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Old 3rd Jul 2010, 11:58 pm   #21
Kat Manton
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi Dave,

I wondered that, too. Then I had a look at some Spice models for drive units and realised I'd need a lot more measurements before I could create models for the Celestion drivers.

So I think I'll go with a combination of some maths, a dash of inspired guesswork, waving a reference microphone around while feeding various noises into them and some trial-and-error...

Cheers, Kat
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Old 4th Jul 2010, 9:43 pm   #22
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Having nothing better to play at last Friday lunchtime, I put the crossover circuit into a simulator with some plain resistive loads, the results looked promising in terms of the amplitude responses. However, whether this has any quantitative bearing on the sound levels generated by the drive units, I'm not so sure.....

Would a proper Spice model of a drive unit help with this anyway? It would perhaps be useful for analysing amplifier load stability etc, but actual sound levels?

(Component refs in attached are not as per KM, they're simulator generated.)
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Old 5th Jul 2010, 12:17 am   #23
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi Chris,

Interesting... it looks like my inductance measurements came out right (lacking a bridge I used generator/counter/oscilloscope, finding the resonant frequency with a known capacitor in parallel.)

I wonder if the lump in the tweeter's curve is due to using 10R for the tweeter? Impedance curves plotted by someone on 'diyAudio' suggest 4.8R @ 5 kHz is 'about right' for the one (out of four) which appeared to be working correctly.

I don't think it's worth going as far as constructing proper Spice models for the driver.

What matters is avoiding a lump or dip in sound level at 5 kHz, as well as matching mid/tweeter level to avoid a step up or down where the tweeter 'takes over' from the mid-range.

Unless anyone has any better ideas, in the first instance, what I think I'll do is connect a resistor in parallel with the SEAS tweeter, then examine what happens with generator/amp/microphone etc.

The SEAS driver appears to have an impedance of 7.5R at 5kHz. Assuming 4.5R to 5.0R is 'about right' for the HF2000, a resistor of around 12R to 15R in parallel with the SEAS tweeter would present the filter with the impedance it was designed for. Then I'll measure levels and take it from there.

There are quite a few unknowns; HF2000 sensitivity has been assumed to be similar to the Coles 4001, its impedance is from one sample out of five that's assumed to be "working"...

It looks like the best (only?) way to do this is to try something, measure it, decide what to tweak and how based on the measurements, try that and measure it. Time-consuming, but I tend to have vast reserves of patience for things like this.

Cheers, Kat
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Old 5th Jul 2010, 1:08 pm   #24
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Dropping the tweeter to 5R does get rid of that peak...... though it does end up looking like an inverse of the LF response.
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Old 7th Jul 2010, 4:22 pm   #25
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Kat / Chris,
that exactly what i was thinking of !!
I think there are enough spot measements to estimate the inductance of the speakers, if I get enough time in the near future I'll get the calculator out ...

Of course this would only ever we a starting point for crossover design.

BTW, I think, to a first approximation, the voltage level can be assumed to be related to SPL (given the spkr efficiency) -- accross the frequency range -- since that is the design target as that's exactly what the amplifier will do ...

dave
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Old 9th Jul 2010, 4:31 pm   #26
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Calculator gave :

Base = 4 ohm + 1.6 mH
Mid = 4.8 ohm + 640 uH (Av of 2 Z readings)
High = 9 ohm + 160 uH

dave
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Old 9th Jul 2010, 7:11 pm   #27
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Kat,
When I replaced my 44 tweeters, they were wired with red to –ve and black to +ve as in the phase was inverted. The routing of the wiring suggested that this was how they left the factory, but since I wasn’t the first owner I couldn’t vouch for that. Did you notice how your tweeters were wired?

Cheers
Eddie
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Old 10th Jul 2010, 7:52 am   #28
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi Eddie,

Examine the crossover circuit topology; the tweeter's filter is third-order high-pass; the mid-range filter is second-order band-pass. Hence the tweeter phase is reversed to ensure the tweeter and mid are in phase where both drivers are operating together.

(If it wasn't, the sound from the tweeter and mid-range driver would cancel and produce a dip in the response around the crossover point.)

Quote:
Originally Posted by mhennessy View Post
If you do have access to a measurement setup, check the depth of the notch when you reverse the polarity of the tweeter - this is a classic measure of integration (prove that the phase of the filters are coinciding).
Cheers, Kat
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Old 10th Jul 2010, 3:12 pm   #29
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Thanks for the explanation Kat, that makes perfect sense. Still, I would have thought that they would have just taken care of the wiring at the crossover, so that red and black would have followed a more expected arrangement.

Cheers
Eddie
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Old 11th Jul 2010, 11:25 am   #30
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi,

Quote:
Originally Posted by dave cox View Post
I think there are enough spot measements to estimate the inductance of the speakers, if I get enough time in the near future I'll get the calculator out ...
I still don't think it's worth attempting to produce an electrical model of the drivers. I've been looking into what's involved.

Superficially a speaker is 'just' a coil with DC resistance and inductance.

But the coil is in a magnetic field and it moves. That makes it into a motor/generator. Then it's got a cone attached to it which has mass and a suspension and moves air... it all gets a bit more complicated (fascinating, but complicated.)

This seems to cover it; Modeling the Loudspeaker as a Lumped System (from Engineering Acoustics on Wikibooks.)

I found some spice models on Duncan's Amp Pages... here's the 'generic' one. It's a bit more than an inductor and resistor in series...
Code:
* generic speaker simulation (8 ohm)
* as published in Stereophile Magazine.
*
* Donated by Jaime Arbona, converted to subcircuit form

.SUBCKT SPEAKER G $N_0002
R_R29         $N_0002 $N_0001  8  
R_R30         $N_0003 $N_0002  5  
R_R31         $N_0004 $N_0002  5.6  
C_C9         $N_0001 $N_0005  4.7uf  
C_C10         G $N_0003  3.3uf  
R_R32         $N_0006 $N_0005  0.5  
L_L16         $N_0004 $N_0007  0.5mH  
R_R33         $N_0009 $N_0008  100  
R_R34         $N_0008 G  39  
R_R35         $N_0008 $N_0010  0.6  
R_R36         $N_0008 $N_0011  0.9  
L_L17         $N_0011 $N_0009  1mH  
L_L18         G $N_0010  10mH  
R_R37         $N_0009 $N_0007  0.7  
L_L19         G $N_0006  0.3mH  
C_C11         G $N_0008  500uf  
.ENDS
Progress report:
  • Tweeters haven't arrived.
  • Amplifier repaired/modified/tested. -3dB points <10Hz and 74kHz, less than 0.03 % THD and >120W into 8 ohms will have to do...
  • Reference mic still works.
  • Dug out a preamp with 15V phantom power; response measured as 'very flat' (I can't rememer the figures. 'Adequate'.)
  • To Do: Add a balanced line-driver stage to the pre-amp

(My 'measurement system' consists of a 3.2GHz P4 desktop system, running Linux, with an Edirol FA-101 FireWire audio interface. It's upstairs; measurements are going to have to be done downstairs and/or in the back garden. Long cables are a given, but aren't a problem if it's all balanced.)

I've installed QLoud but haven't played with it yet. Screenshots suggest it does interesting things, some of which I don't understand. Yet. It's fun, is this!

Cheers, Kat
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Old 12th Jul 2010, 8:56 am   #31
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Quote:
Originally Posted by alex_w View Post
I think I've spotted an error in your crossover schematic.
Corrected; original schematic replaced
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Old 9th Aug 2010, 4:01 pm   #32
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

All quiet on the Celestion front Kat. Still no tweeters? Really keen to hear how they sound with your upgrade.

Not too sure how my other half will feel about having 66's in the living room.

Any ideas on how to disguise them?

Bob W
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Old 14th Aug 2010, 10:46 am   #33
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Kat,
This is useful information thanks, as my Spendor BC111s use the HF2000 tweeter. If I can find it I could scan and post the original crossover network. I believe that there is a schematic of it posted in Martin Colloms' book High Performance Loudspeakers, second edition.
Be interesting to see how you get on with the Seas tweeter
Simon
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Old 27th Aug 2010, 2:38 pm   #34
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi,

The SEAS tweeters arrived today. Given how long ago I ordered them, I suspect they came part of the way from Norway on foot. Ah well, better late than never...

I'll finish off a couple of things I started to keep me occupied while I was waiting, then get back to this

(Once the system down here is working again, I'll get back to the EMT turntable as I'll have something to connect it to.)

Cheers, Kat
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Old 29th Aug 2010, 9:09 pm   #35
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

I figured I ought to thoroughly check the original crossover components.

Good job I did, C1 in the RH crossover (72µF non-polarised electrolytic) measured 7.3µF with 13Ω ESR...

C2 (same type/value as C1) measures 80µF, incidentally; with 0.06Ω ESR. While that one seems healthy, the tolerance of these things appears to be rather wide.

(I haven't checked the capacitors in the LH 'speaker; I'm leaving that one alone as a reference for the time being.)

I found a source of 72uF 100V non-polarised electrolytics here; and they're a reasonable price. Four ordered; ultimately I'll fully re-cap both 'speakers.
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Old 30th Aug 2010, 5:41 pm   #36
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Is the amplifier referred to post #30 your Pamphonic, or have I lost the plot there?

David

Last edited by Kat Manton; 30th Aug 2010 at 11:09 pm. Reason: Thread split
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Old 30th Aug 2010, 6:22 pm   #37
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi David,
Quote:
Originally Posted by qualityten View Post
Is the amplifier referred to post #30 your Pamphonic, or have I lost the plot there?
Nope; it's one of these. (In this case, a 120W 'slave' version, which meant I didn't have to bypass tone controls or remove redundant mixer circuitry as it has neither.) Removing the output transformer, tweaks to the biasing (class B originally, now class AB with quiescent dissipation around 40W) and a few other modifications produces a rather respectable amplifier.

Cheers, Kat

Last edited by Kat Manton; 30th Aug 2010 at 11:09 pm. Reason: Thread split
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Old 30th Aug 2010, 8:08 pm   #38
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Very interesting to read of the amplifiers. One of the weaknesses of current technology is that forum members can't hear each others' equipment in any meaningful sense. Must be good if it is on 12 hours at a time.

David

Last edited by Kat Manton; 30th Aug 2010 at 11:08 pm. Reason: Thread split
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Old 30th Aug 2010, 11:08 pm   #39
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

(Capacitor testing discussion split off into a new thread.)
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Old 8th Sep 2010, 3:41 am   #40
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Default Re: Celestion Ditton 44 - HF 2000 tweeter replacement?

Hi,

There is some progress to report; results are very encouraging.

I've fitted one of the new tweeters (this involves some 'adjustment' to the cabinet, more on that later when I've taken photographs. Original tweeters could be refitted and none of it would show. Not that I think I'll ever want to do that...)

The tweeter section components have been removed and re-assembled on a bit of tagstrip, and the wires for the tweeter brought out through one of the original tweeter clamp holes (which aren't in use now anyway.) So I've got the bass/mid drivers and associated crossover circuitry (now repaired) back in the cabinet. The tweeter's crossover section is on top of the cabinet, so it can be easily modified.

I thought I'd give it a quick test with the original tweeter section components. I can "hear the join", it currently sounds like a two-way system that's "not quite right". It's not a system; but it's not far off being one. It sounds like there's a step up in level above the crossover frequency, but much. This, I think, is good (and is what I expected.)

That 'quick test' turned into a several-hour listening session, joined by Cobweb. Given we found it difficult to stop listening to music, in mono, on a badly-placed speaker with a tweeter which isn't properly integrated, I think that's promising.

Cobweb's remark to me was, "I reckon you're about to take our hi-fi system to a whole new level..."

Next... rearrange the living room a bit and try to get response plots...

Cheers, Kat
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