What is speaker load to amp with inline sub-woofer

bbeacham

I have LSi15 speakers that are rated at 4 ohms. I also have the LSiW sub wired inline, between the amp, and the LSi15s.

What is the speaker load presented to the amp in this manner? Is it still 4 ohms, or is it the subs load, or is it the sum of the sub and the fronts?

Dr. Spec

With a high level connection, the AVR will input a full range signal (full power/current) to the subwoofer inputs. The impedance load is typically higher and should not be confused with the impedance of the woofer itself.

This type of connection is very rare; almost everyone goes low level these days with a cable. See my other post regarding this.

George Grand

What you're doing is paralleling, and the amp should see half the load presented. So if you've got 4 ohm fronts inline after a 4 ohm sub, the amp's going to see 2 ohms. Good luck with that set-up.

George Grand (of the Jersey Grands)

Airplay355

do you mean you are using speaker wire to hook up your sub?

like speaker wire from the receiver or amp to the sub and then speaker wire from the sub to the speakers? this would not affect the impedance.

maybe im just confused though.

bbeacham

<B>What you're doing is paralleling, and the amp should see half the load presented.</B>

Actaully, it would be in series. If I connected both the subs L/R inputs and the main L/R speakers to the amps L/R outputs then it would be parrallel.

What I was wondering was if the amp "sees" the load of the sub, the load of the front speakers, or both.

Incidentally, Polk refers to this connection as Option 1 in the maunal and says:

"This is the recommended hookup option for receivers that do not have pre-ampoutput jacks......With this method there is absolutely no reason to connect the LSiW to the sub-woofer output jack of surround receivers."

George Grand

My apologies, I mis-read your original posting. Series it is.

George Grand (of the Jersey Grands)

John K.

BB, this one has gotten a bit tangled-up. Despite outward appearances, the connection is parallel rather than series and isn't actually "wired inline". The feed to the sub and mains, instead of dividing just outside the amp's output terminals, is split just inside the sub's input terminals, going to the input of the sub amp(not the drivers)and to the output terminals to drive the mains. The input impedances of sub amps are in the tens of thousands of ohms(the sub drivers themselves are typically about 4 ohms)and we'll use 50,000 ohms(exact number makes practically no difference)for our calculation. The receiver or amp therefore "sees" the 50,000 ohms of the sub amp input in parallel with the 4 ohms of the mains. The formula for calculating the net impedance resulting from impedances in parallel is that the reciprocal of the total impedance(1/Rt)is equal to the sum of the reciprocals of the separate parallel impedances( [1/R1]+[1/R2] ), in our case [1/50,000]+ [1/4] or [1/50,000]+[12,500/50,000], which equals 12,501/50,000, the reciprocal of which(50,000/12,501)is about 3.999 ohms, nearly unchanged from the nominal 4 ohms of the mains alone.

With no extra charge I'll also point out that the very high impedance of the sub amp input results in nearly zero power being used by the sub from the external receiver or amp(only the audio signal frequencies are used). This is because one form of Ohm's Law is that current is equal to voltage divided by resistance(I=V/R)and where R is 50,000 ohms, for example, the current would be extremely small. Since another form of Ohm's Law is that power(in watts)is equal to voltage times current(P=VxI), where the current, I, is barely above zero, almost no power is used. So, the receiver or external amp is essentially driving the mains alone.

Dr. Spec

Great post, John. Already tagged and saved in my archives for future reference.

I'll also add that the speaker outputs may be filtered at 80 Hz, or may be allowed to pass through full range.

If the user leaves the high pass filter engaged, the proper low pass setting on the sub is around 80 Hz. While the slope of the high pass filter is not specified, it is typically 1st order on subwoofer plate amps. I'm sure Polk would supply this information if asked.

If the user allows the mains to pass through full range, the proper low pass setting on the sub is around the -3 dB point of the mains. Of course the lowest LPF setting on the LSiW is 50 Hz, so I guess you are stuck with that. This will likely result in a large mid-bass emphasis with speakers like the LSi15 (which can get down to 40 Hz).

Doc

bbeacham

John, thanks for the answer. That is the type of info I wanted.