RSL's Subwoofer Line In intended for preamp?

[RSA]

I have a simple preamp (Bellari PA555, 100 Ohm output impedance, 11.1 dB maximum output gain) which has only one stereo output (i.e. no separate LFE/sub output). This preamp is connected to an old Hafler DH-220 power amp driving Klipsch Heresy I speakers (‘1.5’); more recently I’ve also acquired two Fosi Audio V3 monoblocks (one per channel). Regardless of which power amps are used, I’m primarily concerned about avoiding potential damage to any of this equipment, and my question below involves confusion over use of the term “line level”.

I just ordered a RSL Speedwoofer 10s MkII subwoofer (has yet to arrive). Per RSL's online literature, the subwoofer affords two means of connection: line level or speaker level. In the former, RSL describes this as “Line level RCA inputs and RCA pass-through signal outputs”, where these pass-through terminals would presumably connect to a power amplifier. My confusion is over the use of “line level” here, as my preamp output voltage is obviously not fixed at line level but depends on the preamp’s volume setting. Is there any danger in connecting the output stage of the preamp to the sub’s line level inputs?

Alternatively, is it safer to use speaker level outputs? Also, because this subwoofer has no speaker level outputs, if I go this route can I just daisy chain the sub’s speaker inputs to my main speakers? I read through an ASR post that contains similar content, that thread also involved using BTL, whose details and application I didn’t really understand. Sorry for the long post and likely stupid questions.

 

[TimW]

There is no danger in connecting pre-outs to line level inputs. The subwoofer's line level inputs are intended to be used with a variable pre-out so that its volume can be controlled with the main speakers. They just refer to this input as "line level" to differentiate it from "speaker level". The pass-through RCA output of the subwoofer is just that, a direct connection from its line-level RCA input connectors to the pass-through output connectors. Using the pass-through RCA output for your power amplifiers would be a good way to connect them.

The speaker level input of the subwoofer basically just lowers the voltage from "speaker level" down to "line-level" internally with resistors. Connecting the sub with the speaker level inputs the way you describe is fine but it would create extra wire length and connections going to your speakers. In this scenario I would prefer to run regular speaker cable from your amps to your speakers and then run a separate set of speaker cables (which can be small gauge wires since it is just for signal, not power) to the subwoofer from the same connections on your power amps. I believe this connection is less ideal because the added resistors in the signal path can add distortion of their own and potentially cause your amplifiers to distort more.

 

[RSA]

There is no danger in connecting pre-outs to line level inputs. The subwoofer's line level inputs are intended to be used with a variable pre-out so that its volume can be controlled with the main speakers. They just refer to this input as "line level" to differentiate it from "speaker level". The pass-through RCA output of the subwoofer is just that, a direct connection from its line-level RCA input connectors to the pass-through output connectors. Using the pass-through RCA output for your power amplifiers would be a good way to connect them.

The speaker level input of the subwoofer basically just lowers the voltage from "speaker level" down to "line-level" internally with resistors. Connecting the sub with the speaker level inputs the way you describe is fine but it would create extra wire length and connections going to your speakers. In this scenario I would prefer to run regular speaker cable from your amps to your speakers and then run a separate set of speaker cables (which can be small gauge wires since it is just for signal, not power) to the subwoofer from the same connections on your power amps. I believe this connection is less ideal because the added resistors in the signal path can add distortion of their own and potentially cause your amplifiers to distort more.

Hi Tim, perfect, thanks very much for the speedy and detailed answer. I will definitely run with the first option, simple and straightforward.

 

[Palladium]

I don't get why the L/R line-outs on this particular sub are full-range passthrough and not high-passed by the crossover.

It makes no sense. Isn't the whole point of an active sub in a non-HT receiver stereo setup is to NOT allow LF under crossover frequency to hit the main speakers?

 

[DVDdoug]

In the former, RSL describes this as “Line level RCA inputs and RCA pass-through signal outputs”, where these pass-through terminals would presumably connect to a power amplifier.

That's the best way to go. The speaker has a crossover to not only block the higher frequencies from the subwoofer but to block the bass from the main speakers.

It makes no sense. Isn't the whole point of an active sub in a non-HT receiver stereo setup is to NOT allow LF under crossover frequency to hit the main speakers?

I checked online and it does have a pass-through crossover (which can be bypassed for "pure "pass-through).

Line level is "defined" but in consumer audio it's not calibrated. Plus, there are loud songs and quiet songs, and often there's a volume control. Line outputs usually have plenty of extra signal, and line inputs plenty of extra gain, so that everything works together. But pro line-level is higher than consumer line-level so sometimes a pro amplifier (usually with balanced XLR inputs) doesn't have enough gain to be used with consumer equipment.

 

[Palladium]

That's the best way to go. The speaker has a crossover to not only block the higher frequencies from the subwoofer but to block the bass from the main speakers.


I checked online and it does have a pass-through crossover (which can be bypassed for "pure "pass-through).

Line level is "defined" but in consumer audio it's not calibrated. Plus, there are loud songs and quiet songs, and often there's a volume control. Line outputs usually have plenty of extra signal, and line inputs plenty of extra gain, so that everything works together. But pro line-level is higher than consumer line-level so sometimes a pro amplifier (usually with balanced XLR inputs) doesn't have enough gain to be used with consumer equipment.

I have the actual unit on loan with me right now. The line outs on the sub are permanently full-passed no matter the configuration, even when the sub completely powered off.

The adjustable "crossover" only affects the low-pass bandwidth going to the sub driver.

So as far as I'm concerned its practically pointless without being fed a dedicated LFE channel which I don't have, so its going back. Sigh, I thought it would help relieve LF duties off my iLoud Micros.

I mean even something like a fixed 80Hz active HPF with bypass switch should be trivial a long time ago with op-amps, right?

 

[MarkS]

Sadly, there are very few subs that include high-pass filters for the main speakers.