Need help from electrical gurus... summing line level signals

Discussion in 'Home Theater Projects' started by RichardHOS, Jun 11, 2003.

  1. RichardHOS

    RichardHOS Second Unit

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    I'm sure the circuit must be rather simple, but I've never seen the layout for one, nor have I read any discussions on what the "best" way to do it is (if there are in fact more than one).

    Of course I'm talking about summing two channels of information into one, so that you don't get crosstalk between the channels. Is it a simple resistive network, or do you need active buffering? I'm summing the low frequency output from several outboard active crossover channels into a common channel, and I want to make sure I don't introduce crosstalk into the high frequency portions of those channels.
     
  2. Jonathan M

    Jonathan M Second Unit

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    If you are summing the low-passed outputs from several active crossovers, you won't get any crosstalk, as the signals are buffered by the opamps used in the active xover stages.

    If you want you could always add a simple unity gain buffer first to each signal THEN resistively sum. Or you could be flash and introduce a summation using active circuitry.

    I seem to recall you are using Rod's P09 boards? If so, I've summed both low passed channels on mine. I added the summation after the final buffer stage (Which I kept in to provide a bit more gain range for the low-passed signal - my subamp didn't have enough gain compared to the the amps doing the highs). Basically I replaced the output 100ohm resistors with 22k resistors and tied both ends together before passing through the output cap. This works fine.

    Generally you can just sum two signals in the same way - just run them through 22k or so resistors with their ends tied together. The crosstalk you get is then determined by the output impedance of the pre-amp which is usually low enough not to be an issue.

    Hope this helps. Rod may have some other ideas if you've used his boards.
     
  3. RichardHOS

    RichardHOS Second Unit

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    Hmm. So the output buffers in the XO should provide isolation? I thought that might be the case.

    What about something a little more insidious: say, running the LFE output of a pre/pro through a gain stage for adjustable level, and then summing back with the L/R mains? In that case, the LFE signal will be mono, but you're summing it with both left and right channels. The ~22kohm resistors should be enough to provide adequate isolation?

    Thanks for the comments. Perhaps I'll shoot Rod an email.
     
  4. Jonathan M

    Jonathan M Second Unit

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    The gain stage for the LFE (If opamp based) will give the isolation needed - so summing with 22k resistors should be fine. (I assume you are summing left, right and LFE signals?) Any gain stage should have a low output impedance anyway, so cross contamination won't be a problem. Also, the LFE output on the pre-pro will likely be buffered as well.
     
  5. RichardHOS

    RichardHOS Second Unit

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    Actually, I'm talking about two separate things. One, I will definitely be summing front/center/surround low pass outputs. Sounds like that isn't going to be an issue.

    Also, I'm considering summing the LFE output with the L/R mains before the XO network. The sole reason for this would be the ability to insert an adjustable gain in the LFE channel, in case whatever pre/pro I may have at any point in the future decides to leave that out, or gets confused about LFE level setting when it is set to "none" and routed through the mains. The reason I want to sum the LFE with the mains before the XO network and not with just the low-pass outputs is that the low-pass XO point will chop off the potential frequency range of the LFE channel (which can go to 150Hz, 180Hz, or even full range depending on what source you read). Once I started down the bi/tri-amping path and had an external crossover required, it only makes sense to completely remove all bass-management duties from the pre/pro and handle them with a fixed solution of the that handles all aspects in the desired manner.

    So my biggest concern, really, is making sure I don't introduce crosstalk when summing the mono LFE channel with both the left and right full-range signals.
     
  6. Jonathan M

    Jonathan M Second Unit

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    Ahh - now I understand! (Sorry for missing it the first time - a re-read of your post makes things clear!)

    You should be fine summing the LFE into the L and R channels if that is what you are desiring to do. I'd probably do this actively seeing as we are talking about the full range left and right signals - you want as little crosstalk as possible! I think Rod has a mixing circuit on his site - I'll see if I can find it.

    The LFE channel is a bit of a mystery as quoted around the web. Basically it IS brickwalled at 120Hz. The DD encoder handles this. Most encoders thus introduce a steep (8th order or so) lowpass at around 80Hz to prevent the phase issues with a brickwall filter. Generally there is very little above 80Hz - I have some frequency response graphs somewhere measured off my T752 - I'll see if I can find them on the weekend.
     
  7. RichardHOS

    RichardHOS Second Unit

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    Yeah, I never have really found a definitive source on the frequency range of the LFE channel. If it really does have little content above 80Hz, then summing with the low-pass sections of the crossover network would have the least chance of audible crosstalk issues. In that case, I guess the simple resistors after the output buffers would be adequate.

    If you find a good active summing circuit, let me know. I think one of the benefits of having external crossover networks is the ability to easily reconfigure how your signal routing is done. If I want to sum with the mains before the XO instead of after, I will be able to just swap a couple of interconnects around. [​IMG]
     

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