Sub Woofer Hum... This One Puzzles Me
Hi Scott, Are you saying that the hum occurs even when the sub is unplugged from the AC and the amp supplying the signal to the sub is also turned off? If so, are you sure that the hum continues to occur after several minutes have elapsed, when the power supplies in the amp and in the sub's internal amp can be presumed to be fully discharged? Also, a way of implementing Marakanetz' suggestion about lowering resistance, without changing to the main outputs of the amp, would probably be to put a suitably chosen high power resistor between the + and - terminals of the sub inputs, for each channel. Let us know the make and model of the amp that is driving the sub, and I should be able to recommend a specific resistor. Best regards, -- Al |
Hi Scott, Yes, that's certainly a puzzler. I see that the 2w series of subs is described as using some kind of feedforward error correction. Perhaps that is allowing small amounts of leakage between the AC line and the circuit ground of the MC7100 to find its way past the sub's amplifier to its driver. When the MC7100 is turned on, the low output impedance it has when powered up kills that leakage. BTW, I see that the MC7100 was first released about 20 years ago. If by any chance it has a non-polarized two-prong power plug, I would try it with the opposite orientation from the one you have been using. If it has a polarized two-prong power plug, temporarily try reversing its orientation using a cheater plug that has non-polarized (identical) prongs. Beyond that, assuming that you are using the MC7100 in stereo (non-bridged) mode, for which it is rated at 100 watts into 8 ohms, purchase 2 of the following 100 ohm 20 watt resistors, and connect them between the + and - input terminals of the sub (1 resistor on each channel). I suspect that this will resolve the problem, with no sonic side-effects: http://www.digikey.com/product-detail/en/HLW20R1Z100R0JF/HLWB-100-ND/713741 Best regards, -- Al |
Hi Scott, I hadn't been envisioning that the sub only accepts bananas. It looks like at least some versions of the MC7100 have screw type terminals, although I see that you referred to banana terminals on it. If the terminals on the amp are either screw type or multi-way binding posts, I suspect that if you connect a resistor there it would be as effective as if it were connected at the sub. Otherwise, you'll probably have to kludge up some connection arrangement, as I doubt that a resistor lead and banana plug would fit into a banana jack simultaneously, at least in a secure manner. Also, another thing to try (first) would be shorting the + and - terminals of the unused sub input channel together. Although the manual seems to imply that leaving an unused input open is ok, I wouldn't consider that to be definitive for all systems. It may depend on the output impedance characteristics of the amp when the amp is in the powered down state, which are unpredictable. Best regards, -- Al |
Hi Scott, I have no idea why Richard would consider shorting the unused input to be inadvisable, as long as the connection is applied several minutes or more after power has been removed. The sub would see the short as a zero volt signal being applied to it through a zero ohm source impedance. That is essentially no different than how it would see the output of a solid state power amplifier that might be connected to that input, with that amplifier being in the powered up state while no music is being played. His suggestion was to run a ground wire from the ground terminal on the sub amp to the amp or preamp. Is this just a matter of taking a piece of unterminated wire and securing it under the ground terminal on the sub amp on one end and any screw on the back panel of the main amp on the other end?Not sure. If the ground terminal on the sub is the same as the negative signal input terminal, or if it is connected directly to that terminal, and if the chassis of the amp or preamp is not connected directly to its circuit ground (for instance, if that connection is made through a resistor or inductor or capacitor in the amp or preamp), doing that would amount to an alteration of the grounding configuration of the amp or preamp, possibly affecting sonics. I don't think it would hurt to give it a try, though, PROVIDED that you are NOT running the MC7100 in bridged mono mode. And if perchance you are running the MC7100 in bridged mono mode, it is conceivable to me that the hum problem might go away if you were to change the connections and the stereo/mono switch on the amp such that you are running in stereo mode while using just one channel. Hope that helps. Best regards, -- Al |
