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Old 19th January 2011, 10:33 AM   #21
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Yes, thin film are pricey, those must be thick. They're 5% types though, not the ideal choice for crossovers. Here in China, 1% thick films are around $5 a reel (5k) at retail in 0805. $10 for 1206.
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Old 19th January 2011, 11:16 AM   #22
mt490 is offline mt490  Australia
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Quote:
Originally Posted by s3tup View Post
Thick film are cheap and common. Could be bought over ebay for 100$ for whole E24 values set, 100pcs for each value.
20pcs/value goes for 16$ (don't know whether it's thick or thin film, but the price tells us it's thick)
170 Values 0805 SMD Chip Resistor COMPLETE Kit (0R~10M) - eBay (item 170470904131 end time Feb-03-11 15:01:03 PST)

Thin film are pricey.
Could you share yours thin film resistors source?
Thanks.
Trouble with thick film is that purportedly the noise performance is even worse than carbon film TH resistors, along with even a worse thermal stability coefficient. Although having said that I haven't had that much of an issue with carbon film (still trying to deplete my stocks on all my trial projects, haha)

RS components carries MELF thin film/metal film resistors, which are 'only' twice as expensive as TH metal films that they sell (though compared to local prices both options are far from bad). I'm looking at DIY quantities only though.

I had a re-browse and it seem Digi-Key carries some Susumu 0805 thin films which aren't priced that badly in quantities of 100, but shipping fees have to be considered.
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Old 19th January 2011, 11:23 AM   #23
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Quote:
Originally Posted by mt490 View Post
Trouble with thick film is that purportedly the noise performance is even worse than carbon film TH resistors, along with even a worse thermal stability coefficient. Although having said that I haven't had that much of an issue with carbon film (still trying to deplete my stocks on all my trial projects, haha)
I've built a few crossovers with thick film resistors, no noticeable noise problems on them so far. Quoted tempcos aren't too hot admittedly, perhaps 200ppm. But if they were really poor in thermal stability then they'd not make them in close tolerances, and they do (0.1% is available). I've never heard of carbon film in 0.1%
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Old 19th January 2011, 11:36 AM   #24
mt490 is offline mt490  Australia
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Ah, but there's a catch - the 200ppm thick films are not the ones boasting 0.1% precision. The 0.x% thick films will have a better tempco rating (for e.g Rohm's MCR10 datasheet that I'm looking at now has 50ppm for 0.5% vs 100ppm for 1%, and 200ppm for 5%)

ED- datasheets are all good and that, but only if you can buy the product advertised :/

Last edited by mt490; 19th January 2011 at 11:50 AM.
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Old 19th January 2011, 12:09 PM   #25
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Its a good point, though Ralec do 100ppm right across the board, they don't do 0.1% in 200ppm. I'm not sure whether the 1% ones I have are 100 or 200ppm.

http://www.ralec.com/download/produc...p_resistor.pdf
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Old 19th January 2011, 12:30 PM   #26
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Well your 1%'s would be 100ppm or 200ppm based on the chip size and resistance value, as there aren't any overlapping bands.
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Old 23rd January 2011, 09:42 AM   #27
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As for the tweaking et cetera - variable resistors Vs board space:

You can buy a bunch of bourns-like trimmers off fleabay,
20k ohm 3296 Variable Resistor Precision ,40 pcs - eBay (item 310285395860 end time Feb-03-11 18:26:18 PST)
short the center pin of the trimmer to one of the adjacent pins,
bend the 1 and 3 pins so they would fit the holes in the board and solder them slightly,
and viola - you got a variable resistor, which doesn't takes much space.

When you have settled on specific resistor values, you just buy the required values and replace the trimmer.

Still the board space is an issue - these trimmers wouldn't fit in standart configuration - you'll need to place the caps on the opposite side of the board (if you use full-sized caps).

Or quite the opposite - temporarily mount the trimmers underneath the board, tweak them, order specific values.
I'll make some program (or even web service) for finding a "composite" resistor by series-paralel connection of lower grade (E24) resistors. Say you need 6.31oHm resistor? Take 27ohm and 8.2ohm (27|8.2) and you get 6.29, pretty close. Or (100|(6.8|68)), and take 6.31.
Here are some calculations i've made for 10kOhm stepped attenuator in E24-series (bottom tables):
http://www.vegalab.ru/forum/attachme...3&d=1277238017

I'll do the silkscreen printing on both sides of the board.
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Old 23rd January 2011, 05:17 PM   #28
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Quote:
Originally Posted by s3tup View Post
I'll make some program (or even web service) for finding a "composite" resistor by parallel connection of lower grade (E24)
I already have an early Excel sheet with this on it.
You can usually find any value to closer than 0.1% to what is required. Then it's down to resistor tolerance for absolute value.

Email & I'll send it.
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Old 30th January 2011, 07:24 AM   #29
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AndrewT,
Thanks for the excel sheet




I'm doing some DSP based xover too... and thought of implementing the scheme of this analog filter in biquads. Should be simple... As long as i'll study the DSP biquads voodoo, and take the values from s-plane multipliers of the filter's transfer functions (i've already calculated them for the xover blocks in the software).
Then, having the biquad values, doing basic multiplication in computer should be easy = you get computer based simulator of analog circuit... I've already tried the directsound library awhile ago - yet just in stereo mode. Now it's 5.1 time

Looks way too ambitious... I'll need to split the development steps, so i could pass the waypoints during development with working device in hands.
I've setup a web page for this project
s3t.it> Active crossover: AXO3
and for the digital one
s3t.it> Digital crossover: DXO4

Last edited by s3tup; 30th January 2011 at 07:29 AM.
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Old 30th January 2011, 08:56 AM   #30
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The TI DSP you've chosen looks to be good bang for the buck at $5 or so but I'd be a bit concerned about its lack of flexibility. If it doesn't sound much good, what is there to tinker with? In particular, the dither feature is a little worrying - it says that an LFSR "can be used to dither the audio". Doesn't sound like much attention's been paid there.
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