Interference from cassette player
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From: Adrian Tuddenham on 19 May 2010 08:02 Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > Woody <usenet(a)alienrat.co.uk> wrote: > > > Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > > > > > Woody <usenet(a)alienrat.co.uk> wrote: > > > > > >if it is a mains loop, > > > > > > Earth loop, you mean - via the mains wiring is the usual way it happens > > > in domestic audio. > > > > Earth loop, yes. > > For those who don't know: > > If you get a circle of wire and change the magnetic field inside the > loop, you'll induce a current in that loop. > > The bigger the rate of flux lines cutting the loop, the bigger the > current change (basically, the more changing magnetic field, the more > the induction). > > So that means the bigger the loop, the bigger the current change. > > Big loops pick up lots. > > Think about a loop involving your mains wiring and the signal wires > connecting units in your music gear. > > Big loops. > > So loops via the mains wiring have been known to add /loud/ hum. > > Professional audio gear uses what they call `balanced' lines, aka > differential signalling, where you don't refer the signal to earth but > refer it to the other line (one line goes up while the other goes down, > and vice-versa). This avoids any chance of an earth loop (and other > problems). Also, if there is a current flowing down the mains earth wire of a piece of kit, it will cause a small voltage difference to appear between the ends of that wire. If your signal wiring picks up its signal from one end of that wire and its reference earth from the other end, hum will appear to be superimposed on the signal. Even if the signal wire contains an earth wire, it is still possible to get hum because the unwanted current will also flow along the signal earth wire. There are various tricks for getting around the problem in some circumstances, but, as Rowland says, the correct way to guarantee hum-free operation of complex set-ups is to use balanced signal connections. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
From: Rowland McDonnell on 19 May 2010 08:30 Adrian Tuddenham <adrian(a)poppyrecords.invalid.invalid> wrote: > Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: [snip] > > Professional audio gear uses what they call `balanced' lines, aka > > differential signalling, where you don't refer the signal to earth but > > refer it to the other line (one line goes up while the other goes down, > > and vice-versa). This avoids any chance of an earth loop (and other > > problems). > > Also, if there is a current flowing down the mains earth wire of a piece > of kit, it will cause a small voltage difference to appear between the > ends of that wire. If your signal wiring picks up its signal from one > end of that wire and its reference earth from the other end, hum will > appear to be superimposed on the signal. That too - I've often wonderered which of the two problems is the more commonly evil. > Even if the signal wire contains an earth wire, it is still possible to > get hum because the unwanted current will also flow along the signal > earth wire. All sorts of fun and games can occur. I put it like that because I've met some of 'em - but out of me and Adrian, he's the one who bothered to understand it all properly. > There are various tricks for getting around the problem in > some circumstances, but, as Rowland says, the correct way to guarantee > hum-free operation of complex set-ups is to use balanced signal > connections. Oh yeah. You can spend forever trying to get an earth-referenced-signal rig to work well, but take the small extra `bother' to go to differential and suddenly all your problems evaporate and there are skylarks singing in a clear sky. Or something like that. Actually, opto-isolators can work wonders too and you can slap them in all over the place without bothering engaging brain, at least for slow control signal work... Rowland. -- Remove the animal for email address: rowland.mcdonnell(a)dog.physics.org Sorry - the spam got to me http://www.mag-uk.org http://www.bmf.co.uk UK biker? Join MAG and the BMF and stop the Eurocrats banning biking
From: Rowland McDonnell on 19 May 2010 08:30 Adrian Tuddenham <adrian(a)poppyrecords.invalid.invalid> wrote: > Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > > [...] > > Any decent-quality cable for the job has to be screened. > > I recently built a piece of kit for a community radio station which > allows them to take the commentator's position away from the studio on a > cheap length of unscreened cable. So far it has only been tested with a > few hundred yards of mains flex strung across the village, but up to 10 > kilometres of bell wire (or even a short length of farmer's fence) > should still give a prefectly satisfactory high-fidelity signal. > > A stereo adaptation of it gave a studio-quality signal with 200 yards of > burglar alarm cable strung along the hedgerow from a remote chicken > house, where the 'performers' were making their night-time murmers, to a > nearby cottage where the DAT recorder was safely kept away from the damp > evening air. > > That demonstrates that it is possible to get away without screened cable > in very special circumstances (and by using specialist kit that cost > quite a bit more than a drum of good screened cable) - but the basic > need for screening most ordinary signal leads still applies. Yeah, but the only way you got that to work was by arranging for an effectively balanced signal, I assume? Hard to do that with earthed-case domestic audio gear, innit? Rowland. -- Remove the animal for email address: rowland.mcdonnell(a)dog.physics.org Sorry - the spam got to me http://www.mag-uk.org http://www.bmf.co.uk UK biker? Join MAG and the BMF and stop the Eurocrats banning biking
From: Adrian Tuddenham on 19 May 2010 08:54 Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > Adrian Tuddenham <adrian(a)poppyrecords.invalid.invalid> wrote: > > > Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > > > > [...] > > > Any decent-quality cable for the job has to be screened. > > > > I recently built a piece of kit for a community radio station which > > allows them to take the commentator's position away from the studio on a > > cheap length of unscreened cable. So far it has only been tested with a > > few hundred yards of mains flex strung across the village, but up to 10 > > kilometres of bell wire (or even a short length of farmer's fence) > > should still give a prefectly satisfactory high-fidelity signal. > > > > A stereo adaptation of it gave a studio-quality signal with 200 yards of > > burglar alarm cable strung along the hedgerow from a remote chicken > > house, where the 'performers' were making their night-time murmers, to a > > nearby cottage where the DAT recorder was safely kept away from the damp > > evening air. > > > > That demonstrates that it is possible to get away without screened cable > > in very special circumstances (and by using specialist kit that cost > > quite a bit more than a drum of good screened cable) - but the basic > > need for screening most ordinary signal leads still applies. > > Yeah, but the only way you got that to work was by arranging for an > effectively balanced signal, I assume? Yes ...and very low impedance. > Hard to do that with earthed-case domestic audio gear, innit? Oh yes, this was no domestic gear. It used three transformers at each end. The cable was four-core, so each pair of diametrically-opposite conductors could be used as a balanced circuit. Then the centre-taps of the two balancing transformers were connected to a third balanced transformer to give an extra 'phantom' circuit across both pairs. The windings on each transformer could be arranged to give 600 ohms or 75 ohms line impedance - I measured burglar alarm cable characteristic impedance as 100 ohms, so 75 ohms was near enough. With audio at that low an impedance, electrostatic screening is usually unnecessary, but you do need to use twisted pairs. Inside equipment where there are stray mains transformer fields, even short interconnections of low impedance wiring have to be magnetically balanced and it is sometimes necessary to use the 'starquad' configuration to keep hum out of very low-level signal circuits (e.g. in moving coil pickup pre-amps). For the stereo chicken recording I didn't need the 'phantom' circuit. For some future project it could be used in either direction, so I could have an extra mono mic circuit, or I could use it for talkback or intercomm from the recording position to the mic location. (A bit of cross-talk to the stereo circuits wouldn't matter because the talkback would be heard acoustically anyway). -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
From: zoara on 19 May 2010 09:09
Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > zoara <me18(a)privacy.net> wrote: > > > T i m <news(a)spaced.me.uk> wrote: > > [snip] > > > > You might be able to tell if it's screened if either plug can be > > > opened (ie, not moulded) or, if you can feel a screen / braid > > > under > > > the outer sheath (as opposed to just 3 wires). > > > > Anyone know where I can get one of those tape adapters (with a 3.5mm > > stereo jack at the other end) which is screened? > > Any decent-quality cable for the job has to be screened. > > When I've needed such a thing, I've gone on-line and just got on with > it. That's what I did. Couldn't find anything, which is why I asked. > <shrug> I assumed that what with the advert saying I was buying gold > plated connectors and an OFC cable (argh), I'd be getting screened as > well. And I did. And it fitted the G4 iMac audio o/p socket > properly, > unlike the soldered-on gold plated plug I had been using. > > Assuming you've got a tape player using conventional crappy home audio > RCA connectors, just look for shopping results on stereo 3.5mm jack > plug > to stereo RCA plug cable. And if it's not screened when it turns up, > send it back as `not fit for purpose' :-) If it was a stereo 3.5mm jack plug to stereo RCA plug cable, it definitely won't be suitable for purpose as neither end would fit into my car's tape deck properly. I'm after a tape adapter. http://www.google.com/search?q=tape+adapter -z- -- email: nettid1 at fastmail dot fm |