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Subwoofer volume vastly different based on amp. Why?

Summary: I am trying to better understand why my powered subs have very little available volume (i.e. I have the gain dial set to 80-100% gain) when using some amps and a lot of available volume (i.e. I have the gain dial set to 10-30% gain) when using others amps. After doing some research of my own, I think it may be due to the output voltage on the amps, but I would appreciate some confirmation. Below is a breakdown of my experience.
Sub 1 = Monitor Audio R360(Product Page)
Sub 2 = Klipsch RSW-10(Spec Sheet)
- Not used at the same time, but same experience with either sub.- I am only using line-level RCA connections. Both subs have L/R RCA inputs.

Amp 1 = AudioSource Amp100VS using Main Out = lots of volume(Spec Sheet)
Amp 2 = Cambridge Audio CXA61 using Pre-out or Sub-out = lots of volume(Product Page)Amp 3 = Pioneer VSX-824 using Subwoofer Pre-out = not much volume(Spec Sheet)Amp 4 = Rotel A12 using Pre-out = not much volume(Product Page)
Conclusion: Is this difference in volume due to different output voltages on the pre/main/sub outs? The Rotel A12 is the only amp that I can find an output voltage for. They list the pre-out as 1V. If this is indeed the issue, it seems strange to me that more emphasis isn't placed on these voltages. I have seen this article show up over and over again when trying to learn more about this subject, but honestly, I find it hard to follow.

Possible Solutions (assuming output voltage): Use an RCA splitter from the passive pre-amp? Use an active crossover such as the miniDSP 2x4? Other ideas?
Thank you so much for your input.
ohmy
erik_squires
16,061 posts
 
While the gain of a preamp may vary from model to model (the ratio of output to input) the gain at 20-40Hz should be the same as at 1 kHz.

I'm not sure what is going on in your system but a miniDSP is always an excellent idea when using a subwoofer. The ability to integrate with the mains and adjust for room modes is really amazing.
ohmy OP
8 posts
 
@millercarbon It's not that I think where the volume knob is positioned matters, it's that 100% volume is not enough with my prefered pre-out sources.
ohmy OP
8 posts
 
@erik_squires I might have explained it poorly. The gain isn't changing at different frequencies, it's just that the maximum volume level changes based on which pre-out source I use. Maybe this is obvious to most, but it took me by surprise. Unfortunately, some of the pre-outs appear to have so little gain that 100% volume on the sub is inadequate. Thanks for the input on the miniDSP. I might end up having to go that route.
erik_squires
16,061 posts
 
Ahhh.  It is possible that the combined input impedance of your amp and sub is more taxing on some preamps.

You will be very pleased with the capabilities the MiniDSP will bring to your room. :) 
millercarbon's avatar
millercarbon
12,531 posts
 
Then you have a problem with certain sources not having enough output voltage to drive the sub to the level you want. You simply need more input voltage. The cheapest, easiest, and most sure-fire solution is called a Line Out Converter or LOC. The LOC is a simple circuit that enables you to connect the speaker level output from your amp speaker terminals directly into the line level RCA on your subs.   

All the LOC does is add a couple resistors, typically 20k to 50k ohm. This drops speaker voltage down to line level. It does not affect the amp at all. It can also be done as a simple mod (you can DIY) which is the way I ran mine. But the LOC is cheap, paying more does not get you anything, parts-express.com has them starting at around $10.    

Here is the circuit along with the best explanation I know. https://www.epanorama.net/circuits/speaker_to_line.html/  


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