Powered sub-woofers - is it just me?

[mhennessy]

Originally Posted by stevehertz

Like I say, I've used a digital processor to make adjustments in my listening room (that is on the dead side of middling in terms of liveliness) and I'm very pleased with the results. I did also say in my original post that I recognised that such a process is not perfect for a number of complex reasons, one of them being the listening/testing position. But hey ho.

Tiny bit tetchy there, Steve

I have no problem with equalisation - I have several units myself, and obviously there's not many recordings made without equalisation. I'm also not passing judgement on your equaliser, or your use of it. I'm simply explaining that in general, the best results are reliably achieved by using speakers that are flat under anechoic conditions, then treating the acoustics of the room. If anyone finds themselves having to put large amounts of EQ into nominally flat speakers used a difficult room, then that should set the alarm bells ringing, as now you're into territory that really needs acoustic treatment to cure the problems that can't be fixed in the electronic domain.

Of course, it's not for everyone, but for the "dedicated" who think nothing of spending 3-figure sums on phono cables (and even mains leads!) in order to "tune" their systems, it's most peculiar that acoustic treatment (which actually does something!) isn't more widespread. I've yet to find a listening room that couldn't benefit from acoustic treatment.

Naturally, to get a loudspeaker that is indeed flat under anechoic conditions, equalisation is needed. Good ones have most or all of that built into the crossover; others benefit from an external crossover. The real difficulty is that most people don't have the equipment necessary to measure their loudspeakers, and not all that do are able to correctly interpret the results.

If you follow the writings of The Absolute Sound's Robert E Greene (aka REG), you'll know that he's a big fan of EQ - which is normally an anathema to the usual hi-fi crowd. He runs a Yahoo Group and has his own website: http://www.regonaudio.com/ - there's lots of good info there about room/speaker correction - this is an interesting start: http://www.regonaudio.com/Digital%20...orrection.html

[mhennessy]

Originally Posted by Edward Huggins

This thread may have gone a bit esoteric for the non crtical application the OP has in mind. Given the little 3 + 3 watt Tripletone and 2 x 8" x 5" (Celestions?) Decca satellites, I would have thought that any Budget Sub would do (e.g. Gale) as long as you can trim the crossover frequency.

The OP has a sub but isn't happy with the results. So the posts in this thread have explained the difficulties with sub-woofers, which should go some way to helping the OP - and anyone else who stumbles across this in the future - understand what the issues are.

Also, we've explained why cheap budget subs in particular are a problem. They are invariably high-Q boom-boxes that are only (just about!) fit for movies. The OP's existing sub actually isn't as bad as these (it is at least a sealed box, but Q is unknown), so advising he should go down that route is peculiar, to say the least...

Luckily, the DIY route - as I described earlier - is very cheap and easy, and as this is a forum for engineers, I'd like to think that's helpful to someone - whether that's the OP or someone else.

[RojDW48]

The 'OP' has read all of this with great interest and a reasonable level of understanding. I recognise the issues around room size, shape, placemement of speakers etc but they are not concerns I am prepared to address other than in the most rudimentary way - I'm just too lazy!

My relationship with stereophonic reproduction started in 1958 when I was 10 years old. My old dad was an electronics engineer with STC and he put together a system with an early Lenco deck, Decca FFSS arm and cart, Tripletone pre-amp, home made 15W monoblocs (which looked a lot like Tripletone monoblocs) and huge built in corner speakers with Wharfedale twin cone drivers and sand filled baffles a la Wharfedale SFBs. It was spectacular and people came from all over to sit and listen. I guess, in a way, I have just kept on trying to reproduce that lovely sound.

The system I sit and listen to is basically Decca, Radford, Woodside. It comes from the eras I most admire and I don't think I could do much better. The system under discussion is the audio side of my computer set-up, which is in a different room and, obviously, I am not expecting as much from it. I have had a play around with the signal generator and fiddled with the settings on sub as advised and it sound OK. By no means perfect but I shall probably stick with it because it is such an easy way of using the little Decca Satellites.

Thanks to all for a fascinating exchange of views.

[Phil G4SPZ]

I've read this thread with considerable interest. I actually have three sub-woofer systems, none of them 'professional' in any sense, but they do make a simple system sound better.

The first is a small Harmann-Kardon unit that I bought at a radio club junk sale for £2. It sits behind the Sony 36" flat screen TV and makes a dramatic improvement to the built-in speakers. The second is a Philips SPA 1300/00 which my son gave me when it stopped working. I replaced a shorted electrolytic and it now works fine, adding a modest but very welcome improvement to the bass performance of a small Aiwa stereo system - the biggest SWMBO will allow in the living room.

But in the workshop sits the best system of all. I happen to have a spare Quad II and QCII (mono) and a Tannoy Monitor Gold, which were looking for something to do. So they became a home-brew powered sub-woofer. The difference is that it is fed by a home-designed passive second-order low pass filter with a nominal cut-off frequency of 30Hz. There isn't much 'down there' in most types of music, but as I happen to like cinema organs, this system really comes into its own. Relatively little power is required, and perhaps the process should be seen as an extension of the low frequency response with minimal roll-off.

My favourite test track is "The Light of Foot March" played on a Wurlitzer and contains three notes at 32Hz, which can be clearly heard (and felt). However, going back to the KEF powered sub-woofer mentioned by the OP, I repaired one of these for a friend recently and was impressed by the build quality, but disappointed by its lack of response much below about 60Hz. It was certainly loud, but contributed to the 'false bass' effect that many have observed.

True, deep bass is best reproduced by a big 12" or 15" speaker in a large enclosure, not these little long-throw 6" units squeezed into a small cube!

[stevehertz]

Originally Posted by Phil G4SPZ

True, deep bass is best reproduced by a big 12" or 15" speaker in a large enclosure, not these little long-throw 6" units squeezed into a small cube!

Is that an opinion or fact? I was under the impression that - properly done - either of those methods were good for achieving deep bass; they both shift a lot of 'air'.

[Phil G4SPZ]

Purely my own opinion, Steve, but based on my own experience. Perhaps I should have said that it's easier to achieve decent bass with a big speaker; other methods are, of course, available, but may be more difficult. A friend of mine has built a pair of horn-loaded speakers which only use 6" drive units, and they sound superb but took years to build and dominate the room!

[ribbonmicsrus]

Build one of these by Richard Lord, I did, fed with a 100watt Mosfet amp, comes in on my DM14s at about 32Hz, flat down to about 15Hz, sounds superb, only problem being fixing things to stop them vibrating!

http://www.mediafire.com/file/3mg6qh...er+Circuit.pdf

My window bay was slightly bigger so lengthened the cabinet slightly.
I was told by Richard that possibly only 2 people including myself had built one, would be nice to hear how another one goes.

[ToneArm]

I found getting good bass out of smaller speaker drivers requires the volume to be turned up quite high.

At low volume it's not so easy. I tried a powered subwoofer - it was a good one 10 inch driver in a sealed enclosure & 150w amp, and it could sound impressive - but it was a twiddle-fest that I couldn't stop adjusting, I couldn't relax and just enjoy the music. Getting it to integrate, ie "disappear", proved to be REALLY difficult in my room. In the end I gave up on the sub.

I now follow Phil's school of thought and have ridiculously large speakers that sound nice and full at low volume. Plus they provide useful surfaces for putting stuff down on.

I understand a room's low frequency response is limited by the length of the longest wall, which is disappointing given the effort I've put in over the years toget deeper notes out of my speakers! If it's right then there's only vibration below 43Hz in my 13ft long room, and might explain the bass sweetspot about 10 feet down the hall when the door is open!

[ribbonmicsrus]

My listening room is 38 feet by 15 feet and bass extension is good. When running a sound test CD, I do get a peak at about 17Hz out in the hallway.

The Monochord sub woofer is the only one I have used, other being Yamaha that has actually given me true transparency, most of the time you are not sure it is switched on but when the appropriate frequencies occur it is excellent.

I have a track with a Lightning P1 doing a fly by and it is near as to being in the real world as I have heard. Needless to say, the neighbours have to be out when doing these tests!

[PJL]

From what I can see the better sub-woofers use active crossovers inserted between the pre-amp and power amp. Theoretically, this should improve the sound quality from the main speakers as they will no longer see the lower frequencies which they can't reproduce. Lower frequencies require more air displacement for the same power so matching an amp that works down to 10Hz with a speaker that has limited output at 50Hz can only bring trouble.

[stevehertz]

Originally Posted by PJL

From what I can see the better sub-woofers use active crossovers inserted between the pre-amp and power amp. Theoretically, this should improve the sound quality from the main speakers as they will no longer see the lower frequencies which they can't reproduce. Lower frequencies require more air displacement for the same power so matching an amp that works down to 10Hz with a speaker that has limited output at 50Hz can only bring trouble.

That's a very important point. Nonetheless, even with subs that do not use active crossovers as you describe, they still 'help' the main speakers. Why? well, in my case - and especially having been in bands and listened to lots of live music - I am looking for a 'healthy amount' of bass that reflects the punch and weight of such. Not artificial for sure, but 'for real'. So without a sub, I generally apply a degree of bass boost as most main speakers do not go down to the bottom end of the audio spectrum ie bass drums, lower notes on bass guitars, organs, etc. But using a sub I no longer have to apply this bass boost, so my mains are receiving less 'bass' than they would without the sub, and as a result probably coping better with the programme material and with less distortion. Heavy bass = more distortion. How much difference it makes - admittedly - is open to debate. But nonetheless, it's there.

[Edward Huggins]

I d0n't think I would only ever consider a Sub if my main speakers were lacking in Bass per se. In my case, I can't see what a Sub could add to my floor standing Tannoys. I think they are far more suited to home cinema were they can add real imapct, but for straight music I find extended listening to them can be very tiring, even irritating.

[mhennessy]

REL always took the approach of running the main stereo pair as full-range, and they're not alone in that. The main reason, from their point of view, is simplicity. Which, commercially speaking, is fair enough.

Assuming the main pair already played loudly enough, the gains from crossing them over are hard to quantify. In fact, doing so might alter the sound quality in the midrange - and although that would technically be a change for the better, not everyone would interpret it that way. On the other hand, if the main stereo pair are on the small side, then crossing them over would allow a higher playback level. It's why - to pick one example - the "subs" available for the LS3/5A include a high-pass filter.

Getting more technical for a moment, the harmonic distortion from a drive unit is basically related to the movement of the voicecoil (ignoring diaphragm breakup, etc). The more it moves, the more distortion it creates. If it reaches Xmax - which is not the mechanical limit - then it becomes grossly non-linear (not unusual in practice).

Quite apart from bass distortion, don't forget that the midrange is riding on the bass, and so the mid-range will become distorted whenever the bass component moves the voicecoil. Hence, using a high-pass filter to reduce the bass will decrease the distortion in the midrange. This can be audible. But as mentioned above, whether a reduction in distortion is perceived as an improvement is debatable. People like valves and vinyl, and both of these have lots of "nice" distortion. What might be "clean, accurate midrange" to one might be "clinical" to another.