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Old 7th June 2002, 07:40 AM   #1
Vivek is offline Vivek  India
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Join Date: Nov 2000
Location: Bangalore, India
Default Line out

Hi all,
How does one make a line out connection from an amp to add an active subwoofer to the system? Can I just solder wires to the volume control and send the signal to another amp?
Thanks for any help.
Regards,
Vivek.
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Old 9th June 2002, 06:59 PM   #2
MBK is offline MBK  Singapore
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No, that alone won't do it in all likelihood. The volume control will likely not be able to drive the line. Basically you want a buffer to drive your line out.

If you don't have a tape but you have a "tape monitor" type configuration on the integrated, just use the "tape out": that one is a line output that will use the same source as selected for the main output.

Option 2 if that is impractical or no tape loop on your machine: chances are, after the volume control in your integrated amp there is an op-amp buffer before the power stage. You could take the outputs of that buffer, add a 47 to 200 Ohms resistor in series, connect to a pair of RCA connectors, and use this. Don't forget to connect ground to the RCA's :-) . This solution is not 100% engeneerically correct (to be perfectly pedantic one would want the first buffer to act as a distribution amplifier driving as many final buffers as there are outputs) but I guess it will work.

MBK
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Old 11th June 2002, 09:38 AM   #3
yeti is offline yeti  Germany
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Default subwoofer output

If you want to connect a sub,it would be suitable to combine the buffer with a lowpass filter.
You may construct a lowpass filter with high input impedance,possibly > 100Kohms and connect it to the pot's wiper connection.
The output then follows the level adjusted with the volume pot, and the load is well separated from it.
Additionally you can preserve that the sub doesn't produce sound in the midrange.
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Old 11th June 2002, 10:33 AM   #4
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Quote:
Originally posted by MBK
If you don't have a tape but you have a "tape monitor" type configuration on the integrated, just use the "tape out": that one is a line output that will use the same source as selected for the main output.
Beware, I've never seen a tape loop/monitor on an amp that wasn't before the volume control. If you use it, your sub will not change in level with the mains when you adjust the volume. If they are after the volume control, they're marked as "pre outs"

Quote:
Option 2 if that is impractical or no tape loop on your machine: chances are, after the volume control in your integrated amp there is an op-amp buffer before the power stage.

MBK
Beware again, as this buffer may not be there. Because the designer of the amp has complete control over the gain structure and the load each stage will work into, which is not the case with seperates, there is often (usually) no buffer. Why add another unnneccessary stage with all the added cost, complexity, noise and distortion issues, when you almost certainly wont need any gain either.

You will probably have to add a line buffer/driver, especially if your sub has a plate amp installed and is any distance away from the amp. But try it without first, as it might be OK.
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Old 11th June 2002, 11:32 AM   #5
MBK is offline MBK  Singapore
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Quote:
Originally posted by Brett


Beware, I've never seen a tape loop/monitor on an amp that wasn't before the volume control. If you use it, your sub will not change in level with the mains when you adjust the volume. If they are after the volume control, they're marked as "pre outs"
Agree. My mistake.

Quote:
Originally posted by Brett
Beware again, as this buffer may not be there. Because the designer of the amp has complete control over the gain structure and the load each stage will work into, which is not the case with seperates, there is often (usually) no buffer. [/B]
Agree again, you have to check carefully how your amp circuit is configured and you may not get away with it.

One quick and dirty option is to take the final speaker output and add a resistive voltage divider (read volume control) to take your line out at the speaker level. This applies if your sub already has a crossover (likely). Since the speaker output is designed to drive heavy loads, you can use low resistor values to keep noise low, BUT in any case this will entail some experimenting to match levels and is not ideal: you're going through full gain in your integrated first, then hammer the signal down, only to re-amplify it in the sub.

To experiment with this you could do the following:
- get two, say, 1kOhm potentiometers (I guess 100 Ohms to 2k will give ok results, take 1W units)
- get a CD with test tones recorded at different transfer levels (it will say something like "-10 dB, -20dB, -30 dB" etc.)
- get a DVM (digital voltmeter)

then do this

- disconnect all speaker leads
- play a sinusoidal test tone of low frequency. (This is because DVM's usually will not display correct voltage at high frequencies. Mine will start levelling off above 400 Hz but 1000 Hz is still reasonable correct.). Use a track that has been recorded with a low transfer level (say -30 dB, or -20 dB, but NOT one of "0dB" because your amp may clip with a full signal at full volume setting.
- measure the AC voltage of the line level (output of CD player), write it down
- plug the CD player in the amp and play the same test tone
- measure speaker output AC voltage at full volume. The ratio of speaker to line level AC is the gain of your amp and will be about 20-30. This for reference.
- wire up one potentiometer to the speaker output (pot input to speaker out, pot output to ground, pot wiper is your attenuated signal). Measure the AC voltage of the same test tone at the pot wiper to ground. Play with the pot setting until it matches what you measured for line level voltage. This is now your line level output after volume control. Repeat for the other channel.


Now ... what you REALLY want is a preamp and an active X-O after it. That guarantees (well.... make much more likely) a good integration of your main speakers with the sub.

MBK
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Old 12th June 2002, 06:03 AM   #6
Vivek is offline Vivek  India
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Thanks to all of you for your help. I will try your suggestions.
Regards,
Vivek
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