Are these speakers worth refurbing?

[Rare Bird]

They sure are Audax units Alex

[dowser]

Hi All,

Does anyone know anything about the Audiomaster P202 speaker? There's some on a local auction site I like the look of, but I can find zero info on them.

I also asked on PFM, where they thought MS woofers and Audax tweeter. Pictures here.

Thanks, Richard

[spikeanded]

Hi, I know I'm resurrecting an old thread but I have a pair of these which I picked up over the summer. I was expecting great things, as I used to have a pair of MLS-1 which I liked a lot and have also owned (and still do, in some cases) various Monitor Audio and Epos models, some of which are also Robin Marshall designs. What troubles me with the MLS-4 isn't the midrange dip, but the rising treble which sounds pretty obvious to my ears. I would like to resolve this. Interestingly, I don't remember the MLS-1's suffering from this, and I think the tweeters were common to both models, so it gives me hope. Is there a quick and easy fix for this (other than swapping out the tweeters which I am reluctant to do, and which I think is unnecessary as it isn't the quality of treble, but the quantity I am not keen on). I am considering trying either 2.2R or 3.9R 10W wirewound resistors in series with the tweeters in the first instance, but other tips and advice would be appreciated. If possible I would like to leave the mid-range dip unworsened but I am not sure if this occurs at the crossover frequency or just below it.

[system7]

Yup, very easy to fix an overly bright treble. Add a Zobel network to the tweeter shunted across the tweeter output. This works with the input resistor to the treble filter to roll off the top end slightly. 7.5R and 1uF should do it. Technically it corrects the rising inductive impedance of the tweeter to a flattish 6 ohms at the top. Usually creates about 3dB attenuation.

http://www.wilmslow-audio.co.uk/10-d...tor-1119-p.asp
http://www.wilmslow-audio.co.uk/cera...tors-205-p.asp

It does need some input resistance to work with, but I'd assume it is there, though I'd need to see the schematic to be sure.

[system7]

FWIW, I've deconstructed the Audiomaster MLS4 crossover schematic as best I can from the photos supplied earlier:



With the values I can read, and the guesses in red, it looks like this:



Uploaded with ImageShack.us

I have shown the Zobel which goes across the tweeter to tame the treble. This schematic is quite like the old KEF Calinda, if memory serves. (I always thought that Calinda schematic was not quite right, but that's another story...LOL)

I won't attempt to explain it beyond saying the bass unit gets an LCR trap to create a notch around 2kHz for some typical bextrene horror resonance, and the treble is a third order butterworth filter quite a la KEF acoustic butterworth.

The centre pair of wires is input, left pair is bass output, and right pair is tweeter output.

[system7]

BTW, does anyone notice, as I do, the terrible alignment of coils at the bottom?



That bafflestep coil is crosstalking very badly with the tweeter coil. The tweeter coil should really point out towards the viewer here. Keep them at right angles is the rule.

http://www.troelsgravesen.dk/coils.htm

[spikeanded]

Thanks for that, I will try the Zobel network. In the meantime even a 2.2R series resistor offers some improvement to my ears.

[system7]

More serious attenuation is provided by an L-pad at the (6 ohm DC) tweeter terminals:

http://www.diyaudioandvideo.com/Calculator/LPad/

2.2R (5W wirewound) series and 15R (10W wirewound) shunt will knock the tweeter level down quite a lot. Maybe 4dB. You can buy them at Maplin along with a good 35-50W soldering iron suitable for modern lead-free solder.

L-pads are good because they preserve frequency response. They also improve the sound of the tweeter towards smoothness and low distortion. Particularly with older non-ferrofluid designs.

I'd sort out that awful tweeter coil alignment while I was at it.

[spikeanded]

More serious attenuation is provided by an L-pad at the (6 ohm DC) tweeter terminals:

http://www.diyaudioandvideo.com/Calculator/LPad/

2.2R (5W wirewound) series and 15R (10W wirewound) shunt will knock the tweeter level down quite a lot. Maybe 4dB. You can buy them at Maplin along with a good 35-50W soldering iron suitable for modern lead-free solder.

L-pads are good because they preserve frequency response. They also improve the sound of the tweeter towards smoothness and low distortion. Particularly with older non-ferrofluid designs.

I'd sort out that awful tweeter coil alignment while I was at it.

Hi, thanks again for the advice. 2.2R in series seems about right in terms of attenuation. 3.9R was too much and they sounded dim. The reason I want to try the Zobel network is because, if I understand it correctly, they will counteract the rising treble progressively (a sort of pivot around the crossover frequency), whereas attenuation, whether by a series resistor or L- Pad will create a kind of "step down" at the crossover frequency and then a gentle rise from there as per the original frequency response - is this correct? To my ears, they only need the slight edge taken off at the higher frequencies. What vales of L-Pad would you recommend for this effect, and do you think this is preferable to the Zobel network approach?

[system7]

Looks to me like the Zobel is going to do the right thing here to the top end, but 1R/22R ought to do similar overall 2dB attenuation. Your ear is quite sensitive to small level adjustments at the top, but it may be these speakers should not be toed in for good listening. A bright top end falls off quite rapidly off-axis due to diffraction.

Let's reiterate things:





Personally I'd measure the cutouts and rebates and see what other tweeters we might fit here since the art has come on. The 104mm Morel CAT 298 ought to be a slot in replacement:
http://www.wilmslow-audio.co.uk/more...t298-500-p.asp

The SEAS 29TFF/W is more efficient, but quite well suited to a modified 2 kHz crossover which would cure the midrange suckout caused by the steep rolloff of the woofer:
http://www.falconacoustics.co.uk/sea...ge-series.html