[rookster]

Can the AKSA55 drive speakers that dip to 3.7ohms, but average just over 4oHms?

Regards,

Rookster

[AKSA]

Hi Rookster,

Welcome to our little forum!

Yes, the 55W AKSA will drive speaks down to 3.5R with ease.  The 100W AKSA will do it down to 2.5R.

Just don't short the leads!!

Cheers,

Hugh

[SamL]

Hi Hugh,

What about high impedance? Series crossover tent to have high impedance, is there a limit?

Sam

[Malcolm Fear]

I am using 16 ohm Diatones. They work very well.

[AKSA]

Sam,

Generally good crossover design should not allow impedance to rise above about 20R at any frequency for an 8R speaker.  For this purpose Zobels across the driver are extremely useful;  they correct phase shift as well and promote superior imaging.

As an aside, Zobels (a resistor in series with a capacitor across the driver terminals) improve the imaging in active systems by correcting the phase shifts introduced by a highly inductive voice coil at higher frequencies.  Most with active systems do not use this correction, for purist reasons, but there is absolutely no doubt in my mind that they improve imaging.  In properly designed passive crossovers, such Zobels are routinely used, and the reason is quite separate and apart from bandpass control.

The AKSonics, for example, are nominal 4R speakers, but do not dip below 3.5R or exceed 12R at any frequency.  This aspect of crossover control is very important, as speaker impedance influences two critical parameters at the amplifier; phase margin, and feedback ratio.  Anything excessive can make the amplifier unstable, and is the cause of many problems in the real world.  Impedance issues are one of the reasons speakers should be designed as a system, with the amplifier very much in mind.  In recent years there has been a tendency - often with very expensive speakers - to deliver sub-two ohm impedances around the driver resonance points.  This is horrific to drive, and can sound terrible.  The usual rationale is that one should use a very expensive amplifier, but this actually disguises the truth, and highlights the significance of good design.

Cheers,

Hugh

[rookster]

Hugh,

Thankyou for the reply.  I like the idea of the Aksonics, but the low bass level is my only slight concern.  I know the cabinets can be altered in volume, but does this alter the bass response, ie lower it?  If the user is happy with the volume, can the back panel be screwewd in place permanently?  What is the best supplier for drivers in Australia?

If I have a tiled floor, how should the base be coupled to it?  Can it be sealed on a plinth, or should there be a lossy seal, like on carpet?

Thanks in advance.

[rookster]

Hugh,

Just for note, my current speakers incorporate a Vifa P21 Woofer with a Vifa D27TG Tweeter in a 42 litre bass reflex box.  They are good speakers, but would like somthing with tighter, but not necesarily lower bass and a smoother mid-range.  I really like Vifa drivers, I have used them for 10 years now, but maybe something a step up now would be nice.

I have played with the crossovers a lot and maybe I have spoilt the sound by altering the design too many times without proper test gear.  As such a properly designed and tested crossover suited to these drivers is very appealing.  I noticed that Malcolm has set a post for a Valhalla version of the Aksonics.  What components can be upgraded?  I imagine caps and inductors, but would it be worth the cost?  I imagine the existing components are very high quality anyway.

[AKSA]

Hi Rookster,

I've not actually considered upgrades to the AKSonics, and I'm sure Ron (the designer, my associate) hasn't either.  The AKSonic is filled with high quality components;  it uses SCR caps (made in France, bless 'em), quality bipolar electrolytics in shunt mode, quality metal film resistors (Beyschlags and Philips).

You could likely extract small enhancements with Hovlands or Auricaps for funny money.  However, audiophile cap upgrades are not always to advantage because you sometimes find that the design was optimized for the existing caps.  This is not something we've the time for right now;  the GK-160 is consuming a lot of time and effort.  Both Ron and I prefer to design right with top quality industrial parts, and leave the tweaking to others for reasons I've explained at length before (kitsets have to be competitive in price;  absolutely critical for sales).  Nonetheless, this is not to stop us developing a premium crossover in the weeks and months ahead.......

The caps I'd be replacing first would be the series beasts on the tweeter.  They definitely would be sensitive to cap quality, notably dielectric absorption, but the SCRs are VERY good.

Hope this helps,

Hugh

[rookster]

Hugh,

Thanks for the response.  Can you have a look at my previous post with regard to bass response and suppliers and let me know what you think.

[AKSA]

Hi Rookster,

Couple of points I'd make;  first, it's probably asking for trouble to make recommendations about crossovers without having the speakers on hand to actually test and audition, and secondly, I'm not really a speaker man!!
I know the basics, but not too much more, Ron's your man.

I'll email his phone number and you can call him;  he's not on the internet....... (smart man??)

Cheers,

Hugh

[rookster]

Hugh,

Thankyou for your help and prompt answers.

[AKSA]

A pleasure, Sire, I enjoy chatting on a keyboard......

Cheers,

Hugh

[DVV]

Can the AKSA55 drive speakers that dip to 3.7ohms, but average just over 4oHms?

Regards,

Rookster

Rookster, low impedance is the GOOD part of the deal. Most half-competently designed amps will come to some sort of grips with a simple impedance drop to 3.7 ohms.

The problem is far more complex than that. The trouble is that impedance variations in ANY speaker, no matter what the price and the source, will not only drop to say 3.7 ohms, it will also show quite significant phase shifts as well, and worst of all, typically at the point where the impedance drops.

Ideally, an electrical signal will have both of its constituent parts, the voltage and the current, in ideal phase, ideally following each other's changes. A phase shift occurs when voltage leads and current follows, or vice versa, i.e. when they are not ideally synchronized.

If voltage leads, we say we have a positive phase shift; if current leads, we say we have a negative phase shift.

And this is where the amplifier's troubles REALLY begin. We measure and express phase shift in degrees. Not to bore you with the maths, let me just say that a phase shift of 45 degrees is very common in speakers, and 60 degrees is not unheard of, though not very common either. When you work it out, -45 degrees is a factor of 1.41, and a -60 degrees is a factor of 2.

So, if by some misbegotten chance that 3.7 ohm drop is coupled with a common phase shift of -45 deg., the actual equivalent impedance the amp sees as its working load is (3.7 : 1.41) 2.62 ohms. And if it should be -60 deg., the equivalent impedance will be halved, just 1.85 ohms. This spells trouble for the amp; it will have much difficulty riding over that one, no matter what it is.

To ease your mind, the now legendary Apogee speakers had an impedance drop of 2.4 ohms with a -60 deg. phase shift, making the amp see 1.2 ohms as its load - practically a short circuit. No wonder they required behemoth amps to make them come on song.

Now, Hugh's approach is to let his amps work into low loads and he doesn't fear that for two reasons:

1. His output stage and power supplies can ride it out, and
2. He knows this is not a steady state signal, but in effect short term pulses.

If this was steady state, Hugh's amps would overheat at best, as would the vast majority of amps on this planet, or their overheat protection circuits would trigger and siconnect them. As things stand, they will ride out such dips and go on working.

I ran into this problem myself some years ago when I developed a power amp which was then, and still is today, my favorite project. It is capable of riding out peaks of as much as 130 amps (the largest Krell can do "only" about 85 amps), assuming the power supply could deliver it, which of course it couldn't. But in the end, the problem boiled down not to the power supply, which you can beef up any time you want to, and I was already using 800 VA toroids for a nominally 100W/8 ohm amp (did I mention I was a power supply freak? No? Well, I mention it now!). The problem was in the heat produced - I ended up needing inordinately sized and priced heat sinks, or go the forced air cooling (fancy term for fans). I hate fans, so I had to make do with the knowledge that it could theoretically deliver this.

The last say I had on the subject was in writing the design brief for my speakers. They were developed by a friend of mine, then in the speaker manufacturing business but sadly no more today (see the test at my site, http://www.zero-distortion.com , B&M Acoustics 1041). Anyway, that speaker has no less than 6 ohms, no more than 14 ohms, and the worst case phase shift is -25, +20 degrees. Coupled with an efficiency of 92 dB/1W/1m, a teeny-weeny tubie job of 8 watts can drive them easily, let alone 50+ watt per side jobs.

The speaker took 3 months to come to life, but developing the crossover for it to deliver such ease of drive took 6 months all of its own.

Cheers,
DVV

[AKSA]

Dejan,

Thanks for your welcome comments on speaker/amp design.  You explained it very clearly, particularly the math of real impedance 'seen' by the amp.   Never liked the design concepts behind the Apogee anyway!

Rookster,

You asked questions about the AKSonic I did not answer;  my apologies.

I like the idea of the Aksonics, but the low bass level is my only slight concern. I know the cabinets can be altered in volume, but does this alter the bass response, ie lower it?

The AKSonic uses the Vifa M18WO-09-08 6.5" bass driver.  Bass is just amazing for a driver of this size, with plenty of substance and slam.  Altering the size of the cabinet beyond the optimum (a Theile/Small setting is given in the instructions) does effect bass volume;  more volume gives more bass, but at the slight expense of control.

If the user is happy with the volume, can the back panel be screwed in place permanently? What is the best supplier for drivers in Australia?

Absolutely.  The back panel can easily be secured.  Six wood screws from the sides will do it very nicely!

I find WES Components, of Ashfield NSW offer good prices, at www.wescomponents.com.  Look under the Classic line for the woofer;  the XT line for the tweeter (XT25TG30-04).

If I have a tiled floor, how should the base be coupled to it? Can it be sealed on a plinth, or should there be a lossy seal, like on carpet?

You can place it on a carpet square (lossy coupling) or fit rubber (or neoprene) tape to the underside of the box for better sealing.  In practice, while the carpet square is lossy, the rubber tape seal on a tiled floor creates some reflection effects, and so I tend to prefer the carpet square on tiles.  On a carpeted floor, of course, there's no issue either way.

Hope this is helpful,

Cheers,

Hugh

[DVV]

Dejan,

Thanks for your welcome comments on speaker/amp design.  You explained it very clearly, particularly the math of real impedance 'seen' by the amp.   Never liked the design concepts behind the Apogee anyway!
...

Let me be very selfish for once and say God bless Apogee and God bless my friend who bought them and still has them.

If that were not so, where would I go to see/hear amps make it or flunk it, and under real world conditions?

But seriously, they are a gruelling test for any amp. As for their sound, let me say I have heard better for less money, but as ever, this is a matter of taste as much as technology.

Cheers,
DVV