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Old 28th July 2007, 07:41 AM   #71
sba is offline sba
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Join Date: Jun 2006
Here are some pics from a recent Beovox 5700 sale on ebay. It's very similar to the Ditton 66.



I've been following this thread and want to thank all of you for sharing your expertise here. I have 2 pairs of Ditton 66's, and the attention that that speaker is getting here has only increased my fondness for it.

My speakers seem to be running okay....but then I really don't know what a pristine 66 should sound like.

Should I assume that by now, after about 35 years, that the crossovers no longer meet specs....and that they need to be refurbished?
Or might they still be fine?

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Old 28th July 2007, 01:52 PM   #72
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Hi sba,
thanks for posting the link to the pics on ebay, they look the same version of the 5700 model as in post #66 of this thread, but a larger pic we can see more clearly !
Final bid price was a lot of money for old speakers that will most likely need some reconditioning work, and perhaps a lot of work.

If your 66s sound OK and don't have any of the buzzing sound in their mid-domes, that tonedef2 explained about -{I think in this thread, though there are two other, much shorter threads about Ditton 66s that he and I have posted a bit in also}- then you will only have to check the cross-overs.
The inductors will probably be OK, but the capacitors could be leaking in part, and may have drifted too far in value from their original spec. value.
See posts #30 and #38 on page 2 of this thread for what I wrote about the capacitors, then follow-up with what I have written on page 3.

I will be posting more about replacement capacitors soon, thus you may like to follow that and buy the caps and E.S.R. simulating resistors and install these yourself, then hear what your 66s sound like.
Do one pair first, and compare the sound to the older parts sound of the other pair.

Thus, do keep looking here for what we may discuss next.

best wishes,
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Old 28th July 2007, 01:58 PM   #73
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Originally posted by tonedef2
Have a look at this page i spotted........bye for now, taking the boat out.

Hi Pete, great pic. ! Its almost unbelieveable what some enthusiasts have tried over the years, and I wish I'd heard those monsters in action !!

I hope you had a good venture, or an adventure ? , out in your boat !
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Old 28th July 2007, 03:13 PM   #74
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Default partial error in my post #56

Originally posted by alan-1-b

The E.S.R. values of the electrolytic capacitors will be significant also, and likely need duplicating when replacing with the much lower ESR polypropylene capacitors, especially for the 72uF cap that is in direct parallel with the woofer, and perhaps for the other 72uF, and less likely for the 24uF -{which will have a lower ESR anyway}, and highly unlikely to be necessary for the 4uF black cap. <-- UP-DATE following this -->
I've changed my mind after thinking about an audible effect that tonedef2 described earlier. Now I think there may be need to add the resistance here to reduce to negligible level a possible audible resonance that a very low ESR cap could cause to occur here, as it seems it may be with the low ESR cap later put in his x-overs instead of the black 4uF electrolytic.

E.S.R. = Equivalent Series Resistance

It is so long since I worked with electrolytic caps in a wide-band audio-critical application that it slipped my mind that the ESR,
{which is dependant on the Dissipation Factor of the dielectric}, varies with frequency !
Basically, dissipation factor in electrolytic caps is higher at low frequencies and lower at high frequencies, BUT, do not confuse that with the Capacitive Reactance, which is of course higher at low frequencies and lower at high frequencies.
Dissipation Factor represents a loss, it is not useful, and it does not have the same phase angle in circuit as capacitive reactance, thus we cannot allow for it by simply increasing the value of capacitor when we substitute with a low dissipation factor plastic film cap.

Thus we have a different ESR at 500 Hz in the 24uF cap than is at 5 kHz !
So, how to simulate that with a fixed resistor, to try to keep the Impedance at what it is at 500 Hz region and 5 kHz region ?
Well, it can't be done simply, thus one has to compromise and select an in-between value, put it in and listen, and if not liking the sound then either slightly increase or slightly decrease the resistance value, but I hope if I estimate close enough, and am lucky, then you may not want to change the resistors !

The ESR in the 72uF caps will probably be about 1 ohm at 500 Hz, possibly a little less and hopefully not much greater.

The ESR in the 4uF black cap will probably be about 1 ohm, or slightly less, at 5 kHz.
ESR at 500 Hz is not relevant for the 4uF cap.

The 24uF cap is in Series with the mid-dome and both sections of the x-over filter, thus its ESR is relevant in the regions around both x-over frequencies.
At 500 Hz it could be about 2.5 ohm or slightly larger, and at 5 kHz it will be quite low, probably less than 0.2 ohm.
I recommend using no less than 1.5 ohm for the resistor, because less will shift the lower x-over point up a bit too much.
1.5 ohm will have minimal effect on the higher x-over frequency, but you might notice slightly less upper midrange brightness. If that is audibly not wanted, then reduce resistor to 1 ohm or 0.5 ohm, and yes that will have shifted the lower cut-off up a little, but will be part compensated by being a little bit louder at the low end of the mid-dome's output, for about an octave, approx. 600 - 1200 Hz.
As I said, this will involve a compromise, a trade-off, but probably not an excessive one.

Polypropylene caps have almost negligible ESR, as their dissipation factor is very low.
Thus we add a series resistor to each polyprop. cap to simulate the ESR of the electro's so the impedances, including the phase angles, will be the same as Celestion's original, so that the drivers will cross over at the original design frequencies.

So, why not simply use new electrolytic caps which have the higher ESRs ?
Well, electrolytic caps also have high Dielectric Absorbsion.
What is that ?
It is a Memory effect. Basically when the electro cap is charged by the signal {music} it retains part of that charge and does not fully discharge and charge to the opposite polarity when the signal changes polarity.
It does charge and discharge and re-charge etc, but always lagging the signal by a much larger time interval than the Time Constant of the circuit.
The audible consequence is a muddying of the music owing to the mixing of delayed release information being added to new information.
{this is a simplification of the full process, but it's sufficient to give you the idea}

Polypropylene caps have very very low dielectric absorbsion. That is one reason why they are so good for audio signal use.

I write too much !
Now I have not enough time remaining to post about the brands of polyprop. caps which you both asked about a long time ago, and recommend sizes and values, etc ...
I am sorry. I will try to do that next time.
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Old 28th July 2007, 03:32 PM   #75
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Hello all,

Allen you are the man! And have no reason to say sorry, your help with this project has been amazing, rather there would be no project without you! I will speak for us all and say thank you very much in bring our 66's / joy of music back.

Im looking for one orig 66 version 2 xover board to hack up for this next phase of our project......

News> I finished messing around with the diffraction problem> net result was, attaching the audio wool to the upper grill works best and did have a good affect, I will describe as less blurring of the high freq energy, note: if you listen with grills off........then I simply used a custom cut wool with very little Velcro to slip under the top over hang (your grill will fit a little tighter), works great and am now working on the aesthetics.....

Hello sba!

1-Did Graham ever get you the remaining measurements?
2-I will post some picks of the grill soon for analysis as im pretty sure as the audio wool loosens over time we have a ringing affect at moderate attenuation.

Pete / ToneDef2
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Old 28th July 2007, 03:51 PM   #76
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Hi Pete,
No Grahame hasn't sent me anything, nor posted recently.
When-ever you happen to open your 66s next, if you could measure the DC resistances of the woofers such may be helpful.
I think they will be nominally 4 ohm, thus expect to see about 3.3 ohms on your meter, if your meter is suffciently accurate at measuring low resistance values.

Grahame measured his MD500s at 6.7 and 6.9 ohms, if I'm remembering correctly, and that is posted somewhere here-back, but I've forgotten if the HF2000's DCRs are posted anywhere. {Yes I will scroll back and look} .
From the x-over components' values it seems the HF2000s are nominally 8 ohm, thus would measure about 6.7 ohm or so .

I'm more interested in the more time consuming Impedance measurements at the x-over frequencies and 1 octave each side of for each of the drivers, especially if we are to consider replacement tweeters later, to integrate such as well as possible with the MD500s, so we'll wait to see what Grahame may be able to come up with.

__________________________________________________ __

sba, if you are reading this, are you able to do Impedance measurements on drivers at midrange frequencies ?

I posted something about such earlier in this thread .
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Old 28th July 2007, 04:00 PM   #77
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LOL i was right at the spell check/edit point and was timed out on my last post, re-read for changes if any....lol

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Old 28th July 2007, 04:05 PM   #78
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You caught me at a good time...

1- I will remove the driver and take DCR's
2- My HF2000's are brand new...so the measurements should be perfect.

3-I will post back in 30 mins with results.......:-)

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Old 28th July 2007, 04:52 PM   #79
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Hello Alan/all,

Here are the DCR's with drivers removed:

Woofer 4.1 ohms.
Mid 6.3 ohms > hmmmm?
Tweeter 4.4 ohms.

TonDef2 / Pete
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Old 28th July 2007, 04:54 PM   #80
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funny? this board has lost an entire post of mine????

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