Hi all.
My first post here, so please forgive any unwitting transgression of the rules written or unwritten. I’m also hoping this is the correct forum. I appreciate its about a subwoofer but the issue feels more like an amplifier one. Please feel free to set me straight on any of the above.
So preamble over, here’s the issue:
A friend of mine is trying to connect a single subwoofer to two monoblocks. The high level connection to the SW has three leads and in his stereo amp, with common negative terminals, it is quite happy connected to separate left and right +ve and a single -ve terminal. With the monoblocks, this setup immediately kicks in the power protection circuits on both amps. Conventional wisdom, other forums and the SW manufacturer all state that you can’t do this and two subs is the only option. The problem is these things are £2.6k each and occupy a significant volume of his lounge. So while he would consider buying another one, he’d sooner not just now.
As I see it the issue is that both amps are somehow being connected inside the subwoofer’s amp and the fact that their -ve lines are not connected is producing an imbalance. My thoughts are that if I can isolate the two channels from direct contact with the sub’s amp before they are combined then the problem should go away. With apologies for the crudeness of the diagram, the attached is a suggestion for using two isolation transformers to decouple the monoblocks from the sub, while allowing the signal to pass through.
Thanks in advance to anyone who can spot the school boy error(s) I’m sure are lurking in this somewhere. If it works, I can’t believe I’m the first person to think about it.
Cheers
Alan
My first post here, so please forgive any unwitting transgression of the rules written or unwritten. I’m also hoping this is the correct forum. I appreciate its about a subwoofer but the issue feels more like an amplifier one. Please feel free to set me straight on any of the above.
So preamble over, here’s the issue:
A friend of mine is trying to connect a single subwoofer to two monoblocks. The high level connection to the SW has three leads and in his stereo amp, with common negative terminals, it is quite happy connected to separate left and right +ve and a single -ve terminal. With the monoblocks, this setup immediately kicks in the power protection circuits on both amps. Conventional wisdom, other forums and the SW manufacturer all state that you can’t do this and two subs is the only option. The problem is these things are £2.6k each and occupy a significant volume of his lounge. So while he would consider buying another one, he’d sooner not just now.
As I see it the issue is that both amps are somehow being connected inside the subwoofer’s amp and the fact that their -ve lines are not connected is producing an imbalance. My thoughts are that if I can isolate the two channels from direct contact with the sub’s amp before they are combined then the problem should go away. With apologies for the crudeness of the diagram, the attached is a suggestion for using two isolation transformers to decouple the monoblocks from the sub, while allowing the signal to pass through.
Thanks in advance to anyone who can spot the school boy error(s) I’m sure are lurking in this somewhere. If it works, I can’t believe I’m the first person to think about it.
Cheers
Alan
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If the monoblocks can drive the low impedance, why not use the trick used in car audio. Invert the signal to one of monoblocks. Reverse the connections to that full range speaker. The other channel remains as is. This then gives you the option to bridge the sub woofer. A passive crossover can then be added if required to split the bass frequencies between full range and subwoofer. First, stumbled across this in the 90s with Kenwood's tri-mode.
Paul
Paul
A transformer is a bad idea for connecting a subwoofer. In the secondary market, you can inexpensively purchase a subwoofer from a home theater. For a responsible recommendation, you need to know the brand names of monoblocks and subwoofers.
Thanks for coming back to me. I'll have a look at the inverted input idear but I have a feeliong the protection may object again.
In terms of a bad idea to use a transformer, can you elaborate? The monoblocks are Rotel, the Subwoofer is a Rel - and the cheap subwoofer is not going to wash I fear 🙂
In terms of a bad idea to use a transformer, can you elaborate? The monoblocks are Rotel, the Subwoofer is a Rel - and the cheap subwoofer is not going to wash I fear 🙂
Are you using an active or passive subwoofer? To connect an active subwoofer, use additional resistors at the output of each monoblock from 100 ohms and above. Connect to the high level input of the subwoofer. Or a resistive T-divider for the low-level input.
(The amplifier is limited in load resistance (current) and power.
In BTL, the load resistance for each is defined as 1/2. The main speakers are included in parallel. It also reduces load resistance.
One channel is inverted.)
(The amplifier is limited in load resistance (current) and power.
In BTL, the load resistance for each is defined as 1/2. The main speakers are included in parallel. It also reduces load resistance.
One channel is inverted.)
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You mention in your post the "high level input to the subwoofer". Is there a line level input? If there is use it instead. The ve- speaker term label kind of tells me it is not gnd, but a driven inverted phase of the input. If you have a DVM, test the resistance from the amp gnd to the ve- output with the amp unplugged. It should be near 0 to within 1 ohm. If not, you'll need to use the line input of the subs, or if the sub is passive (no amp) you'll need an amp to drive the sub. And of course the line level is from the preamp out.
Thanks Mike. The Rel is active. There are line level outputs from the Pre amp but these are currently in use for LFE output from the processor. I'll see if I can get a measurement on the -ve output.
Why is your friend trying to use a zero budget cludge to drive a subwoofer in an expensive audio system? Do it right, if the preamp or AVR doesn't have a line level sub out then insert a crossover between that and the monoblocks. If he is still determined to use high level signal then there is no need to connect to both amps, low freq energy is typially the same on both channels of a recording anyway so connecting to a single amp will do. The reason the amplifiers go into protection is because thier outputs are shorted together.. don't do that. I suspect the reason it works with an AVR but not the monoblocks has to do with the differences in amplifier topology, but there is nothing you can do about that.