[Vlad_Bath]

Hello, Vinnie and all!
I have an important question regarding subwoofers.
As one of many audiophiles who are hooked on full-scale fullrange sound, I tend to augment even big speakers with good quality sub or subs.
Traditionally, it is believed by many that the high-level (speaker level) signal input is the only way of proper sub integration for music purposes, as it shares the sonic character of the amplifier driving the speakers, and of the other upstream components, for more organic integration. That is the official position of REL and some other reputed manufacturers. Another reason is that many line-level crossovers degrade the sound more than the ones built-in in the sub, while most preamps lack dedicated mono subwoofer output. And without a dedicated sub out, using the main outputs on equipment that doesn’t have separate isolated stereo outputs, the long subwoofer cables add capacitance and impedance that get paralleled with the cables going to the amplifier thus degrading performance, also introducing cross-channel contamination if the stereo signal is bridged into mono at the inputs of a single subwoofer.
Now I get to the point. In many subwoofers the high-level "-" inputs are united. For example, in REL subs there are only three speaker wires: left +, right +, and common ground.
Some amplifiers don't have the speaker "-" outputs connected to ground and will short if connected together. That's why I didn't get the excellent Ayre integrated - no subwoofer compatibility.
And I am getting to think that the LIO mosfet amp module is of the same type - to quote from V.R. answer in the mosfet amp as the HPA topic: "You can't use the LIO MOSFET AMP speaker outputs to connect to a TRS jack, as you must not connect the L- and R- speaker posts together (they are not internally tied to GND, and must not be connected or else you'll short out the amp)."
I believe that such an important issue as subwoofer high-level input incompatibility should be clearly stated in the description of the amp module, as well as in the manual.
As for me, I would immediately short the amp at turn on, having connected it to a subwoofer, had I not read the topic about using amp module as hpa.
And other than informing people about the issue, it would be very nice to read about the proper way to connect single or stereo subwoofers to LIO, provided that high-level connection is not an option.

Regards,
Vlad

[Vinnie R.]

Hi Vlad,

Here is some info that might be helpful:

1) It seems that most subs do not connect their L- and R- speaker level inputs together.  If you have a DMM, you can measure
OHMS between these two jacks and confirm.  Ideally, there should be at least 100 ohms between them.  It's usually higher than
that.

2) If you measure that the sub does tie the L- and R- speaker inputs together, you can still use them if you put a resistor in
series between the L- cable and L- sub's input jack, and the same value resistor in series between the R- cable and the R- sub's
input jack.  I'd try 100 ohm (1/4W is fine) resistors, so the resistance between the negative speaker inputs is 200 ohms.  This
is plenty and should work well, and the speaker level input impedance on subs is still quite high.

3) If you are running two separate subs, then you can connect the LEFT channel speaker outputs to LEFT sub, and the
RIGHT channel speaker outputs to the RIGHT sub.  The negative speaker posts of the subs would not be connected in this
case (as long as they are not internally connected to the power cables earth GND prong - which would be very unlikely),
as you are connecting to two different subs.

4a) If you are using the REL sub with the 3-wire speaker level inputs, I believe the best way to do it is to connect
the L+ to L+, R+ to R+, and the GND to the RCA GND of LIO (e.g. use an RCA plug with the REL GND wire connected
to only the GND of the RCA, and do not connect the signal (hot) of this plug to anything).  Then just plug it onto one
of the unused RCA jacks on the back of the LIO.

4b) Others with REL subs have reported that they connected the GND wire of the speaker level cable of the sub to either
the L- or R-, and it works fine that way as well.

I believe that such an important issue as subwoofer high-level input incompatibility should be clearly stated in the description of the amp module, as well as in the manual.

It is certainly mentioned in the LIO's owner's manual already.  I will check the website and add it if it is missing.  Thank you.

I hope this helps,

Vinnie

[Vlad_Bath]

Vinnie, thank you very much!
This resistor trick solves the problem that have existed for years!
Why, why no other manufacturer published this information? If Ayre had it out, I would have got their amp some 5-7 years ago...
Although I don't care about Ayre now and looking forward to LIO 
But at the very least, subwoofer manufacturers like REL should have this info for their clients.
Thanks again, this info is very valuable, I hope the word gets out   

[yardbird4]

Hi Vlad,

Here is some info that might be helpful:

1) It seems that most subs do not connect their L- and R- speaker level inputs together.  If you have a DMM, you can measure
OHMS between these two jacks and confirm.  Ideally, there should be at least 100 ohms between them.  It's usually higher than
that.

2) If you measure that the sub does tie the L- and R- speaker inputs together, you can still use them if you put a resistor in
series between the L- cable and L- sub's input jack, and the same value resistor in series between the R- cable and the R- sub's
input jack.  I'd try 100 ohm (1/4W is fine) resistors, so the resistance between the negative speaker inputs is 200 ohms.  This
is plenty and should work well, and the speaker level input impedance on subs is still quite high.

3) If you are running two separate subs, then you can connect the LEFT channel speaker outputs to LEFT sub, and the
RIGHT channel speaker outputs to the RIGHT sub.  The negative speaker posts of the subs would not be connected in this
case (as long as they are not internally connected to the power cables earth GND prong - which would be very unlikely),
as you are connecting to two different subs.

4a) If you are using the REL sub with the 3-wire speaker level inputs, I believe the best way to do it is to connect
the L+ to L+, R+ to R+, and the GND to the RCA GND of LIO (e.g. use an RCA plug with the REL GND wire connected
to only the GND of the RCA, and do not connect the signal (hot) of this plug to anything).  Then just plug it onto one
of the unused RCA jacks on the back of the LIO.

4b) Others with REL subs have reported that they connected the GND wire of the speaker level cable of the sub to either
the L- or R-, and it works fine that way as well.

It is certainly mentioned in the LIO's owner's manual already.  I will check the website and add it if it is missing.  Thank you.

I hope this helps,

Vinnie

Vinnie:

Looking at your example of connection two subs to the speaker outputs (which is my preferred method of connection).  If your subs are two RELs with the three wires for hi-level connection....is there a way to connect them to the speaker outputs as opposed to the variable out?  If so what would you do with the third (ground wire).

[Vinnie R.]

Vinnie:

Looking at your example of connection two subs to the speaker outputs (which is my preferred method of connection).  If your subs are two RELs with the three wires for hi-level connection....is there a way to connect them to the speaker outputs as opposed to the variable out?  If so what would you do with the third (ground wire).

Hi yardbird4,

In this case, use the REL's "BAL HIGH INPUT" of each sub (with the black wires tied to the LIO chassis GND).  I can help you with this when the time comes.

Vinnie

[Odal3]

Good info. Thanks for sharing

[Vinnie R.]

Thanks, guys!

I hope it helps!

Vinnie

[Vlad_Bath]

Yes, Vinnie, it helps indeed.
In fact, I think this info is really priceless!
If you don't mind, I would share (translate) your advice on subwoofers in some local forums (where some folks don't know English enough to understand a direct link/quote).
Of course, I'd cite you as the source and provide a link to your post.

Vlad

[Vinnie R.]

Yes, Vinnie, it helps indeed.
In fact, I think this info is really priceless!
If you don't mind, I would share (translate) your advice on subwoofers in some local forums (where some folks don't know English enough to understand a direct link/quote).
Of course, I'd cite you as the source and provide a link to your post.

Vlad

Hi Vlad,

Sure - feel free to use it where it can be of help to others. 

Cheers!

Vinnie