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Old 30th April 2012, 09:43 AM   #31
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Self's latest filter implementations go the whole hog and reduce the resistances such that paralleled opamps are required to drive these low loads.

But if you go back to Ben Duncan where he adopted 8k, 10k and 12k as typical resistances to keep noise lowish, then you probably would be OK.

I would tend to look at the range 5k1 to 12k for the filter resistors in a wideband filter set up. Bass only may tolerate much higher noise.
regards Andrew T.
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Old 30th April 2012, 11:00 AM   #32
godfrey is offline godfrey  South Africa
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Location: Cape Town
Fair point about the noise. The thermal noise of a 100K resistor is 10dB higher than a 10K resistor i.e. about twice as loud. That's not the whole story, though.

Firstly, there are plenty of other sources of noise in the circuit besides the filter resistors so reducing them, even by a factor of 10 won't have that much of an effect on the noise unless everything else is tackled as well.

Secondly, and more importantly, is that the lower the load impedance on an opamp, the higher the distortion. That is particularly important in this circuit because the opamps aren't just driving the resistors. The last 4 opamps are directly driving the filter capacitors.

As it is, you're using 68nF caps, which is already a very heavy load for the opamps, and if you want to reduce the resistors, you will need to increase the capacitors.

Bear in mind that while 68nF represents an impedance of 33K at 70Hz, it is only 330 Ohms at 7KHz, and not much over 100 Ohms at 20KHz. That's quite a nasty load for an opamp.


edit: Oops, I forgot to mention - It looks like there's a couple of errors in the schematic:
A) R15 (the OUT-L1 connection), should be connected to the other side of R9, i.e. directly to the output of IC6B.
B) The center pin of the potentiometer (R10) should not be connected to ground. It should be connected to either pin 1 or pin 3.

Last edited by godfrey; 30th April 2012 at 11:07 AM.
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Old 30th April 2012, 12:10 PM   #33
NebuK is offline NebuK  Germany
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Join Date: Jan 2011

thanks for your replies! I think in the case of the resistor range i'm going to be in ... best would to be just to build it and toy around?

I'll also try to get around and find so informations on error accounting like this that a beginner like me can understand ;P maybe i can come up with a nice way of finding the right balance on this tightrope walk?

For the schematic errors, i've fixed them here i think ... as i'm still very unsure about all of this, could you maybe check them again?

Thank you so much for your help! Regards
- NebuK
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Old 30th April 2012, 12:27 PM   #34
godfrey is offline godfrey  South Africa
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Join Date: Nov 2009
Location: Cape Town
Originally Posted by NebuK View Post
For the schematic errors, i've fixed them here i think ...
Yes, that looks right, and I agree about build now, fiddle around later. It's too easy to get trapped into "paralysis by analysis".
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