I reverse engineered the schematic here and I'm struggling to understand all the circuits and how to modify it to work with low level RCA inputs.
I believe I need to remove the 12.1k &1uF caps on each input because they're low pass filters, increase the value of the 5.11k resistors to increase the amp gain and remove the 12m & .0039uF and .033uF caps? correct?
Link
I believe I need to remove the 12.1k &1uF caps on each input because they're low pass filters, increase the value of the 5.11k resistors to increase the amp gain and remove the 12m & .0039uF and .033uF caps? correct?
Link
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I have the same sub and wanted to make a similar mod. Thanks for the schematic, but I don't know enough to verify your suggestion. Have you been able to make any progress? I was just going to find an amp driver stage circuit to place in front of the spkr inputs to avoid having to understand and change the circuit.
I agree. All you need is a simple pre-amp that can bring up the line-level signal from the RCA inputs enough to feed to the sub's speaker-level inputs.
I don't think there's an actual technical definition of "speaker level", i.e., I don't know how much signal voltage the sub actually needs. While it takes 30 volts peak-to-peak to deliver 14 watts to an 8 ohm speaker, that would be extremely loud. Subwoofers are often used as part of a home-theatre setup, and a lot of TV listening is done at around 65 dB SPL.
65 dB SPL corresponds to only 0.01 watts into typical speakers with a sensitivity of, say, 85 dB@1m@1W.
And 0.01 watts inot 8 ohms needs about 400 mV peak at the loudspeaker terminals!
So I suspect all you need is a single op-amp gain stage. Now all we need is an estimate as to how much voltage gain it will need.
Consumer audio line level is nominally -10 dBV, which is a sine with a peak voltage of 447 mV.
A typical op-amp running on the usual +/- 15 volt supply rails can probably manage about 12 volts peak output voltage before clipping.
That means a voltage gain of (12/0.447), or roughly 30 times, will bring up a nominally line level signal to about the maximum an op-amp can put out.
So, lacking any further information, I think a volume pot, followed by a non-inverting op-amp gain stage set for a gain of roughly 30 times, will probably work (to bring up a line level signal enough to feed to speaker-level subwoofer inputs).
The volume control at the input is necessary, so you can dial down the signal to keep the op-amp from clipping even with the strongest line-level input voltage from your audio source.
If you find that 30x voltage gain is too much, or insufficient, all it takes is one resistor change to alter the gain, so it will be easy to tweak the circuit to get the gain you actually need.
-Gnobuddy
If you lower the values of the 12.1k, 130k and 5.11k resistors and pot, then you could feed in low-level line level voltage directly into the 2W's RC4136 op amp.
The other option is to add an extra op amp and use that to boost the line level voltage to speaker level as @Gnobuddy suggested.
The amplifier doesn't output a linear voltage since it expects the signal to be high-pass filtered so you must add a 80hz first-order high pass filter @Gnobuddy's op amp circuit.
http://www.vandersteen.com/media/files/Manuals/2wqmanual.pdf
I ended up basically leaving the amplifier as stock but I replaced the RC4136 with 2 NJM2068MD and I changed the value of the 5.11k resistor before the pot to increase the overall gain. I'm using it in a 2 channel setup and not all 2 channel amps have RCA line-level outs so high level is better in my current configuration.
Thanks @Gnobuddy for the circuit parameters, which I will try. It also means I have the option to revert back (or sell unmolested) . Are there opamps you would recommend? I presume an NE5532 would do? I could use the other dual to do the filter (a -6db) as highlighted by @Ryeno before the volume pot.
Thanks @Ryeno for the update.
Thanks @Ryeno for the update.
Will you add the op amp inside the 2W or outside? If inside then measure the voltage from the bridge rectifier and ensure that your op amp can handle it.
This link has audio op amp reviews. http://nwavguy.blogspot.com/2011/08/op-amp-measurements.html