[bmwpowere36m3]

I built a Gainclone amp a while ago to power a set of full-range speakers.  Now I'd like to add a separate subwoofer.  Is it possible to use the Bantam DAC as the source for two amplifiers.
 
The gainclone uses the Bantam DAC as the Input > 10k pot > stereo amplifier
 

 
 
I was thinking of tapping into the signal, post the 10k pot (to maintain volume of both amps with one pot) and send the signal to another amplifier. I was looking at using this powered sub (tying into its line-in) with the signal mentioned earlier.
 
Any issues?

 

[KimLaroux]

I have a bad feeling about taping directly to the input of an op-amp like shown on the schematic. It has the potential of destroying the equilibrium of the circuit and make the op-amp operate in all sort of unwanted ways. If the "output" isn't connected, the RCA connector will essentially be an antenna. When connected, the input of the sub's amplifier circuit will modify the impedance of this amp's input circuitry. The gainclone may not care and actually work even with the modification, but I would never try it, just because it seems wrong.

 

[wakibaki]

The sensitivity (gain) of the subwoofer may not match the sensitivity of the gainclone. You really want another preset pot in there to allow you to do a one-time adjustment of the sub volume. You could use a simple opamp buffer after the volume control but before the preset, to make sure that the feed from the volume pot to the gainclone will be unaffected to all intents and purposes..
 
Of course you may find that the sub is too quiet, relative to the GC. A way round this is to build your opamp buffer in such a fashion so that you could swap in (or jumper in) a change to the feedback to give it some gain. Then you would have the capability to adjust the sub volume both up and down.
 
Edit:-
 
The other issue which just occurred to me is that you're probably dealing with a stereo system. This really makes using a buffer in each channel mandatory to pick off the signal to maintain the isolation between the 2 channels, then you have to sum the outputs of the 2 buffers. You can just do that with a resistive combiner.
 
w

 

[bmwpowere36m3]

Quote:

I have a bad feeling about taping directly to the input of an op-amp like shown on the schematic. It has the potential of destroying the equilibrium of the circuit and make the op-amp operate in all sort of unwanted ways. If the "output" isn't connected, the RCA connector will essentially be an antenna. When connected, the input of the sub's amplifier circuit will modify the impedance of this amp's input circuitry. The gainclone may not care and actually work even with the modification, but I would never try it, just because it seems wrong.

 
That was my concern...
 
Quote:

The sensitivity (gain) of the subwoofer may not match the sensitivity of the gainclone. You really want another preset pot in there to allow you to do a one-time adjustment of the sub volume. You could use a simple opamp buffer after the volume control but before the preset, to make sure that the feed from the volume pot to the gainclone will be unaffected to all intents and purposes..
 
Of course you may find that the sub is too quiet, relative to the GC. A way round this is to build your opamp buffer in such a fashion so that you could swap in (or jumper in) a change to the feedback to give it some gain. Then you would have the capability to adjust the sub volume both up and down.
 
Edit:-
 
The other issue which just occurred to me is that you're probably dealing with a stereo system. This really makes using a buffer in each channel mandatory to pick off the signal to maintain the isolation between the 2 channels, then you have to sum the outputs of the 2 buffers. You can just do that with a resistive combiner.
 
w

 
The subwoofer has its own gain setting and two inputs (line & speaker level, left/right).  Yes, the gainclone is in a stereo configuration.

 

[bmwpowere36m3]

If I understand correctly, then I want something like this:
 

 
Any recommendations on a simple buffer circuit?

 

[wakibaki]

Quote:

The subwoofer has its own gain setting and two inputs (line & speaker level, left/right).  Yes, the gainclone is in a stereo configuration.

 
Ah, right. Chances are the line inputs are already buffered.
 
You can do this, as long as the PSU voltages do not exceed +/- 18V
 
 

 
...but why not just connect the speaker outs from the gainclone to the speaker ins?
 
w

 

[bmwpowere36m3]

Quote:

 
Ah, right. Chances are the line inputs are already buffered.
 
You can do this, as long as the PSU voltages do not exceed +/- 18V
 
 

 
...but why not just connect the speaker outs from the gainclone to the speaker ins?
 
w

I could use the speaker inputs on the subwoofer, however I'd like this to be more universal if I end up using a different subwoofer.  IIRC the trafo is a 22+22 VAC, so ~ 32 VDC is what I have readily available within the amp.  I could also build a small voltage regulator to drop the voltage...

 

[wakibaki]

Quote:

I could use the speaker inputs on the subwoofer, however I'd like this to be more universal if I end up using a different subwoofer.  IIRC the trafo is a 22+22 VAC, so ~ 32 VDC is what I have readily available within the amp.  I could also build a small voltage regulator to drop the voltage...

 
A voltage regulator (each side) is good, or you could just use a series Zener diode of ~15V in each rail to bring the supply voltage down to ~17V.
 
w.