Restoring Monitor Audio R300 bookshelf speakers.

[system7]

Warm greetings, gurus of DIYAudio! Would like your advice on what to do with these (47X25X30 cm) Monitor Audio R300 oldies but goodies, which I picked up second-hand for £60.



Currently working OK, but I've had to strip out the dried-up ferrofluid gunge from the tweeters with a blotter to get any top-end, and they sound a bit harsh and sibilant at the top, and a bit lumpy in the Infinite Baffle bass for my bluegrass style tastes.

The front end, Rotel RA-931 25WPC Amp, RT-950BXTuner and RCD-965BX LE CD player:



The speakers have a simple inductor on the bass, and a 2nd. order RCL filter on the metal domes, with 2.2 ohms series resistor and 3.3 microfarad capacitor plus coil, @ 90dB efficiency:



I could just slot in a 8" Fostex FF225WK fullrange unit and a 65mmX190mm bass reflex tube in the tweeter hole and there is little carpentry involved according to my measurements with a cabinet volume around 28L, or just replace the 94mm tweeter with a Morel Classic CAT308-94. I COULD fit a reflex port on the back if I stick with existing bass unit, but know nothing about what would be a suitable size or whether it is feasable. Because I like bass reflex, and the way it sounds more alive. Or perhaps I should just replace the tweeter ferrofluid and get it sounding smoother?

Any thoughts or advice? It would be much appreciated.

[system7]

I'm just adding this useful info for anyone brought here by google.

The 94mm metal dome tweeters turn out to be SEAS Prestige H0414-08 19TAF/G , and available here in UK.

SEAS info: H0414-08 19TAF/G

There is a similar fabric dome alternative too, SEAS H0737-08 19TFF 1, which I think was fitted to earlier Monitor Audio R300 models: H0737-08 19TFF 1

As it goes, I've got them working nicely now, because the Rotel RA-931 amp speaker fuses turned out to be suffering from corrosion, which I've cleaned off. I'm just missing the ferrofluid.

[AllenB]

Initial thoughts would be to try padding the tweeters down a tad with resistance, and moving the boxes around to try to 'fix' the bass.

[system7]

Turns out they're called Monitor Audio R300/MD speakers. MD for metal dome tweeter, no doubt.

Yes, you're right, placement matters with these babies. Getting them off the ground makes a big difference to the bass, which is then actually quite deep and uncoloured.

I've learned huge amounts hanging round here, and think I can do something about the MD tweeters:



The tweeters have a 2.2 ohm series resistor, but not the parallel arm of the attenuator. I'm going to try a BBC monitor-style Zobel RC network in parallel with the tweeters, as explained in the Arpeggio Loudspeaker article to flatten the impedance curve to 6 ohms. Failing that, I'll replace the tweeters with the fabric domes, which have a more extended HF and will restore the missing ferrofluid.

Seems like a plan?

[AllenB]

Fixing the impedance this way makes a speaker an easy load for an amplifier. Morgan has done this especially with a full range driver that is intended for use with valve amplifiers.

The problem in your case is that you have a crossover between the amp and the speaker. This crossover is probably having some degree of influence in the region to be affected by your RC circuit.

Generally speaking, this means that attempts to 'fix' the impedance at the speaker will not guarantee that it looks this way to the amp. Secondly and more importantly it will change the nature of the impedance for which the crossover was designed.

Specifically it may cause a little top end rolloff. If it happened to be the amount you were hoping for it would be pure coincidence, but there's no harm in trying.

Another possibility would be to put a resistor in parallel with the tweeter to add the second leg of an L-pad, as you were saying. This would have the dual effect of easing the top end as well as applying some damping to the tweeters resonance from the point of view of the existing crossover.

[system7]

I was thinking of a R=8 ohm Zobel that kicks in around 5kHz, which I'd hope is sufficiently above the crossover point. I've yet to work out exactly what the crossover is doing beyond being second-order. But I take your point that a straight resistor is going to be much easier and do much the same thing really.

It might be a resonance on the metal tweeter in the end. They always tended to be a bit harsh. I think I've got hours of fun here trying stuff. Thanks, AllenB.

I'll update with the findings.

[AllenB]

You will have your work cut out for you Just to clarify it was the lack of ferrofluid I was thinking about. The resistance could make changes there as well as what the RC would do...and if you were thinking of trialling an RC, don't limit it to just the Zobel values, other values of R and C might help. Good luck.

[mdocod]

My understanding is that the removal of the ferrofluid would effect the impedance rise and position of Fs as much or more than the impedance rise from coil inductance. The x-over will be effected by the impedance changes around Fs of the tweeter, not so much by the rising impedance from inductance far above the x-over point. A Zobel network is not normally used to correct the impedance rise at tweeter Fs (unless I am missing something?), what you need IMO, is a series notch filter, and/or a re-evaluation of x-over component values. I would not be surprised if the "harshness" you describe is caused by a combination of sensitivity rises around tweeter Fs (due to new higher impedance) and the 8" drivers beams and peaks

[system7]

I want that ferrofluid back! I've always liked fabric domes over metal ones. Bit mushy, but they don't have that horrible ringing. Look at that SEAS Metal dome again:



Horrible things happening at high frequency. Goes up and down like crazy.

Seems this Zobel correction is a doddle. There's a calculator at DIY Audio Projects:

Speaker Zobel / Impedance Equalization Network Circuit Calculator

Putting in 6.2R and 0.05mh for the SEAS Fabric H0737-08 19TFF 1 , gives 7.75R and 0.832uF. How easy is that.

Reckon 1uf audio grade polypropylene capacitors and 8 ohms wirewound resistors can't be far wrong. I'll order the fabric dome tweeter replacements and have a go at this.

[system7]

Apologies if this is turning into a BLOG, but good progress on these speakers.

Had them apart, and decided to go with the 8R2 7W Wirewound/680nf Capacitor Zobel:



This leaves the crossover looking like this according to my multimeter, with LP 1.56mh, HP 2R2, 3.3uF, 0.27mH plus the added Zobel Network which is 8R2/680nF:



Quite easy to do since the capacitor fitted exactly in the HP coil:



The metal tweeters are a bit ropey, IMO. 1700Hz Fs is kinda stoneage, but it's sounding extremely good now! The top end has improved DRAMATICALLY with the Zobel. Very smooth and laid-back, a taste of the high-end.