# Hum at amplifier input



## mikee55 (Dec 11, 2009)

Hi all.
My AV amp's rca sub out connects to my Cleanbox II. The balanced out goes just over 2metres upstairs through the ceiling to my DEQ2496. I then have a Studiospares bi-directional DI-Box, which feeds my BK Electronics amp module and 15inch speakers. If I separate the Studiospares DIBox from the amp, that is the only time I loose the sub. I have temporarily disconnected all the Earths, this does nothing. A 4.7k pot across the input of the BK Electronics amp reduces the hum. Can anyone shed some light here.

Thank you

Mike


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## Wayne A. Pflughaupt (Apr 13, 2006)

You might want to wade through the BFD hum thread. Lots of explanations of the reasons for hum, troubleshooting them, and remedies.

Regards,
Wayne


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## EarlK (Jan 1, 2010)

Hi Mike,

IMHO, it's critical in cases like this to provide web-links to all the magic "black boxes" mentioned within your text message .
Since the world is full of these boxes , it's wrong to assume the majority of us readers ( who may want to help ) are familiar enough with them to give proper guidance .

Anyways, for those who may want to help you out, here are some needed links . BTW, you failed to mention which BK Bass Module you are using .





mikee55 said:


> Hi all.
> My AV amp's rca sub out connects to my Cleanbox II. The balanced out goes just over 2metres upstairs through the ceiling to my DEQ2496.


- I've scoured the official ART literature for this "Cleanbox II" and nowhere does it state that it uses transformers as its signal balancing scheme . 
- Can you confirm that indeed it has in-line balancing transformers ? ( perhaps take the cover off the unit & post a pic here for us to see / Thanks ! ) 
-* If ( somehow ) it doesn't use balancing transformers *, then it's not useful in your application and *is not offering the  galvanic isolation that your system needs between the two locations .* IOW, this balancing box ( if it lacks ground-lifted isotransformers ) could be the source of your hum !




mikee55 said:


> I then have a Studiospares bi-directional DI-Box ( after the DEQ ), which feeds my BK Electronics amp module and 15inch speakers.


I am forced to guess that you use the "Red502" to unbalance ( & isolate ) the signal between the DEQ2496 and the sub amp/module ( which I suppose, must be one of the models without a balanced input ). 
- Is my "guess" correct ? 




mikee55 said:


> If I separate the Studiospares DI-Box from the amp, that is the only time I loose the sub.


These statements are quite unclear to me .

- What do you mean by "separate" ? Do you mean , "disconnect" or "physically move to a different location" ?

- If the hum goes away because the RedBox was removed from close proximity to the subamp then your Studiospares box is having hum induced into "its audio transformesr" by being in close proximity to your amplifiers' power transformer ( or maybe even the woofers' magnet ) . 
- Power transformers for some cheaper amps can put out quite large magnetic fields which can sonically pollute other normally clean pieces of equipment .

- Also, what do you mean when you say "loose the sub" ? Did you mean , "lose the hum" ?
- FWIW ; I would expect that simply unplugging the Red502 would kill the hum / since merely unplugging it will kill any signal going to the sub amp ( ie; no interconnect = no hum or signal ) .
- OTOH ; if you unplugged the Red502 and "reconnected the signal chain up to the sub amp"( ie ;bypassing the Red502 ) & the sub-bass signal became clean / then I could understand your bafflement at this strange turn of events . ( I've already offered one explanation as to how that could happen, see above ) .



mikee55 said:


> I have temporarily disconnected all the Earths, this does nothing. A 4.7k pot across the input of the BK Electronics amp reduces the hum. Can anyone shed some light here.


- Putting that 4.7K pot across the existing input merely ( partially ) attenuates the sub amps input / even when the new pot is fully open. You can get the same ( dubious ) effect by adding a 4.7K resistor between the sub inputs' "hot" & "ground" connection . FWIW ( as a bit of education ) , try a 1K resistor here, it'll kill even more hum as well as limiting the signal going into the sub amp. Anyways, this is a dead-end path to pursue any further / just get rid of the 4.7K pot / it's not part of any long term solution .

<> cheers


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## mikee55 (Dec 11, 2009)

Hi, Ive changed things. I have a Cleanbox, not Cleanbox II. My mistake, sorry.

1: My LFE rca out off my AV amp feeds the input to the Cleanbox.

http://www.subwoofer-builder.com/cleanbox/cleanbox.htm

2: The balanced xlr out, goes to my DEQ2496, sat near the amp

3: The xlr out goes upstairs to the RED502

4: The RED502 converts Balanced back to unbalanced, to drive the MF450

http://www.bkelec.com/Modules/mf450.htm

The output from the AV amp, (turned up in the settings, crossover 60hz), doesn't drive the DEQ2496 with a Passive DI Box, hence the Active Cleanbox.
However, turn up the Cleanbox, gain is achieved so is hum.

Is this clear? Downstairs you hardly hear the hum, but Her Indoors goes into the bedroom to do her stuff and moans:dontknow:

Mike


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hi Mike,

There's no need for the Clean Box or the RED502, if the run between your receiver and sub is only 2 meters. A signal run that short of a distance can be done unbalanced:

AVR -> DEQ -> MF450

Regards,
Wayne


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## mikee55 (Dec 11, 2009)

Hi sir, The AVR LFE socket and Balanced input are less than 1 metre apart. It requires a line to balanced conversion, is this where I just need a lead http://www.hometheatershack.com/for...-solution-do-folks-use-dreaded-bfd-hum-5.html
but with XLR? 

DEQ to MF450 is over2m. 1metre from DEQ to corner of walls where Central heating pipes have been boxed around. The cable travels up here through the ceiling, across the floor another metre to the amp.

Thank you

Mike


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## Wayne A. Pflughaupt (Apr 13, 2006)

You can use the connection scheme pictured in the link for a ¼” TRS plug or an XLR. It’s functionally the same thing. It’s not a “line to balanced converter;” you can’t do that with just a cable. It’s just a connection scheme to get an unbalanced signal into a component with balanced inputs. 

Regards,
Wayne


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## mikee55 (Dec 11, 2009)

Hi Sir, Have made a lead, sent RCA Signal to XLR pin 2 and RCA Shield to XLR pin 3. Hum, no signal????????
Thank you 

Mike


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## Wayne A. Pflughaupt (Apr 13, 2006)

Check continuity to see if you have a short or bad connection in your cable wiring. If not, try installing a jumper between Pins #1 and #3.

Regards,
Wayne


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## Wayne A. Pflughaupt (Apr 13, 2006)

Also, don’t forget that the pins on the male vs. female XLRs are mirror-image. Make sure one of your signal (+) connections didn’t end up on Pin #1.










Regards,
Wayne


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