Impedance Query (Help)

[Nitefly]

Hello

Total nooblet here, just trying to understand something that I don't think is very clear at all... ffff

I have a Marantz PM6005 Amp, which has two sets of output terminals:



I'm currently running a set of cheap Wharfdale 220 speakers. I think it sounds pretty good!

But the idea of a sub has always appealed to me, so I've bought one: a BK XLSS400 (forward firing). Very exciting.

Now, the connection terminals on the Marantz are absolutely ****ing horrible if you are using bare wire. You sort of have to twist them shut and they don't stay tight very well. So the Wharfdales are currently connected via Banana Plugs on the A terminals. So far so good.

The BK electronics sub is a bit like a REL, it's supposed to connect to your high output speaker terminals. The obvious approach, as per RELs, would be to connect the bare wire from the sub to the very same connection terminals on the amp that are going to the 220s. But ugh, since I'm already using banana plugs I'd have to connect the sub using bare wire. This is worse than death.

Ah, I have an idea. Wouldn't it be lovely if I could just connect the sub via banana plugs to the B terminals... avoiding the bare wire nuisance? So I'd end up with the 220 speakers connected to A terminals, sub to B terminals - both via banana plugs. Let's have a look.

Marantz manual says this:



220s manual says this:



BK XLSS400 website says this (not dealt with in the manual):



Huh. Well that all seems fine then! I look forward to using both sets of terminals.

But wait, what's this from Marantz....

However for most amplifiers, if the device has an A / B switch for the front speakers and you use the Connection speaker pair A + B and want to run both pairs together, both speakers must have a minimum impedance of 8 ohms or higher.

i.e. if you wish to run both pairs together then

• speaker A must be no less than 8ohms
• speaker B must be no less than 8ohms

If you use speaker pair A or B, you can also use 4 ohm speakers - but not both together - or this can potentially cause damage to the amp.

Wut?! So I can't do it then? Why is it so goddamn vague in the Marantz manual (╯°□°)╯︵ ┻━┻

Am I stuck with having to use the god awful bare wire connections (to tie the sub to the same terminals as the speakers)? Help

Many thanks in advance!

 

[hornetstinger]

The current draw from the high level is negligable. It's fine using speaker B for the subwoofer

The sub has it's own amplifier. The 4 ohm impedence is the internal characterstics of the driver, that isn't what is seen by your amp.
It'll literally use 500mw of power from your marantz for the signal to the sub in high level

 

[Nitefly]

The current draw from the high level is negligable. It's fine using speaker B for the subwoofer

The sub has it's own amplifier. The 4 ohm impedence is the internal characterstics of the driver, that isn't what is seen by your amp.
It'll literally use 500mw of power from your marantz for the signal to the sub in high level

Thanks champ. This makes complete sense... but I'm still a bit nervous. Does the BK Sub not give an impedance level because it's self-amplified? I suppose this is why the suggestion for Rels and BKs is to connect to the very same speaker terminals...

I think I could deal with blowing up the old amp... but not damaging the sub as that's more expensive.

 

[lucid]

Thanks champ. This makes complete sense... but I'm still a bit nervous. Does the BK Sub not give an impedance level because it's self-amplified? I suppose this is why the suggestion for Rels and BKs is to connect to the very same speaker terminals...

I think I could deal with blowing up the old amp... but not damaging the sub as that's more expensive.

The BK driver impedance is irrelevant because it sits behind the BK's own sub amp. As far as the amp is concerned, the BK sub via its high-level inputs is an invisible load.

The reason for the minimum load requirements when running speakers A + B is that the speakers draw current from the amp. That might not make much sense at first, so here's what happens. Folk think that amps drive speakers, but that's not really what's going on. The amp puts a voltage across the speaker terminals. That voltage is the music signal. The louder the volume control then the bigger the wiggly wave of voltage that represents the music signal. The speaker responds to the voltage wave by trying to move the drive units. This requires energy, but voltage isn't energy, it's something called potential. You can think of potential a bit like a hill. The further up the hill one goes then the bigger the potential. If a kid takes their go-kart a little way up the hill then jumps on, the kart will travel a small distance. The further up the hill then the bigger the potential and so the further and the faster the kart will travel.

When it comes to speakers, the voltage potential causes the speaker drivers to react. They draw power in the form of current from the amp. In effect, speakers suck current from an amplifier. The lower the speaker impedance the more energy it sucks from the amp. Conversely, the higher the impedance the less energy the speaker sucks. The thing that supplies this energy is the set of output transistors for each speaker channel.

When you have one set of speakers at 8 Ohms, you've got the speakers sucking current from the output transistors well within the the performance envelope of those transistors. But when you add a second set of speakers, and have them connected in parallel, then the impedance those same transistors sees changes a lot. (There's only one set of output transistors, so connecting speakers A+B shares the output potential of that one set. i.e. there isn't a second set of transistors just to drive the Speakers B connection.)

The nett result for two sets of 8 Ohm speakers connected in parallel is a 4 Ohm load on the amp. In very simple terms, the energy drawn doubles as the effective impedance halves, so now the transistors are under strain. When you move energy through a bit of silicon, then one of the side effects is that it gets hot. That's where nearly all the heat generated inside an amp comes from, and it's why the transistors are bolted to a bloody great radiator affair known as a heat sink. When they get too hot then that's when the amp blows up.

The lower the impendence of the speakers then the closer the electrical circuit looks like to a short. That's the bad thing that happens when a piece of wire connects directly between the +ve and -ve speaker terminals when the amp volume control is not set at 0. It's a 0 Ohms load, and so the current drawn is infinite for a fraction of a second until the transistors melt and there's a brief flash followed by a puff of magic smoke disappearing.

It follows then that if zero Ohms draws a huge amount of current, but 8 Ohms is a safe impedance, then 100,000 Ohms will draw barely any. As far as the amp is concerned, the sub is virtually invisible as a load. That's why the BK high-level impedance is safe to connect across the speaker terminals.

Connecting the sub to the Speakers B terminals won't add any appreciable load on to the amp. Your amp will be safe.


I don't know if the Marantz amp has any software or circuitry to limit the current output when Speakers B is engaged. That would be the only potential downside of your Speakers A+B plan, but I doubt the amp has anything like that. If you're still worried though, then a possible solution is to add the BK high level wiring to the speaker wires going into the back of the banana plugs.

Banana plugs will usually accommodate up to 4mm cable. The fixing is one of two grub screws to grip the cable. The BK sub lead wires are not thick. (They don't need to be as they're not passing much current.) It's possible then that you could whip off the banana plugs and feed both the speaker wire and the BK wire in to the respective fittings before tightening the screws. That way, both the 220 speakers and the BK sub would be connected to Speakers A terminals without having to faff around with the bare wire connections or use Speakers B.

 

[Ormy1]

What you actually care about is the input impedance of the subwoofer amplifier. You will probably struggle to find this, but it will be high, well above the 8ohm minimum.

Actually, I just looked at the manual, the sub's amplifier has a high-level input impedance of 100Kohms, you're good to go.

 

[Nitefly]

Thanks such much @lucid - that was very generous of you to explain that so patiently! Also thanks to @Ormy1 for the bonus digging, I'm very grateful.

Sadly the current banana plugs are QED Airlocs that have been crimped shut... so attaching wires to those is out of the question. No bother, I'll just buy a set of screw locks for the sub connections and plug into the B terminals (like these).

Looks like I'm good to go then. My final musing... I have seen someone else attempting to use the B terminals as I've mentioned but he didn't get any luck. Obviously, we'll never know what caused it to fail for him, but I have a sneaking suspicion that he forgot to turn on the B terminals - there is a button to select A, B or both on the front of the amp

I'll report back with hopefully a working sub

Thanks again!