- This topic has 28 replies, 7 voices, and was last updated May 30, 2012 at 9:27 pm by .
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As I mentioned in other posts I was facing a issue with noise and ground loops when connecting two laptop sound cards together (even USB sound cards) from different laptops, caused by a variety of issues.
On one laptop for instance had such bad interference on the PSU I have quarantined it from any of the other audio equipment (no amount of ground loop removers, ferrite beads or the like would stop the interference, and I checked the PSU without load and it was clearly coming from there and much of it in the audio frequencies)…
The better one has much less interference (its not totally non existent as you will see below) but using the ground loop adaptor I can get it to a point I am happy with…
I also found this excellent software which this brainy Italian dude has written and given away as freeware – it is hardcore stuff – you’d pay about £20000 for separate test equipment what could do this and it can also do other electronic circuit design like building passive filters etc (TBH I dont completely understand some of the functions myself!)
Managed to set up a basic test rig using a Soundblaster USB Card as line input (not the best, but I’ve had it hanging around since 2005) and an Alesis IO2 as the output. (in case anyone is curious about why I don’t use the IO2 as the input monitor, its because it seems to use exactly the same pre-amp for mic and line without much attentuation so it gets overloaded way too easily).
Anyway here is an illustration of the effect of the ground loop isolator..
This is the recording output of the amp without any signal going through the mixer feeding it
The first bit contains the harmonics and subharmonics of 50Hz which is of course our electric mains frequency in Europe, and some of this getting into your audio equipment is unavoidable – but if kept to this level can hardly be heard.
But there are plenty of nasty spikes at harmonics of 1kHZ! These are right in the sensitive part of human hearing – and if you turn the amplifier up these are heard as a ticking noise. If you (or someone else in your house) are careless and send a distorted signal to your amp as well as all this interference, you may well blow your speakers!
The noise comes from the circuits in the computer, and get sents in from the ground of the USB cable (which is also the audio ground) as thats the ground wire of the metal case of the computer and goes to earth connection in the mains plug – and its international safety law that this must be done. (if it didn’t you can get electrocuted, as a piece of faulty equipment might put 230V on everything metal)
Now here is the signal again with the ground loop isolator in..
notice that all the bad noise is mostly gone, and the remaining amount buried below the level of the mains hum raaaraaa
Now there are lots of expensive devices you can also use to achieve this, but the one cheeseweasel found was barely more than a fiver…
OK here’s whats inside it.. its is just two proper audio frequency transformers on a decent bit of PCB and shielded (was pleasantly surprised, expecting far worse. Ok the build quality is your average Chinese standard (though neater than what I would have come up with) – and it worked perfectly after I put it all together again.
Apologies for the colour balance being all over the place on these photos, was rushing a bit trying to get something useable online..
and this is how it all fits together in the box (note the metal screening box to keep out radiated interference).
I have no idea what the transformers are / their specifications but I’m old enough to remember when all hi-fi equipment had many more audio frequency transformers (this practice only fell out of favour by the late 1970s). They do not appear to be filtering excessively as I am currently feeding a FM audio signal into a soundcard via one of these and the 19KHZ stereo pilot tone is getting through…
Daftfader is correct inasmuch that there will be an insertion loss and perhaps change in frequency response, but It all sounds OK to me, and I don’t have the brains to do the proper test (even though that italian software should do it – it hurt my brain enough just getting it to work!)
What I expect it does do is present high impedance to frequencies well over 30 kHZ (including all the nasties from the computer soundcard) which isn’t a bad thing as thats the noise you want to get rid of!
Frankly its a bargain for what you get. In the old days folk would have built one inside a tobacco tin, but its hard to get both baccy in tins and decent AF transformers and my old mate what told me about this was in addenbrookes for some time with throat cancer :yakk: (he recovered though)
OK this is the spectrum without the filter
and with the filter added
apologies for not having the scale zoomed in but TBH I’m not 100% sure how to do that and get consistent results for both tests!
So yes there is some insertion loss and a slight change in frequency response (it actually slightly boosts the treble frequencies) but TBH I’d rather put up with that instead of the interference from the PC noise!
going by my ears I would say the frequency response is good up to about 18-19 kHZ (I can still hear up to about 18.5-19 kHz) – TBH that graph isn’t scaled very well anyway for looking at HF response – out of curiosity I managed to run one with a easier to read scale..
TBH the two soundcards used for the test haven’t been calibrated/balanced either so more errors are creeping in there. They definitely can pick up higher frequencies, as if I try and use the record output from the amp/receiver, the damn 19kHZ pilot tone from the FM tuner stage gets into it! (the obvious solution is not to record from the mixer output, unless you are of course recording from the radio (in which case there are ways of filtering it as FM audio only goes up to 15kHZ)
That said I’ve now got everything to a quality I’m happy with – even the basic stuff is way better than all the audio kit I’ve had in the past,…
many old style analogue modems have such a shiekl on them as well as isolation transformer (the Communications Ministry in most countries will refuse to approve them for connection if they are sending interference back down the telephone line, as it causes problems to other users in the multipair cable especially with ISDN and broadband being common nowadays),
What Daftfader may have is a dirty SMPSU though… this is what the Tosh PSU is putting out without any load on it (and with the DC filtered via the capacitor in the scope). This is only a cheap scope unfortunately, was a mission to get it to capture the spike properly (had to keep pressing “hold”) – I can’t afford a top range multi colour Agilent one and my sources at British Telecom have took early retirement so I couldn’t even try and blag the PSU into Adastral where they have loads of nice toys like that..
thats a square wave pulse of 40 mV if not 50 so definitely going to find its way into any audio..
@General Lighting 455326 wrote:
OK here’s whats inside it.. its is just two proper audio frequency transformers on a decent bit of PCB and shielded (was pleasantly surprised, expecting far worse. Ok the build quality is your average Chinese standard (though neater than what I would have come up with) – and it worked perfectly after I put it all together again.
Apologies for the colour balance being all over the place on these photos, was rushing a bit trying to get something useable online..
and this is how it all fits together in the box (note the metal screening box to keep out radiated interference).
I have no idea what the transformers are / their specifications but I’m old enough to remember when all hi-fi equipment had many more audio frequency transformers (this practice only fell out of favour by the late 1970s). They do not appear to be filtering excessively as I am currently feeding a FM audio signal into a soundcard via one of these and the 19KHZ stereo pilot tone is getting through…
Daftfader is correct inasmuch that there will be an insertion loss and perhaps change in frequency response, but It all sounds OK to me, and I don’t have the brains to do the proper test (even though that italian software should do it – it hurt my brain enough just getting it to work!)
What I expect it does do is present high impedance to frequencies well over 30 kHZ (including all the nasties from the computer soundcard) which isn’t a bad thing as thats the noise you want to get rid of!
Frankly its a bargain for what you get. In the old days folk would have built one inside a tobacco tin, but its hard to get both baccy in tins and decent AF transformers and my old mate what told me about this was in addenbrookes for some time with throat cancer :yakk: (he recovered though)
Looks awfully like the boxes I’m putting in the car when I eventually install the bass bins and amp – I’m using 2 to go from the head unit to the power amp in the boot (setting them up back to back at either end of the cable essentially gives us a balanced line between the 2 devices – doesn’t need an “earth” point like a normal sound system as the car doesn’t have a standard earth like a house or genny etc, it uses a negative grounding point system…)
No sure how well they’re gonna work yet (I’ll post here when we eventually get round to it), but I’m told they work really well like that (transformer balancing works as well as if not better than servo etc IMO – most of the reason it doesn’t get used much is the transformers are usually pretty expensive especially when compared to an SMD servo chip)…
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