M&K subwoofer blew it's 125W amplifier, found replacement but...

[Techlord]

Hello everyone,

I have an M&K LCR 750-THX 5.1 speaker system and my M&K V-1250THX subwoofer blew it's 125W amplifier, being an older model I wasn't able to get a direct replacement as they are no longer available! I have never cared for any THX modes for my subwoofer as I have kept it set to Variable for Bass Level and the Low Pass Filter at 80Hz 24db/OCTAVE.

I have however found a replacement M&K VX-1250 250W amp (newer model) except it's the non-THX version. Now for my question, how well will the integration of this new non-THX version subwoofer change how it blends in with the rest of my speaker system? Here is a picture of my older dead amp and the second the new amp. The newer M&K amp is only $139.95, better than spending $1200 for a new SB-1250!




I would appreciate any insight and thank you,
Techlord.

[MStrickland1988]

So what exactly does THX certified mean for a subwoofer amp? There should be little to no difference as long as it fits.

[TLS Guy]

So what exactly does THX certified mean for a subwoofer amp? There should be little to no difference as long as it fits.

It seems to me Lucas labs collect a hefty fee, otherwise nothing else. It seems Lucas labs have turned this certification into a racket. I have never liked the outfit, and think they are wrong in a lot of their positions.

[Techlord]

Will I be able to setup my new amp to match the Low Pass Filter at 80Hz 24db/OCTAVE? Also what's an 24db/OCTAVE anyways? I'm looking for identical bass output, sound exactly the same!



Thank you guys & gals,
Techlord.

[TLS Guy]

Will I be able to setup my new amp to match the Low Pass Filter at 80Hz 24db/OCTAVE? Also what's an 24db/OCTAVE anyways? I'm looking for identical bass output, sound exactly the same!



Thank you guys & gals,
Techlord.

A filter that rolls off 24 db per octave is fourth order so in your case the voltage from the filter will be down by 24 db at 160 Hz if you set the crossover to 80 Hz

[Techlord]

A filter that rolls off 24 db per octave is fourth order so in your case the voltage from the filter will be down by 24 db at 160 Hz if you set the crossover to 80 Hz

Uh huh, what? Language translation please, will I be able to achieve the Low Pass Filter of 80Hz 24 db/octave with the newer amp? I won't pretend any longer to know what any of that means! Lol

[MidnightSensi]

Uh huh, what? Language translation please, will I be able to achieve the Low Pass Filter of 80Hz 24 db/octave with the newer amp? I won't pretend any longer to know what any of that means! Lol

A low pass filter cuts sound above a set frequency. 80Hz is where the cutoff is set, 24 db/octave is how fast it rolls off (the slope). So, with an 80Hz crossover setting, the output decreases 24dB/octave below 80Hz. This means that at 40Hz (1 octave) the receiver's speaker output will be down 24dB (roughly 4 times less volume)

THX specifies a 4th order Linkwitz/Riley low-pass, but a THX receiver can do the same thing, and you should set the crossover with one or the other.

Your mains are have a roll off of about that of a 80Hz 2nd order Butterworth. When you add in the receiver having a 2nd order Butterworth high-pass filter, the overlay of the rolloff and crossovers equals a THX spec 4th order Linkwitz/Riley. The result is flat, and if the subwoofer level isn't cranked, good integration, assuming good placement.

That's why your mains are THX select and designed to be used with a THX processor and subwoofer.

[Techlord]

A low pass filter cuts sound above a set frequency. 80Hz is where the cutoff is set, 24 db/octave is how fast it rolls off (the slope). So, with an 80Hz crossover setting, the output decreases 24dB/octave below 80Hz. This means that at 40Hz (1 octave) the receiver's speaker output will be down 24dB (roughly 4 times less volume)

THX specifies a 4th order Linkwitz/Riley low-pass, but a THX receiver can do the same thing, and you should set the crossover with one or the other.

Your mains are have a roll off of about that of a 80Hz 2nd order Butterworth. When you add in the receiver having a 2nd order Butterworth high-pass filter, the overlay of the rolloff and crossovers equals a THX spec 4th order Linkwitz/Riley. The result is flat, and if the subwoofer level isn't cranked, good integration, assuming good placement.

That's why your mains are THX select and designed to be used with a THX processor and subwoofer.

That put things into (crystal clear) perspective, I have never cared for THX processing even though I have it on every component accept for my Oppo BDP-83SE. I purchase these components because of there performance when aditioning them in a treated room, not because of the THX logo. All of My speakers have built-in crossovers cutting off everything below 80Hz.

However my Denon THX Ultra receiver has a fixed 80Hz crossover for the subwoofer, I guess that's what people meant when saying my Denon is outdated! So since my receiver has a fixed crossover for the subwoofer at (below) 80Hz, what will happen to the signal once it gets to the subwoofer? Also is it even possible to bypass the sub amp in the picture? One more question, how does one go about setting the reference level of the bass volume in conjuction with the speaker level adjustment in the receiver?

Thank you,
Techlord.

My new subwoofer amplifier




My burn up subwoofer amplifier

[BillCinLR]

Your sub will only get frequencies 80hz and below. If you leave all of the settings on the amp exactly as shown in the photo that you posted, you will be just fine.

The sub level is set with the BASS LEVEL knob and a Sound Pressure Level (SPL) meter (available from Radio Shack) to set the sub to the other speaker's levels (usually about 75db).

Bill C

[MidnightSensi]

THX equipment is designed to run at a 80Hz crossover, so when the LFE (subwoofer) output is set on the receiver to 'THX' it will set it at 80Hz. That is most likely defeatable in your receiver by switching the subwoofer to non-THX (then you can still use THX modes).

Your speakers and subwoofer are designed to be run at 80Hz crossover, and I'd leave them at that. Higher crossover points can increase localization issues. 80Hz is actually, in my opinion, a fairly good starting point for a subwoofer crossover.

When you use your receiver to provide the satelite to subwoofer crossover, make sure on the subwoofer you turn that knob to 'bypass.' That will bypass the internal circuit on the subwoofer. If you set the receiver to 80Hz and the crossover to 80Hz, they will interfere and give you a steeper crossover slope than you intended.

For setting the bass level, the best thing to do is to use a SPL meter and test tones.

Your receiver, depending on the model, may also be able to set the levels for you as well.

What Denon do you have?