SpeakerTalk

[iso]

I bought kit Cantata from late 70.s.

Crossovers seems to be original, so planned to make cap overhaul but keep original crossover boards.

Alcaps seems to be good choice if stock performance is appriciated. Falcon seems to carry 3 different lines of Alcaps, 50VDC, Low loss 50VDC and 100VDC, constructions of diffrent lines are also different.

Do you have any idea wich line is ideal for positions in series to signal or paralell to signal. Also bass section may need different caps than high frequency section.

It would also be nice to know if PP, Mylar or polycarbonate caps will improve performance in critical positions. As there is not too much space available, it seems that box style caps might be easier to fit, and first of all to know wich are the most critical positions.

There also seems to be at least 2 different cantata crossover boards as mine is quite different than one Colin has pictured. Layout is considerably different and I can only see 2 pcs 2R2 resistors, but level controls seems to work similar way in scematic Colin has provided. Macig, I don´t understand how this is possible.

[ColinR]

The first incarnation of the Cantata (the ones that blew up Quad 405s) had the switches on the PCB rather than fly leads found on the later ones, so there will be slight differences in componentry and the circuit.

The original Cantatas used the following 100V 100uF bass section, C2 100V 50uF, the rest were 50V low loss apart from 0.1uF C5 polyester.

Using Solens in the T52 section can brighten up the beasties, C2 can be "Solenised" up to 30uF in parallel with a 20uF electrolytic, it does hang off the PCB but it is the maximum sized component that can be used so that everything fits back through the B110B orifice.

If the switches are on the PCB the clearances will be such that only replacement electrolytics and small poly components will fit.

In all cases series components affect the sound, parallel ones are "engineering" so do to a much lesser extent.
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[iso]

I have now tried to evaluate Cantatas.

Overall they seems to present ambience quite nicely, but sound is somehow nasal.. When I placed 2 pcs ordinary bathroom towels ea side of tweeter and mids to controll reflections from cabs, this improved performance considerably. Do you any idea what might cause this nasal sound as I don´t believe this is how they sounded in 70:s.

The caps are original, but I somehow believe this is not cap related problem.

This kit has upper crossover board glued to metal front plate, so level control switches and fuses are directly wired to plate. Input wiring and driver wiring is attached to crossover board by one multiway connector.

Other gear I use is 90:s Sony CDP-XA30ES, Conrad Johnson Premier 7B and 11A, Wilmslow ATC K100 kit.

Best Regards

PS you mentioned early Cantatas blew Quad 405:s, what was actually the problem?

[audiolabtower]

The first Quad 405s were designed for true 8 ohm speakers and could drive 6 ohms resistive but not reactive loads and went into current limiting early with speakers of low impedance and large reactive swings.

The famous example was the Spendor BC3, where Spencer Hughes eventually put a slight mod in the crossover to get rid of a reactive 3 ohm dip in the midrange which caused the Quad to sound "hard".

The Mk2 Quad addressed this problem by putting a couple of vertical modules on the pcb to replace the simple current limiting with a time averaged limit, allowing large current transients through, and only starting to limit when the safe operating area of the output stage was close to being breached.

I actually built Mk1 405s when they came out and used them with Spendor BC1s for a couple of years. This was a good match since the BC1s were an easy 8 ohm drive.

[ColinR]

[iso]

Thank for your advice about caps. One 50uf measured 66uf others seems to be mostly within 10%. It was funny but all seems to be over specified values.

There was so little space on PCB that Sollens would not fit and box style had wrong wire spacings, so I ordered AlCaps. I suppose as Elcaps from 1978 did have smooth sound, exept considerable honk or nasality, new ones must be OK.

It was also funny to note that main PCB and bass section is attached quite close to each other. Main PCB is located in well behind front plate and bass section is screwed on back side of main PCB well, so B110 crossover inductors are quite close to B139 inductors. This simplifies construction but anywy, it would have been quite easy to reduce crostalk between inductors as there is 2 crossover board anyway.

Re Quad 405 and reactance. I noticed from Kef Cnatata bulletin that they had engineered Cantata be easy load to amplifiers. Resistance was said to be at least 65% from 8 ohm. From published impedance/reactance curve you can see that reactive nature of speaker is mostly between 30-100 hz and it isn´t too bad there.

So... was 405 protection circuity so sensitive for rectance... or were Cantatas driven so hard in those days. I know that Quad refined protection circuit later, when audio press started to measure available output currents.

[ColinR]

[proffski]

I remember the early MKI 405s well, not fon memories I'm sorry to say.

"A piece of wire with gain" and some nasties thrown in!
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill

[audiolabtower]

The Mk1 was "ok" for its time as long as you used competently designed 8 ohm impedance speakers. Later designs had a "blacker" background because the TLO opamps of the time did not have a superb signal to noise ratio. The Ludwig/Snook type mods made it a lot better, but not better than say Audiolab monos or even integrateds.

The irony was it drove electrostatics, which were supposed to be the most difficult loads of their time. Nevertheless the BBC used them in active designs, as did Kef in the KM. And did not Peter Walker do a blind test with 303 and bridged valve IIs including Martin Colloms and Paul Messenger of the subjectivist brigade where no one got real statistical evidence they were consistently identifying the correct amp?

[proffski]

You mean they were Ok driving eight ohm non inductive resistors?

I briefly used them to drive the legendary and much unappreciated B&W DM70s until I discovered Amcron, Adcom and Krell!
Just look at their impedance curve above 10kHz!
Here: http://loudspeaker-repair-service.reromanus.net/B+W-DM70_Manual-1974.pdf

I still remember the burnt PCBs under the emitter ballast resisors?
Inside the MKIs.

I have literaly had the ESL HF units catch fire whilst using MKI 405s.

Apologies to the moderators, we have meandered off the original KEF topic, sorry!
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill

[audiolabtower]

I never had any speakers that went to 0 ohm In those days it drove 104aBs, Harbeth HLs, and Spendor BC1s. It was replaced as more transparent amps came along in the hi-fi, but still works today driving Cresta 10s for the synthesiser

[iso]

Maybe we should transfer this topic to other forum, but this issue is becoming quite interesting.

I rebuilt last year Quad II/22/FM1 set p and discovered that it was wery nice amp when driving Tannoy Little Red monitors. But not too good when driving Wilmsow ATC K100 monitors. I quess that Quads decission to use low anode load for KT66:s was reason for this. Output impedance wasn´t low enough for ATC:s.

This brings us to question what kind of damping factor figures we should expect for proper control of speakers.

We can use Cantata as example. Woofer series coils (7mH and 3,5mH) have dcrs in order of 1 ohm + 0,5 ohm. Slightly higher than Thiel has proposed, but does this realy matter as inductors reactance at 50Hz is roughly 2ohm and 1ohm.

Also B139 is long coil in short magnetic gap design, so there is roughly 2-3 ohms coilwire outside magnetic gap. This portion of coil is not working and can propably be considered to be in series to signal.

Consequently there is roughly 6-7 ohms in series to signal at 50Hz, so does it realy matter if amps outputimpedance is 0,3 or 1,5 ohm. If we also think about Thiels recommendations to use series coils with dcr of less than 5% of woofer coil. Is this advice also sound, especially when using long coil in short gap woofers with low crossover frequencies.

Situation is propably worst when using reflex loaded woofers. Impedance minimum is at resonace frequency where most damping is needed and where actually minimum electrical damping is available. IB:s are better in this respect due impedance maximum at resonance frequency.

After all, it seems that short coil/long gap woofers with high crossover frequencies are better alternatives in this respect.

As my technical expertice is limited and only diy based, I hope that you don´t laugh on my ideas. But in my opinion, Loudspeker books have not discussed about this topic toroughly enough.

[proffski]

Pretty Please! I want to stay on the friendly side of the moderators.
So can we move this to where it belongs guys, indeed it is getting interesting!

Link: http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1008
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill

[iso]

I have now replaced old Elcaps with new Alcaps. Also most leaks in cabs and bafflebords have been corrected.

Most of honky nature of speakers have now dissappeared. Tonal balance and imagining is pretty good, but low level resolution is not even close to present standard. I suppose that this something to do with crossover caps.

Colin recommended Sollenising treble and mid sections but warns about messing with inductors... I understand that inductor DCR has effect on crossover slopes, but nobody has made it clear if capacitor ESR and series inductance can be ignored in crossover calculations.

I ques they can ignored if they ESR and inductance is less than 2-5% of reactance of capacitor at crossover and, say 2 octaves below and above.
I do not know how to read cap datasheets, so can caps replaced pretty freely with other types without any ill effects.

I suppose that pretty sharp imagining quality of Cantatas has something to do with original crossover calculations... and I do not want to lose this quality.

[ColinR]