Interference from cassette player
in [UK Mac]
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From: Peter Ceresole on 18 May 2010 14:21 Chris Ridd <chrisridd(a)mac.com> wrote: > I don't think the TiBook power adapters had a metal earth pin. No, they don't. -- Peter
From: T i m on 18 May 2010 15:28 On Tue, 18 May 2010 16:45:06 +0100, usenet(a)alienrat.co.uk (Woody) wrote: >> There is no reason (that I can think of) why you (especially for this >> exercise) wouldn't have mono coming from the cassette playing feeding >> both the L and R inputs on the Mac. > >That would be the ideal. Well, idealy it would be stereo! ;-) > >> I'm thinking that even if you 'suppressed' one input (as you would >> with a mono jack in a stereo socket) it (the Mac) might ramp the gain >> up and therefore be picking up more 'noise'. Given a signal it might >> AGC back to a more normal sensitivity <shrug> > >There isn't any AGC on the input of the mac, it is nominally line level, >although adjustable. Ok ta, wasn't sure and I have had AGCs 'ramping up' and causing issues in the past. T i m
From: Rowland McDonnell on 19 May 2010 06:18 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). > I can't remember if the iMac actually uses the earth > though. I guess I should have looked when I had mine apart. > I know the macbooks don't. -- 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 07:01 T i m <news(a)spaced.me.uk> wrote: > usenet(a)alienrat.co.uk (Woody) wrote: [snip] > >> I'm thinking that even if you 'suppressed' one input (as you would > >> with a mono jack in a stereo socket) it (the Mac) might ramp the gain > >> up and therefore be picking up more 'noise'. Given a signal it might > >> AGC back to a more normal sensitivity <shrug> > > > >There isn't any AGC on the input of the mac, it is nominally line level, > >although adjustable. > > Ok ta, wasn't sure and I have had AGCs 'ramping up' and causing issues > in the past. Hmm - is there a software AGC anywhere? Rowland. (who likes AGCs, when properly applied) -- 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 07:18
Rowland McDonnell <real-address-in-sig(a)flur.bltigibbet.invalid> wrote: > Adrian Tuddenham <adrian(a)poppyrecords.invalid.invalid> wrote: [...] > > A ferrite ring on the audio lead might help too. > > Sounds like a more fundamental problem than that. I was thinking that if there was a lot of common-mode HF hash coming from the computer into the output socket of the recorder, it could be being rectified by the output stage if there was an earth on the rest of the recorder chassis. A ferrite ring might reduce it to the level where it is no longer rectified (less than 0.6 volts). The ring wouldn't affect wanted signals or differential-mode interference. > > > Is the 'earth' of the system actually earthed at all? > > If the signal's inside a Faraday cage the whole way, that shouldn't > matter. Should it? You're quite right, it shouldn't. In practice the screening of domestic devices is far from perfect so, for instance, the exposed ends of the wiring on the back of the tape head can 'see' the operator's hand and other earthed objects in the vicinity. With capacitive coupling, the effect gets worse as the frequency goes up, so the sharp edges of the square waveforms on computer bus bars or from switched-mode power supplies at many Megacycles could be turning up at the level of several volts in a circuit designed for a few kilocycles at a few tens of microvolts. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk |