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|5th November 2006, 07:06 PM||#1|
Join Date: Aug 2005
lesser of two evils, high level -> line level, digital volume control
I'm building some real nice speakers for my dad, 2 x all active towers, parts cost only.
These speakers have balanced XLR in, and 12 volt triggers to turn on and off. I though I would have to make some rca->XLR adapters and maybe buy a 12 volt ac/dc adapter to put on a switched outlet.
I asked him what his amplifier is, it turns out to be the yamaha 5840
This thing lacks any preouts. I have the following options to make thing work.
make some high->low level converters like on this page, possibly I could put in an op amp or two and make some active converters:
buy a surround receiver for < $400 with digital inputs and preouts
buy a usb dac/preamp and let him use his HTPC for volume control. I could put in some jumpers in the speakers to ensure they have a low enough gain not to accidentally blow up with full volume.
He has a quiet htpc in the room he wants the speakers, so that could possibly be a preamp. Also, I am debating whether he should be able to use these loudspeakers in his surround system (old paradigm), or suggest a two channel only solution.
What do you guys think is the best idea here?
|18th November 2006, 12:40 PM||#2|
Join Date: Sep 2006
Sorry that nobody responded earlier. . .this seems like a simple
solution to your problem of getting an amplifier without preamp
outputs to work in your setup.
You could always wire into the headphone output. . . either with
an 90 degree angle headphone plug or go inside and hard wire
the connection. I've used the technique to run three receivers
without any preamp outputs. Back in the days of poor DJ. . . .I
would borrow three receivers and use the first as master. . .the
next would be just an amp and the third we would EQ to make
a low pass amp and run the sub. . .poor man's DJ. . . .
The headphone output is just a preamp output able to deliver
more power at low impedance. . . A preamp output might have
a output impedance of 47 ohms (my C-85). . .the headphone
output impedance might be able to drive 8 ohm speakers into
several milliwatts. . .I often use the preamp output to drive
my headphones . . .when I don't want the cables sticking out
the front of the preamp. . .
Converting unbalanced to balanced to drive long lines. . . .look at
what the professionals use. . .converting boxes. . .some are all
passive and some require power (allowing you to adjust overall
gain). I use the passive variety. . .works great. . .the good ones
have great frequency response. . .then use good microphone
cables to run your long cables to the powered speakers. . .
I have never used the powered versions but assume they will
work on the same principle but with some transistor arrangement.
A quick search of google shows:
Rane makes a Balance Buddy 22 that is high quality for less
than $150. RCA in and 3-pin XLR output
EBtech makes a cheaper unit that I use:
Ebtech LLS2 Line Level Shifter (2 channel) that uses 1/4"
jacks. . . make up your cables. . .mount the box behind the
receiver with short cables between the receiver (now preamp)
and you are set.
If I had the $$ or was doing a high end install I would use the
Rane. . .they make some great equipment (I love my VP12).
That should get your signal out of the receiver and converted
over to unbalanced.
As for the 12V trigger. . . .there are kits for remote sensing of
signal to turn on units. . . I don't like to run long DC cables. . .sets
up all kinds of noise. . . so. . . .
Use a 12VAC (note the AC aspect of the wall wart) plugged into the
switched outlet on the back of the receiver. . . now run twisted pair
wire (catagory 5 ethernet will work great. . . to the speaker. Now
convert to DC with a single diode to create DC at the input jack. . .
or internally wire a diode. . .now all the DC is inside and not running
around your house. . . It would be very bad to run DC house
mains instead of AV house mains. Noise is one! If your amplifier
on the speakers can't handle 'more than 12V' then use a 9VAC
adaptor. . . when the diode converts to half-wave DC. . .there
will be a slight increase of DC. . .especially if you use a 470 micro
farad 25V cap to clean up the DC.
Let me know how it works. . .
|18th November 2006, 02:35 PM||#3|
Join Date: Aug 2005
Thanks a lot for the detailed reply. System integration is one of the interesting aspects of the project I am working on.
I didn't know about using headphone outputs as pre-outs. They are rated at 150 mV, which should be plenty. That would allow him to toggle between my speakers and his existing surround system. He wouldn't be able to integrate the two speaker setups that way, but I'm not really sure if there would be a point to that.
I might go and purchase the Balance buddy now, I'd like to test these with actual balanced input, I only have rca->XLR cables for now.
About your point where you suggest that running long DC lines is not good from a noise perspective. For the trigger switch, I am using some Omron G6K low signal relays, coil is 12 Volt DC rated, 9mA load. I opted for these rather than a transistor because it seemed like a good way to keep everything separate. Do you still think AC is best?
I considered putting unbalanced to balanced transformers right in the loudspeaker plate amp, but the Jensen transformers I was looking at were quite expensive, also space is at a premium in the plate amp. I also opted not to do a signal detect for now, signal detecting amps I've had before have sometimes turned off when played to quiet, I may leave a shunt in the circuit to add one later.
I'm still thinking about the different options you proposed, and will post back with progress.
|19th November 2006, 12:25 AM||#4|
Join Date: Sep 2006
You might be surprised by how much the old headphone jack
can actually supply. . .you won't be using the mains out. . .just
the headphone jack. . .terminated with between 10k to 100k amplifier
input impedance; the rating on most headphone jacks it volts across
some low impedance. . . ultimately for the dissipation of watts. . . you
just need many volts. . .something it might be able to provide since
you will be terminating it with a much higher impedance than normal.
Try it first. . .then build a L-Pad for the speaker outputs if it doesn't
drive the amps with enough voltage. I've used three receivers all
driving each other in the past.
You will want to limit it to about 8V (the general max of most preamps).
A 100W amp can supply about 28.3 V into 8 ohms. . .somewhat higher
into a higher impedance load. . . all very doable with a correctly
designed L-pad; quite safe also. . .
You always want to convert to balanced lines as close to the
unbalanced source as possible. . .then run those long runs with
the low impedance balanced lines. . . you may not know this but
you can run microphone cables hundreds of feet with very low
signal loss or noise introduction.
You can try the DC since the signals for the amps will be shielded
and low impedance balanced lines. . .if it doesn't work. . .switch
to AC. . .I just would rather run low voltage AC. . .less fire or
Using low voltage AC to switch a relay to switch high voltage AC
mains seems like the easiest way to go. Converting to DC seems
extra work unless you already have the DC coil relays. . .
Let me know.
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