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Old 5th October 2011, 07:00 AM   #1
neazoi is offline neazoi  Greece
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Default AF filter using opamps - substitution of opamps

hello I consider this filter
http://www.hanssummers.com/images/st...es/article.pdf
I guess I could I use another opamp with the same values of external components without affecting it's frequency response?

I think I am going to try the single opamp versions, since it will be easier to align the components and also it will be easier to add more stages or less, to achieve variable bandwidth. Will something like the cheap ne5534 prove better opamp?
Which opamp would you recommend for very low noise?
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Old 5th October 2011, 07:07 AM   #2
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Changing to the 5534 would probably be noisier, as the allpass stages are working at 100k impedance. To get lower noise you'd need to scale down this resistor value and multiply up the capacitors accordingly. Or you could use (much more expensive) low noise JFET opamps like OPA827. Definitely ditch the differencing 741 if you want it to sound half-way decent.
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Old 5th October 2011, 07:10 AM   #3
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Quote:
Originally Posted by abraxalito View Post
Changing to the 5534 would probably be noisier, as the allpass stages are working at 100k impedance. To get lower noise you'd need to scale down this resistor value and multiply up the capacitors accordingly. Or you could use (much more expensive) low noise JFET opamps like OPA827. Definitely ditch the differencing 741 if you want it to sound half-way decent.
So the OPA827 could be proved better without any changes in the circuit? (except from pinout maybe, I have not checked)

any equivalent DIP version?
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Old 5th October 2011, 07:13 AM   #4
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Well I think the TL074 is probably low noise enough for something acting as a filter for radio signals - those signals are already far noisier than the opamps here. The difference when switching to OPA827 would probably only be heard on hi resolution audio formats with better than 16bit noise floors.
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Old 5th October 2011, 07:14 AM   #5
neazoi is offline neazoi  Greece
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Thank you. That make sense
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Old 5th October 2011, 07:17 AM   #6
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For single (or dual, or quad) opamps I would go with the TL071 (or TL072 dual, TL074 quad).

The 5534 is power hungry and you would need to ensure stability. The TL0's would drop right in place with no problems.

tbh for it's intended use a 741 (or LM324) isn't going to worsen the overall snr of what comes out all that much.
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Old 5th October 2011, 07:20 AM   #7
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SNR - no, but I bet the THD suffers owing to 741's very restricted bandwidth. No reason to use that device these days with the price of 5532s
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Old 5th October 2011, 07:26 AM   #8
neazoi is offline neazoi  Greece
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Could you give me a review about the AD847 or the ad825 in use in this circuit as well please?

If it is ok, could ththese be used with single supply as well in the circuit? (I see a balanced supply in the datasheet)
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Last edited by neazoi; 5th October 2011 at 07:31 AM.
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Old 5th October 2011, 07:32 AM   #9
Mooly is offline Mooly  United Kingdom
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neazoi... the signal you are dealing with is extremely limited.

Anything better than a TL071/2/4 is overkill... you gain nothing performance wise and may lose out in terms of cost, stability and power consumption.

If power consumption is an issue then the low power TL061 and TL062 would be fine.

On the random swapping of opamps see this,
Swapping Op-Amps... you have checked to see it's stable haven't you ?
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Old 5th October 2011, 07:35 AM   #10
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Thanks, that makes sense!
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