Rotel RA-820 - mystery issue

[mattm1234]

Hi

I don't know is someone has seen this before but any advice would be really welcome!

My Rotel RA-820 works absolutely fine on all inputs (cd , tape, radio )when connected to an ipod or similar device but when connected in the same way to my desktop PC it picks up quite a bit of background noise. This is not just static hiss but cracks and pops etc which can be heard over the music .

I've checked the cables inputs,outputs etc and have confirmed that it is defiantly the source (PC or ipod) that makes the difference. I've also had my PC running through another amp and tried another sound card and i don't get this issue.

My guess is that it is a combination of the PC having a more "noisy" output than the ipod and the amp not filtering as well. I'm wondering if changing out the smoothing capacitors for some new ones would help?

Any advice welcome!

M

 

[Ian Finch]

PC noise

I'll guess that you mean simply to say that the RA 820 has a problem with the PC sound card output whilst another amplifier doesn't. Most likely that will be a dirty earth issue, common to most PCs.

Compared to an amplifier that is double insulated rather than fully earthed, as I imagine the RA-820 is, you may find that this noise follows the mains grounding around. Using powered PC speakers, which aren't earthed nor very noisy, is mostly quite successful.

The simplest sure-fire solution is to use an outboard DAC, as this can, via an optical or USB link, provide sufficient isolation and most free-standing DACs will still sound better than on-board, cheap PC sound cards. It is unsafe and illegal in many countries to remove earthing from domestic appliances but it may be possible to reduce the noise by improved RF filtering on the input. You can find service manuals for all Rotel models - http://www.bwgroupsupport.com/manuals/service

I recently added a mid-range DAC to my system and it certainly cleaned up some residual noise that my very fancy sound card couldn't - basically because it is situated in a very noisy, high power SMPS and intensive digital/ EMI environment.

 

[mattm1234]

Thanks for the reply, that an interesting idea it certainly sounds like the issue!

I've tried using the PC with a USB sound card (only a cheap one from ebay) but it made no difference, is this the same as a DAC?

i will try fitting a non earthing plug (only temporarily and with an RCD so no major safety concerns) and see if that helps might point me in the right direction

thanks

M

 

[monty78pig]

If I were you I'd get a DAC and connect it to the TOSLINK output on the PC's sound card (if it has one). That way your sure that the PC is completely isolated from your sound setup. I use this method with a DacMagic and it sounds great .

Or alternatively you can do it on-the-cheap and get a ground loop isolator. I wouldn't use a plug without an earth as that could be potentially dangerous if the PC is not connected to any other earthed appliance, and it also means that all the crap and SMPS noise that would have been going through the PC's ground will be going straight through your system. Not good .

 

[tvrgeek]

I have run my PC into my 840 with no problem, but I have pretty good power filters on both. Of course, the internal sound card is garbage, so it was just a test. I run an external USB DAC ( e-bay Chinese special) and it works quite well. Actually, a toss up between it and my NAD 565. I also run the internal sound card PCM (coax) out to me DCX on my speaker analysis bench and have no issues. Toslink would guarantee isolation of course.

Now, as much as I love old Rotel,s (I have 5) they are not the best on noise rejection.

Yes a USB sound card is a DAC. Digital to Analog Converter.

 

[monty78pig]

Rotel amps are generally not grounded via an ESR, so you have to be very careful to watch out for ground loops. I had (and still have) an RA-930BX that had grounding issues when hooked up to a vintage tuner, fixed it by opening it up and splicing in an ESR made with two diodes a resistor and a cap. Works fine now, definitely worth a go .

 

[mattm1234]

What's an ESR?

M

 

[jitter]

A quick google revealed some images of the back of the RA-820 in which the square-in-square symbol can be found. This symbol indicates that the RA-820 is a double insulated appliance which will not be connected to protective earth (PE), also known as safety earth.

What does the sound from the Rotel sound like when only the soundcard is connected and every other source is unplugged? Does the problem disappear or remain?

 

[mattm1234]

jitter - My RA-820 is not double insulated, not sure plastic had been invented when this made

I've ordered a ground loop isolator will see if that does the trick

M

 

[McDaisy]

Preferable you should use a usb or optical dac between your pc and amp.
For now try to use different outputs from your pc if you have more than one and max the volume on your computer and control it from the amp instead.

 

[jaycee]

Another possibility is that the longer signal cable to get to your pc is simply acting as an antenna. There is now lowpass filter on the CD input of this amp. Try connecting it to the Tuner input (which IS filtered, go figure) and see what happens.

 

[tvrgeek]

What's an ESR?

M

As he did not answer, I am guessing an electrical safety return. A third wire ground. Yes, it is a two wire cord. In the US this third wire is a redundant path tied back to neutral in the panel. Don't know how they do things over in two phase land.

 

[jitter]

jitter - My RA-820 is not double insulated, not sure plastic had been invented when this made

I've ordered a ground loop isolator will see if that does the trick

M

If it is not double insulated, it must have a three-wire power cord and a three prong plug. In the amp, the third wire must be connected to the chassis close to the entry point.
The technical manual linked to in a previous post only shows a two prong plug for all versions (120/220/240 V).

If your amp really does appear to be connected to earth via a third wire, look at the back. If the double insulation symbol is present, it must have been modified at some point in time after it left the factory.

The ordering of a ground loop isolator may have been premature.
Did you try with only the PC connected to the RA-820 yet?
If your amp really is earthed, then you must also temporarily disconnect the earth wire to break a possible loop. If this does not solve your problem, it is not a groundloop.

 

[jitter]

As he did not answer, I am guessing an electrical safety return. A third wire ground. Yes, it is a two wire cord. In the US this third wire is a redundant path tied back to neutral in the panel. Don't know how they do things over in two phase land.

On this side of the ocean, we're in three-phase land. Because of the phase shift of 120 degrees per Live conductor (L1, L2 and L3), they measure 400 V between eachother, but each of L1, L2 and L3 measures 230 V with respect to Neutral (N)
Light and domestic appliances all run on single phase 230 V.

The Safety Earth or Protective Earth (PE) is kept fully separate from L and Neutral N. In a building the PE conductor is earthed to a long rod driven into the earth. N comes from the power grid.