Hate all those electrolytic bi pass caps

[Bean]

We all know that those cheap electrolyic coupling caps are no good. Most of them are ranging 10u to 47u in value. It is hard to replace them with the same value with MPK caps becasue there will not be enough space. Can anyone help let me know what is the min value if I like to replace those elect cap with MKP...is 1u enough?

Thanks
Bean

 

[sam9]

what is the min value if I like to replace those elect cap with MKP...is 1u enough?

First it depends on what you are "decoupling". If you mean power supply rails I think you are in for disapointment. For opamps just follow the info on the datasheets. Usually 10uF is the minimum, less is not enough. For power amps 100uF is more like the minimum.

If you mean input and output caps, there are a few 10uF and greater caps available, but not MKT (I think). You can use the big ones that are typically used in speaker crossovers such as Solen, Auricap, etc. Check www.Madisound.com or www.parts.express.com for a selection. However, they are rather large, but I have seen some people use them.

 

[DSP_Geek]

And, as mentioned on other forums here, sometimes a cheap electrolytic is exactly what you want for decoupling, as low-ESR parts might resonate with the series inductance of the power trace.


Francois.

 

[Magura]

For power amps the rule of thumb AFAIK is 1000/1, so if you have 47.000uF PSU caps, you will need 47uF bypass caps.

Magura

 

[Jocko Homo]

It depends on what you are trying to bypass, and why. You have not given us an application, frequency range that it has to be effective over, or anything. Just that you do not like cheap electrolytics.

Tell us something that we don't already know.

Francois is right........and depending on whether or not this is for a 3-terminal regulator........you may get the same answer for a slightly different reason.

Jocko

 

[sam9]

You probably don't like ceramic caps either, yet for some purposes they are exactly what you need. There are far too many urban legends floating around about caps.

 

[macboy]

First it depends on what you are "decoupling".

From what I read, the word "decoupling" wasn't mentioned at all. Beam was asking about coupling caps, not decoupling caps.

 

[Mr Evil]

From what I read, the word "decoupling" wasn't mentioned at all. Beam was asking about coupling caps, not decoupling caps.

He does say "bypass" though, which is often used interchangeably with "decoupling" (although they don't mean quite the same thing).

 

[AndrewT]

input coupling caps

Hi,
for my two pence worth, I think he is asking about input coupling caps.
For low bass he needs an RC time constant of between 80mS and 200mS. But even a big film cap here will need a bypass (22nF to 220nF) to help the high audio frequencies through without phase errors.
Bipolar or back to back electrolytics also need a bypass but of a higher value maybe 470nF to 1.5uF.
MKP (polypropylene) is probably the best at an affordable price.

 

[sam9]

He does say "bypass" though, which is often used interchangeably with "decoupling" (although they don't mean quite the same thing).

That's why I asked. The terms are interchanged more or less loosey-goosey. I would rather get clarifaction babble on about the wrong thing.

 

[sam9]

Bipolar or back to back electrolytics also need a bypass but of a higher value maybe 470nF to 1.5uF.

I've been using 47uF bi-polars at the input as the most sensible thing wityh regard to space. Although I use either Panasonic or Nichicon, if I start suffering from audiohypochondia a BG will fit there as well.

I don't detect any HF rolloff in the audio range. The choice of 47uF appears to be near optimum with regard to reducing LF noise and preserving LF flatness to 20Hz. That is to say there is a clear improvement over the more common value of 10uF (even when comparred to film types) but increasing it to 100uF doesn't do anything I can measure unambiguously.

I actually connected a huge 20uF Solen in this position once; there may have been some advantage but I would have had to increase the area of the board by about a third!

Query: these high value film caps are typically rated for very high voltage as well. Could a 16V 50uF film cap be made and how big would it be? If one already exists - where? If there are small emough to be practicle I could have my audio-cake and eat it, too.

 

[Bean]

Hi Guys
I am new to this forum. I left this question coupke of days ago and forgot about that. When I came back there are so many replies. Thank you so much guys.

I am sorry that I didn't make myself clear enough. What I was refereing to those audio opamp circuits, where they have 10u or 47u caps in the input or output positions to block off DC but letting AC signal to pass to the next stage.

Thanks Sam9 and AndrewT who gave me some good answer. (Thanks to Macboy, Mr Evil, Jocko, Magura, DSP_Geek...)

Most of the coupling cap used in Tube circuits are ranging from 0.1u to 0.47u..to the extreme maybe up to 1u. It was just the other day I was trying to modify my DAC which contain many inexpensive electrolyic 10u caps in the input and output of the opamp.
First I bypass them with a MKP 0.047u just for curiousity reason, I found the HF sound much better. I was surprised how just a few caps made such a different. I haven't even changed the opamp yet...but the unit did sound better. Then I took all the electrolytic (bi pass, not the supply decoupling)cap out and replaced each with a 1u 50V MKP, and paralleled it with a 4.7u 50V metal polyester cap (total value was then 5.7u). The unit sound very very different in high and low freq...... (due to the much bigger size of the MKP & film caps, I have to use leads to connect them outside the board).
I know if I just say it sound very nice now...many will say that I had just fallen into one of those myth about expensive AUDIO caps....However, the unit does sound much brilliant and tighter. The MKP I used was only US$0.66 each, and the 4.7u metal poly was US$0.95 each.

My question is what is the sensible way to replace those electrolytic cap. I will not pay for those inflated price for AUDIO grade cap. Sam9 mention about Black Gate....I already found them quite expensive. Solon is a bit more reasonable but they are huge.

MKP or flim caps usually start from 50V, and are so much bigger than electrolytic type. That is why I ask what should be the optimum value as it is hard to replace the electrolytic with the same capacitance value.
I don't have instructment to really measure the low LF roll off.

Bean

 

[Jocko Homo]

You don't have to measure the LF roll-off, you can calculate it. At least close enough for your use.

Jocko

 

[sam9]

As far as roll off goes, unless you are a fanatic about pipe organs and big subwoofers, I think when I looked into this for myself 4.7uF at the input is big enough. At any rate it is a reasonable value. (I'm nutso about pipe organs and deep bass so I tend to go overboard.)

There other thing that motivates me noise floor. Below approx 250-500Hz the value of the input cap can dominate the noise floor. The bigger the cap the lower the floor. At 4.7uF the effect is probably not audible for 99.5% of the people, (but I'm nutso on the subject.)

Here are some digikey (www.digikey.com) part numbers that may be a reasonable compromise for you -- they seem to be just about the smallest film caps avaiable at these values (4.7 -- 10uF)

EF1106-ND
P10987-ND
495-1131-ND
P4675-ND (use 4 or 5 in parrallel)

I appreciate your frustration on this problem -- I've been through the options myself and find all choices are compromises.

The one remaining "solution" is no cap at all. It has one big problem -- if the signal source has a failure that applies DC to you input, considerable damage can result. Many people advocate this but I'm not brave enough -- I've never had this kind of failure but feel that once I expose myself to it, it will be nearly certain to to happen.