What value cap to bypass power supply cap?

Hi Kijanki, I think what you're talking about is putting a single cap at the input as an AC line filter. That is a great place for Mundorf Supreme. People have already tried this out and got good result:

http://www.laventure.net/tourist/caps.htm

However, as you can see in the link, there a caveat with Mundorfs as AC filter...It is note rated for use in AC application.

I have been told by several amp designers that bypassing each power supply cap with a 0.1 microF (or other small value cap) is good design practice to filter out noise in the hundreds of kHz to megaHz range, which I was told can cause unwanted oscillations in the amp.

Thanks for hanging in there to guide me.

Hi Davehrab, thank you for your input. When I started this thread, I was confusing several things about bypassing. Here is what I've learned so far from helpful people like you and Kijanki. Please correct me if I'm wrong.

1) The 1% bypass cap recommendation is to reduce ripple in the power supply. This helps with the frequency response of the amp (not exactly sure why) and decreases noise generated by the power supply.

2) Film caps have higher self resonant frequency than electrolytic caps, which is a good thing because it is farther out from audible frequency range. Film caps have much lower ESR (and ESL?) than electrolytics. Do these properties of film caps mitigate the need for the 1% bypass cap recommendation that is employed on electrolytics?

3) The 0.1 microF bypass on each power supply cap and also on the amplifier end helps filter out the really high frequency noise (hundreds of kHz to MHz) range that can cause unwanted very high frequency oscillations in the amp. Some have argued just filtering out the high freq hash may even affect sound quality (not to sure about this argument).

I hope someone will correct me if any of my statements are incorrect. I am learning so much on this forum. Thank you.

BTW, I have replaced the carbon pot on my amp with stepped resistor attenuator. A significant improvement. But the most significant improvement came from treating the room acoustics...took me over two years with various combinations of absorption and diffusion. Phase Linear?...Bob's solid state amp. I'm a fan.

Dracule1, no I would not recommend Mundorf for AC line filter. What I meant is that all your caps are most likely connected in parallel and then to the amp's circuitry. At the input to this circuitry I would put 1uF cap.

When you connect caps in parallel wire going to circuit should be from the very last cap and not the one close to rectifier.

Dracule1, 1% cap won't do much to reduce ripple but will deliver current when demanded during high frequency transitions where main caps is too slow (inductive).

It will also work to suppress high frequency interference. Imagine that high frequency interference contains 90kHz and 100kHz frequencies. When they appear on non-linear element like output transistor (noting is perfectly linear) sum and difference of them is created (modulation). Sum is inaudible 190kHz but the difference is 10kHz. It gets worse when interference contains a lot of frequencies making a lot of new audible junk. Almarg posted few times on this subject stressing importance of protecting audio system from high frequency interference.

Oh I see. That makes sense, but I thought you want the bypass caps as close to the power supply caps as possible? The further away the bypass cap is, the less effective the filtering, no?

I thought the 0.1 microF cap will handle those high frequency noise you just talked about.