Don't know if there is a better forum for this, feel free to move topic if you feel there is.
Anyhow, when using active filters do you feel there would be any significant advantage to attenuating after the filters rather than before? The idea is naturally that the filters are handling a higher level signal with the aim to lower noise and possibly crossover distortion. I can see practically that with good quality op-amps this may not be relevant, but theoretically it seems sound; the attenuated level can be many times lower than the original signal level.
There is also the issue of headroom, but I know that with a typical op-amp +/-15v supply a CD plyer or other source won't get anywhere near there.
Incidentally, does anyone have any opinions on this chip:
http://www.rapidonline.com/producti...stereo+digital++volume+control&moduleno=35600
It appears to act as a passive in the signal chain and the resistor networks in the chip will naturally be highly accurate. Seems like a good option for volume control at a modest cost. My thinking is that these could easily be interlinked if the above idea is worthwhile, 3 chips would be needed for 3 way for example.
Just to throw in another idea, balanced active crossovers. High end amplifiers and sources often use balanced connection so it would seem a shame to break that at the crossover stage. How would one implement a balanced active crossover; would 2 crossovers (1 for each line in the pair) be needed?
Discuss
Anyhow, when using active filters do you feel there would be any significant advantage to attenuating after the filters rather than before? The idea is naturally that the filters are handling a higher level signal with the aim to lower noise and possibly crossover distortion. I can see practically that with good quality op-amps this may not be relevant, but theoretically it seems sound; the attenuated level can be many times lower than the original signal level.
There is also the issue of headroom, but I know that with a typical op-amp +/-15v supply a CD plyer or other source won't get anywhere near there.
Incidentally, does anyone have any opinions on this chip:
http://www.rapidonline.com/producti...stereo+digital++volume+control&moduleno=35600
It appears to act as a passive in the signal chain and the resistor networks in the chip will naturally be highly accurate. Seems like a good option for volume control at a modest cost. My thinking is that these could easily be interlinked if the above idea is worthwhile, 3 chips would be needed for 3 way for example.
Just to throw in another idea, balanced active crossovers. High end amplifiers and sources often use balanced connection so it would seem a shame to break that at the crossover stage. How would one implement a balanced active crossover; would 2 crossovers (1 for each line in the pair) be needed?
Discuss
That chip looks fine, but i'm afraid you'd need a microcontroller of some sort, to be able to command/control it. 
In my active crossovers, i'm going to use some Dallas/Maxim DS1802's, as per Mr. Pedja Rogic advises ( http://www.pedjarogic.com/spot/index.html ). You can easily get a few of them as free samples (i got 4, for a pair of 2-way crossovers). All you'd need is a few TL431's, a few film caps and a few pushbuttons, and voila
And regarding balanced crossovers... I don't see much point in keeping them balanced, unless they're some distance away from the power amps. Myself, i designed my boards with a balanced input and single-ended outputs. Full-balanced would've ment an extra 3 dual opamps and 2 singles, plus a whole lotta "real-estate"
In my active crossovers, i'm going to use some Dallas/Maxim DS1802's, as per Mr. Pedja Rogic advises ( http://www.pedjarogic.com/spot/index.html ). You can easily get a few of them as free samples (i got 4, for a pair of 2-way crossovers). All you'd need is a few TL431's, a few film caps and a few pushbuttons, and voila
And regarding balanced crossovers... I don't see much point in keeping them balanced, unless they're some distance away from the power amps. Myself, i designed my boards with a balanced input and single-ended outputs. Full-balanced would've ment an extra 3 dual opamps and 2 singles, plus a whole lotta "real-estate"
Balanced connections are advantageous for sending signals, but inside the box, the advantages are minimal. Inside the crossover box, I'd convert the balanced signal to single-ended, do the crossover thang, then convert the output to balanced.
Of course, unless you're running very long leads or are in an unusual environment, there's no real advantage to balanced transmission at line level, either, but I know there's no way to talk you out of that.
Of course, unless you're running very long leads or are in an unusual environment, there's no real advantage to balanced transmission at line level, either, but I know there's no way to talk you out of that.
Thanks for your replies
Thats interesting about the attenuators usually being after the filters, I didn't realise that. I think I would do it that way myself if I assembled a design. I have a freind who may be able to sort out a controller for that chip.
Your probably right about balanced transmission generally being unrequired at line level. One reason I found it attractive is in how it may help with earth loops. If you were running 3 earthed power amplifiers it could definately cause a problem, but with balanced you could simply break the shield connection at one end of the interconnect (theoretically you can do this anyway but last time I tried this in practice I found that not to be the case, humming was worse than the earth loop).
Thats interesting about the attenuators usually being after the filters, I didn't realise that. I think I would do it that way myself if I assembled a design. I have a freind who may be able to sort out a controller for that chip.
Your probably right about balanced transmission generally being unrequired at line level. One reason I found it attractive is in how it may help with earth loops. If you were running 3 earthed power amplifiers it could definately cause a problem, but with balanced you could simply break the shield connection at one end of the interconnect (theoretically you can do this anyway but last time I tried this in practice I found that not to be the case, humming was worse than the earth loop).
Generally, a shield is only connected at the sending end (assuming balanced transmission).
My own setup is triamped, no balanced lines, no hum; I did need to use an input transformer on my preamp because of the hum induced from my satellite dish, but for all other sources, hum loops are a non-issue. It's just a matter of getting the grounds right in each of the other components. Bill Whitlock's papers are terrific guides as to how it's done.
My own setup is triamped, no balanced lines, no hum; I did need to use an input transformer on my preamp because of the hum induced from my satellite dish, but for all other sources, hum loops are a non-issue. It's just a matter of getting the grounds right in each of the other components. Bill Whitlock's papers are terrific guides as to how it's done.
Hi Dr EM,
I know it's not the greatest idea but I've actually got an active filter with a 2 channel volume control on the input.
It was a quick-n-dirty build, so after weighing up the pros and cons I figured that having a slightly lower noise floor just wasn't worth the additional bulk and the risk of problems with mechanical tolerances due to a 6-channel pot.
As for a high quality design, I'm a bit iffy about digital volume controls because their ratings are only valid for high gain values. Have you checked out Texas Instruments, Asahi Kasei, or Analog Devices?
I know it's not the greatest idea but I've actually got an active filter with a 2 channel volume control on the input.
It was a quick-n-dirty build, so after weighing up the pros and cons I figured that having a slightly lower noise floor just wasn't worth the additional bulk and the risk of problems with mechanical tolerances due to a 6-channel pot.
As for a high quality design, I'm a bit iffy about digital volume controls because their ratings are only valid for high gain values. Have you checked out Texas Instruments, Asahi Kasei, or Analog Devices?
Actually I don't know if it's "usually" done that way... My thinking was that ideally the signal voltage should only be attenuated just before the input to the power amplifier, so that the noise produced by the op-amps is also attenuated. IIRC there are even certain guidelines about the preferred order of filter stages to minimize their noise.
One thing that I've been thinking about is building a binary-weighted array of reed switches, which would be controlled by a flash 8 or 10 bit ADC. The analog input can be anything...
Click reduction could be implemented with silicon switches* that are connected a few milliseconds before the reed switches to "soften" the transitions.
Edit: "silicon switches" being a generic term because that's likely to be the hard part. They have to be suitable for bi-directional usage.
One thing that I've been thinking about is building a binary-weighted array of reed switches, which would be controlled by a flash 8 or 10 bit ADC. The analog input can be anything...
Click reduction could be implemented with silicon switches* that are connected a few milliseconds before the reed switches to "soften" the transitions.
Edit: "silicon switches" being a generic term because that's likely to be the hard part. They have to be suitable for bi-directional usage.
This is worth a read: A Differential Op-Amp Circuit Collection, TI Application Report by Bruce Carter
http://focus.ti.com/lit/an/sloa064/sloa064.pdf
http://focus.ti.com/lit/an/sloa064/sloa064.pdf
Thanks for that BWRX, very interesting read
Now the challenge is to implement those differential filters in this filter topology :
http://sound.westhost.com/project09.htm
I know we pretty well agreed it was pointless for line level but it would be a beautiful thing . I like the idea of a compact surface mount design too, though the capacitors need to be through hole types as far as I can tell (mounted on top or through hole).
Do most people feel the Linkwitz-Riley active filter design presented there is the best? I've been very impressed with it so far and can't see why people use other arrangements; its not especially complicated to build the Linkwitz-Riley arrangement.
I am going to be using an attenuator before my filter when I implement my subwoofer, i'm building the passive attenuator/selector now. Was an unforseen expense as I planned to use my amps pre-out and feed back into tape monitor; this did NOT work
Now the challenge is to implement those differential filters in this filter topology
:http://sound.westhost.com/project09.htm
I know we pretty well agreed it was pointless for line level but it would be a beautiful thing
. I like the idea of a compact surface mount design too, though the capacitors need to be through hole types as far as I can tell (mounted on top or through hole). Do most people feel the Linkwitz-Riley active filter design presented there is the best? I've been very impressed with it so far and can't see why people use other arrangements; its not especially complicated to build the Linkwitz-Riley arrangement.
I am going to be using an attenuator before my filter when I implement my subwoofer, i'm building the passive attenuator/selector now. Was an unforseen expense as I planned to use my amps pre-out and feed back into tape monitor; this did NOT work
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