Bypass caps on power supply

[digital desire]

I have seen a few threads on modding amps, (gfa 555 ) and adding small bypass caps on top of the main ps caps was suggested a few times.
Can someone explain how this does anything? A link or something? How does this improve bass as was purported?
Thanks!

[Toobstheway]

Read part 4.0 here: http://sound.westhost.com/articles/capacitors.htm#4.0

[AndrewT]

Hi,
I was a believer.
Until I read Eva's posts.
Now I am inclined to not believe until I am capable of testing/measuring the outcome (a bit like Thomas & his finger).

It appears that oscillation can be promoted and HF garbage sent to the output if bypassing is done incorrectly, or not at all if Eva is to be believed.

Go find his/her posts and extensive test results/graphs/diagrams.
Then make up your own mind.

[digital desire]

O.K., Thanks. Kinda what I figured...
"" .................................................. .............................................The concept of 'self-resonance' in this context is flawed thinking, and leads to silly designs (100nF caps in parallel with 10,000uF electros for example) that generally achieve nothing useful, other than using more components.."


I guess I will leave it the way adcom designed it, and file this away with the notion of expensive speaker cables.

[Piercarlo]

Originally posted by digital desire
I have seen a few threads on modding amps, (gfa 555 ) and adding small bypass caps on top of the main ps caps was suggested a few times.
Can someone explain how this does anything? A link or something? How does this improve bass as was purported?
Thanks!

These small capacitors are pretty cosmetics and essentially useless in normal working condition. In abnormal situation they can also led the output stages to plain faults due to transient oscillation on power supplies which may trigger SOAR and overvoltage problem in bipolar output stages and plain destructive self oscillation in MOSFET output stages...

At my knowledge, the only real improvement on power supply capacitors which may improve sound quality is reducing ESR and ESL of capacitors itself. This may be reached with proper selected components (using the same capacitors employed in switching PSU) or splitting the existing required capacitors in more components with lower nominal values but with roughly the same phisical size).

By the way: the main improvement obtained with "increasing" capacitors values (provided that they don't kill the rectifier...) is just a (relatively) reduced ESR and, in my opinion, is this reduction that "sound better" because assure, also at high frequencies (above a couple of kHz), a better "closure" in AC of the rails to the ground, so diminishing the risk of intermodulation at high audio frequencies, just where the efficiency of NFB (and of the PSRR) start to fail.

Other improvements in low frequency audio, especially when original capacitors were of adequate value (roughly a 1 or 2 mF for each ampere of output capability of *single* channel), and - not to be forget - ground connections are properly designed, are, in my opinion, somewhat questionables.

Hi
Piercarlo

[djk]

"Can someone explain how this does anything?"

Since none of the big brains can think of a reason why adding small caps in the range of 10µF~100µF to the top of the 15,000µF caps in the Adcom GFA555 makes a hugh difference in how the bass sounds, I guess I would have to say it's 'magic'.

"How does this improve bass as was purported?"

As above, I guess I don't really know. Big computer grade caps just sound 'slow'. Paralleling them with small values (as above) makes them sound 'fast'.

Here is a clue though:
I used to service a brand of amplifier that was regarded as having a lot of 'punch'. When the owners would bring them in claiming they had lost their 'punch', I would find a bunch of 10µF power supply bypass caps (a pair per channel) rolling around inside the chassis. The cans had blown themselfs clean off the board. This would imply an enormous amount of current going through them, and not through the much large main filter caps.

The same thing happens with my Leach. The cans don't blow off, but the two pair of 22µF go open on a regular bassis (every few years).

This behavior suggests that on transients that the small caps must be providing large currents, despite what the EEs proclaim.

The change in sound is so dramatic that some will not like it. I had someone that owned a Bryston 3B that was in this camp, he liked the stock, muddy bass without the bypass caps. He listed mainly to RAP music though.

The small film caps on top of the big cans in the Adcom are cosmetic, they do nothing. Any effect they have is negated by the inductance in the wiring to the output stages. It is very dangerous to try and add small values of film caps (below 1µF in this context) as close as possible to the outputs. Oscillation is a very real possibility.

One last story to relate:
I modified a McIntosh for a recording studio at Cornell. The head of the music department and also the head of the studio came and talked to me after a few days of listening to the 'new' amp. "You added too much bass and treble boost" was their response. I showed them that it was ruler flat from 20hz~20Khz. I played them some real good recordings from Shefield Lab, and they compared the sound vs five other amps reputed to be good for studio use.

At that point they asked: "Why does so much of recorded music sound so bad?". My answer was: "Because their monitors and speakers can't tell them what is being recorded, and how bad their entire mic>mixer>recorder chain really is.

The quality of their studio recordings continued to improve as the speaker and mixer were worked on.

I suggest you spend $5~$10 on some caps and find out for yourself.

[carlmart]

Originally posted by djk
I suggest you spend $5~$10 on some caps and find out for yourself.

This is very good advice that we should all follow.

And come back here with some answers, if possible.

The figure you mentioned is also very logic, so we can still stay within good parts and not go crazy on stupid prices.

Just a warning: if the amp is well designed we should hear very little differences.

[Zero Cool]

Originally posted by Piercarlo
Other improvements in low frequency audio, especially when original capacitors were of adequate value (roughly a 1 or 2 mF for each ampere of output capability of *single* channel)
Piercarlo

Ok you are using 1m or 2m as is 10,000uf or 20,000uf? is that correct?

So a 200 watt per channel @ 8 ohm amp = 5 amps RMS, then By your statement above, you are saying that at least 50,000 to 100,000uf should be used per channel? and double that for 4 ohm ratings?

I dont know any MFG's that follow that.


Zc

[sixtek]

Zero Cool, 1mF=1000uF!

[Zero Cool]

Originally posted by djk

The small film caps on top of the big cans in the Adcom are cosmetic, they do nothing. Any effect they have is negated by the inductance in the wiring to the output stages.

It is very dangerous to try and add small values of film caps (below 1�F in this context) as close as possible to the outputs.


Oscillation is a very real possibility.

I am not sure i understand your statment the way it is worded. are you saying it is a bad idea to put bypass caps at the main filter caps, and that they would be better located as close to the output devices as possible???



Sorry for misunderstanding the comment.

Zc