Modular Active Crossover Boards in development

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  • charlielaub
    Seasoned Veteran
    • Jul 2006
    • 2345

    #31
    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
    Charlie's Audio Pages: http://audio.claub.net

    Comment

    • BobEllis
      New Member
      • Dec 2010
      • 19

      #32
      Re: Modular Active Crossover Boards in development

      For main L/C/R speakers, unbalanced only is usually fine, since most of us put our equipment rack in the front of the room and signal runs are short. Those who put their equipment in the back of the room may want balanced runs to their amps in the front of the room.

      I'm working on a set of active surrounds and the signal runs to the rear speakers will be around 50'. The power for the surrounds is on a different circuit than the fronts, so a ground lift may be awfully handy.

      I like the idea of a plug in INA134 module for balanced input.

      Deward, funny that you think Jens' layout was crowded. I was pushing him for a few more features. :eek:

      Comment

      • Deward Hastings
        Seasoned Veteran
        • Feb 2006
        • 3680

        #33
        Re: Modular Active Crossover Boards in development

        Originally posted by BobEllis
        Deward, funny that you think Jens' layout was crowded. I was pushing him for a few more features. :eek:
        I seem to recall getting pretty "3-D" over the board a couple times ;) . . . it sure wouldn't have passed a NASA vibration test . . .

        Charlie . . . you're running head on into the "all things to all people" problem . . . and what you end up with is a large board with a lot of functional blocks and jumpers between them.

        As for socketed op-amps . . . it was fine for the older limited gain-bandwidth parts, but some of the newer stuff just doesn't like the extra lead length. Plus, when you switch op-amps it is often also necessary to change overall circuit impedances, making the whole socket exercise pointless. It's actually pretty easy to make the case for surface mounting the "gain blocks" (op-amp and bypass) components, while remaining through-hole for the networks . . .
        "It suggests that there is something that is happening in the real system that is not quite captured in the models."

        Comment

        • BobEllis
          New Member
          • Dec 2010
          • 19

          #34
          Re: Modular Active Crossover Boards in development

          Not to mention the cost of the socket is the same as the cost of upgrading the opamp.

          Comment

          • charlielaub
            Seasoned Veteran
            • Jul 2006
            • 2345

            #35
            Re: Modular Active Crossover Boards in development

            Originally posted by Deward Hastings
            Charlie . . . you're running head on into the "all things to all people" problem . . . and what you end up with is a large board with a lot of functional blocks and jumpers between them.

            As for socketed op-amps . . . it was fine for the older limited gain-bandwidth parts, but some of the newer stuff just doesn't like the extra lead length. Plus, when you switch op-amps it is often also necessary to change overall circuit impedances, making the whole socket exercise pointless. It's actually pretty easy to make the case for surface mounting the "gain blocks" (op-amp and bypass) components, while remaining through-hole for the networks . . .
            Deward,

            Don't forget, with the modular design, the functionality is actually spread over several boards. This allows people to only buy what they need/want AND permits later expansion with additional boards. People who don't want some parts of the total functional capability can just not buy those boards. For that reason I'm not too worried about the board size growing too large with lots of jumpers, etc.

            Regarding the socketed ICs... if I have one rule, that is that a normal guy with average soldering skills should be able to assemble everything in a straightforward fashion. As a result, I am ruling out anything that comes only as small SMD device. There are still plenty of options left open for audio. But if the op-amp DIPs are soldered directly to the board, I am afraid that average soldering skills may lead to overheated and dead parts, so I will try as much as possible to use the sockets, if only to protect the ICs. And a DIP14 socket is pretty cheap! The design includes 0.1uF ceramic bypass caps at the PS pins, and electrolytics where the supplies connect to the board.

            -Charlie
            Charlie's Audio Pages: http://audio.claub.net

            Comment

            • Æ
              Seasoned Veteran
              • Dec 2005
              • 6695

              #36
              Re: Modular Active Crossover Boards in development

              Even though my soldering skills are superb, I still prefer my to "DIP" my ICs into sockets. Whenever I build a kit that comes without a socket, I go ahead and install one anyway.

              Originally posted by charlielaub
              Deward,

              Don't forget, with the modular design, the functionality is actually spread over several boards. This allows people to only buy what they need/want AND permits later expansion with additional boards. People who don't want some parts of the total functional capability can just not buy those boards. For that reason I'm not too worried about the board size growing too large with lots of jumpers, etc.

              Regarding the socketed ICs... if I have one rule, that is that a normal guy with average soldering skills should be able to assemble everything in a straightforward fashion. As a result, I am ruling out anything that comes only as small SMD device. There are still plenty of options left open for audio. But if the op-amp DIPs are soldered directly to the board, I am afraid that average soldering skills may lead to overheated and dead parts, so I will try as much as possible to use the sockets, if only to protect the ICs. And a DIP14 socket is pretty cheap! The design includes 0.1uF ceramic bypass caps at the PS pins, and electrolytics where the supplies connect to the board.

              -Charlie

              Comment

              • charlielaub
                Seasoned Veteran
                • Jul 2006
                • 2345

                #37
                Time Delay / Soft-Start / Relay Board

                Thinking about what other kind of circuitry might be useful, I thought that something that could prevent turn-on and turn-off thumps might be useful. After playing around with a few ideas, I realized that a general circuit that can implement turn-on and turn-off time delays via LM555 timers would be very useful for:
                * delaying the start-up of the power amps(s) while the line-level crossover circuitry PS stabilized
                * delaying the turn-off of the line-level crossover circuitry while the power amp(s) shut down
                * implementing soft start using a series power R that is switched out of the AC line after a few tenths of a second
                * power amp mute
                The circuit is activated (turned on and off) via a single pole AC switch. All switching is done via 12VDC coil "ice-cube" relays that are external to the circuit board, to keep AC away from the DC power as much as possible. Both the delay-off and delay-on times are adjustable for each timer via potentiometers. The present design has three independent timers, however the number of timers could be increased.

                This would be a very useful circuit for a number of DIY applications where power amp and line level circuits operate together.

                -Charlie
                Charlie's Audio Pages: http://audio.claub.net

                Comment

                • selder
                  New Member
                  • Dec 2010
                  • 15

                  #38
                  Re: Modular Active Crossover Boards in development

                  If you build it I will come...EXACTLY what I am looking for

                  Comment

                  • rogoll
                    Seasoned Veteran
                    • Oct 2008
                    • 1070

                    #39
                    Re: Modular Active Crossover Boards in development

                    Originally posted by charlielaub
                    Hi Phil,

                    I suggest getting a couple of these very handy helpers:

                    I have several of them! You can hold parts with one, and the circuit board with another. They are currently on sale, too!

                    These will be initially sold as a parts kit - everything must be soldered on to the board. But it's all thru-hole components THERE ARE NO SURFACE MOUNT DEVICES! Op amps are in DIP packages. Connections are made via terminal blocks. Nothing the average guy with a basic soldering iron can't handle.

                    -Charlie

                    Harbor Freight has an even better one (they carry the one pictured, as well), that has LED lights to illuminate what your working on, and also has a soldering iron holder and sponge. I HAD to have it to be able to work on the Dayton T-Amp!

                    Comment

                    • jonpike
                      Seasoned Veteran
                      • Sep 2005
                      • 1790

                      #40
                      Re: Modular Active Crossover Boards in development

                      Originally posted by charlielaub
                      Deward,

                      Regarding the socketed ICs... if I have one rule, that is that a normal guy with average soldering skills should be able to assemble everything in a straightforward fashion. As a result, I am ruling out anything that comes only as small SMD device. There are still plenty of options left open for audio. But if the op-amp DIPs are soldered directly to the board, I am afraid that average soldering skills may lead to overheated and dead parts, so I will try as much as possible to use the sockets, if only to protect the ICs. And a DIP14 socket is pretty cheap! The design includes 0.1uF ceramic bypass caps at the PS pins, and electrolytics where the supplies connect to the board.

                      -Charlie
                      I'd agree with the flexibility and part protection goals... If one feels the need to be a purist, just solder the chip straight to the board. I would recommend the round pin, "machined" type of IC socket, they make a much better (and lasting) connection, then the cheap, folded spring types. Gold plating is worth the few more cents, too.

                      And FWIW, I've been using the DIP LME497xx in a socketed, hand soldered prototype board at work, (lab built transimpedance microammeters) with ugly layout but proper cap bypassing... and haven't had problems with oscillation. They DO, without the bypassing, though! :eek:

                      Comment

                      • marscoast
                        Midrange Member
                        • Jan 2009
                        • 300

                        #41
                        Re: Modular Active Crossover Boards in development

                        I'm very interested in these. Even at a slightly higher price-point (with the upgraded op-amps and balanced input option) I think they'd be well worth it - not to mention fun!

                        Regarding the balanced input option - would the INA137 be a better choice? It has a gain of .5 to bring down the voltage from the balanced outputs of our preamps/crossovers/dacs.

                        Comment

                        • charlielaub
                          Seasoned Veteran
                          • Jul 2006
                          • 2345

                          #42
                          Balanced input and output board

                          For balanced input: it looks like I would lean towards the topology proposed on page 11 of the THAT Audio TH1200 series datasheet:


                          This allows dual compatibility with the THAT 1240 ICs, as well as the INA134 and the SSM2141 ICs that have the standard 8-pin assignments. This provides many options in terms of performance.

                          I will likely only supply the PCB for the balanced inputs and outputs and let people populate it with what ever IC they desire.

                          -Charlie
                          Charlie's Audio Pages: http://audio.claub.net

                          Comment

                          • charlielaub
                            Seasoned Veteran
                            • Jul 2006
                            • 2345

                            #43
                            Development Update

                            Thought I would post an update on the active filter boards... I am eagerly awaiting the prototypes for the input and filter boards that I ordered. Once they arrive I can build them up, check functionality, and then move on to the other circuits that I am planning.

                            All of these will be offered as kits = PCB plus parts that you assemble (solder). All parts are thru hole.

                            Here is a list of the kits I plan to offer:
                            input board
                            filter board
                            biquadriatic filter (AKA Linkwitz Transform) board
                            delay board
                            timer board

                            The input and filter board are described here:
                            MODULAR CROSSOVER OVERVIEW

                            I recently did some circuit design and modeling for the LT and delay boards. Here is a summary:

                            DELAY BOARD: This permits constant analog delay that is actually useful, because the delay remains constant across the entire crossover region. SL touches on delay in he web site, but the single first order section that he describes is inadequate for most applications because either not enough delay can be achieved or the delay is changing (decreasing) across the crossover region. The circuit I have designed will be continuously adjustable, will offer higher delays and higher corner frequencies, so that constant delay can be applied.

                            LINKWITZ TRANSFORM: The LT is a very useful circuit for active crossovers. It makes is possible to change the high pass behavior of a driver in a sealed box, either boosting or cutting power and changing the phase behavior so that the driver effectively has a new Q and Fb. Since the driver's HP function can be incorporated in to the crossover as a second order stage, being able to change this at will is very powerful. Unlike SL's LT circuit, which has a high sensitivity to component variation and can't be adjusted once built, the circuit that I have developed will be able to create both boost and cut type of transforms, and has adjustable parameters for the Q and Fc of the physical and transformed high pass functions. This means you can pretty much do anything with it, and reconfigure it as needed.

                            TIMER BOARD: This will have three independent timers for which the turn-on and turn-off delays are adjustable up to about 10 seconds. These can be used to sequentially switch on/off low level circuitry, power amp supply, implement soft start, mute/unmute amplifiers, etc. Relays are external to the PCB and will be ice-cube 10A DPDT type or similar. These are reliable, inexpensive, and universally available. 10A will permits switching of just about anything.

                            I hope to be releasing these during the Spring, as development and testing permits.

                            -Charlie
                            Charlie's Audio Pages: http://audio.claub.net

                            Comment

                            • jpomann
                              Been Around Awhile
                              • May 2009
                              • 74

                              #44
                              Re: Modular Active Crossover Boards in development

                              Charlie,

                              When trying to do "everything", active XO boards can become needlessly large. For example, adding a single passive cap on the tweeter after the amp can often result in a much simpler lower-order active XO solution (smaller active board with fewer parts). Dont forget that you can also use a 1st or 2cd order passive-line-level filter (PLLXO) between the active XO and the power amp. Don't forget that you can use feedback from the PLLXO back to your last op-amp (great opportunity for a PLL shelving filter?).

                              I encourange you to make your main board as simple as possible (?limited to 2cd order active filters?), and expand upon ways to achieve higher orders and shelving (when needed) with PLLXO's and post-amp first-order passive filters. Of course, the Linkwitz transforms and phase shift time-delays are then appreciated as add-on boards.

                              JP

                              Comment

                              • charlielaub
                                Seasoned Veteran
                                • Jul 2006
                                • 2345

                                #45
                                update on modular active crossover boards...

                                Well, a month has passed me by and unfortunately not much has progressed with the boards. I sent off the PCB design for the modular crossover board with the HP/BP/LP filters before the Xmax holiday to a company that makes cheap prototypes in Bulgaria. Well that was a mistake, since I still haven't gotten any thing from them and I am out 60 Euros. That's the last time I use them for anything. My next choice fab is shut down until next week, but the turn around time should be much faster, and I am hopeful that these will get off the ground soon. Here is an update on what it coming down the pipeline...


                                UNIVERSAL ACTIVE FILTER BOARD
                                In the mean time, I have come up with a "universal" active circuit. I call it universal because the same circuit footprint can be configured as any of the following:
                                inverting and non-inverting gain stages
                                first order HP or LP filter stages
                                second order HP or LP filter stages (Sallen-Key)
                                second order filter stage plus notch, HP or LP
                                notch filter
                                first or second order all-pass filters
                                This means you can create just about any crossover function that exists with these boards, including elliptical filters, Chebyshev type II filters, Hardman filters, etc.

                                The "universal" circuit can do all that, but it is not adjustable after you build it. The user would need to determine and source all the components for the design. As a result, these will be sold as a bare board only.


                                MODULAR ACTIVE FILTER BOARD
                                For something more user friendly, the modular crossover board has a fixed design that is adjustable for frequency and Q after it is built. So between these two products, I think that the bases will be covered nicely. See the overview pdf file at this link for more information.


                                OTHER CIRCUIT BOARDS IN DEVELOPMENT
                                In addition, the new plan is to offer the following:
                                regulated power supply kit
                                Equalizer board (use with BP output from the modular crossover board)
                                6dB/oct shelving filter board for baffle-step / open-baffle compensation
                                Biquadratic filter to implement a "Linkwitz transform" style equalizer
                                Some of these were previously lumped together on what I called the "main board", but I got many requests to have these available separately, so I decided to break each part out on to its own PCB.

                                All PCBs will have an identical footprint, so multiple boards can be stacked up using standoffs to make a compact package.

                                I will update this thread with new details as they come up. Please post any questions or comments and I will be happy to answer them. Thanks

                                -Charlie
                                Charlie's Audio Pages: http://audio.claub.net

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