Cambridge BassCube 12 blowing relays

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  • blargg
    Senior Member
    • Feb 2010
    • 51

    #1

    Cambridge BassCube 12 blowing relays

    I got a broken Cambridge Soundworks BassCube 12 someone was giving away. Powered on, and sub suddenly went all the way in/out (not sure which). Disconnected and I measure -49V on speaker output. Ouch.

    I trace things and find that it has a DPDT relay that connects the +/-50V rails to the amplifier, and that one side is stuck closed, and the other stuck open, so the amp was only getting -50V. I had also noticed that one of the transistors was hot, but the other wasn't. With it off, I didn't measure any shorts between the rails and the speaker output, so the transistors aren't shorted.

    Oddly there was a second relay inside whose coil was in series with the main one, but had NOTHING connected to the contacts. So I rewired the power rails to this one, and it promptly blew out the contacts for the +50V; they're open permanently now.

    I found another SPDT relay and connected it in series to control that rail, and it didn't destroy it. I got ~0V at speaker output, so I connected it to the speaker and powered on. Loud pop, then the same -50V again, with the speaker not liking it at all. As best as I can tell, the protection circuit is kicking in when it's turned on, but only turning ONE relay off, perhaps just lowering voltage until both rails aren't on. So the speaker gets -50V again.

    At one point, only the +50V rail was connected, and the OTHER transistor got hot. So the first time one got hot was clearly a result of having only one rail.

    If I UNPLUG the speaker from the amp, power in, THEN plug the speaker in, it works beautifully. Been running it for hours now, with georgeous room-filling bass and no hum or anything. I checked and none of the amp transistors is getting hot now. I'm going to get a really beefy DPDT relay and see if perhaps the relays in it were just old or maybe got worn out by all the bass. They were 5A 250V, so it's hard to believe the contacts would blow that easily. There aren't any capacitors either on the amp side after the relay, so no inrush current to fuse the contacts.

    Also a side question about improving the voltage regulator design: With this thing on for a while now, I'm noticing that almost all the heat comes from what seems to me a terrible voltage regulator design: power resistor to zener diode. They employ FOUR of these, for two +/-15V supplies from the +/-50V filtered/rectified output of the transformer. Would there be any downside to just putting some 15V regulators in place of this? The PCB is really toasted/brown from all the heat these wasteful regulators generate. I'd rather not subject it to more heat, and save some electricity as well. I'm thinking that there's one possible snag: regulators would put less constant load on the transformer, which might result in a higher +/-50V supply since it's unregulated off the transformer. I'm wondering why they don't use a switching supply in subs like this, since switcher noise is high frequency and therefore totally filtered out by the nature of the sub being very low-frequency.
    Last edited by blargg; 02-10-2013, 05:28 PM.
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Cambridge BassCube 12 blowing relays

    I am surprised that this unit does not have speaker relay protection that will detect DC present at the speaker and shutdown the speaker relay. To drop 50V down to 15v, the regulator IC will have 30V voltage drop across it, if the 15V circuits draw lets say 100mA, it means that the regulator will have to dissipate about 3 Watts of power. This setup will be pushing the Vin limit of the regulator.
    The power relay contacts may expose to high inrush current, so what ever you put in, you should find out the inrush current spec if they can provide it.
    By the way, is this thing using Class D amplifier or linear amplifier?
    Last edited by budm; 02-11-2013, 12:03 AM.
    Never stop learning
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    Comment

    • blargg
      Senior Member
      • Feb 2010
      • 51

      #3
      Re: Cambridge BassCube 12 blowing relays

      I wonder whether this is relay system is related to detecting large DC on the speakers, and was failing to work. I could always put the speaker on one of the relay switches, as that would have protected it from these stick DC episodes.

      I researched the regulator a bit and found that I could use a 317/337 adjustable regulator, which has an absolute maximum drop of 40V from in to out. In this case it'd be dropping 50V to around 16V, a 34V drop. I'll have to see if I can measure how much current the preamp is using, and the amp.

      I hadn't thought about the regulators having to dissipate a lot. Perhaps I could drop to say +/-20V, then use normal 7816/7916 regulators to get to the two +/-16V supplies. Or heck, once it's dropped to +/-20V, use their resistor-zener regulators, since they'll be dissipating a lot less.

      But maybe I'm not paying attention to the fact that both of these use resistive means to drop the voltage, and thus might be hardly any different in terms of efficiency. Switching regulators maybe? This is getting involved. My Kill-A-Watt says it uses about 20 watts (@120V AC).

      I don't think the relays had any inrush because they're after the power supply filter caps.

      It looks like a linear amplifier. It's over a decade old. No inductors on the output either. It's got a huge metal-cased power transformer inside.

      Comment

      • blargg
        Senior Member
        • Feb 2010
        • 51

        #4
        Re: Cambridge BassCube 12 blowing relays

        After fighting with this protection circuit all day, mapping it out and seeing how poorly it operates, I just bypassed the relay so the amp is always powered, then put the speaker on the relay and manually switch it in after the amp has been on for a few seconds. This eliminates all popping and nasty stuff, and it works perfectly. I also moved the power resistors from the underside of the board by the zeners to the copper trace side on top, which gets the heat away from the zeners and other sensitives there. Either this thing is way overpowered, or I'm just running it at very low volume compared to what it can do. I think an amp one quarter this size could easily drive it plenty loud. Oh well, I've spent way too many hours and it's working and hopefully won't fail and kill the speaker...

        Comment

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