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Old 10th August 2007, 02:28 AM   #1
jarthel is offline jarthel  Australia
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Default paralleling film caps with electrolytic caps

I've seen various website modding their gear where they parallel 0.1uF metallized polypro with the electrolytic cap.

I understand that this is called bypassing.

But when I searched the net, it seems bypassing is commonly done on the supply pins of the chip (opamps for example).

Is there any value to paralleling film caps to electrolytic caps? If yes, is film caps the best type to use?

Thank you very much

ps. I'm not sure if this fits with power supply design but in most instances, the power supply caps are the ones that are bypassed.
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Old 10th August 2007, 07:31 PM   #2
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Those high speed bypasses are normally to be used at the point of load, that is at the chip or output transistors. If you bypass the 'lytics in the PSU the way to the point of load still has wiring inductance which degrades the effect of the bypasses. That inductance is also the reason why bypassing both the supplies and the point-of-load is a no-go, this forms a C-L-C circuit that can easily ring at RF frequencies. If bypassing is used at the PSU to filter incoming RF noise, there should be some resistance on the way to the load (ferrite bead, e.g.) is needed when the load is also bypassed. Any good film capacitor will do, ceramic capacitors will also be ok. The return point (GND connection) is critical, when the GND gets disturbed by high/fast current pulses from the bypasses not much is to be gained. One will have to measure things (a lot of things) to make sure that bypassing actually impoves perfomance, there a quite a few gotchas...

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Old 11th August 2007, 03:25 AM   #3
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the issue is that there is significant ESR and ESL in the larger caps, they make poor caps at higher frequencies. So some designs call for the addition of film caps, which act like caps to a higher frequency.

in industry, ceramic caps are used for the smaller size. because the ceramic caps are not in the signal path, there is less concern about possible issues.
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Old 11th August 2007, 08:45 AM   #4
jarthel is offline jarthel  Australia
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I have read that bypass caps should/must have higher voltage that the electrolytic. true?
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Old 11th August 2007, 10:28 AM   #5
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by jarthel
I have read that bypass caps should/must have higher voltage that the electrolytic. true?
no,
each cap must be rated to exceed the highest operational voltage.
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Old 11th August 2007, 02:58 PM   #6
Eva is offline Eva  Spain
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Quote:
Originally posted by theChris
the issue is that there is significant ESR and ESL in the larger caps, they make poor caps at higher frequencies. So some designs call for the addition of film caps, which act like caps to a higher frequency.

in industry, ceramic caps are used for the smaller size. because the ceramic caps are not in the signal path, there is less concern about possible issues.
This is a myth. Larger caps exhibit lower ESR and overal lower impedance. Also, electrolytic capacitors are much less inductive that things such as the PCB traces connecting them (unless power planes are employed).

Furthermore, parallel capacitors usually resonate with parasitistic inductances leading to increased impedance.
There are only a few situations in which paralleling capacitors is actually advantageous.
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Old 11th August 2007, 03:27 PM   #7
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The concept is simple, but the reality is anything but. Yes, electrolytics are far better than their reputation, but at some tens of kilohertz, they can become more inductive than capacitive. I have some nice manual capacitance and inductance bridges, but at work we have a fancy HP LCR meter that covers 20hz to 2Mhz, a difficult range for most of the equipment a hobbiest can get his or her hands on. It's also fast, so I ran measurements on a bunch of different caps and put the results in an Excel spreadsheet. If I did the math right, you'll see ESR in ohms vs frequency for each cap. Note the log scale. If you were going to parallel caps, you'd need to convert the series model to a parallel model, but it's still interesting to see the differences in ESR between say, a big tubular polypropylene, a dipped tantalum, and a low value mil spec that's either polypropylene or teflon- not sure which.

Excel Capacitor Comparison (.xls file)
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Old 12th August 2007, 03:04 AM   #8
jarthel is offline jarthel  Australia
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thank you all for the help
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Old 12th August 2007, 04:23 AM   #9
john65b is offline john65b  United States
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Quote:
This is a myth. Larger caps exhibit lower ESR and overal lower impedance. Also, electrolytic capacitors are much less inductive that things such as the PCB traces connecting them (unless power planes are employed).
If you look at the 47Lab Gaincard, there are no bypass caps. At $3000, you would think if it really would matter, they would have added a bypass cap....

I agree, most time, bypassing large caps with smaller caps at 1/100 the bigger cap is a myth.

If you can hear the difference with the bypass and it is better, then go for it. Otherwise leave 'em out...
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Old 13th August 2007, 01:57 PM   #10
KSTR is offline KSTR  Germany
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At $3000, one would also expect to get a decent amplifier... the Gaincard doesn't seem to be one (I'd say it rather is a money making machine)...
http://stereophile.com/solidpoweramp...47/index4.html
I guess that bypassing would not have improved it's performance too much, as it is limited by other design factors.

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Old 13th August 2007, 03:44 PM   #11
AndrewT is online now AndrewT  Scotland
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Hi,
I didn't realise that 47 Lab's gain card was that bad.
They only ask for 24W into 8ohms and it can't even manage that easy bit of the spec.
Then the output falls 2.0dbV into 4r0. Terrible.
Talk about brass neck and profit margin.
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Old 13th August 2007, 05:16 PM   #12
john65b is offline john65b  United States
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Now I would never pay $3000 for an original Gaincard, but if you look at all those clones, they all have bypasses, snubbers etc added to the original design. No more 9 components.

I have alway heard that Gainclones (at 1/50 the cost) don't sound as good as the original Gaincard, but could it be that it is those that acually own the Original are the poor blokes that are saying this???

I like Gain Clones. For $60 in parts, they sound great. I just don't expect or claim it to be one of the best sounding amps on the planet, like the Gaincard.
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Old 12th September 2007, 01:04 PM   #13
klitgt is offline klitgt  Denmark
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Quote:
Originally posted by theChris
the issue is that there is significant ESR and ESL in the larger caps, they make poor caps at higher frequencies. So some designs call for the addition of film caps, which act like caps to a higher frequency.

in industry, ceramic caps are used for the smaller size. because the ceramic caps are not in the signal path, there is less concern about possible issues.

Some claim that the power supply is a part of the signal path and therefore one would believe that caps in the power supply should be as "audiophile" as possible.

In my SiriuS power amp (now marketed under the name of GamuT) the power supply elcaps are bypassed by film caps.
I understand that copper foil paper in oil signal capacitors sound better as signal coupling caps than any type of film cap. Does this predict that copper foil paper in oil signal capacitors would be good for bypassing power supply elcaps?
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Old 12th September 2007, 02:11 PM   #14
Eva is offline Eva  Spain
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To demonstrate that high inductance in modern electrolytic capacitos is a plain myth, check the following datasheet:

http://www.epcos.com/inf/20/30/db/aec_07/B43501.pdf

Self inductance is rated at approx 20nH (0.125 ohms reactance at 1Mhz) for those medium sized high voltage electrolytics. Big film capacitors exhibit similar inductance values. Electrolytics should be considered good up to 1Mhz.

Contrary to popular wisdom, the main purpose of local bypass near high speed ICs is to compensate for PCB inductance (at frequencies considerably higher than 1Mhz). Remember that wire self inductance figures are in the 5nH per cm range (so one of those medium sized electrolytics is not more inducive than two inches of plain wire or PCB track).
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Old 12th September 2007, 04:03 PM   #15
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"....the Gaincard doesn't seem to be one (I'd say it rather is a money making machine)....."

Yes.....business ia a money making machine !

This fact has bugged me for a long time. I've been on both sides of the business. Manufacturing and buying.
I've also been lectured ( in a university ) a lot on this by professionals from Switzerland.

My conclusion is that one cannot complain about costing . You aren't being forced to buy anything and it isn't essential for living comfortably. The maker might have spent a lot of man hours perfecting 'their implementation' and might want to get back their 'man hours money' over a 'limited sales' that they project.
If you like what they make then you might need to cough up for their product. If not , there are several other products to choose from.

You will never get to know how much they spent on developing their product and so it's very hard for outsiders to determine what the product really should cost . It can never be just the cost of parts. In fact that is just a fraction of the overall cost.

That's what DIY is all about . You spend a pittance ( not always !)on parts and don't have to plonk down on your time ( far more expensive ) and help ( others time and hence money ) that is not visible at all. But it does make us happy and think we got it done cheap !

So maybe we needn't clobber the makers for thier prices . Just clone them and be happy ! You don't have to buy it of course.
Cheers.
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Old 12th September 2007, 05:29 PM   #16
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Eva, I've been measuring various caps on a $16K Agilent LCR meter that is accurate to 2Mhz. Though I believe electrolytics have gotten a bad rap, they aren't that good either. Smallish low ESR electrolytics that I'd expect to be quite good are still all done in terms of capacitance and ESR well below 100khz. Even a similar value OS-CON is in trouble at 200khz. Larger caps will lose effectiveness at surprisingly low frequencies like 5 or 10khz. Yes, they still display reasonably low impedance and will shunt high frequencies to ground, but their ability to store and return energy is greatly diminished. IOW, they act as a low value resistor with a phase angle closer to -45 or even 0, than to -90 degrees. I don't believe in bypassing filter caps at the cap- this is far better done at the circuit where the cap can do some good, or at the bridge, where the RF can be shunted, but the electrolytic can't do the job alone. FWIW, though I've been measuring caps for decades, I've changed my thinking on this matter greatly since having access to a meter that can do both high frequencies and low impedances at the same time. Traditional bridges, though I love 'em, aren't up to the task.

edit/addition- Notice how in that cap data sheet they concentrate on impedance and ESR, but make no mention of effective capacitance at high frequencies- nobody does.
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Old 12th September 2007, 06:35 PM   #17
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I ran some equations here:

http://www.diyaudio.com/forums/showt...83#post1229783

which show that bypassing isn't a matter of spraying random smaller capacitors on top of big ones. You do want to know the parasitic values for the large cap, and low-ESR capacitors paradoxically can get you in trouble if you bypass them with too small a film cap.

The math gets a bit involved, but the end result is simple: find the equivalent series inductance (ESL) and equivalent series resistance (ESR) of the large capacitor and apply the equation

Cb = ESL/(ESR*ESR)

to find the optimum value of Cb, the bypass capacitor.


Some folks avoid the whole mess by running a few smaller electrolytics in parallel instead of a single large one, which increases capacitance while also decreasing ESR and ESL. It's a neat trick if you have the room.
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Old 12th September 2007, 09:52 PM   #18
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Quote:
Originally posted by Conrad Hoffman
Eva, I've been measuring various caps on a $16K Agilent LCR meter that is accurate to 2Mhz. Though I believe electrolytics have gotten a bad rap, they aren't that good either. Smallish low ESR electrolytics that I'd expect to be quite good are still all done in terms of capacitance and ESR well below 100khz. Even a similar value OS-CON is in trouble at 200khz. Larger caps will lose effectiveness at surprisingly low frequencies like 5 or 10khz. Yes, they still display reasonably low impedance and will shunt high frequencies to ground, but their ability to store and return energy is greatly diminished. IOW, they act as a low value resistor with a phase angle closer to -45 or even 0, than to -90 degrees.
We are more or less telling the same story. People must understand that electrolytic capacitors are *not* inductive, which would result in phase shift becoming +90 (positive) at very low frequencies rather than remaining at -45 or close to 0 as you mention, and in impedance rising at 6dB/oct. However, as frequency increases, capacitive reactance becomes negligible and ESR dominates progressively resulting in a phase shift that sweeps to 0 and an impedance that tends to become flat. Thus electrolytic capacitors behave as very low value resistors at least up to a few Mhz, *not* as inductors.
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Old 12th September 2007, 10:40 PM   #19
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Agreed- I think the idea may come from the fact that most capacitors are wound, but the geometry is not such as to form an inductor. More important is keeping the leads short.

I don't have the answer, but wonder what it would take in terms of caps and their type, to create a high value (100uF or so) composite cap that would maintain all it's parameters to 100khz or so? That means it would show close to full capacitance value and close to -90 degrees over the full range. I did a brief paper exercise with some known caps, and it's harder than one might think!
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Old 13th September 2007, 11:50 AM   #20
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...simply measuring.. that's it. Much better than guessing!


Conrad,
I would be quite happy to get your paper exercise!
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Old 13th September 2007, 12:28 PM   #21
AndrewT is online now AndrewT  Scotland
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Hi,
many of the cap manufacturers publish impedance Vs frequency plots for their electrolytics.
I have seen many/all that show a flattening out of the impedance as frequency rises and then rising again further up the frequency range.
Some have a pronounced flattening over a wide range of frequency. I guess that is telling me they have become resistive in character.
I also note that these minimum impedances are generally only a decade or so above audio bandwidth, i.e. ~<=200kHz.

I cannot recall a bigger (non speciallised) cap, >=2200uF, that had the minimum into a few Mhz.

What happens with advertised as "low ESR" or "High frequency" types at these higher frequencies?
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Old 13th September 2007, 03:25 PM   #22
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As far as I can tell, low ESR caps intended for switchers have, in fact, lower ESR, and thus, higher ripple current ratings, but they do not have substantially better high frequency performance so far as remaining a capacitor. They still become resistive at sub Mhz frequencies. Some of my favorite caps are the Illinois KXM series, and the OS-CONs, but though they have high ripple ratings, they follow the same pattern as the higher ESR caps. As a side note, I haven't seen any really low ESR high voltage electrolytic caps, making high frequency switchers for tube amps more difficult than one might first think. Illinois does make some really fantastic high voltage polypropylenes, up to several kV, but they're not cheap or easy to get. I've yet to find my holy grail of a 100uF cap that shows better than -80 degrees at 100khz.

edit/addition- The impedances are so low that what I seek may be impossible to realize or impossible to measure even if it did exist. Or maybe they all do it, but I can't prove it- gotta think about this some more.
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Old 13th September 2007, 03:57 PM   #23
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Best capacitors I've ever seen for high voltage switchers are the power rings, made by Sprague-Barre: http://www.sbelectronics.com/powerring/index.htm

I used a pair of 500 uF versions this past year in a compact 20 kW boost converter for a hybrid racecar...certainly overkill for any (most?) audio application, but they performed exceptionally well at high frequencies. I don't remember how they measured, but if I can dig up a datasheet later on I'll give some specs.

The downside? They start at around $400 a pop, and they're not exactly small for an amp.
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Old 13th September 2007, 04:33 PM   #24
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Hi,
how woulld polypropylene motor run caps do in trying to meet that 100kHz target?
I've seen upto 40uF @ 400Vdc at nearly sensible prices.
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Old 13th September 2007, 06:44 PM   #25
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Great idea! I actually have one and will test it, though it may be a few days before I can get to it

FWIW, my reality check is to parallel a bunch of film caps together, just to confirm that whatever bridge, vector analyzer, or LCR meter I'm using, is telling me the truth. Invariably, the parallel bunch gives the expected low DF, so the problem lies with the electrolytics, not the measurements, at least not in the 100uF regime. Above that, it probably gets more difficult.
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