Basic Sub-woofer addition question for stereo music

[loafeye]

How to gently ask: Is the following true or false?
Setup: 1 pair of active speakers.
I add a powered sub-woofer, or a pair of subs (the subs would match, as best as manufacturing tolerances allow).
So now I have a stereo sound system with 2 amps, or 3 amps if I've added a pair of subs.
I have zero confidence that the gain stage of the speaker amp matches the gain stage of the sub-woofer(s). (OK, I could get lucky, but...)
The system is controlled by a single pre-amp (say, an internal pc sound card, or some iteration of the miniDSP Flex).
I hook up the subs, turn on the "system", crank to a happy listening level, and then match the listening level (volume) of the subs to the listening level (volume) of the speaker pair, so that I realize a balanced output.
My system sounds great to my ears (let's just imagine that this is true).

Now, when I raise or lower the volume knob on the pre-amp, the sound from the sub-woofer(s) will be louder or softer than the sound from the speakers,
because the amp in the sub(s) doesn't match the amp powering the speakers, and the music will be bass-heavy or bass-light, and as a consequence, my system will not sound so good. IOW, my system will only sound great at that initial (single) volume level.
Is this true?

Thank you for any insight.

 

[voodooless]

The Flex will volume control all outputs, including the subs. You should be fine.

As long as you have one point to control the volume of all the outputs you need for your system, you should be good.

 

[loafeye]

Thanks for your response.

I was unaware that "most" power amps (or power amp stages in integrated amps) use engineered gain at 28-32 dB, though I guess the gain is also dependent on the voltage it sees from the preamp. So this now makes sense to me that in choosing a sub or subs, that the purchasing guides I've looked at don't ask the buyer to agonize over gain mismatch between the power amps, because presumably they're all close, or to care about 100w RMS vs. 300w RMS as a starting point, as there's more important considerations, like room size, etc. (OK, this is incomplete, but whatever, it's not my focus). And if a manufacturer subscribes to the THX certification, then their units apply ~ 28.5-29 dB gain (from some common reference signal) so they'd all be fairly closely matched. I think I understand that low frequency volume differences are more difficult for me to distinguish than mids and highs, so if my system was slightly mismatched at a higher or lower volume than my "optimal setup" (so the balance is off), I probably couldn't tell the difference, or it wouldn't be off enough for me to care, or I could use the remote sub volume control (if I had one) to fiddle with it.
It reminded me of the features of a powered (Yamaha?) speaker pair I looked at a couple years ago, where the LF driver used a 50w RMS (?) amp and the tweeter had its own ~35w RMS (?) amp; the gain of the two amps would be matched, and of course they both use the same incoming signal, so that the balance between the lows and mids/highs would be preserved at differing listening levels (my guess).
I'm not interested in home theater anything, but there was some good reading on some of the sites I looked at.

 

[DVDdoug]

There are lots of ways to do it. Most powered (active) subwoofers have a gain control and a built-in low-pass filter. Most power amps also have a gain control. Speaker sensitivity varies too so it's pretty useless to "calculate" the dB gain.

Usually the power amp gain is set-and-forget with the daily volume adjusted at the source (a preamp, etc.).

The easiest setup is with an AVR (audio video receiver). An AVR has speaker outputs for passive main (and surround) speakers and a line-level subwoofer output. Most people use an active subwoofer but you can use a separate amp and passive sub. The AVR has a crossover ("bass management") to optionally route the bass away from the main speakers (and surround channels) to the sub. The AVR will have a separate adjustment for the sub-level in in case your sub or sub-amp doesn't have a gain control. With movies the "point one" channel (LFE = Low Frequency Effects) goes to the subwoofer only, so that's the only way to hear the LFE. (The regular bass can still go to the other speakers).

You can configure an AVR for stereo if you don't have (or want) surround speakers. It's still an economical solution for stereo.

Not all AVRs have volume-controlled line-outputs ("preamp outputs") for the other channels so you'd need to look for that feature if you wanted to use a separate stereo power amplifier.

Another option is an active crossover or something like a MiniDSP. Pro setups used for live music or in dance clubs usually have an active crossover.

You can also get "pro" power amplifiers with DSP built-in so you can set the main amp to high-pass and the sub amp to low-pass.

Some active subwoofers have a full-crossover built-in. There are line-inputs passed-through a high-pass filter to line-outputs. Some have optional "speaker level" inputs and line-outputs (for use with a separate power amplifier). That would work if you have a receiver without preamp outputs.

Or, for a simple setup you can use a Y-splitters to feed your main speakers and the sub from a regular stereo output. The bass still goes to your regular speakers but if the sub is active it will have a low-pass filter so it doesn't try to reproduce the higher frequencies.

 

[loafeye]

There are lots of ways to do it.

Boy, I guess.
My thanks to you.