Op-amp choices, balanced inputs, separate EQs???
I wanted to touch on a few items and get feedback from those people who are interested in the modular crossover boards. I have gotten some good input and questions over the past week, and I am considering making some changes that make possible some of these issues:
1. EQ - I came up with a combination input and EQ board for the first design iteration. This seemed to make sense and was a good use of the board space. But it was brought up that EQ for each driver might be desired. To address this, I wouldn't need to make any changes actually, but I would like to know if what I am proposing here sounds OK - It's possible to supply the input board without the input or power supply section and connect the input via a jumper wire directly to the EQ section. This reduces the part count significantly and because the EQ section uses its own dual op amp, it is more or less independent of the input and baffle step circuitry. So if people want a separate EQ for one (or more) drivers, they could buy the board in an "EQ only" version, and this would be relatively inexpensive. This means I would not need to develop and make yet another PCB for standalone EQ. If this approach sounds OK, then this issue seems to be resolved.
2. Balanced inputs - This was brought up a couple of posts ago. I actually agree that this is a good idea, since with active speakers the line-level cable runs will be longer. I am not sure that there really are any problems to be solved by balanced inputs, since I don't imagine people running 50'-100' of cable from their preamp to the speakers. If runs are kept under 20' and the right low capacitance unbalanced cables are used, I believe that things will be fine and there won't be undue noise pickup or high end roll off. But I don't live in an area with lots of RF interference, so I may not be sensitive to these kind of issues.
To allow for balanced inputs, I would probably want to develop a separate board that people could add on to the front end. It's extremely simple, since I would probably just use one of the off the shelf ICs like the INA134s that I mentioned in the last post. I would have room on the board for balanced outputs too, in case one wanted to send a signal externally (to a sub for instance). The board could probably be sold as 1 balanced input and 0,1,2, or 3 balanced outputs, depending on how many I can fit on the PCB. Whatever circuitry is unused would be left un-populated on the PCB.
3. Op-amps: I have to admit, by making everything "flexible" and "modular" I need to use a lot of op-amps in the designs. I was originally planning to use a moderately priced op-amp from OnSemi, the MC33274, which has pretty good but not top notch specs (datasheet link here). But for about $1.75 more per quad amp I can use an excellent quad op amp from National Semi, the LME49740 (LME datasheet). This will definitely raise the cost of each board a few dollars, but the distortion is several magnitudes lower (.003% for the OnSemi MC versus .00003% for the National LME) and noise is almost 10x lower (18 nV/ (sq. root Hz) for the OnSemi versus 2.7 nV/ (sq. root Hz) for the National). I was originally planning on socketing the op-amps, so that two different performance levels could be offered and op-amps could be swapped out, however it was suggested that the LME op-amps might not work well if socketed, because they are faster and have a much higher GBP. This makes board layout and bypassing much more important. It might work socketed or it might not. I just have to try it with the prototypes and see what happens. I would love to get some feedback on this from anyone who has designed and built PCBs using the newer fast op amps to learn about tips and tricks, etc.
For now, I would like to keep the idea of offering two performance levels of op-amps and socketing the devices and take a wait and see approach. How would people feel about not having two options but only having the more costly version available? Honestly, I believe that I need to keep costs down as much as possible without cutting any corners in order to keep these boards competitive with other crossover offerings out there...
So, those are my thoughts for now. I would love to get some feedback on these issues, and anything else that people might think of, so that I can work out everything in the near future.
Thanks,
-Charlie
I wanted to touch on a few items and get feedback from those people who are interested in the modular crossover boards. I have gotten some good input and questions over the past week, and I am considering making some changes that make possible some of these issues:
1. EQ - I came up with a combination input and EQ board for the first design iteration. This seemed to make sense and was a good use of the board space. But it was brought up that EQ for each driver might be desired. To address this, I wouldn't need to make any changes actually, but I would like to know if what I am proposing here sounds OK - It's possible to supply the input board without the input or power supply section and connect the input via a jumper wire directly to the EQ section. This reduces the part count significantly and because the EQ section uses its own dual op amp, it is more or less independent of the input and baffle step circuitry. So if people want a separate EQ for one (or more) drivers, they could buy the board in an "EQ only" version, and this would be relatively inexpensive. This means I would not need to develop and make yet another PCB for standalone EQ. If this approach sounds OK, then this issue seems to be resolved.
2. Balanced inputs - This was brought up a couple of posts ago. I actually agree that this is a good idea, since with active speakers the line-level cable runs will be longer. I am not sure that there really are any problems to be solved by balanced inputs, since I don't imagine people running 50'-100' of cable from their preamp to the speakers. If runs are kept under 20' and the right low capacitance unbalanced cables are used, I believe that things will be fine and there won't be undue noise pickup or high end roll off. But I don't live in an area with lots of RF interference, so I may not be sensitive to these kind of issues.
To allow for balanced inputs, I would probably want to develop a separate board that people could add on to the front end. It's extremely simple, since I would probably just use one of the off the shelf ICs like the INA134s that I mentioned in the last post. I would have room on the board for balanced outputs too, in case one wanted to send a signal externally (to a sub for instance). The board could probably be sold as 1 balanced input and 0,1,2, or 3 balanced outputs, depending on how many I can fit on the PCB. Whatever circuitry is unused would be left un-populated on the PCB.
3. Op-amps: I have to admit, by making everything "flexible" and "modular" I need to use a lot of op-amps in the designs. I was originally planning to use a moderately priced op-amp from OnSemi, the MC33274, which has pretty good but not top notch specs (datasheet link here). But for about $1.75 more per quad amp I can use an excellent quad op amp from National Semi, the LME49740 (LME datasheet). This will definitely raise the cost of each board a few dollars, but the distortion is several magnitudes lower (.003% for the OnSemi MC versus .00003% for the National LME) and noise is almost 10x lower (18 nV/ (sq. root Hz) for the OnSemi versus 2.7 nV/ (sq. root Hz) for the National). I was originally planning on socketing the op-amps, so that two different performance levels could be offered and op-amps could be swapped out, however it was suggested that the LME op-amps might not work well if socketed, because they are faster and have a much higher GBP. This makes board layout and bypassing much more important. It might work socketed or it might not. I just have to try it with the prototypes and see what happens. I would love to get some feedback on this from anyone who has designed and built PCBs using the newer fast op amps to learn about tips and tricks, etc.
For now, I would like to keep the idea of offering two performance levels of op-amps and socketing the devices and take a wait and see approach. How would people feel about not having two options but only having the more costly version available? Honestly, I believe that I need to keep costs down as much as possible without cutting any corners in order to keep these boards competitive with other crossover offerings out there...
So, those are my thoughts for now. I would love to get some feedback on these issues, and anything else that people might think of, so that I can work out everything in the near future.
Thanks,
-Charlie
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