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Old 21st March 2005, 09:54 PM   #1
akunec is offline akunec  Canada
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Default Supply capacitance reduces hf performance? Is that true?

Is it true that using more capacitance on the supply rails can negatively affect high frequency performance? I find it really hard to believe but I heard it a few times on this forum. Can someone offer a logical explanation?

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AK
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Old 22nd March 2005, 01:22 PM   #2
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There are some who think that small capacitance on the power supply rails results in best sound from the chip amps. Keep reading these forums and you will see many, even stranger things...

Large electrolytic capacitors (thousands of microfarads) have relatively low Q at high frequencies. This means the ESR is relatively high compared to the capacitive reactance. The normal procedure is to connect small capacitor(s) (a few 10s of microfarads, and even 1 or 2 uF) with high Q in parallel with the large capacitor with low Q. This makes the overall Q higher and allows the power supply filter to perform better at high frequencies.

Using only small capacitors in the power supply filter/storage will allow the voltage across the amplifier to drop during loud signals (typically low frequencies) that require maximum output from the amplifier. The reduced voltage powering the amp will limit the power available to the load (your speaker). This will result in clipping or other behavior which ultimately results in distortion.

Some people seem to like that distortion and use only small capacitors in the power supply.

I_F
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Old 22nd March 2005, 01:38 PM   #3
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" ... some people like this distortion ... " - many people can't live without distortion ( sometimes they are very well known ).
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Old 22nd March 2005, 01:52 PM   #4
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Default Re: Supply capacitance reduces hf performance? Is that true?

Quote:
Originally posted by akunec
Is it true that using more capacitance on the supply rails can negatively affect high frequency performance? I find it really hard to believe but I heard it a few times on this forum. Can someone offer a logical explanation?

Thanks
AK
i think that someone pulled that thesis out of thin air -- which seems to be standard operating procedure (SOP) for some DIYrs'

the chipamps are generally run at high gain (A=20) and are quite sucsceptible to small hum (60, 120, 180 240Hz at US line frequencies) making its way into the circuitry -- this shows up as increased distortion at the fundamental and harmonics of the line frequency -- 10,000uF as recommended by National Semi is fine -- careful attention to layout is also important.

i can demonstrate the way in which ground loops, inadvertent power cable placement etc. measurable affect distortion.
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Old 22nd March 2005, 02:14 PM   #5
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Quote:
Is it true that using more capacitance on the supply rails can negatively affect high frequency performance? I find it really hard to believe but I heard it
The effect is only one of perception and certainly not measurable. Not because it does not exist, but because we haven't a faintest idea how subjective perception works and what needs to be measured.

Of course, all the subjective benefits of better midrange and high frequency clarity with low capacitance are only possible with sympathetic speakers. I used to be happy with 1000uF driving Lowthers but none of my current speakers are listenable at all with such PS. Clarity? Yes, but with an unhealthy dose of anaemia, especially in the bass.

Has not this horse been beaten to death already? There are no simple explanations and practically no measurements to support half of what we hear. Is this a huge problem?
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Old 22nd March 2005, 07:27 PM   #6
GregGC is offline GregGC  Canada
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Last night I was playing with the GC (lm3875 NITGC).

First changed the PS from dual (220VA) transformer and 2 Bridges per channel to Single transformer (220VA center tap) and a single Bridge (400V/40A). I didn't hear any difference, so I left it that way. Now I have a transformer and extra bridge for another one. Then I decided to double the 1000uF PS caps. The base increased substantially and subjectively the mids and highs became more recessed. It changed the whole picture. It has more base but it doesn't sound as lively. I know that it's because of lack of low frequencies. Because the PS caps are in ser of the speaker I decided to calculate the F-3dB for 1000uf and 4 Ohm load (my speakers are rated at 6 Ohms). Well it's 40 Hz. No wonder it has less base with 1000uF. I think that the same effect could be achieved by decreasing the input cap and keeping the 2000uF in the PS. So, to make sure 20 Hz are not attenuated at all by the GC you need to have at least 2000uF per rail in the PS, just because all speakers dip below 8 ohms in the LF area.
I hope I haven't missed something.
/Greg
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Old 22nd March 2005, 07:44 PM   #7
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Quote:
Originally posted by Upupa Epops
" ... some people like this distortion ... " - many people can't live without distortion ( sometimes they are very well known ).
nice



Quote:
Originally posted by akunec
I find it really hard to believe but I heard it a few times on this forum.
You will find quite a bit of stuff that is hard to believe on this forum.. not necessarily a bad thing, it does make you think and discuss (this picture is the best).
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Old 22nd March 2005, 08:03 PM   #8
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Quote:
Originally posted by GregGC
[snip] The base increased substantially and subjectively the mids and highs became more recessed. It changed the whole picture. It has more base but it doesn't sound as lively. [snip]/Greg

Greg,

This is quite normal. Whenever you get more bass, it is perceived as if there is less mid/high. It is all relative. Conversely, if you change to a more efficient tweeter, you perceive as if you have less bass.

This is perception-related, well documented, no secret.

So, if you decrease you ps capacitance, you may perceive relatively higher levels of mid/high because the bass level relative to the mid/high falls. But saying that less ps cap improves mid/high is just deceiving yourself. You get the same effect with a resistor in series with your woofer, or by using a cheaper woofer, and I don't think you would call that an improvement (unless you have unbalanced speakers to begin with).

In fact, playing with the ps cap in this way seems to me like a rather awkward way of re-introducing tone controls...

Jan Didden
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Old 22nd March 2005, 08:07 PM   #9
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Quote:
Originally posted by analog_sa


The effect is only one of perception and certainly not measurable. Not because it does not exist, but because we haven't a faintest idea how subjective perception works and what needs to be measured.[snip]

... but we DO know how it works in these circumstances. See my previous post. Not all people do know, surely, but a little research will show that the knowledge, experimentattion and etc IS available.

Jan Didden
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Old 22nd March 2005, 08:12 PM   #10
GregGC is offline GregGC  Canada
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Quote:
Originally posted by janneman



Greg,

This is quite normal. Whenever you get more bass, it is perceived as if there is less mid/high. It is all relative. Conversely, if you change to a more efficient tweeter, you perceive as if you have less bass.

This is perception-related, well documented, no secret.

So, if you decrease you ps capacitance, you may perceive relatively higher levels of mid/high because the bass level relative to the mid/high falls. But saying that less ps cap improves mid/high is just deceiving yourself. You get the same effect with a resistor in series with your woofer, or by using a cheaper woofer, and I don't think you would call that an improvement (unless you have unbalanced speakers to begin with).

In fact, playing with the ps cap in this way seems to me like a rather awkward way of re-introducing tone controls...

Jan Didden

Quote:
Originally posted by janneman



... but we DO know how it works in these circumstances. See my previous post. Not all people do know, surely, but a little research will show that the knowledge, experimentattion and etc IS available.

Jan Didden


Thanks Jan.
That's exactly how I see it too.
/Greg
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