Speakers used with Pioneer XC-L7

[Old Sparky]

This late 1990s compact HiFi system comes with two mid/upper range speakers type SL-7A -LR, and a Sub-Wofer model SL-7A-W.  The mid/upper speakers handle up to 20W each, and are 8 ohm nominal impedance, whilst the sub-woofer handles up to 50Watts and is rated at 4 ohms.  I haven't opened either speaker to check what's inside, but a check with an ohmmeter suggests they are straightforward speakers, yet on each is printed a warning that they must not be connected to any amplifier(s) other than the XC-L7's receiver.  Does anyone know why this should be the case?  THe small LH & RH speakers handle frequencies from 200Hz-20kHZ, whilst the Sub handles frequencies up to 200Hz.  I understand that, if you connect the small speakers to an amplifier which doesn't have separate (sub)woofer outputs, the small (ca. 4" diameter) speakers will be fed with the full audio range unless  a high pass filter is wired between the amplifier output and the speaker, and that Bass notes, especially at high volume settings might damage the small drivers used in the SL-7A-RL, but the SL-7A-W could surely be used with any amplifier having a Sub-Woofer output rated for 4 ohms, so I've maybe partly answered my own question.  THe XC-L7 has built in amplifiers for LH & RH channels which output only frequencies above 200Hz, plus a Sub-Woofer amp. for frequencies below 200Hz.

[Murphyv310]

The XC-L7 I'm sure is a Bi amplifier. Feeding the mid and treble speakers with full range is likely to destroy them. I'd suspect there would be no LF filtering in the mid and treble speakers as the reason of Bi amps is to stop phase errors so why introduce them at the speaker.

[Mark Hennessy]

It looks like a full-range driver in the stereo "satellites", but if there is a tweeter, it'll be crossed over internally in the usual way.

With systems like this, where the electronics are matched to the supplied speakers, there is equalisation that compensates for the particular frequency response of the speakers. That will be in addition to the basic crossover between the "sub" - or more accurately "woofer" - and the satellites. There will likely be EQ for the woofer as well as the satellites.

So the dire warnings are not so much about damaging them - which you'd be unlucky to do in practice - but rather the fact the speakers will sound pretty poor when connected to any other system. In other words, it's more about reputational damage rather than actual damage.

Not sure how bi-amping is related to "phase errors". Any filter introduces phase shift, whether done in a low-level crossover circuit (as here), or in a high-level crossover (as is the usual approach). The job of the designer is to ensure that the phase shifts are managed so that the output from the drivers sum together coherently. Reverse the phase of a tweeter in a well-designed system and you'll get a massive notch at the crossover frequency.

Bi-amping means using 2 amplifiers, both of which carrying the whole audio signal, and relying on the high-level crossover inside the speaker enclosure to split the audio. Just one step up from "buy-wiring". What we have here is an "active crossover", where we have low-level filtering, and amplifiers only carrying the relevant part of the frequency spectrum. Might seem pedantic, but there's a world of difference between bi-amping and active operation. One of them is worthwhile and brings many real advantages; the other one only benefits amplifier and cable manufacturers

So what is the question behind the question here? Are you planning to re-use the speakers in another system?

[Murphyv310]

I could be wrong but I'm sure this is the model we used to see a lot of.
The low level filtering was done in a chip that was a A to D to A converter and programmed for the crossover.
It's well over 15 years since I did modern audio so I could be totally wrong with the model number. If it's not sparkys model then I apologise. Certainly the one I'm thinking it is had the bi amp set up and no filters in the speakers. The manufacturers blurb clearly stated the absence of phase errors.

[Old Sparky]

No, Mark, I wasn't planning to use the speakers with anything else, though the (sub)woofer could probably be used with a receiver I have, which has an o/p for a S/W, but that receiver developed a fault some years ago, which I haven't got around to fixing. I obtained the system from eBay to replace a receiver wjose CD player is faulty, and may well sell on the other items

[Mark Hennessy]

Yes, it sounds like you're thinking of the same or similar model. Of course, this approach is not unique. It's actually not a bad plan, and there's a lot to be said for tailoring the amplifier to the speakers, as the components used can be much cheaper, and it's easier to include more elaborate filtering and compensation. Doing it at speaker-level inside the enclosure is not only more expensive in production (because of the need to handle higher powers), but harder to design because of the interaction with the non-constant impedance of the drive unit and lack of flexibility compared to working with op-amps at line-level. Passive speakers are really hard to design well - just look at how complex the crossovers are in the BBC designs.

For a trivial example of EQ'ing an amplifier to a particular (cheap!) drive unit - along with overall measurements made with a measurement mic - see here: http://www.markhennessy.co.uk/activespeaker/mods.htm

It's fair to say that "phase" is one of the most mis-understood and mis-used terms in audio marketing today, and has been for many decades. We're actually very tolerant to phase shifts, and if we weren't, then audio as we know it simply wouldn't work. Every tone control or EQ introduces phase shift, as does every filter, and indeed every loudspeaker enclosure has phase shift along with the LF rolloff (in fact, every peak and trough in the response of a loudspeaker has a phase shift associated with it). Hence, "phase error" is little more than marketing-speak. Along with "linear-phase", et al.

Obviously, signal inversion - often erroneously called "out of phase" - is another matter, quite separate to all this.

When designing loudspeakers, you know the filters will introduce phase shifts, so you design them so the phases align to give the target response through the crossover region(s). Sometimes that calls for the tweeter to be wired in opposite polarity to the woofer, which confuses folk, but many BBC and commercial designs were done that way.

(The mentions of BBC designs is simply because they can be readily looked up as they are in the public domain)

[Murphyv310]

Absolutely Mark.
Glad you can explain phase errors in audio nice and easily

[Old Sparky]

All that's in the SL-7A speaker is a 2.75" (70mm) diameter Pioneer cone speaker type B70AC45-56F. I haven't yet checked what's inside the Sub-Woofer Cabinet.

[Murphyv310]

Hi.
I remember on a Minidisc course I was on when working for Sony UK the subject came up of phase errors.
I think the trainer was Henry McAloon, a local man who had been a trainer for Sony for some years.
His point was that audio phase errors were around us all the time, from listening to sounds emanating from another room or by certain reflections and absorption that would distort the sound, the human ear though and processing in the brain tends to disregard this.
Not being into Audio so much these days I have no idea if Bi Amplification is a thing of the past or still on the go.
During the many years that I did audio I could never hear the difference in Bi Amps and decent Hi-Fi with normal crossover systems.

[Mark Hennessy]

Yes, the benefits of bi-amping are theoretical at best. If there is a difference, that implies there might be a problem with the amps. An "active crossover" has a lot of potential benefits, but not all designers are able to capitalise on that, so active designs don't automatically sound better than conventional passive setups - but the good active systems really are very good indeed.

Despite what Marketing say, most of the sound comes from the design decisions - and mostly in the crossover design. Even the cheapest drive units can sound amazing in the hands of a good designer, and someone like me can make the most expensive drive units sound terrible!