Adcom noise
August 8, 2003
I hooked up an Adcom GFA-555 amp to one of the power
outlets of my GFP-555 preamp. When I turn off the GFP-555 and wait for the capacitors of
the GFA-555 to drain, I can hear a fairly loud noise coming from the speakers (mainly left
channel) at the very end (after front LED of the amp is off). It sounds as if something
else is discharging because the noise goes from high to low and finally disappears. I'm
using high-quality speaker cables with banana plugs. My speakers are Paradigm Titans.
What's causing this noise? What can I do to correct it? Is it affecting my speakers? I
contacted Adcom and their only suggestion was to plug the amp directly into a wall outlet.
I'd appreciate any information you can provide.
Ricardo
To ask an obvious question, Why don't you follow Adcom's
advice? It has been my experience that amplifiers, due to their high current draw,
typically don't do well when connected to surge protectors and other components. I would
suspect the current draw of a large amp like the GFA-555 does need to be plugged into the
wall. I'd suggest you take Adcom's advice and see what happens.
Impedance mismatch
August 7, 2003
I am currently making my first explorations into the world
of home cinema and have decided to utilize the speakers I already have. I have
accumulated:
KEF Q30 fronts, 6 ohm
KEF 70S rears, 6 ohm
KEF Q95C center, 4 ohm
All of these are second-hand purchases, and I have a
Marantz SR4200 receiver. My concern is the impedance of the speakers, the difference
between them, and also the fact that the impedance is lower than the amp says it is rated
at. I won't be driving the speakers particularly hard as we have a small viewing area.
Will the system operate OK despite the differences in the speakers and the rating of the
amp?
Matt
You may be OK, but it depends entirely on the
load-driving ability of the amplifiers in the receiver. If the Marantz is rated to drive
loads no lower than 8 ohms and there is no headroom in that specification, I'd say you'll
likely trip the protection system on the Marantz when you fire up the system. This mode
would simply shut down the receiver for a period of time. I've seen many receivers with an
official rating of 8 ohms that would drive a 4-ohm load comfortably, however. It should be
noted, though, that you could potentially damage the receiver if its protection circuitry
isn't up to par. In the end, you're likely better off getting a receiver rated to drive
4-ohm loads.
Decoupling
August 6, 2003
Would you please explain the theory or theories pertaining
to decoupling audio components from the surfaces they are mounted on, and what happens
when a speaker is decoupled from its stand, using cones or blue tack and then the stand is
decoupled from the floor using spikes. Can we be decoupling uselessly by isolating
sonically all from all? And what are the sonic benefits from all of this?
John Martin
There are multiple questions regarding multiple
issues contained in your letter. I think the primary issue you raise is related to
coupling, not decoupling. When a speaker is spiked to the floor you are coupling the
speaker so as reduce the movement of the speaker as it is playing music (since drivers are
mechanical devices, they are moving themselves, but they shouldn't move the cabinets).
There are also devices used to decouple components and speakers, and this is done for
different theoretical reasons. One is to reduce the transfer of vibration into
components and the other is provide a "path" to release energy away from the
component. There are a wide range of opinions on all this, many differing, even amongst
learned engineers. Although I can't give you absolute answers to your questions, I can say
that I've found it useful to provide a solid, level base for components and speakers.
Beyond that, you can certainly experiment with the myriad devices designed to couple and
decouple your system's components. Hopefully you'll find what benefits your situation the
most.
Replace jumpers?
August 4, 2003
Is it worthwhile to replace those bent-metal jumpers that
connect the pre-out section on an integrated amp? I would imagine that the preamp-to-amp
connection is a pretty important one worthy of a decent interconnect.
Jay Stengel
It is an important connection, but I'm not sure
replacing the manufacturer-supplied jumpers with a set of interconnects would make for an
improvement. It might even be a step backwards. Your proposal seems like an easy one to
test in any case, and easy to reverse if you don't like the results. So hook up a set of
interconnects in place of those jumpers and see what you think. Let us know the results.
Virtual surround
August 1, 2003
I've seen some DVD players that have "virtual"
surround. Could I get one of these and have the same home-theater performance as I would
with a 5.1 system?
Daryl Harris
No. A virtual-surround decoder and two speakers will not
exactly replicate a 5.1 system. But this configuration can add to the enjoyment of movie
soundtracks by creating a more expansive soundstage. The key to virtual surround is to go
in with realistic expectations and realize that the technology, although good, has
limitations. A discrete 5.1 system can create surround effects that are astonishingly
real, and as of yet, there's no shortcut to that experience. Try to hear a
virtual-surround setup and compare it directly with a 5.1 system. That way you'll know
what you're getting versus what you'll be missing.
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