Sallen-Key op-amps or alternatives

Discussion in 'DIY' started by skem, Jun 16, 2018.

  1. skem

    skem Friend

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    I’m thinking of building line-level crossovers/filters to bi-amp a system.

    DIYA people claim modern op-amps are very close to transparent; one person suggests using a mix of op-amps so as not to magnify coloration.

    Any SBAF views on op-amp models to use for unity gain applications, or if I should really use an alternative buffer?
     
  2. purr1n

    purr1n Desire for betterer is endless.

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    You can get away with opamps for lows. Use passive for highs, or maybe passive line level.

    It depends upon the rest of your gear and drivers. If you use modest amps, class D, lower efficiency unresolving drivers, then active opamp XOs will be better than passive.

    On opamps, what constitutes "modern" opamps?

    Unity gain means lots of negative feedback. Also as far as transparent, what does that mean? I never felt that even "old" opamps were colored sounding. And the new opamps were just as colored sounding. Modern or old, they all have their own sound. But the differences are not huge.

    One thing you can do is breadboard a CMOY with unity gain and try with headphones. Actually, throw the unity gain CMOY in front of a headamp.
     
    Last edited: Jun 16, 2018
  3. ultrabike

    ultrabike Measurbator - Admin

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    Not sure if this on-line application is of any use to your efforts:

    http://www.analog.com/designtools/en/filterwizard/

    You can try later use SPICE. Many vendors provide models of their op-amps.

    If I was going to design, I would make sure that the op-amps I choose will work (not oscillate or perform with crap loads of noise or whatever else) for my application. Then I would select from the ones that will work in terms of what sounds best perhaps once I have something build to test them (on a breadboard or so).
     
  4. skem

    skem Friend

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    Thanks guys. This is to feed pass xa30.8 amplifiers, which I find pretty resolving. Source is PS Audio DirectStream with reported output impedance of 100 ohms. I haven’t checked, but I sort of presumed that would be incompatible with a passive network.

    I’ll futz with some circuit-designer software and see where I get
     
  5. purr1n

    purr1n Desire for betterer is endless.

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    I'd try PLLXO with the good gear you have. Use passive after power amp if PLLXO can't get the slopes you need.

    Throwing in opamps in the chain with gear designed by audio Gandalf would be an outrage.
     
  6. bazelio

    bazelio Friend

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    XA30.8 is awesome. What's the bi-amping strategy here, for the curious?
     
  7. skem

    skem Friend

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    The speakers are old B&W 801 series iii. They always sounded disjointed until I acquired the xa30.8. Now they’re fairly coherent and suddenly I have actual imaging. So I became curious — how far can these speakers go.

    Word on the web is original XOs sucked. On top of that, I feel the frequency response is not that flat: something missing around 200-400 Hz, I think. XO is supposedly at 200.

    So I began wondering about the line-level solution and bi-amping. I was intrigued by the possibility of using a 4th order LR filter with an all-pass filter to cope with phase delay. But I was nervous at the idea of op-amps. Hence my post.

    Maybe the best thing to do is just rebuild the original XO and skip the bi-amp strategy. I’m sort of unsure how to proceed.
     
  8. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    Do you know the topology of the stock filters and what was shoddy about them? When you know the exact mathematical model you're shooting for, you could plan for a filtering solution. Generally PLLXO gets you better and cheaper passive components, as you won't need to use coke can caps due to high impedance loads. Inductance generally is a mixed bag, as you need to bump up the Henries, but coil impedance becomes a non-issue. For gentle slopes, I'd say PLLXO all the way; for more creative stuff ALLXO might be better, but will generally be harder to implement. I'd stay away from high level XO, as I don't really see the benefit, when you're doing a line-level box.

    On the other hand, don't sweat too much about the crossover implementation technology. The hard part will be bettering the B&W design. I hope you have a measurement mic at hand and knowledge how to interpret in-room measurements.
     
  9. Priidik

    Priidik MOT: Estelon

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    Ideally you'd need 3 amplifiers to fully take advantage of line level XOs.
    I have never tried PLLXOs myself, how does one integrate mid and tweeter with single amp feeding both?
    Apparently quite complicated to tune the phase xo point, and then would such an abomination sound any good?
    Here is the smaller sibling going over XO overhaul: BW802mk3
     
  10. peef

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    A bit late, but if you're not trying to milk any gain out of your op amp, a discrete Sallen-Key is totally a thing.

    [​IMG]

    With a simple BJT buffer like this, the feedback resistors must be small-ish and it isn't possible to DC couple (which is high for a highpass). A JFET buffer or diamond buffer could get around both problems.
     
  11. murphythecat

    murphythecat GRU-powered uniformed trumpkin

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    yes, rebuild the xo with newer caps...
     

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