Paradigm SubWoofer Low Level Input - Interface Standards?

[Thomas M. Alldread]

Greetings:

As a labor of love I am trying to repair/rebuild a 100W(?)
amplifier built into a subwoofer speaker. Unfortunately I cannot find
a schematic for this "Paradigm Cinema Sub 0.2" subwoofer.

The subwoofer low level input is intended to be fed from a
surround sound, JVC receiver equipped with a sub woofer output. This
subwoofer output is internally fed from both left and right channels
which are mixed together and then low pass filtered to pass only low
frequency audio to the sub woofer output phono jack which resembles a
line output.

This output appears to have a high source impedance as it
would not drive headphones. I have had a chance to measure the level
and source Z yet as receiver is at a different location. I didn't
happen to have my scope with me when I had access. The amplifier
output does work with another subwoofer from a different manufacture
as long as the volume control gain on that woofer is set near max.

In my search to find specitication standards for low level
subwoofer interfaces I ran across a sub woofer amplified speaker with
a specified low level input level of 75 mV rms and input impedance of
50KOhm.

I drew a schematic of the input circuitry of this amplifier
by tracing the circuit board and have concluded the input Z is about
10K in this case. Considering the attenuation ratio from the alternate
speaker inputs a line level input 2V ptp would match the level from 10
watts RMS across 8 Ohms on alternate speaker inputs.

The sub woofer was manufactured by Paradigm and is a model
"Cinima Sub 0.2". I wish for a schematic for the amplifier which is
built from discrete components.

The receiver/amplifier is a JVC MODEL RX-6042
AUDIO VIDEO CONTROL RECIEVER.

Thanks in advance to anyone who can enlighten me on typical
subwoofer low level interface operating levels and impedance or better
still a place from which I can download a schematic for this amplifer.

Tom Alldread
[email protected]

 

[JeffM]

Thanks in advance to anyone who can enlighten me

Stop posting like a moron
http://groups.google.com/groups/search?enc_author=SWIIYw8AAAClmtXxQoQc9bMrk929wwqK&scoring=d

and learn what to avoid doing.
http://www.google.com/search?q=multi-posting

 

[Thomas M. Alldread]

Previous Subject: SubWoofer Low Level Input - Interface Standards?

Greetings:


Many thanks to all those that responded to my post on this
subject. I decided it may be of use to let everyone know that the
subwoofer is now repaired and doing its job nicely.

As I mentioned this was a labor of love repair job as the cost
to have it repaired would not have been economical.

Sometime in this woofers previous life a mouse had crawled in
through the base reflex port and got caught between the power switch
terminals and one end of the audio pre-amp board in the area of the
+/-12V regulators. This could be considered a "high end" mouse trap as
the mouse certainly never survived.

At the onset, before taking this task on, the distributer for
Paradigm audio products was contacted. They advised there were no
replacement boards available for this 4 (?) year old product,
schematics were proprietary and there was not any service depot
available for it.

About 3 square inches of the board was charred beyond
component identification. As there was no other option I decided to
try and repair it. To start I cut the charred area of the pcb away
with my Dremel tool which removed what I thought was probably just the
+/- 12V dual polarity power supply regulator. I built a new power
regulator daughter board using a couple of TO220 style variable
regulators. With this effort I managed to get it back to life, but
without sufficient sensitivity to work with the amplifier it was
purchased with.

Finally a few days ago I decided to byte the bullet and do
some reverse engineering. I drew out portions of the schematic by
tracing the components. The amplifier uses legacy, leaded components
which I found surprising but which made my job easier. The pre-amps
are made up from conventional LM324 and LM741 DIP package op amps. A
Cmos logic device is provided for muting.

The primary power supply is +/- 42V. When I built the
regulators off the pre-amp board I decided to leave this disconnected
from the preamp. Since the preamp is powered from the +/- 12V supply
and I thought the charred area I removed contained only the regulator
circuitry I thought the primary DC voltages were probably no longer
needed on the preamp board. My presumption was proven wrong as it
turns out the +/- 42V was also used to mute the amplifier until the
voltages stabilize during power up.

To make a long story short I ended up bypassing some of the
original mute protection and maximum level protection circuitry which
once had some components in the burnt region of the board. The
sensitivity is now around 20 - 50 mV p-t-p into a Z of 10K and it will
put out 80V ptp into the speaker before clipping which is well beyond
what the speaker can handle. One must now be careful to ensure the
power is off when hooking up the input cables else the speaker could
be blown with 60 Hz pickup.

I had never worked on an amplified subwoofer before and had no
idea that the inputs are so Hi Z and so sensitive! I expected, like
most I think, to find the Z around 600 Ohms and the level around 1V
ptp. I should also mentioned that the receiver is rated for 100W and I
don't imagine we were testing it at more than 10W. But scaling that
back it would still be 300 mV reference 1V at 100W. Had I measured the
receiver output at maximum I think I would have found it around 100 mV
ptp about 20dB down from normal line levels.

It seems odd that the industry has adopted such a low
interface level for a subwoofer which is subject to hum pickup.

In any event happiness is nice music now with lots of good
base!


Thanks again!
Tom Alldread