Today’s question caught our attention because it came to us entitled, “Is This Guy Crazy?” It took more than one round of e-mail for me to understand the question, but it boils down to the following (heavily edited):
“I know a sound guy who is taking the speaker output from a QSC power amp that is driving a pair of speakers and also running it into a Hartke bass amp, then taking the speaker out of the Hartke to a bass cabinet originally designed for automobile use. What are the ramifications of this? I know that speaker outputs of power amps shouldn’t be plugged into other amplifiers. There is a terrible hum on the system as a result of this, whenever he plugs in the Hartke. Please let me know before this guy blows our church sound system. PS-this guy is not in anyway a sound tech, he’s an electrical engineer who just happens to think that this qualifies him to run sound reinforcement equipment.”
Boy, you have several problems here: You are correct that it is bad to connect the output of a power amp to the input of another amp. The only saving grace in this case is that it is in parallel with other speakers. The speakers, due to their low impedance (compared to the amp input) are taking most of the wattage, which means very little power is being pushed in to the other amp’s input. Had this not been the case you would probably already be taking the Hartke to the shop. Presumably the electrical engineer knows this, or he’s just lucky. If it is desired to take a feed to this subwoofer off of the same signal as that QSC power amp, why not just parallel off of the INPUT to it? Then the levels should be matched better, though if the bass amp is expecting the level of a bass guitar you may still be overloading it with this line level signal. You’ll just have to try it. There are ways of padding the signal down to a more suitable level. At least this way there is no danger of damaging the Hartke. The hum could be any number of things at this point. I suggest getting the system connected properly before digging in to it. It sounds like your biggest problem may be this engineer. People who are competent in a given discipline sometimes assume they are competent in another discipline when it appears similar to them. I have encountered this electrical engineer (or worse, the electrician) mentality in audio a number of times. Most of the time they are a big help because they can help find tweaky things like ground loops and other electrical anomalies. But they don’t always have a good sense for quality audio. They see things from an engineer’s perspective and not at all from an artesian point of view. You may be in the middle of a “church politics” issue. I can’t advise you here, but I can tell you that you need to find a way to reduce this guy’s control of the system and give it to someone fluent in connecting and operating audio gear.
My final point is to get the right gear for the job. I know money is an issue, but when people start connecting all sorts of audio gear in unorthodox ways the results are rarely better than just using the part of the system that is designed to do the job. I’m not sure why you are trying to use this subwoofer, but I speculate that given the components and the way they are having to be connected you may be better off without it until you can afford to do it right.