| Leolabs |
| It may sounds crazy,but have anyone try it before??? |
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| tiltedhalo |
| I have actually built a few useing 1% and even a very succesfull design useing 5% tolerance carbon comps in a BPA200 |
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| sangram |
:Hands raised:
Audiosector LM4780 kits, connected as BPA, using the resistors that came with the kit. I don't think those are matched... |
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| OzMikeH |
| Resistors are cheap. buy 20 or so and and measure them. pick the closest ones. |
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| Dxvideo |
Exactly true.
I done that methode for my LM4780 GainClone and have only 0.2mV offsets in each channel while inputz shorted (but I have very balanced SMPS for PSU)..
I bought 40 pcs for 20K feedback and input load and 40 pcs for 1K and matched them with my 4000 count multimeter. I also matched the capacitors with the same technic. It takes only 10 minutes! Thats all. No need for %0.1s.. |
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| Fenris |
| Most 1% are actually within about 0.3%. Should be good enough. |
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| kscharf |
Actually in the bridge circuits I've seen one side (the non-inverting side)
has 1k and 20.5k resistors and the other side (inverting side) has 21.5k resistors. I'm not sure why the difference or how critical this is. I suppose given enough resistors and an acurate ohmmeter one could hand pick 1k and 22k 5% resistors to work, or parallel high value resistors with the 22k to get the desired values. |
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| kscharf |
In the bridge ciruits I've seen one side (non invertng) uses 1k and 20.5k resisters and the other (inverting) uses 1k and 21.5k resisters.
I'm not sure why and how critical this is.
I suppose one could hand pick from 22k 5% resistors with a good ohmmeter or parallel high value resistors with the 22k to trim to the desired value. |
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| Fenris |
| quote: | Originally posted by kscharf
Actually in the bridge circuits I've seen one side (the non-inverting side)
has 1k and 20.5k resistors and the other side (inverting side) has 21.5k resistors. I'm not sure why the difference or how critical this is. I suppose given enough resistors and an acurate ohmmeter one could hand pick 1k and 22k 5% resistors to work, or parallel high value resistors with the 22k to get the desired values. |
Because one side is inverting and one is non-inverting. The formula for calculating the gain is slightly different for the different types. See http://en.wikipedia.org/wiki/Operat...er_applications for the difference. If you plug the numbers in, you'll find they end up with the same gain using different resistors. |
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| FastEddy |
| You might consider making your own Wheatstone Bridge ( http://en.wikipedia.org/wiki/Wheatstone_bridge ) ... I have one I picked up from a trade school inventory close out made in a nice wooden box = works great and can measure and match resistors to 0.01% with a 9 Volt battery as source and an el cheapo voltmeter. :smash: |
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| tlmadsen |
HI LeoLabs
I have build a number of BPA designs (search "Das Modul") using 1% gain-resitors without matching them. I can only recall once where this possible mis-match coursed a problem.
Up on closer inspection of the problem/function is turns out that 1% resitors comming from the same batch are normally within 0,3 % or better to each other (not to nominal value).
So using 1% resistors (without matching) will very offent works without any problems.
The problem is not when you are bridging, since a mis-match just means that one half of your amplitude will be stronger that the other. Not nice, but it will not harme the amplfier or speaker.
However, when you start to parallel amplifierer/chips things starting to get interesting.
I don't have my calculations at hand, but if you are unlucky and one amp end up having the lowest possibe gain and the other ends up having max, gain you for sure have a "sub-optimal" situation that should be avoided.
Today I always measure my gain-resitors on a my multimeter and I think that any good multimeter will be up to the task.
Remember that is is the relative differnece between the resistors that are important. ( you don't have to "search" for resistors that are nominal value +/- .3 %).
The DC-offset is not necessary removed by matching gain-resitors in a paralleled design.
The major course for DC-offset for an op-amp is "mismatch" between the Rin and impedance of your feedback network. If they have the same impedance, first then matching of gain-resistors will also bring your DC-offset down to a "workable" level.
Have fun :)
Thomas |
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| Leolabs |
Thanks for the advice guys!
Will non-matched resistors cause problem to the sound???? |
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