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Old 27th June 2004, 11:23 PM   #1
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Default Simple line-level amplifier?

Hi, I need to amplify the subwoofer/LFE signal coming out of my receiver to my subwoofer amplifer. I am using a 'professional' type amplifer that requires a stronger line-level signal. I am looking for a simple solution as the sound quality doesn't have to be the best (<200Hz). Maybe there is a simple one-chip solution? Thanks.

--Ferdi
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Old 28th June 2004, 12:34 AM   #2
matjans is offline matjans  Netherlands
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yes there is.

it's called an opamp
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Old 28th June 2004, 01:07 AM   #3
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Any schematics you could point me to? I have some OPA627 and AD8610 chips lying around that I could use. Thanks.

--Ferdi
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Old 28th June 2004, 02:01 AM   #4
matjans is offline matjans  Netherlands
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nice chips to have 'laying around'... they're amongst the best opamps available so you need not to be concerned about sound quality.

basic (non inverting) opamp circuit:
Click the image to open in full size.
R1 sets the input impedance, use a 100k resistor for this.

Gain is set with R2 and R3: gain is 1+(R2/R3).
Gain = 2 --> R2=1kOhm and R3=1kOhm.
Gain = 3 --> R2=2kOhm R3=1kOhm
etc...

input capacitor C3 (4,7uF bipolar min. 16V) removes any dc offset from your signal
C1 & C2 are small capacitors (a couple of uF's, rated at least your PSU voltage). bypass these with small (0,02uF) caps if you want.

edit: add a 50ohm resistor to the output of the opamp, this to avoid oscillations of the opamp.
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Old 28th June 2004, 03:49 AM   #5
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Thanks, you meant to put that 50ohm resistor in series with the output, correct? And, I am guessing that it should go after the loop going to R2? So pretty much just before X2?

Also, I've been talking to a friend of mine and he thinks that the output from the opamp should be buffered? Any comments, or is that only necessary when driving capacitive loads like cables?

--Ferdi
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Old 28th June 2004, 09:18 AM   #6
matjans is offline matjans  Netherlands
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yep before x2.

including a buffer will make it more complicated and this will work on almost any cable.
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Old 28th June 2004, 09:27 AM   #7
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Thanks for clearing that up. Now on to the power supply. I see that I need +-12volts. What would be a simple solution? I am thinking about using a small transformer (I have a 28V center-tapped) along with an LM317 voltage regulator to make a 24 volt supply, and then using a rail splitter (TLE2426) to make a virtual ground at 12v. That should give me a cheap and simple +-12v supply. Do you see anything wrong with this? Am I over-complicating the PSU? Thanks.

--Ferdi
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Old 28th June 2004, 09:33 AM   #8
joensd is offline joensd  Germany
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As you have a center-tapped transformer you shouldn´t use a rail splitter.
Have a look at this power supply which would be all you need.
http://www.jcscript.de/projects/eaton.html
The capacitance after the regulator can be much lower that 2200uF but there should be some.
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Old 28th June 2004, 10:01 AM   #9
matjans is offline matjans  Netherlands
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you can get cheap pcb-mounted transformwers with dual secondaries at almost any electronic supplies stores. they shouldn;t set you back more than a few (~5) euros; something with dual 9V supplies should work fine. After rectifiyng this will give you ~ 12V.

For testing purposes connect 2 9v battieries is series; this will give you 18V. You can use something like this:

Click the image to open in full size.

cap + leads are connected to +9v (for the top caps) and gnd (for bottom caps). cap - leads are connected gnd (for top caps) and -9v (for bottom caps)
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Old 28th June 2004, 10:35 AM   #10
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If connect middle of bateries, you can't get there resistors 4k7 .
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