Expert Acoustic Columns, Cross over query. - UK Vintage Radio Repair and Restoration Discussion Forum

RojDW48 · 23rd Sep 2021, 11:57 pm

I am trying to resurrect these lovely omnidirectional speakers. So far I have masked the baffles to fit Kef B200s. One original Lorenz tweeter remained - it's working and the LF was choked off with a 2 uF cap, so I have re-instated that. I tried a little EMI plastic cone tweeter in the other column, similarly restricted. The Lorenz column sounded nicer, so I introduced a simple 1st order x-over into the other (EMI) one and that improved things.

My rookie question is - what level of crossover frequency is achieved by just fitting a 2 uF capacitor? So that I can try to recreate that level with a x-over in the other column and try to match them up.

Herald1360 · 24th Sep 2021, 10:58 am

It'll depend on the tweeter impedance. If this is assumed mainly resistive (R), then the -3dB point will be where 1/(2*pi*f*C) = R. Explicitly, f = 1/(2*pi*R*C). The rolloff will be -6dB/octave.

Whaam68 · 25th Sep 2021, 7:01 am

They look lovely….well saved. As the column tops look to have diffuser cones that the drivers fire up into. They might also suit something like a Wharfedale super 8 full ranger? From memory (could be wrong) they had a rising high frequency response to suit omni directional use. I can see a pair of those going v cheap on the you know where auction site at the mo. Just a thought.

dave cox · 27th Sep 2021, 3:05 pm

Some data on the HF unit is here.

The claimed dispersion seems 'hopeful' so I would ignore that

It is probably a lot more sensitive that a B200, as is the EMI HF unit.

The B200 may also suffer if fed with a full range signal (?) as the cone is a lot stiffer and may go into nasty vibration modes at higher frequencies (>3KHz). It might be worth finding a 'basic' 2 way crossover for the B200 and adapting the HF portion with an L-pad to suit the HF unit used.

dc

RojDW48 · 27th Sep 2021, 5:12 pm

Quote:

Originally Posted by dave cox

Some data on the HF unit is here.

The claimed dispersion seems 'hopeful' so I would ignore that

It is probably a lot more sensitive that a B200, as is the EMI HF unit.

The B200 may also suffer if fed with a full range signal (?) as the cone is a lot stiffer and may go into nasty vibration modes at higher frequencies (>3KHz). It might be worth finding a 'basic' 2 way crossover for the B200 and adapting the HF portion with an L-pad to suit the HF unit used.

dc

Thanks Dave - helpful info. I have just the one Lorenz, but several EMIs, so I shall probably end up with the EMIs in place + the simple 1st order x-overs.

The cab design is interesting. The active column (below the disperser cone) is 36 X 14 X 14". Approx half of the column is damped with what looks like kapok about half way up, retained by hessian top and bottom, Again - half way up there is a plywood 'choke' about 2" deep all round the interior. There are 2 vents of approx 10 X 3" at the bottom. Does this kind of cab have a name? It doesn't seem quite like a Reflex, or horn, or T-line.

RojDW48 · 29th Oct 2021, 6:47 pm

Much fiddling has taken place since I last posted on these speakers. I bought a cheap pair of Peerless tweeters to replace the one Lorenz and fitted a simple 1st order x-over. I have been trying to find something contemporary (more or less) with the Baker 12" drivers, which are rumoured to be original.

I picked up this pair of big old Dynatrons at a clearance auction for next to nothing. I am 90% certain the 12" drivers are Wharfedale. One fitted so far and very promising, crossed over at 2KHz.

Radio Wrangler · 29th Oct 2021, 8:27 pm

Is the tweeter dome purple-ish?
Green baize/felt gasket on the 12 incher, and a big square magnet?

David

roffe · 29th Oct 2021, 9:52 pm

Dynatron LS4038 or Wharfedale Dovedale III.

jamesperrett · 30th Oct 2021, 12:22 am

As roffe says, they are probably Dynatron LS4038's which are Wharfedale Unit 5 kits in Dynatron cabinets. I have a pair here and they sound pretty good as general hifi speakers.

Edward Huggins · 30th Oct 2021, 9:27 am

Quote:

Originally Posted by RojDW48

Much fiddling has taken place since I last posted on these speakers. I bought a cheap pair of Peerless tweeters to replace the one Lorenz and fitted a simple 1st order x-over. I have been trying to find something contemporary (more or less) with the Baker 12" drivers, which are rumoured to be original.

I picked up this pair of big old Dynatrons at a clearance auction for next to nothing. I am 90% certain the 12" drivers are Wharfedale. One fitted so far and very promising, crossed over at 2KHz.

Well done so far.....er, probably the best speakers that Dynatron never "really" made! That crossover point seems a tad low?

RojDW48 · 30th Oct 2021, 3:18 pm

Yes - LS 4038s - with purply tweeter centre. So - all Wharfedale drivers? I wasn't completely certain as there was no impressed 'W' on a basket leg. But I have a 10" which, likewise lacks it. Did they mark them thus earlier or later? Is the tweeter a good one?

The x-overs I am using have 3 settings - I chose 2 KHz because the big, stiff drivers didn't look like they could do a lot of mid range. But I can try 2.5 and 4KHz in the future.

Pity about the cabs, but they are in a parlous state. I had to take a jigsaw to the back to extract the big driver !

23rd Sep 2021, 11:57 pm	#1
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Expert Acoustic Columns, Cross over query. I am trying to resurrect these lovely omnidirectional speakers. So far I have masked the baffles to fit Kef B200s. One original Lorenz tweeter remained - it's working and the LF was choked off with a 2 uF cap, so I have re-instated that. I tried a little EMI plastic cone tweeter in the other column, similarly restricted. The Lorenz column sounded nicer, so I introduced a simple 1st order x-over into the other (EMI) one and that improved things. My rookie question is - what level of crossover frequency is achieved by just fitting a 2 uF capacitor? So that I can try to recreate that level with a x-over in the other column and try to match them up. Attached Thumbnails __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)

24th Sep 2021, 10:58 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Expert Acoustic Columns - x-over query It'll depend on the tweeter impedance. If this is assumed mainly resistive (R), then the -3dB point will be where 1/(2pifC) = R. Explicitly, f = 1/(2piRC). The rolloff will be -6dB/octave. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

25th Sep 2021, 7:01 am	#3
Whaam68 Hexode Join Date: Feb 2007 Location: Luton, Bedfordshire, UK. Posts: 456	Re: Expert Acoustic Columns - x-over query They look lovely….well saved. As the column tops look to have diffuser cones that the drivers fire up into. They might also suit something like a Wharfedale super 8 full ranger? From memory (could be wrong) they had a rising high frequency response to suit omni directional use. I can see a pair of those going v cheap on the you know where auction site at the mo. Just a thought.

27th Sep 2021, 3:05 pm	#4
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Expert Acoustic Columns - x-over query Some data on the HF unit is here. The claimed dispersion seems 'hopeful' so I would ignore that It is probably a lot more sensitive that a B200, as is the EMI HF unit. The B200 may also suffer if fed with a full range signal (?) as the cone is a lot stiffer and may go into nasty vibration modes at higher frequencies (>3KHz). It might be worth finding a 'basic' 2 way crossover for the B200 and adapting the HF portion with an L-pad to suit the HF unit used. dc

29th Oct 2021, 6:47 pm	#6
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Re: Expert Acoustic Columns - x-over query Much fiddling has taken place since I last posted on these speakers. I bought a cheap pair of Peerless tweeters to replace the one Lorenz and fitted a simple 1st order x-over. I have been trying to find something contemporary (more or less) with the Baker 12" drivers, which are rumoured to be original. I picked up this pair of big old Dynatrons at a clearance auction for next to nothing. I am 90% certain the 12" drivers are Wharfedale. One fitted so far and very promising, crossed over at 2KHz. Attached Thumbnails __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)

29th Oct 2021, 8:27 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Expert Acoustic Columns - x-over query Is the tweeter dome purple-ish? Green baize/felt gasket on the 12 incher, and a big square magnet? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Oct 2021, 9:52 pm	#8
roffe Hexode Join Date: Nov 2003 Location: Linkoping, Sweden Posts: 493	Re: Expert Acoustic Columns, Cross over query. Dynatron LS4038 or Wharfedale Dovedale III.

30th Oct 2021, 12:22 am	#9
jamesperrett Octode Join Date: May 2007 Location: Liss, Hampshire, UK. Posts: 1,873	Re: Expert Acoustic Columns, Cross over query. As roffe says, they are probably Dynatron LS4038's which are Wharfedale Unit 5 kits in Dynatron cabinets. I have a pair here and they sound pretty good as general hifi speakers.

30th Oct 2021, 3:18 pm	#11
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Re: Expert Acoustic Columns, Cross over query. Yes - LS 4038s - with purply tweeter centre. So - all Wharfedale drivers? I wasn't completely certain as there was no impressed 'W' on a basket leg. But I have a 10" which, likewise lacks it. Did they mark them thus earlier or later? Is the tweeter a good one? The x-overs I am using have 3 settings - I chose 2 KHz because the big, stiff drivers didn't look like they could do a lot of mid range. But I can try 2.5 and 4KHz in the future. Pity about the cabs, but they are in a parlous state. I had to take a jigsaw to the back to extract the big driver ! __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)