This depends on the resistance of the transformer etc. The second capacitor would have a greater effect on the sound.

To simulate a valve rectifier you would add a series resistor. This reduces voltage, but also lengthens and reduces the height of the charge pulse. So it has a similar effect to a smaller cap, but does it in a slightly different way.

I think your usual practice of using the smallest first cap which will maintain the required voltage is probably the best option. The place to put big caps is in the second position.

Eric Juaneda 10th August 2010 05:41 PM

Hello Rick,

Originally Posted by Rick Miller (
Just wondering what you have found to sound the best in your power supplies...

What kind of equipment are you thinking about: amplifier or preamplifier (dac...)?

Rick Miller 15th August 2010 12:29 AM

Hi Eric,

Low current equipment like Pre-amps and CD players.

AndrewT 15th August 2010 09:54 AM


Originally Posted by Sch3mat1c (
...............Big capacitors worsen power factor considerably. .....

would you explain this to me, either via PM or in the Thread?

abraxalito 15th August 2010 10:13 AM


Originally Posted by AndrewT (
would you explain this to me

Power factor is a measure of how much in phase the current is with the voltage in an AC circuit. Sticking a big capacitor across the mains can draw a large current but dissipate relatively low power - this means a poor power factor as there are losses in the wiring associated with the large current flow even though the load has low dissipation (power consumption). Poor power factors are bad news for suppliers as consumers by and large aren't paying for the wasted energy in the wiring.

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