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Precision resistors
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Precision resistors

by Soldermonkey on Wed Feb 04, 2015 3:54 pm

I am needing 200k 0.1% resistors to replace some badly out of range cheapo 1% that are in one of my Xoxes,
Do any UK builders know of any UK component suppliers that stock these and dont require me to spend £150 on a minimum buy of 1000?

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Re: Precision resistors

by MMM on Wed Feb 04, 2015 7:16 pm

I usually buy them off ebay. Couldn't find right away a good match on ebay.co.uk (too expensive for my taste); but the German supplier I bought mine from ships also to the UK.

http://www.ebay.co.uk/itm/360650441233

You'd need two of them (each is 10 resistors); including shipping you should still stay under 10 UKP (shipping costs would be higher than the cost of the resistors...).
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Re: Precision resistors

by Soldermonkey on Wed Feb 04, 2015 9:03 pm

Hi thanks for that, nowhere in the uk seems to sell them.
was about 8.50GBP with postage for those resistors, they are now ordered.
I have 2 other Xoxes that need those resistors swapping, the scale difference is noticeable compared to the one you fixed.
Started another xox kit, hopefully it wont end up being full of terrible soldering and fake parts like the last one.
Might still require your services if this one has problems. :)

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Re: Precision resistors

by Luap on Sat Feb 07, 2015 11:42 pm

1% should be plenty good enough if they are genuine good quality parts.

This is a really useful UK company for getting small orders of parts: www.bitsbox.co.uk

If you really want to get ultra accurate ones, then you could buy a bunch more than you need and check them with a decent multimeter, and pick out the best ones.
Again though, if they are good quality parts, then you'll find very little difference between them. Even 5% ones tend to be pretty close these days (in my experience)
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Re: Precision resistors

by MMM on Sun Feb 08, 2015 5:28 pm

Luap wrote:1% should be plenty good enough if they are genuine good quality parts.


No, they are not. You really need 0.1% resistors for the DAC. There is a reason the TB303 used 0.1% resistors there, too.

Using 1% resistors (within their tolerances) one often gets poor tuning. Usually this manifests itself with a tuning "gap" in the non transposed octave between G# and A. This is where the highest bit becomes active and hence this is the point with the highest voltage in the ladder. That's why this is also the point where any problems with the resistor tolerance become most obvious.
If you tune the non transposed octave C to C to about 0 cent precision and you get more than 3 cent difference for the G# and A tuning you have resistors that are not precise enough.
Last edited by MMM on Tue Feb 10, 2015 6:32 am, edited 1 time in total.
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Re: Precision resistors

by Luap on Mon Feb 09, 2015 5:12 pm

^ Apologies if my info was incorrect. Its been 4 or 5 years since I built mine so I don't recall what I put in there now. But I thought I remembered 1% throughout (i'll recheck next time im poking about in there)
Unfortunately, I don't think Bitsbox sell .1% resistors. So Soldermonkey might be out of luck there..
Still, checking out a bunch with a meter can't be a bad idea?
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Re: Precision resistors

by MMM on Tue Feb 10, 2015 6:58 am

The BOM for the xoxbox specifies 1% resistors. However the TB-303 service manual explicitely states 0.1% resistors for R74 to R90; actually the wording is

"MATCHED PAIR .1% SELECTED MFR: R74-R90"

My guess is that this somehow got overlooked when the xoxbox was designed.

One could try to get to this precision by selecting them from a bunch of 1% resistors; from my own experience this is a study in futility, though. You need a very precise ohmmeter for this; and the setup also needs to be very precise (for example: grabbing the resistor with your hands is a no-no). And in the end you'll need a very large sample (probably over 100 resistors) to get enough resistors that give the same reading within .1% tolerance. We tried this first before we switched directly to .1% precision.

Various suppliers like Mouser have them in stock; price usually seems to be around .15 EUR per resistor for the cheapest type though some suppliers seem to charge about 10 times as much. As I wrote earlier I usually buy them off ebay.

In my experience using those resistors can really make a big difference in tuning. There is probably not much to gain elsewhere in the circuit so 1% is fine for the rest.
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