Richard Allan Three Way - Rebuild
Posted 27th January 2015 at 03:07 AM by googlyone
Updated 26th January 2015 at 09:02 AM by googlyone
Updated 26th January 2015 at 09:02 AM by googlyone
I recently picked up a pair of, I guess, 1970's Richard Allan three way speakers. The line-up are RA drivers with which I am not familiar. Bass, LP10B, midrange LP5B and tweeters - long dead and replaced with mismatched dome tweeters.
Richard Allan was reasonably popular in Australia in the 70's and 80's, and did some pretty good gear. I was interested to see how these went, but did need to do something about the tweeters.
On pulling the drivers out, I noted a few things:
- The LP10B is a 10 inch bextrene cone woofer, using a 1.5" voicecoil rather than the HP10B's 2" coil. It is also very much an "acoustic suspension" driver - read on.
- The LP5B is very much like a 5" version of the KEF B110 - as used in the LS3A. I read somewhere that Richard Allan made a version of the LS3A under license, but that might be an "internet fact".
- The crossover was made by KRIX, a local speaker manufacturer who is now at the top quality end of the business, and doing a bunch of professional gear for cinemas. This crossover must have been very early for them indeed. I assume it was a standard stock item, and may have even been provided to a hobbyist to use themselves. I know I bought components from Krix in the late 80's / early 90's.
- The crossover explains why the old tweeter had failed. It was a simple first order, and included no attenuation on the tweeter (so I guess it was either very insensitive or was going to be rather bright), and also had no impedance correction for any driver, nor handling of the resonance of the tweeter. Hmmm. My experience is that ignoring htese aspects in a crossover leads to a pretty poor result.
So what to do?
I elected to stick with the retro scheme, and pulled out some Philips AD0160/T8 dome tweeters I had in the cupboard.
On looking on the net, I see these described at the "Worlds Worst Dome Tweeter". I actually disagree, but would concede that they are going to be "Really Hard to Use if You Are Not On Your Game". Why? Check the impedance!
I measured the impedance (mag and phase) and imported this into Excel. The plot is from Excel. The yellow line is the raw impedance. Look at the peak, 60 Ohms almost! Ouch! A first order crossover would do nothing to save this tweeter, and a second order crossover would almost certainly sound awful, with a huge peak from the tweeter blasting through at the 900Hz resonance.
Adding a serious LCR trap fixed this. Adding this sort of thing is ok for a serious speaker head or engineer like myself to do, but for the average hobbyist this sort of design is just too much and would result in misapplication of this driver. The "trap" circuit is also pretty serious, including a 2.7mH inductor, 12uF capacitor and 8R2 resistor. Quite chunky, but it does get the impedance under control.
Given this start, looking at the woofer and mid, neither the bass driver nor the mid were well enough behaved into the crossover band that I felt happy using natural rolloff. I added zobels to these, as I wanted to use a first order crossover, and did not want to wind up fighting increasing impedance with the XO.
Similar tuning in zobels for the LP5B and LP10B resulted in neater impedances such as this rom the midrange, note the Lp5B has a resonance at about 60Hz:
So how did the Richard Allan / Philips drivers measure?
Richard Allan LP10B Frequency Response, 1m, 1/6th octave smoothing, i.e. just enought to get the fluff off the graph.
Richard Allan LP5B Frequency Response, 1m, 1/6th octave smoothing.
Philips AD0160/T8 Frequency Response, 1m, 1/6th octave smoothing.
Notes:
-1- The LP10B and 5B both show a big peak at 70Hz and some artefacts particularly on the LP10B above this. This is the room and floor bounce. This is a result of the measurement approach of placing the speaker box at about halfway between the ceiling and floor and placing the mic on a stand at 1m distance.
-2- The midrange is actually pretty damn well behaved. Whatever commonality in heritage this driver has to the B110, it does not exhibit the screamingly awful response of the B110! This driver does have a bit of a dip between 1-2KHz, but not so much that I am going to need to address this in the XO.
-3- The tweeter is fairly well behaved, certainly for its vintage!
-4- The woofer measurements show a mass of peaks and dips, but I am pretty comfortable that most of this is the room.
Looking at the impedance plot for the woofer, there are a few clues though:
Just above 2000Hz is a slight peak in impedance. There is a resonance either in the box or the driver, and is probably associated with part of the peak at 300-400Hz.
Integrating this lot together required the following crossover:
OK, it is a lot more complex than the old one. And in terms of a saleable product, is overcooked in that the zobel networks on the bass and midrange could likely have been addressed by a single capacitor, saving a couple of resistors and board layout / space.
I do think however that the impedance trap on the tweeter is essential - though the original tweeter may have been really well behaved. The double failure however suggests not.
How was the final response? Pretty good actually!
One of the boxes:
And comparing the two boxes to one another (in the same measurement location):
Which shows a couple of things:
- The drivers still behave very much alike.
- The measurement location has probably got very consistent foibles, and I managed to get the speaker in pretty well exactly the same spot!
The bottom line however is that the LP10B needs to be in a sealed box, and is only just suited to that.
The Thiele and Small parameters are:
Qt: 1.16
Qm: 10.6
Qe: 1.31
Fs: 51.5 Hz
Vas: 40.8 litres
So these have a 3dB point of 50Hz, but there is a 3 odd dB peak in the response between 50 and about 150Hz in simulations. I used an awful lot of padding in the boxes, as had the original designer, which has probably dropped this peak a fair bit.
How do they sound? Pretty good actually. Which makes sense - apart from a few compromises in the bass area, the response is pretty smooth, and within 3-4 dB across the entire audio range.
Subjectively?
- Smoother than I would have expected.
- Not tiring
- Imaging is "ok" but not up to the svelte and slimmer designs we prefer today.
Richard Allan was reasonably popular in Australia in the 70's and 80's, and did some pretty good gear. I was interested to see how these went, but did need to do something about the tweeters.
On pulling the drivers out, I noted a few things:
- The LP10B is a 10 inch bextrene cone woofer, using a 1.5" voicecoil rather than the HP10B's 2" coil. It is also very much an "acoustic suspension" driver - read on.
- The LP5B is very much like a 5" version of the KEF B110 - as used in the LS3A. I read somewhere that Richard Allan made a version of the LS3A under license, but that might be an "internet fact".
- The crossover was made by KRIX, a local speaker manufacturer who is now at the top quality end of the business, and doing a bunch of professional gear for cinemas. This crossover must have been very early for them indeed. I assume it was a standard stock item, and may have even been provided to a hobbyist to use themselves. I know I bought components from Krix in the late 80's / early 90's.
- The crossover explains why the old tweeter had failed. It was a simple first order, and included no attenuation on the tweeter (so I guess it was either very insensitive or was going to be rather bright), and also had no impedance correction for any driver, nor handling of the resonance of the tweeter. Hmmm. My experience is that ignoring htese aspects in a crossover leads to a pretty poor result.
So what to do?
I elected to stick with the retro scheme, and pulled out some Philips AD0160/T8 dome tweeters I had in the cupboard.
On looking on the net, I see these described at the "Worlds Worst Dome Tweeter". I actually disagree, but would concede that they are going to be "Really Hard to Use if You Are Not On Your Game". Why? Check the impedance!
I measured the impedance (mag and phase) and imported this into Excel. The plot is from Excel. The yellow line is the raw impedance. Look at the peak, 60 Ohms almost! Ouch! A first order crossover would do nothing to save this tweeter, and a second order crossover would almost certainly sound awful, with a huge peak from the tweeter blasting through at the 900Hz resonance.
Adding a serious LCR trap fixed this. Adding this sort of thing is ok for a serious speaker head or engineer like myself to do, but for the average hobbyist this sort of design is just too much and would result in misapplication of this driver. The "trap" circuit is also pretty serious, including a 2.7mH inductor, 12uF capacitor and 8R2 resistor. Quite chunky, but it does get the impedance under control.
Given this start, looking at the woofer and mid, neither the bass driver nor the mid were well enough behaved into the crossover band that I felt happy using natural rolloff. I added zobels to these, as I wanted to use a first order crossover, and did not want to wind up fighting increasing impedance with the XO.
Similar tuning in zobels for the LP5B and LP10B resulted in neater impedances such as this rom the midrange, note the Lp5B has a resonance at about 60Hz:
So how did the Richard Allan / Philips drivers measure?
Richard Allan LP10B Frequency Response, 1m, 1/6th octave smoothing, i.e. just enought to get the fluff off the graph.
Richard Allan LP5B Frequency Response, 1m, 1/6th octave smoothing.
Philips AD0160/T8 Frequency Response, 1m, 1/6th octave smoothing.
Notes:
-1- The LP10B and 5B both show a big peak at 70Hz and some artefacts particularly on the LP10B above this. This is the room and floor bounce. This is a result of the measurement approach of placing the speaker box at about halfway between the ceiling and floor and placing the mic on a stand at 1m distance.
-2- The midrange is actually pretty damn well behaved. Whatever commonality in heritage this driver has to the B110, it does not exhibit the screamingly awful response of the B110! This driver does have a bit of a dip between 1-2KHz, but not so much that I am going to need to address this in the XO.
-3- The tweeter is fairly well behaved, certainly for its vintage!
-4- The woofer measurements show a mass of peaks and dips, but I am pretty comfortable that most of this is the room.
Looking at the impedance plot for the woofer, there are a few clues though:
Just above 2000Hz is a slight peak in impedance. There is a resonance either in the box or the driver, and is probably associated with part of the peak at 300-400Hz.
Integrating this lot together required the following crossover:
OK, it is a lot more complex than the old one. And in terms of a saleable product, is overcooked in that the zobel networks on the bass and midrange could likely have been addressed by a single capacitor, saving a couple of resistors and board layout / space.
I do think however that the impedance trap on the tweeter is essential - though the original tweeter may have been really well behaved. The double failure however suggests not.
How was the final response? Pretty good actually!
One of the boxes:
And comparing the two boxes to one another (in the same measurement location):
Which shows a couple of things:
- The drivers still behave very much alike.
- The measurement location has probably got very consistent foibles, and I managed to get the speaker in pretty well exactly the same spot!
The bottom line however is that the LP10B needs to be in a sealed box, and is only just suited to that.
The Thiele and Small parameters are:
Qt: 1.16
Qm: 10.6
Qe: 1.31
Fs: 51.5 Hz
Vas: 40.8 litres
So these have a 3dB point of 50Hz, but there is a 3 odd dB peak in the response between 50 and about 150Hz in simulations. I used an awful lot of padding in the boxes, as had the original designer, which has probably dropped this peak a fair bit.
How do they sound? Pretty good actually. Which makes sense - apart from a few compromises in the bass area, the response is pretty smooth, and within 3-4 dB across the entire audio range.
Subjectively?
- Smoother than I would have expected.
- Not tiring
- Imaging is "ok" but not up to the svelte and slimmer designs we prefer today.
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