Speaker level sub woofer hook up

[Tom Jirschele]

I have a few older pre amps and integrated amps. These have no provision for a sub woofer such as a sub out jack or jacks. I have been hooking a sub to these using the speaker binding post, either doubling up the speaker cable with the main speakers or using the speaker B terminals and then to the powered subs speaker level in hook up. Does this method of hooking up a powered sub put any excess strain on the amplifier? The end result seems fine but I often wondered what effect this has on the amplifier. I should mention my integrated amp has no pre out.

 

[XEagleDriver]

Tom,
First off, welcome to the forum. I had a system using subwoofer speaker level inputs/outputs and have a separate system using an external Parasound subwoofer crossover to divide the amplifier's speaker level output signal into separate low and high frequency signals for the the sub and main speakers respectively.

Both methods you describe (doubling speaker cables from the main posts or using the B terminals) result in the same parallel electrical connection and do reduce the impedance by approximately 1/2.
- B terminals are connected in parallel to the same amplifiers as the A terminals, so no advantage in lowering the load on the amp.
I caveat with approximately, because I do not know the impedance of the subwoofer speaker-level inputs or the speakers you are using.
- FYI: A parallel connection resistance is calculated as "product over sum" (i.e. (R1*R2)/(R1+R2). Therefore, if you had two 8-ohm speakers connected in parallel the impedance is (8*8)/(8+8) = 64/16 = 4 ohms.

If it is working fine, you are probably OK. However, most subwoofers with speaker level inputs also have speaker level outputs ran through an internal crossover that diverts low-frequency content to the sub and passes high frequency content to these speaker outputs.

Using this sub crossover avoids doubling up on mid-bass frequencies (sub + main speakers) and lessens the demand on the main speakers to attempt to produce very low frequencies.
- Is there a reason you are not connecting this way (amp - to sub input - sub output - to main speakers)?

Cheers,
XEagleDriver

 

[ski2xblack]

I caveat with approximately, because I do not know the impedance of the subwoofer speaker-level inputs or the speakers you are using.

Typically, the input impedance on a speaker level input on a sub is very, very high, which results in it being essentially "invisible" to the upstream amp. The OP should have no issues hooking the sub up this way.

 

[Tom Jirschele]

Thank you, your response has been very insightful. First off, the reason I am not using the sub out to main speaker connection is mostly due to speaker location. Also there is just something about seeing the speaker wire coming from the amplifier. (aesthetics)

I just want to be sure, this configuration is not putting any undue or excessive stress on the upstream amplifier.

In closing, if I understand this, using the speaker level input on a powered sub, the amplifier essentially does not see the sub woofer thus it does not effect the performance of the amplifier itself. Correct?

Thanks again

 

[KEW]

In closing, if I understand this, using the speaker level input on a powered sub, the amplifier essentially does not see the sub woofer thus it does not effect the performance of the amplifier itself. Correct?

Thank again

Technically, this is a bad statement. As xEagleDriver points out, the impedance is effected!

However, your statement is "close enough" from a practical standpoint. Ski2XBlack's post adds more detail which points out that the magnitude of this effect is inconsequential!

 

[Tom Jirschele]

Although I believe I am getting the gist of all this, It still appears to me that there is some conflicting info here. If I am interpreting ski2Xblack post right, because of the high impedance of the sub, Then the formula put forth by XEagleDriver for calculating the resistance for a parallel connection does not apply here because the amp is not seeing the same thing as it would if there were two sets of 8 ohm speakers connected.

So lets see if I can rephrase my last closing statement.

In hooking up a powered sub with the speaker level inputs, the amplifier does see a very slight difference in impedance due to added resistance from the speaker wire etc. but the difference is so insignificant it is not worth considering. Also this connection should suffice in a set up that is not your main one and kept around for nostalgic reasons in addition to still sounding pretty good!

Would this statement be more technically correct?

 

[KEW]

Although I believe I am getting the gist of all this, It still appears to me that there is some conflicting info here. If I am interpreting ski2Xblack post right, because of the high impedance of the sub, Then the formula put forth by XEagleDriver for calculating the resistance for a parallel connection does not apply here because the amp is not seeing the same thing as it would if there were two sets of 8 ohm speakers connected.

So lets see if I can rephrase my last closing statement.

In hooking up a powered sub with the speaker level inputs, the amplifier does see a very slight difference in impedance due to added resistance from the speaker wire etc. but the difference is so insignificant it is not worth considering. Also this connection should suffice in a set up that is not your main one and kept around for nostalgic reasons in addition to still sounding pretty good!

Would this statement be more technically correct?

Look at it like this. Say the load from the speaker level connection of the sub is 42 Ohms and your speakers are 8 Ohm. Using the formula, we have

8X42 = 336/50 = 7.72 Ohms
8+42

So it did not change much from 8 and thus is not a big concern!