AW: To earth, or not to earth...
magnus at analogue.org
Thu Dec 4 00:26:47 CET 1997
>>>>> "BZ" == Bob Zimmer <bzimmer at voicenet.com> writes:
BZ> At 07:46 PM 12/3/97 +1100, you wrote:
>> This balanced/unbaslanced thing might be a bit Quixotic...
>> after all, there is a hell of a lot of modular stuff out there
>> produced over the years and I havn't noticed any of it suffering
>> from hum pickup or crosstalk to any degree (OK the switch matrix
>> on the ARP2500 does, but that is a different thing). Most of this
>> gear has suffers more from IC noise in my experience...
>> Which makes me think that just using commercially tried construction
>> methods and layout might be all that is needed, or worth doing.
>> paul perry melbourne australia
BZ> This is my recording background kicking in!
BZ> I agree with Paul's comment all this balanced/unbalanced stuff being overkill!
BZ> Pro audio gear lives in the balanced world. But their equipment costs
BZ> mega-bucks and requires the utmost in quality and low noise. Most
BZ> recording studios think nothing of spending $100 for a transformer to
BZ> balance a line.
BZ> To me, balanced lines for microphone level signals is a no-brainer. Most
BZ> consumer gear does not, but most semi-pro gear, and all pro gear does!
BZ> Realize also that consumer and some semi-pro gear uses -10dbm signals while
BZ> pro gear uses +6dbm levels.
BZ> WE ARE SPEAKING ABOUT +5 V SIGNALS!
BZ> This is a much hotter level that even pro audio gear! You have to put some
BZ> serious attenuation on that level or you will overload your mixer's
BZ> preamps! Loading that +5 V level into a typical line input which expects a
BZ> signal of maybe 100-150 mv or so, requires attenuation (15X attenuation).
BZ> This attenuation will also reduce the level of any crosstalk or noise
BZ> picked up by the cables to the same degree.
BZ> Ground loops are a problem even with pro gear, but it's not an area I know
BZ> much about, and therefore will shut up!
The reason pro-gear uses hot balanced signals is that you like to have
a very high dynamic range to work with. For instance, if you build a
PA for a theater then you can see how you can peak at very high levels
like 139 dbSPL but you would also like it be so quite that when you
press your ear to the speaker front (still with full gain on speakers
and everything at normal levels from the house console) you should
just vaugly hear some noise and certainly no hum - this is quite
irritating so we want to remove it as much as possible. Now, with
something like 120 to 130 dB of needed dynamic range you will use if
not all but many tricks in the book to kill noise, crosstalk, hum and
other unwanted signals.
As we start to use 24 bit recording and similar things these things
will crawl up onto us. Getting just a decent sound does not require
as many tricks, but surely we can use some of these for semi-pro
equipment. Actually, you can do a lot of just sound things for quite a
little extra expense but compensate this with cutting away several of
all these unnecessary functions that we see so much off.
BTW. I have done the above stuff with the speakers... it can be done
with not that much of work - really!
So, in my mind is balanced lines, transformer isolation and hot
signals valuable tools and they does not even have to be expensive to
I do agree that a modular with balanced lines for all patchpoints is a
luxury, but there is a hell of a good chance that it is much more
quite than any other around!
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