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- Thread starter Madmike2
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DigiKey has 6,800uFd 35V NP.

I have designed a mike-amp where that was a limiting factor. The gain-set resistor should be under 100 ohms, and must vary more than 100:1, which leads to a minimum value of about 1 ohm. It also should be DC-blocked, yes has no predictable DC, so it has to be NP. Is this design "impossible"? With the 6,800uFd NP it is just-reasonable. Bass limit is higher than 20Hz at the highest gain, but that actually works out well with typical sources and studios. The lowest-gain settings are above the 100 ohm goal, but that isn't a serious problem.

>

Two of the 6,800uFd in parallel puts you in the ballpark for I think about $13.

>

Yes, be suspicious of incredible answers. Check your math. Look at the situation several different ways. 10,000uFd is about 1 ohm at the bottom of the audio band and much less than 1 ohm at higher frequencies: can you possibly be working at such insanely low impedances? Seems unlikely. If you are, can you really build it? In my super-low resistance gain-set scheme, fractional-ohm parasitics in the wiring and switch really screw-up the results.

I don't know if 8.38uFd is the right answer, but 14,000uFd is probably a wrong answer, or the right number for the wrong solution to the problem.

Remember if two approaches give two answers, at leat one is wrong, but maybe BOTH are wrong.

PRR said:DigiKey has 6,800uFd 35V NP.

Got a part number for that? I was recently looking for an alternative to a Black Gate 4,700uF 35V non-polar and I wasn't able to find anything from anyone else above 2,200uF at that voltage in a non-polar let alone 6,800.

se

jeez, I knew someone would ask.

http://digikey.com

Put "6,800" in the Search-box, you don't get any cap.

Put "6800" in the Search-box, you get many things including "200 capacitors".

Click that, highlight "6800uF" and "Bi-Polar", Apply Filters.

Bingo. Digi-Key Part Number P1151-ND

I wuz wrong. It is

Indeed, at Bi-Polar 35V, DigiKey only has up to 470uFd, P1194-ND. They are only $1.08 each and cheaper for 10, so you could parallel-up a 4,700uFd 35V NP cap for $7.72. I found one listing for Black Gate 4,700uF 35V non-polar and it shows $120. So neglecting any special magic in a Black Gate, in effect you do 20 minutes soldering for a $112 savings, $336/hour, which is a fine labor-rate.

PRR said:I wuz wrong. It isonly 6 volt, and only $2.44.

I was pretty sure there had to be something wrong. I'd looked through the catalogs of about a dozen different electrolytic cap manufacturers.

Indeed, at Bi-Polar 35V, DigiKey only has up to 470uFd, P1194-ND. They are only $1.08 each and cheaper for 10, so you could parallel-up a 4,700uFd 35V NP cap for $7.72. I found one listing for Black Gate 4,700uF 35V non-polar and it shows $120. So neglecting any special magic in a Black Gate, in effect you do 20 minutes soldering for a $112 savings, $336/hour, which is a fine labor-rate.

Oh sure. But I rather liked the idea of a single case solution.

se

PRR said:>i have never seen NP bigger then 440.

>I went to another calculater and got 8.38 uF which makes much more sense.

Yes, be suspicious of incredible answers. Check your math. Look at the situation several different ways. 10,000uFd is about 1 ohm at the bottom of the audio band and much less than 1 ohm at higher frequencies: can you possibly be working at such insanely low impedances? Seems unlikely. If you are, can you really build it? In my super-low resistance gain-set scheme, fractional-ohm parasitics in the wiring and switch really screw-up the results.

I don't know if 8.38uFd is the right answer, but 14,000uFd is probably a wrong answer, or the right number for the wrong solution to the problem.

Remember if two approaches give two answers, at leat one is wrong, but maybe BOTH are wrong.

It was to build a zobel for a passive x-over. One online calculator is on crack because the other two gave me nearly identical answers.

http://www.apicsllc.com/apics/Misc/filter2.html#TOP this one gave me the 14k cap zobel with a 8.3 ohm resistor.

http://www.mhsoft.nl/spk_calc.asp and this guy and a formula and a calculator gave me nearly identical 7.78 or 8.1 cap results with approx 8 ohm resistor.

So the top site has been removed from my bookmarks just in case

Go ahead and enter 6.5 RE and .61 Le and see what you get. Tell me that someone has a different formula and that the top link is for designing anti gravity units or something.

Madmike2 said:Go ahead and enter 6.5 RE and .61 Le and see what you get. Tell me that someone has a different formula and that the top link is for designing anti gravity units or something.

The result is 8R1 and 14.4uF. It would seem that you did not enter Le in Henries!

quasi said:2 x 10,000uF polarised capacitors in series and back to back will give you 5,000uF non-polarised etc...

Back to back means either the negative terminal or postive terminals tied together.

Cheers

There is a very interesting articly by W Jung on this. He measures THD of the caps he shows that for a back to back pair polerising the connected points reduces the THD.

This may not be possible in a crossover, but it is generally a good idea.

Brian

bscally said:There is a very interesting articly by W Jung on this. He measures THD of the caps he shows that for a back to back pair polerising the connected points reduces the THD.

What do you mean by poliarizing the connected points? You mean apply a bias voltage at that point?

se

I never saw a ~8Ω speaker with

One

> BAHH ! ! How am i supposed to know that?

It clearly says "Henries" next to the box.

In fact if you read the Instructions instead of skipping to the pictures, it says:

I don't trust a programmer to write the right units, but I will try it his way and see what I get.

>

I agree that, since this is clearly a SPEAKER calculator and mentions 4 ohm speakers, and uses mH elsewhere, it would be more obvious and convenient, less error-prone, to use mH here. But I didn't write it, I didn't pay for it to be written, I'm glad it was written even if it isn't "exactly how I would like it".

Also: it is JavaScript and you can modify it yourself. (For personal fun only: don't share your hack of ApICS LLC's work without permission!) View Source gets everything including the math, save it to your machine. Find the Zobel script, the equations are fairly clear, including a number with a lot of zeros in it. Take 3 zeros out of that,

An externally hosted image should be here but it was not working when we last tested it.

>

I have a room-acoustic calculator, scaled to a specific old concert hall, with input in milli-furlongs, output in micro-fortnights.

PRR said:>enter 6.5 RE and .61 Le and see what you get

I never saw a ~8© speaker with0.6 Henriesof inductance.

OnemilliHenry is more typical. Maybe you mean 0.61 mH? Or 0.000,61 H?

Nice! abuse the new guy And i clearly typed .61 Le which anyone would know is milli I think.

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