Help with this RCA-TRS speaker setup...

  1. salanos
    I'm back on Head-Fi for some help I need pretty quickly... I just went ahead and splurged on some powered 2.0 speakers to replace the passive satellites on an Altec Lansing 2.1 system.
    The 2.0 speakers have a 3.5mm input, while the Altec Lansing is basically everything done by the subwoofer as a hub, and it outputs sound to its satellites via RCA (hardwired to the satellites).

    So I went and got myself a red-white RCA to 3.5mm cable - usually, I understand, for sending a 3.5mm output to an RCA input. However - I'm doing the reverse here - an RCA output to a 3.5mm input.

    Now the sound seems to be transmitting - but the huge problem I'm having is that the left channel is also playing through the right speaker (obviously this has bad effects on sound) and I can't figure out how to fix this, if it's even possible. If it's not I probably have to look for some passive RCA speakers instead.

    It'd be a shame if this doesn't work out as the new speakers sound pretty good for the price and outperform the original satellites.
    Do I need to rework the cable? I went and dug up a red-white-yellow cable and tried using that on the supplied RCA-3.5mm adaptor with the speakers, but that has the same effect.
     
  2. JohnSD
    This is interesting, I would expect it to work assuming the cable is not wonky. You would be feeding from the woofer output a low impedance, amplified signal into essentially a preamp (your powered speakers) so its not ideal but I would expect it to work.

    The only problem I can see is if the L and R subwoofer outputs were differential amplifiers, so sending a L+ and L- and R+ and R- to the tiny speakers. If this was true any 3.5mm TRS cable would short the R- to the L- which could result in you hearing the other channel on each speaker. But I mean come on I would not expect to see this. If you can turn the sub off and probe the shells of the RCA connectors on the subwoofer with an ohm-meter or a short circuit detector you can test for this. If the shells on the subwoofer measure 0 ohms or short to one another then they are probably not balanced and you will still be confused because it should work. If they measure a high resistance or open circuit then you may have a balanced output and need to use passive speakers.

    If you cannot get into the tiny gap where the shells are on the subwoofer use a standard L/R RCA cable connected to the subwoofer at one end and nothing on the other end and probe the two shells there. Make sure the unit is unplugged and turned off for all probing procedures.
     
    Last edited: Jun 25, 2018
  3. drtechno
    ok, so you need to get a 3.5mm stereo "y" cable that has one plug and two sockets. plug the y cable into the computer, one output goes to the altec, the other, to the 2.0 powered speakers.

    The reason why its doing what its doing is the satellite speaker - is not dc ground, its an ac common only referenced in the altec unit on that channel, and this common is actually an inverted signal. So when you place the unbalanced powered speaker in that chain, you essentially summed the inverted signal on both sides and that signal is the common signal reference of the 2.0 speakers, so the end product of the signal for the left ended up with 2L - (-L + -R) and right ending with 2R- (-R + -L)

    The one thing i have to point out to everyone, Ground is only in DC. In AC, "common" is used because it is just the common signal reference reguardless if its the DC power, DC ground, an inversion of the signal, or zero crossing in AC.
     
  4. salanos
    If it helps, I'm trying to hook up a Gigaworks T40-II here.

    I was pretty much expecting everything to function as, well, expected - the right channel works perfectly fine, but I can't figure out what is causing the left channel to also play on the right channel.
    So what's happening exactly is:
    Right channel audio is normal; plays only on right
    Left channel audio plays on both left and right speaker

    I unfortunately have no probing tools I can use, so I'm not sure how to check what you've described.
    If it also means anything, the white and red plugs on the RCA-TRS cable seem to be inverted - I had to plug the white connector into the red socket and vice-versa before R wound up on R and L wound up on L (and R), which was not the case with the actual red-white-yellow composite cable I tried to use.


    So what you are suggesting is to split the output from the source with a 2x 3.5mm Y-adaptor to the Altec subwoofer and the Gigaworks?
    I am hoping as much as possible to output the L/R to the speakers from the Altec subwoofer as this model has the SFX and Loudness DSP. The former feature is the main reason I have kept this system for so long and was hoping to be able to use on new satellites.


    Worst case scenario I suppose is looking for appropriately sized passive speakers, but most these days are bookshelf-sized and would have difficulty fitting in my space.
     
  5. drtechno
    well there is two solutions if you really want that connection:
    1. you can bypass the electronics and run them as raw speakers (soldering required)
    2. Get a set of signal transformers to establish an isolated connection that can be used as an unbalanced connection.

    method 2 is simple. as we do things like this all the time, even with higher wattages (guitar re-amping). This might cost you more than your 2.0 speakers are worth.

    so, I suggest a passive stereo di box like this one:
    https://www.sweetwater.com/store/detail/ZdirectDual--art-dualzdirect-2-channel-passive-direct-box

    with a cable like this, just add a coupler:
    https://www.sweetwater.com/store/de...inch-trs-to-2-xlrf-y-cable-10-foot-xlr-female
     
  6. ProtegeManiac Contributor
    How exactly did you reconnect the system? I'm having a hard time visualizing how exactly you rewired them. Most 2.1 speakers I've used had a 3.5mm stereo jack on the sub module and the 3.5mm cable goes into the left speaker, hardwired, then another hardwired cable goes from the left speaker to the right speaker. If your subwoofer module's amplifier outputs are independent Left and Right RCA then that's two separate signals, but if your new 2.0 speakers have a similar wiring where the cable goes into the left speaker then out to the right, then that could be where the problem is. Have you tested it hooked up directly to the soundcard or any other stereo 3.5mm output? If you still get the same problem then there's something broken with the wiring on the 2.0 speakers.

    Also if you're using a desktop computer and not a laptop you can just use the FL/FR jack on the motherboard/soundcard to directly feed the satellites. You won't have unity gain on the sub module's main volume control (if it's even on the sub module as this is usually on the left satellite) but if it gets around the bigger problem, not to mention this will not have double amping, then you can just use Windows volume control.


    Barring opening up the speakers and tearing out the active components and rewiring the cables to the drivers, this is somewhat difficult to do since most speakers with RCA inputs are powered speakers that need to take in a line signal or are passives from a set that has the amp on the sub module.

    And independent passive hi-fi speakers might not have a high enough sensitivity to work properly with the output power of the sub module's amp, which tend to be around 5W or lower, as they were intended for 3in or smaller fullrange drivers with very high sensitivity (with trade offs to the linearity of the response).
     
  7. JohnSD
    This is what you should do, one 3.5mm into the PC, one Y 3.5mm into the sub, other Y 3.5mm into the actives. Can't believe I did not think of it, good thing there are smart people here.
    I don't think you are going to be able to do this very easily with your active speakers. Either way, the SFX and DSP are probably customized to the altec passives so might not be suitable for your new actives.
     
  8. salanos
    Here's the best illustration I can give at the moment. If it's not clear then do let me know what needs elaboration.
    [​IMG]
    At present, the source is a 96kHz optical signal being fed from a dedicated sound card into an amp, which the Altec is connected to by its supplied 3.5mm cable. Then the Altec subwoofer also has connections for its control module (volume/bass/treble/Loudness/sound field xpander) and there are a white and red RCA output which at present I've reconnected the original satellites, which are passive.
    The Gigaworks are 2.0 active speakers with their own power source and I connected them via a RCA-3.5mm cable to the outputs on the subwoofer. The left speaker is slaved to the right speaker (hardwired to the left speaker but not the right speaker), which is likely part of the problem. All inputs/power go through the right speaker.
    If it helps, I believe the original satellites specification is 2x 10W@4Ω, so I was thinking that if I could find some similarly specced passive satellites it should be safe. That said I never play these things at max volume, though at this point I would hate to accidentally blow out the subwoofer by trying to drive something too strong through it. I'm not particularly familiar with all these electrical sides of things, too.

    Again to emphasize, right channel audio comes out normally through the right speaker on the Gigaworks but the left channel comes out both sides.
    They of course work perfectly normal with a normal line-in (3.5mm) input.

    The main reason I'm trying to do this is the sfx (sound field xpander) DSP on the Altec Lansing. It has a genuine stereo expansion effect - the kind that actually makes the speakers feel like they're 10m apart, rather than the kind that adds some weird tinny echo effect.
    Were I to split the signal via Y-jack (I have one on hand, that said, and even considered this idea in advance), I would lose the ability to control the Gigaworks with the Altec and the ability to apply sfx to the satellites, which is the reason I am trying to do this in the first place.
    That said, while connected to the Altec, the sfx function worked on the Gigaworks.

    Admittedly I should have probably done some research on this beforehand and I don't think I'm too optimistic this will work.
    The PC speaker market is pretty quiet these days, so I'm not sure what out there would work. Most bookshelf speakers are too wide to fit in the space and I don't know what's out there among passive speakers with RCA connections that are slim and good sounding. Another option I suppose would be to cannibalize the satellites off another 2.1 system, if there's one that's suitable.
     
    Last edited: Jun 27, 2018
  9. ProtegeManiac Contributor
    Wiring is otherwise correct and if they work normally direct to the source then the problem seems to be on the subwoofer module, given another RCA cable with an adapter also has the same problem. That said, if it's on the subwoofer module, it should have been happening with the original satellites that came with it. Looks like you didn't have that problem until you had to use a 3.5mm cable so it's baffling how there can be severe crosstalk from the left channel when the new satellites are otherwise working normally when not using that sub module.

    One possible explanation though is that the problem really is the cable and the adapter are both not going in properly into the new satellites. Try pushing them in and see if it helps. If it does, and there's something near the mini TRS plug like a knob or whatever, try another cable with a very thin TRS plug barrel.

    One possible reason is there might really be something different in the wiring on the sub module. Where is the main volume control on it? That might be what's making the left channel bleed across both channels.

    What you can do is just hook up the sub to your motherboard's sub out and the new powered satellites direct to the FL/FR jack on the motherboard, and then just use the OS volume control.


    Well it's not the speaker that is "powerful" or not, but the subwoofer module's amplifier. And higher power handling on the driver hooked up to it doesn't necessarily mean the amp output is inadequate. Sensitivity and impedance are a better indicator of that since even if a speaker is rated to be capable of 100watts or 25watts if the speaker has a sensitivity that's high enough to get to decent volume with power to spare that way it won't clip, for example if the sensitivity is 93dB/1mW at 1M then you should be able to get to 93dB with power to spare for dynamic peaks.
     
    Last edited: Jun 27, 2018
  10. drtechno
    Well the satellite speaker outputs on the Altec Lansing is the inverted signal on its negative connection instead of it being a dc ground. shorting this connection to ground may lead to damage of the satellite outs because of this balanced signal connection. Hopefully, Altec added a loading resistor in series just in case you short the negative on the rca to a ground (which would prevent this damage).

    I think you are observing a secondary effect of these loading resistors mixing the inverted signal. It will not be symmetrical as this would change by a m-s stereo field subtraction of sorts.
     
    Last edited: Jun 27, 2018
  11. salanos
    Bass, treble, volume, sfx/loudness controls are on a wired control pod.
    Currently I'm considering purchasing a preowned MX5021 and using the satellites from those with speaker-wire-RCA convertors, or perhaps getting Harman HKTS 30SATs, if those use RCA connectors, if this setup turns out to be a failure.

    I'm hoping I've not caused any permanent damage in the experiment, but at present everything seems normal with the original satellites connected as before.
    You suggested previously; "Get a set of signal transformers to establish an isolated connection that can be used as an unbalanced connection.", could you please elaborate on this for a dummy?
     
    Last edited: Jun 27, 2018
  12. ProtegeManiac Contributor
    Or just connect them direct to the motherboard. Your new satellites on the FL/FR jack and the subwoofer on the C/Sub jack.
     

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