Power supply bypass cap values.
- Thread starter kgoss
- Start date
adamct said:
I'm not quite sure what you mean by voltage influencing volume. A capacitor of the same capacitance will be larger in physical volume as the voltage increases.
To understand the time constant, consider the noise to be a 60Hz signal, and that the RC filter is a low pass filter. As the time constant goes up, the effective level of the filtering also increases. If you go down far enough, you may not attenuate the 60Hz signal at all!
Paul,
Maybe I misunderstood. When you wrote "A higher time constant will really shave down the noise, but at the expense of potentially dropping voltage or paying a lot for caps", I assumed the higher time constant was due to a larger capacitor, and that the downsides were reduced voltage and the higher cost of the larger caps. And then I was trying to figure out what the implications of lower voltage would be. My understanding is that lower voltage results in reduced volume, all else being equal. But maybe I misunderstood your statement, and a larger cap does not reduce voltage, you were just saying that if you keep the cap the same size and want to reduce the noise, then you have to reduce the voltage. Is that right?
And do I understand correctly that the noise is coming primarily from the power supply, which is why it is 60Hz?
And my final question is: how/why does the cap act as a low-pass filter?
Best regards,
Adam
Maybe I misunderstood. When you wrote "A higher time constant will really shave down the noise, but at the expense of potentially dropping voltage or paying a lot for caps", I assumed the higher time constant was due to a larger capacitor, and that the downsides were reduced voltage and the higher cost of the larger caps. And then I was trying to figure out what the implications of lower voltage would be. My understanding is that lower voltage results in reduced volume, all else being equal. But maybe I misunderstood your statement, and a larger cap does not reduce voltage, you were just saying that if you keep the cap the same size and want to reduce the noise, then you have to reduce the voltage. Is that right?
And do I understand correctly that the noise is coming primarily from the power supply, which is why it is 60Hz?
And my final question is: how/why does the cap act as a low-pass filter?
Best regards,
Adam
Ah, you would lose voltage by increasing the R value, not the C value.
Yes, the noise is primarily 60Hz or 120Hz from the power supply, depending on the design.
If you distill your model of a capacitor down to the resistive component, your RC filter simplifies to a voltage divider. The actual impedance to across a capacitor depends greatly on the frequency of the AC voltage, and on the capacitor itself (construction, temperature, lifetime, etc.). This value, therefore, has its own separate name (capacitive reactance).
If we look at a good quality electrolytic cap, the ESR at 120Hz may be around 1 Ohm. Having a 270 Ohm resistor in series with a ~1 Ohm resistor is a giant reduction in the AC signal, but better yet would be a 500 Ohm resistor in series with the same cap (at the expense of heat and voltage loss).
Some good reading can be found here, under Low Pass Filter.
-PB
Yes, the noise is primarily 60Hz or 120Hz from the power supply, depending on the design.
If you distill your model of a capacitor down to the resistive component, your RC filter simplifies to a voltage divider. The actual impedance to across a capacitor depends greatly on the frequency of the AC voltage, and on the capacitor itself (construction, temperature, lifetime, etc.). This value, therefore, has its own separate name (capacitive reactance).
If we look at a good quality electrolytic cap, the ESR at 120Hz may be around 1 Ohm. Having a 270 Ohm resistor in series with a ~1 Ohm resistor is a giant reduction in the AC signal, but better yet would be a 500 Ohm resistor in series with the same cap (at the expense of heat and voltage loss).
Some good reading can be found here, under Low Pass Filter.
-PB
The only caps I haven't replaced or bypassed in my S.E.X. are the following:
A --- 2x 100uf 450V
B --- 2x 220uf 250V
C --- 2x 22uf 450V (currently bypassed with .22uf film caps, but see below)
With regard to (A) and (B), I initially didn't plan on making any changes to these caps, since they aren't directly in the signal path and weren't recommended changes. But I recently replaced all 3 220uf 250V electrolytics in the power supply of my Speedball'd Crack with 200uf metalized polypropylene film caps. I can't explain it, but I feel like the change is distinctly audible, much bigger than I ever would have expected. Frankly, I was really sort of shocked, since I wasn't expecting much of an audible difference. Bass seems much stronger and resolution (if possible) seemed to get even better than it was before. Best of all, the cost was surprisingly low.
So I am now revisiting my decision to leave the 100uf and 220uf electrolytics listed above for (A) and (B) in. I've been considering replacing the 100uf electrolytics with oil-filled 100uf motor run caps, and replacing the 220uf electrolytics with 200uf metalized polypropylene film caps. I would have to mount the caps externally and run fly leads to them. The voltage ratings would be equal to (or exceed) the voltage ratings on the stock electrolytics.
My questions are as follows:
1. Putting cost aside (again, the cost is fairly minimal), is there any reason not to do this? Is there a chance the fly leads will cause problems?
2. With regard to (A), is there any reason why oil-filled motor run caps are a bad idea in this position?
3. With regard to (B) is there any reason why films caps are a bad idea in this position?
4. Again with regard to (B), are 200uf film caps sufficient, or should I use 250uf caps instead? The price difference is negligible. Or do the caps really need to be exactly 220uf?
5. With regard to (C), rather than just bypassing them (as is currently the case), I plan on replacing them with 20uf 630V film caps. Is 20uf acceptable or is the 22uf crucial?
6. Again with regard to (C), should I couple the 20uf caps with 2.2uf caps wired in parallel to get closer to 22uf in total capacitance? (I already have all of these caps, so extra expense is not an issue.)
Best regards,
Adam
A --- 2x 100uf 450V
B --- 2x 220uf 250V
C --- 2x 22uf 450V (currently bypassed with .22uf film caps, but see below)
With regard to (A) and (B), I initially didn't plan on making any changes to these caps, since they aren't directly in the signal path and weren't recommended changes. But I recently replaced all 3 220uf 250V electrolytics in the power supply of my Speedball'd Crack with 200uf metalized polypropylene film caps. I can't explain it, but I feel like the change is distinctly audible, much bigger than I ever would have expected. Frankly, I was really sort of shocked, since I wasn't expecting much of an audible difference. Bass seems much stronger and resolution (if possible) seemed to get even better than it was before. Best of all, the cost was surprisingly low.
So I am now revisiting my decision to leave the 100uf and 220uf electrolytics listed above for (A) and (B) in. I've been considering replacing the 100uf electrolytics with oil-filled 100uf motor run caps, and replacing the 220uf electrolytics with 200uf metalized polypropylene film caps. I would have to mount the caps externally and run fly leads to them. The voltage ratings would be equal to (or exceed) the voltage ratings on the stock electrolytics.
My questions are as follows:
1. Putting cost aside (again, the cost is fairly minimal), is there any reason not to do this? Is there a chance the fly leads will cause problems?
2. With regard to (A), is there any reason why oil-filled motor run caps are a bad idea in this position?
3. With regard to (B) is there any reason why films caps are a bad idea in this position?
4. Again with regard to (B), are 200uf film caps sufficient, or should I use 250uf caps instead? The price difference is negligible. Or do the caps really need to be exactly 220uf?
5. With regard to (C), rather than just bypassing them (as is currently the case), I plan on replacing them with 20uf 630V film caps. Is 20uf acceptable or is the 22uf crucial?
6. Again with regard to (C), should I couple the 20uf caps with 2.2uf caps wired in parallel to get closer to 22uf in total capacitance? (I already have all of these caps, so extra expense is not an issue.)
Best regards,
Adam
Paul Joppa
Moderator
[see my reply in the Smack thread as well]
Electrolytics are usually +/- 20%, sometimes even more. So a film within that range is fine.
Electrolytics are usually +/- 20%, sometimes even more. So a film within that range is fine.
Thanks for the recommendation. I just read a couple of threads on diyaudio about PSU Designer II, and it is fairly obvious I would be in way over my head. Even if I figured out how to work it (highly doubtful), I wouldn't be able to interpret the results of various simulations. Still, I'll bookmark it and play around sometime when I have more time and knowledge....
I'm going to order 200uf and 250uf, and experiment to see if there is a difference.
Separate question: for the 100uf 450V electrolytic caps, I was going to use Temco oil-filled motor run capacitors. These are rated for 440VAC. They don't provide a DC rating. The Wikipedia entry for film capacitors indicates that with a nominal AC rating of 350V, the DC rating should be 1000V. Elsewhere I've seen references to a rule of thumb that the DC rating is generaly 1.4x the AC rating.
Will the Temco 440VAC caps be OK to use in this position?
Separate question: for the 100uf 450V electrolytic caps, I was going to use Temco oil-filled motor run capacitors. These are rated for 440VAC. They don't provide a DC rating. The Wikipedia entry for film capacitors indicates that with a nominal AC rating of 350V, the DC rating should be 1000V. Elsewhere I've seen references to a rule of thumb that the DC rating is generaly 1.4x the AC rating.
Will the Temco 440VAC caps be OK to use in this position?
