Audax HD-3P Gold Dome replacement - Page 3 New Strategy

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Audax HD-3P Gold Dome replacement
Page 2 Crossover
Page 3 New Strategy
Page 4 Final Listening
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New Strategy

One thing I noticed, whilst listening, was that the midrange performance was totally different to that with the old crossover. This gave me the clue that I was looking for. It was pretty obvious that, if I tinkered around with the midrange unit crossover, the whole speaker was going to change in character.


Now this wasn’t what the owner wanted. I knew, from a previous conversation, that he was very happy with the overall sound of the KLS3 Gold and he just needed an ‘upgrade’ and restoration of the treble performance, not a whole new speaker! So, back to the drawing board – actually LspCAD6 – and re-look at the crossover details.


To start with I decided to put the treble unit back into phase with the midrange. The reason is one that I’ve found works for me, although looking at other people’s crossovers it apparently doesn’t work for everybody.


This shows the final plot of the midrange/treble crossover.


My philosophy is that the ear is sensitive to the time arrivals of transients. I know that there is a considerable body of evidence to show that the ear does a summation of the total transient response from a speaker and then a type of mechanical Fourier analysis of the waveform to realise the tonality of the sound. But I’m still convinced that the ear also reacts to the initial transient of any sound in a particular way.


It is the initial transient, for example, that gives us directional clues and, hence in hi-fi reproduction, helps position the image in the stereo plane. Thus if the phase is inverted between the drive units, especially at high frequencies, there can be a little confusion for the ear/brain in that it will see the transient from, say, the treble unit going in the positive direction and then that from the midrange unit going in the negative direction.


Effect of reversing the treble drive unit phase indicating excellent phase integration when in phase!


This is particularly important where the drive units are phase aligned (which doesn’t happen very often in modern speakers) but, in my view, is still noticeable even when the output from the treble unit reaches the ear slightly in advance of that from the midrange. Back to the drawing board


So part of the ‘back to the drawing board’ process was aligning the crossover to work with the drive units in phase. A bit more playing around with components and values in LspCAD6 and, hey presto, suddenly I had a smooth response without having to adjust the midrange crossover values far from the originals. In fact the only change is to up the value of the capacitor to ground from 6uF to 8.2uF (8uF was used in the Mk1 crossover).






Again listening showed that this was the right way to go. The character of the KLS3 Gold was now restored plus the missing treble was back again and the stereo image now seemed more solid and realistic.


It took a bit more juggling of component values to elicit the final crossover for the treble section but, at last, I was satisfied with the performance and guessed that the owner would be too. An appointment for collection was made and, while I was waiting, I went back to the listening room for a more extended listen.


Now that the treble was restored to its former glory (as far as I could tell, not being in possession of a pair of correctly working originals for test) it was so beautifully sweet, detailed and transparent that it revealed that the lower midrange had a touch of overt chestiness about it. This clouded female vocals a little and added undue emphasis to male vocals in my opinion.

Now this seemed to be coming from the mid-bass crossover and there wasn’t much to suspect there other than a hefty 50uF in parallel with the bass unit. I started by adding a small bypass SONIQS polypropylene capacitor across this 50uF. This seemed to help so I increased the value until the chestiness seemed to disappear.


The value I ended up with was another 18uF on top of the 50uF, not enough to seriously disturb the bass/mid crossover point but just enough to clean up the sound and remove a level of coloration I’d heard.


Whilst we’re talking about the mid/bass crossover what about bringing these two units into electrical phase too? Now this wouldn’t, and didn’t, work with the existing crossover and nor does it matter, and here’s why.


At frequencies as low as that of the bass/mid crossover in a three way speaker, the wavelengths are long enough, and the arrival time of the transient slope from the bass unit delayed enough, that the phase of the bass unit is hardly significant at all. I wouldn’t rule it out of the equation completely, because the bass unit is still contributing a small amount of output at higher frequencies, but it’s not worth upsetting the crossover performance just to correct it. So leave well alone!

Comments (1)
Speaker Upgrading
1Tuesday, 31 July 2012 08:36
Greg Ferrari
A very good article with the detail I prefer. The principles of how to improve the old speakers with well known drives units to meet the current quality of the best digital sound is now a necessity to anyone wanting to retain old speakers and get fantastic sound

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