Eish! It hums... Sub Arcam high level setup

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I have the following setup:

an oldish Arcam 6 amp connected to some Fostex speakers and also to a Wharfedale SW150 sub.

The Arcam does not have line level or sub outputs so I have connected the sub to the arcam using the subs high level inputs from 'speaker b' on the Arcam amp. This suits me well because when I do not want sub output then all I do is switch 'speaker b' off on the front of the amp.

The problem I am having is that if I switch speaker b off then the sub starts to hum. Since I only really switch the sub off for late night low level listening the hum is intrusive...

I could switch the sub off with its power switch but the power switch is at the back of the sub under everything so accessibility is a pain...

Do I have any other options?

See attached diagram...

[ludo]

Either use shielded cable from B outputs to sub, or connect a lowish value resistor (1k Ohm / 5 Watt) across the B outputs' + & - terminals. Do this for both channels. Or use both the R and shielded cable on both B channels... If you're having a real serious party, the resistor might get a little hot. The lower the resistance value, the quieter things get, but the hotter the resistor will get too. 1k/5Watt is a safe bet.

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Either use shielded cable from B outputs to sub, or connect a lowish value resistor (1k Ohm / 5 Watt) across the B outputs' + & - terminals. Do this for both channels. Or use both the R and shielded cable on both B channels... If you're having a real serious party, the resistor might get a little hot. The lower the resistance value, the quieter things get, but the hotter the resistor will get too. 1k/5Watt is a safe bet.

Wow Ludo - thanks for the response. I am definitely gonna try that when I get back to Cape Town.

Out of interest sake how are the speakers connected to the amp with the switch. Series or parallel? Also what impedance does the amp 'see' from the sub's high level inputs...

[ludo]

A & B would be connected in parallel. Usually only the one (signal) wire is switched.

Inside the sub, there would be some sort of attenuator that drops the speaker level by some 30dB or so, and does filtering, before applying the filtered signal to the subs built-in amps. The initial attenuators are likely made of high value resistors. If they have high value, they don't draw current and don't get hot, so it's cheaper. It probably presents very high resistance to the Arcam.

When switching off the B speakers, the speaker wire going to the subs has its + side disconnected from the Arcam. This forms a nice antenna that picks up mains humm from everything around. Shielding the cable will bedevil the functioning of the antenna and it will pick up less humm. Placing a lowish value resistor in parallel to the + & - cables will have a similar effect on the "antenna." The lower the resistance, the less good the antenna. 1k Ohm is a safe compromise between heat generated in this resistor by the Arcam when the sub is playing, and keeping the effectiveness of the antenna low when the subs are not playing.

Without the added resistor the entire + wire forms a high impedance node (probably 10-100k Ohm) on the input of the sub. The sub should be very effective at 50Hz, so one hears it. In spite of things having been attenuated as a first step when entering the sub, the 50Hz did not get filtered out before gain was applied again by the subs power amp.

When the B set is switched on, the Arcam drives the speaker wires from a very low impedance (milliohms), so it doesn't pick up humms and buzzes.

If you use shielded cable, it adds capacitance on the output of the Arcam when the "B speakers" are switched in. 5 meters of co-ax might amount to about a nanofarad, which will not bother the Arcam. If it was a multi kW PA amp, you would think about the breakdown voltage of the coax cable, but not for the Arcam. I doubt it will swing 50V RMS.

The Arcam will see 8 Ohm(?) main speakers in parallel to 1000 Ohm, so no real change there.

I haven't seen the insides of those subs. So if everything above is all lies, the experiment will cost you about R2 for resistors. You would of course be wise to try with real cheap coax before acquiring those 30k interconnects for this 

Piccie for clarity:

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Thanks for the lesson there Sensei. ( http://www.merriam-webster.com/dictionary/sensei )

The reason I ask about the amp A/B switching is because I was always wondering what the amp would see... On the back of the amp there is a warning against using speakers less than 8Ω if you connect speakers to both A + B... I assumed parallel as the amp would not be happy to see less than 4Ω

Can I also assume that connecting both speaker A + B would mean no change in SPL output regardless of the on/off status of the switch on the front of the amp. If they (the speakers) were in serial then the SPL output would halve....

Arcam will see 8 Ohm(?) main speakers in parallel to 1000 Ohm, so no real change there.

Excellent! The amp is getting old (by modern electronic standards) so I do not wanna strain it unnecessarily...

[ludo]

( http://www.merriam-webster.com/dictionary/sensei )

Hehe! I have an acquaintance that is very much into his Aikido (the martial art, not the amp) He became somewhat known for the phrase: "Weet jy hoekom is jy nou gemoer?!" Great guy actually, but I'm really not the type.

Speakers (with crossovers) connected in series never really work well. The speakers impedance is firstly impedance, and secondly not stable over the frequency band. So when placed in series this affects pretty much everything in both units then. It should give a nominal 6dB drop in level for each box but a 3dB lower overall SPL result, if my Math is well today(?) Series connection finds its best application in shopping malls and guitar cabs (no xovers) and where weirdness is valuable. I don't know of any hifi amps that have such a feature. They always add things in parallel.

So no change in SPL from the main speakers when adding the B set with the given scheme. Just the subs either singing along, or not.

If you added 2 more of those beautiful horns in "B" to tempt the cats, you'd get an extra 3dB, but the amp would (theoretically) clip fractionally sooner and then the guys 3 houses down will also complain. If you had added just an 8 Ohm resistor on the B outputs, then it would still have clipped sooner, the fire would have started on your premises, but the same neighbours will have been 3dB less unhappy until then.

If the humm doesn't go, keep complaining here. The sucking of the collective thumb is a powerful thing.