I have an external stereo amp that is being driven by a 7.1 receiver with pre amp outputs.
When both units are powered up, no audible hum or hiss is present. However, when the receiver is powered off and the external amp is powered on, a loud audible hum is present.
Both amps are drawing power from a "protected" 120V power strip.
Is this a result of improper grounding? Should I run a ground wire from the chassis of the external amp to the chassis of the receiver?
Thx
Hello Robert
Normally the amp goes on after the preamp and goes off before to keep turn on/off transients out of your speakers. If you follow this regiment do you ever have a problem?? I noticed I had a hum when I accidentally turned off my Pre first. Wasn't bad though and normally everything is hum free.
Rob
Thx.
If I turn the receiver/pre amp on first, no hum is present if the external power amp that is connected to the two front channels is turned off (receiver power amps are connected to 5 other speakers in a 7.1 HT system).
Hum is present in front channels only when the external power amp is on and the receiver/pre amp is off.
....so this is normal and, therefore, to avoid nasty driver damaging transients, I should never turn the external power amp on until [after] the reciever is on for a few seconds?
Originally Posted by robertbartsch
YES!!! this is normal!
Pre on 1st, Pwr on 2nd.
Pwr off 1st, Pre off 2nd.
It is a very safe habit to get into!
Scotty.
One step above: "Two Tin Cans and a String!"
Longtime Alaskan Low-Fi Guy - E=MC² ±3db
A power sequencer is a wonderful thing! If your not the only one that uses your system, and or you want safe one button opreation.
Check out the Furman Sound website, they make great sequencers.
If you don't want the industrial look, or can't hide it, Adcom made some pretty nice ones, the AC515 is usually available on the world wide garage sale for less than $100.00.
Guessing the interconnect is unbalanced (like an RCA...?) this is fairly normal.
By "protected" you mean a fused strip? That is not protection and is actually redundant to the fuse in the breaker box for that circuit.
Possibly. You may avert the condition if you are able to have balanced interconnects, but you are dealing with consumer gear and it typically does not offer balanced I/O. Pity.
Follow the advice you've received:
Power Amps are always LAST on, and FIRST off.
....receiver is a consummer product but the power amp is a Crown XLS 802D - 500wpc. I assume it has balanced inputs....
...the inputs to the Crown are 3-pin connectors which are sliced into RCA plugs for the reciever pre-amp output.
Ah, so the Crown is balanced but the consumer pre-amp is not. Your situation is not uncommon.
Spliced balanced to RCA are no longer true balanced.
None of this matters, though, so long as you Amps on last, and amps off, first. However, if you do develop 60Hz groundloop, you might try one of those Radio Shack in-line isolation transformers. Stereo pair, inexpensive and very effective.
(btw - I've been thinking of your symptom. I get something like it in SR. For instance, when the guitar player unplugs his cord from his guitar without putting the amp on standby. The amp sees that as an open circuit I guess.)
Take a thin strip of wire that is insulated and attach it to the screw off one piece of equipment and attach the other end to each screw while going steadily around each and sooner or later you’ll isolate the humming.
Do you have a cable hookup in the mix anywhere. If you disconnect your cable does the hum go away?
Ok Audio Beer - I'll try that - good suggestion. I assume the answer is yes, since the hum is not present if the reciever is off.
Thx,
On almost 99% of all the complaints such as yours I've experienced it was the Coax coming in fron cable that caused the problem. Mondail used to sell a "little blck box" that eliminated the problem.
Jensen have one called ISOMAX.
http://www.jensen-transformers.com/iso_vid.html
I’ve just soldered up the LF low frequency for the bass mid on the Alesis RA300 with XLR and its reduced humming down a bit further over previous use.
In some instances, stripping back the shield and breaking it's connection can resolve the symptom. Connect only the signal wire.
FWIW, take a multi-meter and see if there is any potential between the coax signal and shield. If there is (I measured +9v at one point...), that will be your source. I subsequently cancelled cable TV and went satellite.
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