LSi25 Subwoofer Upgrade

Interested in your findings.
I was thinking of doing the DB1040 swap as the 15s take the DB840. The DB requires little port modification from what I have read.

Anyway, hope to read more as you finish the second one and port mods.

That would be great. They seem to run around the same price so that's not a concern.
Thanks.

Nice job. Did you ever extend the tube?

Good info on this thread. I thought I'd share what I know.

First, some background. I've been watching these speakers on Ebay for years, and just finally pulled the trigger on (what I thought was) a mint condition set. However, once I got it setup, I realized one of the internal amps is was blown.

I was on the phone with Polk customer service twice today. The first time I was told that I was essentially SOL. So I started thinking about potential workarounds. The most obvious (and simplest) is to bypass the internal amp, via an external amp with signal from the LFE output on the back of my AVR. To do this, you need to disconnect the woofer from the internal amp, disconnect the internal amp from the bottom binding posts, splice-in connectors on the woofer that are compatible with the binding posts, and connect the woofer directly to the binding posts. Next I'd need to connect a 4-ohm stable amp (as the woofer is 4-ohm nominal) to the lower binding posts on the back of the speaker. The amp would be fed information from the LFE out on the back of the amp. This would result is similar be to the 3rd connection method as described in the instruction manual, whereby the the passive drivers are left/right channels and the woofer is setup as a true subwoofer.

The positives of this solution include no need to actively manage a crossover as AVR/Audessey will take care of this via bass management setup. From my opinion the negative is you no longer have the option of running a full range speaker (matters for 2-channel listening, especially if you use PureDirect or a comparable 2-channel only listening mode that turns off all crossovers and bass management).

The second option is essentially the same as the first, but instead of rely on the LFE out to send information to the amp, I'd incorporate a low pass filter via an active crossover. The advantages would be that you could still run the speaker as full range, which I think is optimal for 2-channel listening, and the you don't have to incorporate 2 additional subwoofers into your room. It's worth noting that most modern mid to high level AVRs include 2 LFE outs and each is EQ'd independently for room correction purposes. When you add 4 subwoofers into the mix that becomes more difficult (especially considering you have mismatched subs - e.g. multiple subs of different make).

All that being said - I called Polk back a second time, and spoke with a customer service rep with strong knowledge of these speakers. He said that although the amps are no longer made, Polk would still try to repair for me, so I'm sending the amp in to see what, if anything, Polk can do.

While on the phone with the rep, I took the time to ask about the details of the speaker that I thought I'd share with you:

- The passive drivers (midrange and tweeter) have a 90hz high pass filter.
- The passive drivers are not effected by any tweaking to the crossover on the back of the amp - meaning the high pass filter is static - does not change - stuck at 90hz.
- The crossover on the back of the amp is actually better described as a variable low pass filter. However, since the passive drivers roll off at 90hz, it really doesn't make sense to turn that knob off the factor setting (which the rep confirmed was 90hz). Lowering that knob below 90hz will cause loss of information between the active and passive speakers, and raising above 90hz is redundant, and results in a a "boomy" sound.

That's correct. The passive drivers are on a HPF that rolls off the information at about 90hz. The crossover "factory setting" on the amp is 90hz. In fact, calling it a crossover is actually sort of misleading; it's best described as a variable low pass filter.

I don't think it makes sense to run these speakers as full range and rolling off the entire speaker to a dedicated subwoofer at 80hz (THX recommended) - you'd barely utilize the woofer. Given that, I see two viable options for integration:

1. Put woofer on LFE track - which requires some additional subwoofer EQ if you're like me and have a dedicated sub. I suppose this is probably the way t go if you're mostly watching movies, where the audio mixer developed a separate LFE channel.

2. Use left/right pre-outs and run the speakers full range (set to "Large" in AVR). Probably the better way to go if you like to listen to two channel music in Pure Direct (on Denon AVRs), or another comparable audio setting that optimizes two channel music. For my setup, I'd have to run a low pass filter before sending the signal to the external amps, so this will take some extra integration with an SPL meter, mic and a way to implement the EQ/crossover.

I envision wanting to use the speakers to play full-range 2 channel audio as well as surround sound mixes, so I'm going to integrate an A/B switch to hop back and forth between LFE and L/R signals easily.

Not sure what you are explaining exactly, but to clarify, are you planning on using the Lsi25 AND another sub or two, or JUST the Lsi25's?

Gotcha!
So one still works fine as intended, you could use that as a reference point. Pretty sure the built in amps employ EQ boost.

Dayton makes several stand alone amps, that have EQ built in or variable. Most likely there is a 6db boost at 35 hz in the built in Lsi25 amps or something very similar.

The one odd thing, your "dedicated subs" will be in essence crossing over to another amplified subwoofer at some frequency, and then again the built in subs, to the front mounted speakers again.

What are the dedicated subs you are using?

I only ask because the built in subs in the Lsi25 are fairly decent as they are. Not great, but it may be a case of similar enough to not really need one or the other.

I guess fundamentally, I'm asking is for advice on (quasi) bi-amping and tuning these speakers.

My thought is I could use a MiniDSP 2x4 unit to EQ/integrate the woofer back into the broken speaker. Using a program like REW, I could use the EQ curve of the good speaker to help me model the LPH on the broken speaker (once I am happy with the sub integration on the broken speaker, I'd make the same upgrade to the good speaker).

My current thinking as to how to do this:

1. setting crossover - passive drivers have HPF at 80hz at 12db. I was thinking of rolling off to the sub at 18db and seeing how that looks on the REW graph compared to the stock sub. That said, this all gets a little funky when you start thinking about how these speakers interact with bass management from an AVR. The passive portion of the speaker is connected to the FL/FR speaker level out. Audyssey recommends setting these to SMALL and correctly sets the filter to 80hz. But in doing so, there are now two 80hz HPFs on these speakers; that cannot be optimal. I think better is to set to passive portion of the system to LARGE and allow passive HPF to roll off the signal. Also, consider further that if you connect the LSi 25 woofer to the LFE out on the AVR, you're bypassing whatever LPF Polk designed for the woofer...And so I start wondering then how sacred can the specs of the LPF really be if Polk intended the user to bypass the LPF altogether (in the manual, one of the suggested setups include using AVR's LFE signal to subwoofer). So I think it will come down to tinkering with the crossover in the MiniDSP application until I get a reasonably flat 80hz crossover. That said, I'm happy to hear suggestions about how to set that LPF. Again, ideally I'd like to run these speakers as full range using Left/Right signal as opposed to using LFE signal.

2. setting gain - Polk allows the user to set the gain on the sub (though they have no suggested factory setting), so presumably Polk had no hard and fast rule about subwoofer gain. I think I can dial in the gain using an SPL or some EQ software (like REW). Having never done this manually, I'm looking for advice on how do practically do this. My thought would be to use an SPL meter and to push signals at various frequencies through the crossover range. However, I'm trying to understand how to identify modes/nodes vs gain mismatch. I suppose if I set the mic a few feet away from the speaker (as opposed to seating position) I might mitigate potential modes/nodes, but curious to hear other's thoughts on this.

3. delay - this one is really tricky in my mind because I have no way of knowing if Polk setup some sort of delay in the stock circuitry. What little reading I've done here says it's more important for car audio where tweeter and woofer can be really far apart, but I've also read how speaker designers may create an angled front baffle such that the tweeter is further away from the listener than the mids, and further still from the woofer. Not even sure it's material, so I guess I won't attempt to adjust this unless I can see a compelling reason to do so. Even then I have no idea how to measure/account for any adjustment here.

Thanks to the forum for acting as a sound board for my ideas.