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Old 17th October 2004, 03:48 PM   #11
Bricolo is offline Bricolo  France
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Well, thank you for your explanations.

In fact, my point is that if there's a difference between the theory and the practice, it means that the theory isn't complete enough


BTW, do you have some nice discrete opamps examples?
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Old 17th October 2004, 04:55 PM   #12
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A good place for learning about discrete op-amps is Douglas Self's site, specifically this page, which shows the standard design.
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Old 17th October 2004, 05:28 PM   #13
gembler is offline gembler  United States
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Here are a few suggestions from recent experience designing the Virgin Electronics boomtube ex(tm). These apply to both integrated and discrete op-amp-based crossovers. For the buffer stage, you will probably be stuck with using a non-inverting buffer configuration set to the gain you require. The reason is that you will need to avoid loading or interaction with the previous stage. For an inverting buffer to achieve this, it would require large value resistors that can introduce excessive noise. For the crossover filters, you should try to use the multiple feedback (MFB) configuration. This configuration avoids the common-mode distortion of the Sallen-Key configuration. However, it uses more components and is somewhat harder to design. There are other differences between the two configurations and you might take some time to look into them. Among the many other well-known references, I recommend TI's filter ap notes. TI also has a good filter design program, FilterPro. Try the following TI docs:

SBFA001A - FilterPro MFB and Sallen-Key Low-Pass Filter Design Program
SLOA024B - Analysis of the Sallen-Key Architecture
SLOA049B - Active Low-Pass Filter Design
SLOA062 - Using the Texas Instruments Filter Design Database
SLOA065 - Filter Design on a Budget
SLOA093 - Filter Design in Thirty Seconds
SLOA096 - More Filter Design on a Budget
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Old 17th October 2004, 06:08 PM   #14
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From my perception the AD8620 or AD8610 seem to be a very good solution for low gain applications...
These amps do not have excessive gain overhead, but impressive
speed combined with really nice phase accuracy.

Have fun
Markus
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Old 17th October 2004, 07:09 PM   #15
Bricolo is offline Bricolo  France
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Quote:
Originally posted by Mr Evil
A good place for learning about discrete op-amps is Douglas Self's site, specifically this page, which shows the standard design.
Thanks, instructive link.
Nelson's discrete opamp tutorial is also very nice
http://www.passdiy.com/pdf/diyopamp.pdf


Quote:
Originally posted by gembler
Here are a few suggestions from recent experience designing the Virgin Electronics boomtube ex(tm). These apply to both integrated and discrete op-amp-based crossovers. For the buffer stage, you will probably be stuck with using a non-inverting buffer configuration set to the gain you require. The reason is that you will need to avoid loading or interaction with the previous stage. For an inverting buffer to achieve this, it would require large value resistors that can introduce excessive noise. For the crossover filters, you should try to use the multiple feedback (MFB) configuration. This configuration avoids the common-mode distortion of the Sallen-Key configuration. However, it uses more components and is somewhat harder to design. There are other differences between the two configurations and you might take some time to look into them. Among the many other well-known references, I recommend TI's filter ap notes. TI also has a good filter design program, FilterPro. Try the following TI docs:

SBFA001A - FilterPro MFB and Sallen-Key Low-Pass Filter Design Program
SLOA024B - Analysis of the Sallen-Key Architecture
SLOA049B - Active Low-Pass Filter Design
SLOA062 - Using the Texas Instruments Filter Design Database
SLOA065 - Filter Design on a Budget
SLOA093 - Filter Design in Thirty Seconds
SLOA096 - More Filter Design on a Budget
You forgot this one
SLOA088 - Active Filter Design Techniques

But please, keep this thread on the feedback discuttion
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Old 17th October 2004, 08:29 PM   #16
gembler is offline gembler  United States
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Quote:
You forgot this one
Quote:
SLOA088 - Active Filter Design Techniques
I have seen this one -- it is actually Chapter 16 by Thomas Kugelstadt, from Ron Mancini's Op-Amps for Everyone. It contains good background info but the other references may be better starting points.

Quote:
But please, keep this thread on the feedback discuttion
Sorry if I went OT but in my defense I was focusing on your objective of designing a crossover and not the bigger issue of whether to use feedback or not. The MOR advice is of course "a moderate amount of feedback is OK", and I tend to agree with this in general. Unfortunately I do not have references as readily available as the previous ones for feedback design, although Op-Amps for Everyone probably has some good info. However, the feedback question contains so many variables that it is difficult to make any valid general recommendations. Also, crossover design is interlinked with feedback issues which are in turn interlinked with implementation (discrete vs. discrete op-amp vs. integrated op-amp). OT or not OT, that is the question...
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Old 17th October 2004, 10:16 PM   #17
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"yes P.Lacombe, you need to go back and look at those theories, as they don't say there will be no difference!"

Mr Evil,

I know that. It was a joke... Things are not so simple.

Regards.
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Old 18th October 2004, 03:24 PM   #18
Bricolo is offline Bricolo  France
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Quote:
Originally posted by gembler



I have seen this one -- it is actually Chapter 16 by Thomas Kugelstadt, from Ron Mancini's Op-Amps for Everyone. It contains good background info but the other references may be better starting points.



Sorry if I went OT but in my defense I was focusing on your objective of designing a crossover and not the bigger issue of whether to use feedback or not. The MOR advice is of course "a moderate amount of feedback is OK", and I tend to agree with this in general. Unfortunately I do not have references as readily available as the previous ones for feedback design, although Op-Amps for Everyone probably has some good info. However, the feedback question contains so many variables that it is difficult to make any valid general recommendations. Also, crossover design is interlinked with feedback issues which are in turn interlinked with implementation (discrete vs. discrete op-amp vs. integrated op-amp). OT or not OT, that is the question...

No problem The links were interesting, but I just wanted to keep this thread on topic
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Old 18th October 2004, 05:38 PM   #19
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do you want to use some active filters, which need opamps, or something like buffer isolated passive filters + gainstage

http://home.tu-clausthal.de/~tpa/space/axover.gif ?
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Old 18th October 2004, 06:51 PM   #20
Bricolo is offline Bricolo  France
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Quote:
Originally posted by till
do you want to use some active filters, which need opamps, or something like buffer isolated passive filters + gainstage

http://home.tu-clausthal.de/~tpa/space/axover.gif ?

First, my active XO consists of a 2 way, variable frequency, slope and Q sallen key filter

I'd like to use followers (simple and clean ) but this is giving me headaches.
Look at the equations for K (gain) =1. I haven't found a way to easilly change Q without altering the other parameters (easilly means: without having to change all other part's value)

So I think I'll go for discrete opamps, unless I find an easy way to have the variable Q with the followers. (and I need it, simply for the variable order since for 2nd order LR Q=0.5, 3rd order butterworth Q=1, and 4th order LR Q=0.707 (for both))
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