[Grif]

Ummm...sorry for the stupid question, but how do you 'bypass' an electolytic with a film cap....I've seen them under the board directly beneath the electrolytic...but I thought that was in parallel

 

[grasshpr]

The output from the electrolytic caps are fed directly into the film caps (hence you see the film caps put in parallel with the electro caps). This procedure attempts to do filtering of noisy signals arising from the output of the electro caps. Its the same idea as why we need bypass capacitors.

 

[nikongod]

Quote:

Originally Posted by Grif Ummm...sorry for the stupid question, but how do you 'bypass' an electolytic with a film cap....I've seen them under the board directly beneath the electrolytic...but I thought that was in parallel

it is somewhat of a "misnomer." as you said, it refers to putting an electrolytic cap in parallel with a film cap in hopes of getting closer to the film cap's propertys.

 

[PinkFloyd]

I believe the reason you parallel film caps with electrolytics is to compensate for the rising impedance of electrolytics with frequency and to minimise hf and rf noise. The impedance of all electrolytics rises very sharply with frequency so there is a beneficial effect in fitting film caps in parallel with the 'lytics.

I've just paralleled 220pF polypropylene caps and 220nF polyester caps onto the 100uF electrolytics in my WNA amp and the sound is noticeably "smoother" I'm not sure if I prefer the sound with paralleled film caps but they certainly do make a difference, of that there's no doubt.

If there had been enough space on the board I would probably have paralleled the electrolytics with a 1uF polypropylene cap and a 100pF silver Mica cap... there wasn't enough room so I ended up using 220pF polyprop and 220nF polyester and they make an audible difference to the sound, I may piggyback a silver Mica on as well but I'm kinda limited to using small value caps (and physically small caps)

A pic of my paralleled caps

 

[Sinbios]

So, what uF film cap should I use to replace a 220uF electrolytic?

 

[dsavitsk]

Quote:

Originally Posted by Sinbios So, what uF film cap should I use to replace a 220uF electrolytic?

I think you missed the point here. Use both.

 

[Sinbios]

Quote:

Originally Posted by dsavitsk I think you missed the point here. Use both.

I probably did, but I thought the point was to get rid of the electrolytics?

 

[drewd]

Quote:

Originally Posted by Sinbios I probably did, but I thought the point was to get rid of the electrolytics?

Right, but you need to replace those electrolytics with equivalent value replacements. So, if you're replacing a 220uF electrolytic, you'll need to come up with somewhere around 220uF of whatever the replacement is.

Pinkie had a good point about bypassing the electrolytics with film caps - I believe that I've achieved a very significant portion of the film cap's audio qualities with a good quality electrolytic bypassed with a good quality film cap. It's also a lot cheaper and/or easier than trying to find high value film caps.

Of course, part of the joy of building it for yourself is getting to try out whatever technique you want to use!

-Drew

 

[dsavitsk]

Quote:

Originally Posted by Sinbios I probably did, but I thought the point was to get rid of the electrolytics?

Look at posts 5 and 6 in this thread, and follow the link.

 

[PinkFloyd]

Quote:

Originally Posted by Sinbios So, what uF film cap should I use to replace a 220uF electrolytic?

You don't replace the electrolytic (specially a 220uF) unless you want to replace that tiny electrolytic with a film cap costing $$$$ which possesses the dimensions of a house brick?

By paralleling film caps onto the electrolytic you are effectively compensating for the electrolytic caps shortfalls and are overriding them by adding the qualities of the film cap into the equation.

Electrolytics have a tendancy to rise in impedance with frequency whereas film caps are a lot more stable.... paralleling a film cap onto a electrolytic tends to reduce the impedance and with a 220uF electrolytic I'd be inclined to parallel say, a 220pF silver Mica and a 2.2uF polyprop which should cover the entire frequency range nicely.

Mike.

 

[robzy]

Correct me if im wrong, but didnt you replace 100uf with a 4.7uf cap? Isnt that bad?

Rob.

 

[aerius]

Quote:

Originally Posted by drewd Pinkie had a good point about bypassing the electrolytics with film caps - I believe that I've achieved a very significant portion of the film cap's audio qualities with a good quality electrolytic bypassed with a good quality film cap. It's also a lot cheaper and/or easier than trying to find high value film caps.

It gets you closer to the film sound and as mentioned, it's a heck of a lot more practical and cheaper. But in my amp at least there's still a significant difference between bypassed electrolytics and film caps only, same with my CD player. Results will vary of course depending on the circuit.

Quote:

Originally Posted by robzy Correct me if im wrong, but didnt you replace 100uf with a 4.7uf cap? Isnt that bad?

Indeed I did. But I took a good look at the circuit, traced it out, and checked out the appropriate spec sheets to see if a 100uF cap was really needed or if they just slapped it in there because it was convenient. I determined that it was the latter, and that anything bigger than 2uF or so would do the job.

 

[__redruM]

Just for the record, does bypass mean replace entirely (even with a lower value), or run in parallel with a better cap.

 

[steinchen]

it means to run a better cap in parallel

 

[grasshpr]

Here's a little link showing the benefits of bypass caps.

http://www.seattlerobotics.org/encod...97/basics.html