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
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
akunec said:
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.
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?
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
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
Upupa Epops said:" ... some people like this distortion ... " - many people can't live without distortion( sometimes they are very well known
nice
akunec said:
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).
GregGC said:
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
janneman said:
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
janneman said:
Thanks Jan.
That's exactly how I see it too.
/Greg
I_Forgot said:
Not only that, but because there is riple on the supply (quite a lot with low cap values), with high output levels the amp output voltage will be modulated by double the mains frequency (assuming two-phase rectification), which IS audible and measurable. Again, all well established, no secret. Now, if your speaker lacks robust bass, this may actually give the perception of more bass. Something like turning your amp into a perceptual coder and saving on caps at the same time...
. What was it again that the "fi" in "hi-fi" stood for??Jan Didden
If it would be true that the power supply ripple will modulate the output, then this would show up in the simple distorsion measurements. Why it is that I did not see anyhting of it, with 2200 uF/rail, 5 ohm? What I have seen was diminishingly low distorsion right up to the clipping. Also, I would challenge anybody here, to demonstrate the direct bass - enhancing effect of a bigger cap. Also that should have shown up in my measurements, but I have only seen the high pass effect of the input cap, with the 20 kohm input resistance, and it was ~ 3-5 Hz, now I don't remeber that well.
If we increase the power supply impedance, we only trigger the amp to correct it's own dynamic impedance. Until it runs out of headroom, it will do it. So, with the lower power supply caps, what is really lost is the dynamic range.
george
If we increase the power supply impedance, we only trigger the amp to correct it's own dynamic impedance. Until it runs out of headroom, it will do it. So, with the lower power supply caps, what is really lost is the dynamic range.
george
Joseph K said:
Certainly.
Dynamic range can't be the same.
In my case, with my Epos 11 speakers and 1000uf capacitance I didn't have lack of bass at all.
What I did have was untight bass, undefined, it was like listening to a bad cd player with a strong bass recording.
Untight, unprecise, slow bass always kills midband and treble and this was no exception.
Although very detailed with simple acoustic music, as soon as more instruments came in some sounds got masked behind the mix, unperceptible.
Summarizing: the amp didn't drive the speakers properly.
The results with the low capacitance PSU are very different from speaker to speaker.
With high sensitive (and easy load) speakers the results may naturally be quite different.
But they are never correct.
Welcome back, George.
carlosfm said:
The ultimate performance of the amp chip is defined by the designers who assume and use stable, well bypassed, DC supplies. All bets are off if you don't give the chip a stable, well bypassed DC supply. Small power supply capacitance leads to high supply impedance, which leads to high output impedance. The results are unpredictable.
Would you put diesel fuel in your car's gasoline engine, then complain that the performance sucks?
I_F
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