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Old 2nd April 2011, 05:53 AM   #1
anilva is offline anilva  India
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Default Problems bi-amping F5s

Recently I set up a bi-amped system, using two F5s. The Crossover used is a Behringer DCX2496.

One F5 is driving a pair of Altec 416A 15" woofers in a Onken cabinet and the other is driving a pair of Altec 808HF drivers mounted to 811B sectoral horn assemblies.

The DCX2496 is in turn being fed from pre-outs of my vintage Yamaha DSP-A1 receiver, which has an excellent preamp stage with Phono input as well and has remote input and volume selection. I used the same preamp earlier to drive my F5 paired with Fostex fullrangers.

The problem I am facing is the moment is, when I connect the input signals from the preamp to the Behringer, I start getting small hum/oscillation in the speakers connected to F5s. Not loud enough to intrude when listening, but annoying enough to know that it exists. When I disconnect the inputs, there is no hum/oscillation. The reason I am not sure that it is just hum, is because there is a little bit of HF content, produced through the horns. May be it is oscillation....

One their own the F5s and the speakers are dead quite. Even when I connect the outputs of DCX2496 to the amps, the set up is free from hum, barring slight hiss, which DCX2496 is known for.

How do I eliminate this hum emanating from the input connections. I tried rerouting, replace cables etc. No luck.

Need some suggestions/help.

Cheers.
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Old 2nd April 2011, 09:22 AM   #2
Zen Mod is offline Zen Mod  Serbia
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you need to rearrange connection between audio gnd and safety gnd on one F5

instead of NTC use either pair of anti-parallel connected diodes , or include switch in-line with NTC

that last solution use with care - only when amp is connected with other equipment , which already have that safety link

observe existence of "ground lift" switch on many professional gadgets ; same reason
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Old 2nd April 2011, 10:23 AM   #3
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Wakoo question?

Why not cut the ground (screen) on the imput cables (F5) side.

Maybe you got 4 and you only need 1 as the rest should be grounded with the safety hearth cable going to the mains.

Just chop the signal returns and listen recon 1 and listen recon 1 other chanel and listen ...

Cost noting and easy peasy to do and same times it works.

PS. do not ever cut or mess about with the safety earth cable as that is the most important cable there is
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Old 3rd April 2011, 03:50 AM   #4
anilva is offline anilva  India
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Quote:
Originally Posted by Zen Mod View Post
you need to rearrange connection between audio gnd and safety gnd on one F5

instead of NTC use either pair of anti-parallel connected diodes , or include switch in-line with NTC

that last solution use with care - only when amp is connected with other equipment , which already have that safety link

observe existence of "ground lift" switch on many professional gadgets ; same reason
Quote:
Originally Posted by Bksabath View Post
Wakoo question?

Why not cut the ground (screen) on the imput cables (F5) side.

Maybe you got 4 and you only need 1 as the rest should be grounded with the safety hearth cable going to the mains.

Just chop the signal returns and listen recon 1 and listen recon 1 other chanel and listen ...

Cost noting and easy peasy to do and same times it works.

PS. do not ever cut or mess about with the safety earth cable as that is the most important cable there is
Ok. Though I do not understand this completely, I get the gist. We are basically trying to eliminate multiple loops of earth from source tom the amp i guess.

Will try what you guys are suggesting and report back, though I hate to open a completed & tested F5 and mess around. May be I should add the earth lift switch.

Cheers.
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Old 3rd April 2011, 08:32 AM   #5
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Glad to give same back

Yes you quite right about the eliminate loops.
Small differencies in potential cause same of the electrons to go shopping in the wrong places.

No need to open the case if you Just cut the cables screens to start.
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Old 3rd April 2011, 09:17 AM   #6
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by anilva View Post
...... May be I should add the earth lift switch.

Cheers.
just to be precise ;

not "earth"

GND lift switch
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Old 3rd April 2011, 07:27 PM   #7
anilva is offline anilva  India
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Quote:
Originally Posted by Zen Mod View Post
just to be precise ;

not "earth"

GND lift switch
Got it.
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Old 6th April 2011, 12:08 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Zen Mod View Post
you need to rearrange connection between audio gnd and safety gnd on one F5

instead of NTC use either pair of anti-parallel connected diodes , or include switch in-line with NTC

that last solution use with care - only when amp is connected with other equipment , which already have that safety link

observe existence of "ground lift" switch on many professional gadgets ; same reason
The ground lifting switch will be safer located in parallel with the Disconnecting Network.
Closing the "lifting" switch shorts out the Disconnecting Network and directly connects Safety Earth to Audio Ground with just a few milli-ohms of cable and switch resistance. Able to pass Fault Current.

Opening the "lifting" switch puts the Disconnecting Network in circuit and now Fault Current must pass the inverse parallel diodes. I have proved to my self and reported that this can be done safely.

When there is no Fault Current the Disconnecting Network has a choice of 10r // 10nF // lifting switch // inverse parallel diodes // Thermistor for parasitic currents to pass to Safety Earth.

I cannot recommend and will actively champion against ever fitting a lifting switch in series with the Safety Earth to Audio Ground connection.
This wiring option is potentially FATAL
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Old 6th April 2011, 12:14 PM   #9
AndrewT is offline AndrewT  Scotland
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I have reported my own post.
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Old 6th April 2011, 12:15 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Zen Mod View Post
just to be precise ;

not "earth"

GND lift switch
This relabeling does not make your wiring suggestion any safer.
It is still dangerous to break the connection between Audio Ground and Safety Earth.
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