Amplifier hum

Posted by: grantlyt1

Amplifier hum - 08/25/12 06:56 PM

I have just recently purchased a crown XLS1500 amplifier to be used with my Onkyo 875 receiver to power my front M80s. I didn't realize that the amp could only work with a 120 volt power supply. I only have 240 volt where I live. I then decided to try a cheap step down transformer to step 240v down to 120v. Suprising that this crown XLS1500 amp only draws max 250 watts yet claims to put out over 500 watts/channel into 4 ohms. Anyway that is a separate story.
With everything connected up I am getting a low frequency hum from the fronts. When I disconnect the rca input to the crown the hum disappears. It is only present when the onkyo receiver is on and the pre-amp out is connected to the crown amp.
Is there any way I can eliminate this hum or reduce it.
Should I use the XLR inputs on the Crown. To do this I would need to get an RCA to XLR adaptor. Is it possible this hum is caused by the step down transformer? I doubt it since the hum is not always present. Any ideas? confused
Posted by: MarkSJohnson

Re: Amplifier hum - 08/25/12 07:08 PM

Is everything plugged into one outlet? It sounds like a ground hum which often pops up when interconnected devices are plugged into different outlets.

Try a cheater plug (no ground prong) to see if that makes the hum go away.
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 07:23 PM

Yes everything is plugged into the one outlet. I even disconnected all devices and left only the receiver and amplifier connected but still the same problem.
Is the use of this cheater plug dangerous?
Posted by: SirQuack

Re: Amplifier hum - 08/25/12 07:24 PM

Are all your outlets on the same circuit to the breaker? Might be a ground loop hum. If you have a sub, what happens if you unplug the interconnect?
Posted by: MarkSJohnson

Re: Amplifier hum - 08/25/12 07:25 PM

No, though it's better used as a diagnostic tool than a permanent solution.

Do you have a coax cable (TV, Cable) going into a device that's plugged into the receiver? If so, try disconnecting the coax cable.
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 07:35 PM

Yes the outlets are on the same circuit breaker. Disconnecting the pre-amp out to the sub has no effect on the hum.
Posted by: dakkon

Re: Amplifier hum - 08/25/12 07:37 PM

Originally Posted By: grantlyt1

Should I use the XLR inputs on the Crown. To do this I would need to get an RCA to XLR adaptor. Is it possible this hum is caused by the step down transformer? I doubt it since the hum is not always present. Any ideas? confused


To get the true benefit of XLR connectors you're receiver/processor would have to have XLR outs... XLR's are a 3 wire system, RCA is a 2 wire system... To get rid of the hum, you need a true 3 wire system.

I had the same problem, i think it's the wiring in my house.. I am using all XLR connectors. However, my processor has XLR outputs....

By using a step down transformer, are isolating that outlet.
My money is on a ground loop problem. My house also have a ground loop... So, i have to deal with multiple electrical issues.....
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 07:39 PM

I disconnected all inputs to the receiver but no difference however I just disconnected the hdmi out from the receiver and the hum is significantly reduced. Still annoying but much better. But I need to use this HDMI output.
Posted by: dakkon

Re: Amplifier hum - 08/25/12 07:40 PM

you may not have a good earth ground.... that is one other possibility.
Posted by: Ken.C

Re: Amplifier hum - 08/25/12 08:20 PM

Where do you live? I would wager that this has something to do with the step down transformer.

Guys, if he's got 240V, I doubt they even have cheater plugs...
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 08:50 PM

I live in Australia. I have never seen these cheater plugs.
Posted by: Ken.C

Re: Amplifier hum - 08/25/12 08:58 PM

Yeah, I suspect they don't have them there. It's a legacy thing in the States from before we had grounded outlets--a lot of places still don't have them. Basically it disables the ground.
Posted by: Socketman

Re: Amplifier hum - 08/25/12 09:31 PM

Unless you plan on using the amp in your shower just buck the ground off . Hell my new yamaha reciever came with no ground on the plug. It could also be a polarity problem , house wired incorrectly. Also change out the rca cables, i had a set i used from my kids car that were not that old and they were making my sub hum.
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 09:47 PM

I could easily take the ground pin off the 120v side of the plug. It already looks a bit loose. Is it really not that risky to power without the earth pin? I did already try another rca cable but same result. I could purchase a US cheater plug and use it on the 120 volt plug into the step down transformer.
Posted by: Socketman

Re: Amplifier hum - 08/25/12 09:52 PM

i have no grounds on my kit. i just went through this bs over the past week. You can use it as a test then buy a ground loop isolator if your concerned. You could modify an extension cord also. here are some hum eliminators to look at .

https://www.google.ca/search?q=hum+elimi...120&bih=613
Posted by: Ken.C

Re: Amplifier hum - 08/25/12 09:53 PM

It is significantly more hazardous with 240V than 120V, but if you're doing it on the 120V side, it shouldn't be THAT big a deal.
Posted by: Socketman

Re: Amplifier hum - 08/25/12 09:55 PM

Originally Posted By: Ken.C
It is significantly more hazardous with 240V than 120V, but if you're doing it on the 120V side, it shouldn't be THAT big a deal.


just wear rubber boots when operating the stereo grin
Posted by: JohnK

Re: Amplifier hum - 08/25/12 10:19 PM

Grantly, welcome. If you mean that there was no hum problem previously with the 875 used for power, and if the connections(with the exception of the Crown connection)were the same, then there appears to be nothing new to create a ground loop. It's possible that there's a defect(e.g., rectifier, filter capacitors)in the power supply section of the Crown.

If it would be a ground loop, using a RCA/XLR adaptor wouldn't be helpful. As Alex pointed out, the balanced circuitry is entirely different, and doesn't simply involve the difference between a RCA plug and XLR connector. The sending unit(875)would have to have a balanced output with two voltages(of opposite polarity)rather than one, which the 875 doesn't have. The wiring itself is just one element and is meaningless without both balanced output circuitry and balanced input circuitry.

Incidentally, although you say that the Crown draws 250 watts "max", that's highly unlikely. Unless a power consumption number is specifically termed "full power" or similar language, the number given is an average one rather than max. Typically, 1/8th of full power is used for the average. For example, 1/8th of 500 is a bit over 60 watts, and at 1/8th power class AB amplifiers are on the order of 20-25% efficient(about 50% at full power). 60 watts output would therefore need about 250 watts of consumption. There's no inconsistency when manufacturers show a consumption number lower than the maximum output number.
Posted by: grantlyt1

Re: Amplifier hum - 08/25/12 11:03 PM

Problem solved! I just bought a ground loop isolator and the hum is entirely gone. Thanks everyone for your help. grin

Johnk- I am only using a stepdown transformer rated for 500 watts. Does this mean I may blow an fuse on it if I run the amp close to its upper limits?
Posted by: dakkon

Re: Amplifier hum - 08/25/12 11:12 PM

Originally Posted By: Ken.C
I would wager that this has something to do with the step down transformer.


Ken, this is highly likely...Due to the transformer in the the mix this, in of it self could also be the source of the ground loop as this is a new device, and does not address the grounding of that circuit...

How did you wire the ground? If you could post a schematic of how you wired the outlet with the transformer that would help. At this point everyone is hypothesizing as to what you did...


lol... this is what i was just writing as you posted the solution..... oh well!!


With this being said, i would bet the source of your ground loop is the wiring of the outlet off of the transformer.. Again, if you can post a schematic we could help eliminate the ground loop.....
Posted by: JohnK

Re: Amplifier hum - 08/25/12 11:19 PM

The transformer could be burned out by lengthy operation at that power. What you would "blow" would more likely be your hearing in that case. The M80s are slightly above average in sensitivity and use about 1 watt for a comfortably loud average level. Brief peaks use a lot more, but are unlikely to need more than about 100 watts, which your 875 can easily handle.
Posted by: Socketman

Re: Amplifier hum - 08/25/12 11:49 PM

Which ground loop isolator did you get.
Posted by: grantlyt1

Re: Amplifier hum - 08/26/12 12:03 AM

http://www.jaycar.com.au/productView.asp?ID=AA3084

I bought this one.
Posted by: SirQuack

Re: Amplifier hum - 08/26/12 02:38 PM

try unplugging the sub from the wall outlet.