Archived from groups: rec.audio.tech (
More info?)
Vland wrote:
> "Vland" <vlandw@pacbell.net> wrote in message
> news:01Nud.41456$6q2.5858@newssvr14.news.prodigy.com...
> >
> > "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
> > news:41BB4FE4.C5D0F20E@hotmail.com...
> > >
> > > Vland wrote:
> > >
> > > > "Richard Crowley" <rcrowley7@xprt.net> wrote in message
> > > > news:10rgjucdid35t7f@corp.supernews.com...
> > > > > "Vland" wrote ...
> > > > > > I tested without a microphone since I wanted to compare the
> > > > > > noise level in all eight channels without the distraction of room
> > > > > > ambience. I hear only low-level random noise in channels 1-4,
> > > > > > a very slight hum along with random noise is channels 5-6, and
> > > > > > a higher low-level hum in channels 7-8. I'll make up an XLR
> > > > > > with a 200-ohm resistor and see what difference that makes. I
> > > > > > agree that the power supply is probably the source, so my question
> > > > > > is: are all Octopres like mine in this regard, or is this
> something
> > > > > > that could be fixed?
> > > > >
> > > > > Until you test it with an input load (200 ohm resistor), you
> > > > > don't know if you actually have anything that needs fixing.
> > > > >
> > > >
> > > > I wired up a 200 ohm dummy XLR and tried it with the hummy channels.
> > Loading
> > > > the input succeeds in dropping out most of the random noise but does
> > nothing
> > > > to reduce the hum. I can hear both a 60 hz and a 120 hz component, so
> > per
> > > > other consultations, it may be that the 60 hz is from a badly shielded
> > power
> > > > transformer (physically closest to the hummy channels) and the 120 hz
> > may be
> > > > because of a weak filter capacitor on the branch of the power supply
> > that
> > > > powers channels 7 and 8.
> > >
> > > Most likely it's actually 180Hz. Transformers radiate 3rd harmonic
> nicely
> > too.
> > > Your suggestion doesn't tie in with likely actual circuitry.
> > >
> > >
> > > > At normal monitoring levels, the hum begins to be potentially audible
> > with
> > > > the level controls set a bit above "1 o'clock." I've found that I
> > typically
> > > > record acoustic instruments with levels set between 2 and 3 o'clock,
> so
> > the
> > > > hummy channels are out for such recordings.
> > > >
> > > > I expect my future use of the Octopre will be as preamps 1-6 for my
> MOTU
> > 828
> > > > MkII's first six channels, with my FMR-RNP feeding channels 7 and 8.
> If
> > I
> > > > need two more, the MOTU also includes a couple of fairly decent pres.
> > Not a
> > > > bad scheme, but too darn bad about the Octopre living up to only 3/4
> of
> > its
> > > > potential.
> > >
> > > Why not contact Focusrite and complain ? They may have done an upgrade
> > since you
> > > bought yours.
> > >
> > Thank you for your responses concerning my issue. I had been reluctant to
> > send my Octopre off to Focusrite, but I looked them up and, wonder of
> > wonders, their North American Service Operation is nearby in the San
> > Francisco Bay Area, about a half-hour from my East Bay home. I'll give
> them
> > a call and hand deliver my unit next week.
>
> FINAL REPORT (In case anyone's paying attention)
> The hum problem turned out not to be the Octopre. I found that the hum
> problem disappeared when I re-tested the Octopre after removing it from the
> rack. It turns out that the hum was getting into the Octopre from the
> adjacent MOTU 828 MkII in the same rack. The 828's power supply was above
> the portion of the Octopre's main board that contains the circuitry for
> channels 7 and 8. I moved the two units one rack space apart (with a FMR
> RNP compressor in a rack tray going between), and the problem is solved.
Now why didn't I consider that possibility ?
Pleased to see you've resolved it anyway.
Graham