EDIT
The EMI problem outlined in this post turned out to be due to a production problem with the Oscilloscope I bought to look for noise in my music system. More details of what happened with the scope are given in my post on page two (
http://www.phasure.com/index.php?topic=2404.msg27276#msg27276 ) but I wanted to save anyone wasting time reading thought the post.
Regards,
Nick.
I wanted to post on the subject of the effect of environmental EMI noise. I am on the journey of understanding and diagnosing an audible "noise" problem in my system any pointer / experience that anyone can offer would be greatly appreciated (I know there are some experienced engineers amongst us).
The audible sound quality issues manifest themselves as a number of sound quality symptoms. It is possible that some people may also be experiencing them to greater / lesser extent possibly without knowing it. The audiable symptoms of the problem include:
- Faint playback of music when playing at a -144db XX volume setting (this happens whilst playing with my NOS1, Pauls NOS1 and just this weekend with my friend Pete’s Musical Fidelity NuVista Dac).
- Harshness if the upper frequencies (this is something that may be quite widespread reading some of our posts).
- Volume imbalance between left and right channels (usually about 2 to 5 db differance, estimated subjectively)
- Generally very variable and poor sound quality performance from my system, on occasions sound quality has shone and my NOS was fine playing at Pauls house but 90% of the time my system has refused to perform as well as I know it should (Pauls system set the standard).
Until recently I had worked on the basis that there could be a single rouge bit of equipment in my system causing the problem (eg a part of the PC, the NOS1, Gainclones, speaker cables etc, etc, etc). My approach was therefore to replace the system components one by one until I stumbled on the cause. The problem is that with 60 % of my system changed the problem was still most defiantly there so a new approach was needed. Enter the oscilloscope that I had been promising to buy myself for many years, at last I have gained some “eyes” to look into what is actually happening.
So how did it look with the oscilloscope ? .... not good, that's for sure !
It turns out that on virtually a wet bit of string anywhere in my house I can pick up air bourn interference in the 100Mhz range which happens in short pulses, the pulses have a frequency of about 64khz. This stuff is EVERYWHERE, on the mains, on my speaker cables, in the digital and analogue sections of my NOS1 and my Gainclones (who needs a PC to create noise when you have this much about already) .
When I say wet bit of string mean for instance the 30 cm length of untwisted cable in the picture below which is picking up 0.6us bursts of ~100mhz at an amplitude of 10mv !
The general bursts of ~100mhz do have a pattern of sorts this is a sample of the general profile if the ~100mhz spikes – ouch ! (the vertical scale is 2mv per division).
And a closer look at one of the larger spikes showing the 100mhz wave form.
So for a few weeks I have been trying to understand where the EMI is coming from and what it is doing to my audio system. So far as the system is concerned since basic Gainclones have no regulation on power supply and no HF rejection on their inputs and outputs the airbourne EMI is certainly causing problems. I think the effect on the Gainclones and the interaction of this effect with my NOS1 might well be causing the NOS’s word clock signal to become out of sync with the bit clock as 32bit words are processed by the DAC section. I think this may be resulting in LSB word data from one 32bit word entering later adjacent 32bit words at more significant bit locations. This super jitter however it is happening just ruins music quality. A couple of things it is important to point out are that this defiantly a Gainclone / DAC interaction problem. As I mentioned before, my friends Musical Fidelity DAC (played using SPDIF from a M2Tech) exhibits the problems even more than my NOS1. Whilst using an amplifier with better RF rejection (A Denon midi component as it happens) the problem goes away completely so far as I can tell, the only issue with this solution is that the sound quality of the Denon is way below the potential of the Gainclones.
So where is the RF coming from ??
I checked the house and the immediate area round my village expecting to find an SMPS about to explode somewhere close but nothing so far as I could see. I did a lot of reading about EMI on the net about possible sources and started driving round in my car with the oscilloscope to “look” for the source (fortunately the scope runs from a battery as well as the mains). This evening I drove around half the county trying to work out where the source was. After a couple of hours I ended up parked at the base of a 100m transmitter 20 miles from my house with the amplitudes of the 100mhz bursts 7 X higher than back at the house. I think it is a TV transmitter. The culprit ??
So I have some questions which I need to research now.
- Is it a TV transmitter ?
- Should digital TV signals have this 100mhz modulated pulse structure at about 64Mhz ? is that normal.
- The EMI signal shows as a 5mv pulse on my NOS1 outputs which I know from Peter has a noise spec of about 6uV !!!! That is not good for signal to noise ratio coming from my DAC.
- Are my specific set of Gainclones more susceptible to the interference than they should be ? I plan to build another balanced set to try this theory (another upgrade in the name of EMI
).
No resolution as yet for these these points, I might still be looking in entirely the wrong direction but I think there is smoke here so the fire should be close as they say.
If anyone has any experience of this stuff I would love to hear your thoughts, anything that can shorten the path to sorting this out would be fantastic.
From an EMI buzzing Suffolk.
Nick.
Ps I just hope I done have to build a copper clad faraday cage with no windows and power my system from batteries before I can listen to it at its best
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