Subwoofer Impedance Speaker Out Doubt

RandomBoy

Novice Member
Hi everyone,

I'm trying to set up a 2.1 system which is a Frankenstein of different components.

My amp does not have a sub out port, so I'm forced to use the speaker in/out port of the sub and from there I will go to the speakers. Unfortunately the sub does not state the impedance of the speaker in/out ports. How risky is this set up for the amp?

Amp: 8 Ohm (Hitachi AX-M135)
Speakers: 8 Ohm (2 x Sony SS-MFS500H)
Sub's speaker in/out impedance: unknown (Celestion S10)

Thank you so much

P.s. a similar sub (Celestion S8) has 4 Ohm
 

oscroft

Member
I believe at least some powered subs generally have high impedance on their speaker-level inputs, so as to put no little or load on the main amp - even at speaker level, they still use their internal amp to provide the power. (Edited)

Paul McGowan says: "My favorite method is to tap the output of the loudspeaker power amplifier instead. Some subwoofers have a high-level input that can accept the main power amp’s speaker outputs. (Check to make sure this feature is available before purchasing a subwoofer) In this configuration, the amp’s power is not being used by the subwoofer, just its signal. Internal to the subwoofer are high-value resistors that neck-down the amp’s big output to something usable to the sub’s internal amplifier."
 

RandomBoy

Novice Member
Thanks so much for the answer. What about the speaker outputs though? Do you think that the 2 main speakers will receive the initial 8 Ohm impedance that was coming from the amp?

Also, because of having the speakers connected to the sub, should I need to calculate the impedence of the speakers too as if these were connected in series/parallel with the sub, in order to know the amp load?
 

oscroft

Member
Thanks so much for the answer. What about the speaker outputs though? Do you think that the 2 main speakers will receive the initial 8 Ohm impedance that was coming from the amp?

Also, because of having the speakers connected to the sub, should I need to calculate the impedence of the speakers too as if these were connected in series/parallel with the sub, in order to know the amp load?
Well, 8 ohm impedance doesn't "come from the amp". Amp specs generally tell you what power they produce into loads of different impedances. So if you have an amp specified as, say, 50w into 8 ohms, that's just telling you what power it will produce when driving speakers of 8 ohms impedance.

So what the speakers receive is the power coming from the amp. In the case of a powered sub with high-impedance speaker-level inputs, the sub driver itself should not see any of that amp power - it will only be driven by the built-in amp.

And with such a sub, if its speaker-level inputs are indeed high impedance, it is designed to put effectively no load on the main amp (or, at least, very little load), and so it should have no effect on the impedance matching of your main amp with your main speakers. As an example, I've just measured the DC resistance (which isn't the same as AC impedance, but it's close enough here) of my sub's speaker-level input as 4.5kohms, which will make no real difference at all in parallel with 8ohm speakers. So part of the design aim is that you can just connect such a sub in parallel with your speakers without having to think about impedance matching.

The only thing that concerns me is that you say there's a similar sub to yours with a stated impedance of 4ohms, which sounds like effectively just a passive speaker. So I guess it's possible you have a sub with only a passive connection from the speaker-level inputs to the driver. If that's the case, then nothing I've said will apply. You need to find out about your specific sub, and I can't really help there.
 

RandomBoy

Novice Member
That was incredibly well put and helpful! Thanks so much for such a clear reply.

Regarding the "similar" subwoofer having 4 Ohm... I am currently referring to an info I found on its manual. I'm talking about a Celestion S8, and I own, instead, the S10. The manual is a bit cryptic, though, and I may have got it wrong. Perhaps you could be so kind to double check that I've got that info correctly?

Specs are at page 2:



I have also attached a picture of the back of MY sub, just in case it's useful.

Also, when connecting my sub using its speakers in/out connections, I can still use the volume potentiometer of the sub, which would lead me to believe that the sub it's using its internal amp, rather than being used in a passive mode.

Thanks so much once again
 

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oscroft

Member
Aha, page 3 of that manual, point 15, input impedance... it says 20k ohm for RCA and 200 ohm for speaker level.

Even 200 ohm is high enough that you shouldn't have to worry about impedance matching. In parallel with an 8 ohm speaker, for example, it would drop the nominal impedance to approximately 7.7 ohm overall, which should be of no concern.
 

oscroft

Member
Also, when connecting my sub using its speakers in/out connections, I can still use the volume potentiometer of the sub, which would lead me to believe that the sub it's using its internal amp, rather than being used in a passive mode.
And yes, that does suggest it's still in active powered mode.
 

RandomBoy

Novice Member
That's great news!!! :)

Even if that is not exactly the same model, I'm gonna take the plunge and connect everything during this weekend then!

When used with its own speakers (so no subwoofer and just 2 x matching 8 Ohm speakers connected) the amp becomes a little warm after some time, not hot just warm. Can I use the heat as a reference when I connect the sub? If it is even less warm I guess it should all be safe, and if it becomes hot I may consider disconnecting it? Or am I just exaggerating?

Thank you SO much!!!
 

oscroft

Member
I don't think you have anything to worry about, and if there was anything wrong it would surely sound wrong. But yes, there's no harm checking the amp doesn't get too warm.

Oh, and yes, it looks like your amp is the bigger (10 inch) version of that other one - so very similar.
 

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