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Old 11th October 2009, 10:49 AM   #1
oranda is offline oranda  United States
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Default Looking for advice on Harbeth monitor rebuild (long)

Hi all:

This is my first post to this forum I was hoping to tap the collective wisdom of the group to help me plan the next steps for a recent project I have undertaken. I am rebuilding a pair of Harbeth HL MKII monitors monitors that are about 30 years old and in need of some TLC. The custom Harbeth 8Ē woofers were cleaned, professionally serviced, and now sound quite nice. After much research and a phone call to Madisound, the original Audax HD12x9 D25 tweeters were replaced with Audax TW025A0s, using a custom machined rectangular faceplate so that the new tweeters would be correctly integrated to the existing speaker woodwork.

Here is my dilemma: While the TW025A0 is not bad and a reasonable replacement, among other differences, the fs is about 300 Hz higher than the original HD12x9 D25 (1090 Hz and 800 Hz respectively). Given the low-ish 2 kHz crossover point of the speaker, I thought it might be worthwhile to consider other HF drivers that might integrate better. Also, I feel that I would appreciate a tweeter that was a bit more sonically refined.

As I see it, here are my alternatives:

1. Find someone who can refurbish the HD12x9 D25 tweeters. I have not seen anyone who is doing this, but maybe someone knows of a place that offers this service? The originals are not completely dead, but the output and frequency response of the pair are diminished.

2. Find a high quality tweeter with similar Impedance (8 Ohms), sensitivity (89 dB@1W, 1m), and fs (800 +/- 120 Hz) as HD12x9 D25 and use that. It would also need to have a removable faceplate so that I can machine a new faceplate to fit the original routing. The Morel CAT298 (or possibly the CAT308 or 408) comes to mind, but I am open to suggestions.

3. Replace the tweeter and design a new crossover to better integrate it with the woofer. The problem here is that I donít have any specifications for the custom Harbeth woofer. Maybe someone has the T/S parameters or knows of a service that I can send woofer to get parameters. I have considered purchasing one of the plethora of programs available to do this, but I am not sure it makes much sense to go that route for a one-off project. Although, if that is my best option, I will certainly consider it.

4. Replace all drivers and crossover and just use the cabinets as an open canvas.

5. Do nothing.

So, any suggestions as to what my next step should be? I am not interested so much in restoring the speakers to their original shape and I am in making them sound and work the best they can (without investing an inordinate amount, of course.) The existing crossover uses no electrolytics, and the high frequency end contains a tapped autoformer with one-decibel steps so that the HF unit level can be adjusted. I have not exploited this functionality, but maybe the existing crossover offers enough flexibility to warrant continued use?

Anyway, I look forward to any and all suggestions and thanks for reading.
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Old 11th October 2009, 11:51 PM   #2
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Quote:
Originally Posted by oranda View Post
Hi all:

This is my first post to this forum I was hoping to tap the collective wisdom of the group to help me plan the next steps for a recent project I have undertaken. I am rebuilding a pair of Harbeth HL MKII monitors monitors that are about 30 years old and in need of some TLC. The custom Harbeth 8Ē woofers were cleaned, professionally serviced, and now sound quite nice. After much research and a phone call to Madisound, the original Audax HD12x9 D25 tweeters were replaced with Audax TW025A0s, using a custom machined rectangular faceplate so that the new tweeters would be correctly integrated to the existing speaker woodwork.

Here is my dilemma: While the TW025A0 is not bad and a reasonable replacement, among other differences, the fs is about 300 Hz higher than the original HD12x9 D25 (1090 Hz and 800 Hz respectively). Given the low-ish 2 kHz crossover point of the speaker, I thought it might be worthwhile to consider other HF drivers that might integrate better. Also, I feel that I would appreciate a tweeter that was a bit more sonically refined.

As I see it, here are my alternatives:

1. Find someone who can refurbish the HD12x9 D25 tweeters. I have not seen anyone who is doing this, but maybe someone knows of a place that offers this service? The originals are not completely dead, but the output and frequency response of the pair are diminished.

2. Find a high quality tweeter with similar Impedance (8 Ohms), sensitivity (89 dB@1W, 1m), and fs (800 +/- 120 Hz) as HD12x9 D25 and use that. It would also need to have a removable faceplate so that I can machine a new faceplate to fit the original routing. The Morel CAT298 (or possibly the CAT308 or 408) comes to mind, but I am open to suggestions.

3. Replace the tweeter and design a new crossover to better integrate it with the woofer. The problem here is that I donít have any specifications for the custom Harbeth woofer. Maybe someone has the T/S parameters or knows of a service that I can send woofer to get parameters. I have considered purchasing one of the plethora of programs available to do this, but I am not sure it makes much sense to go that route for a one-off project. Although, if that is my best option, I will certainly consider it.

4. Replace all drivers and crossover and just use the cabinets as an open canvas.

5. Do nothing.

So, any suggestions as to what my next step should be? I am not interested so much in restoring the speakers to their original shape and I am in making them sound and work the best they can (without investing an inordinate amount, of course.) The existing crossover uses no electrolytics, and the high frequency end contains a tapped autoformer with one-decibel steps so that the HF unit level can be adjusted. I have not exploited this functionality, but maybe the existing crossover offers enough flexibility to warrant continued use?

Anyway, I look forward to any and all suggestions and thanks for reading.

Hello,
I've had a pair of these speakers for 30 years. They are pretty well retired from use now. Might have a spare pair of original tweeters for you. BUT, I'm not sure where they might be, or if they've been thrown out. They may be in storage at my brother's house, where I will be visiting around Thanksgiving. They may or may not be any better than what you now have.

As for getting T/S parameters of the drivers, well, have you written Alan Shaw? He most likely will just tell you the speakers are too old to be any good, and just try to sell you a new pair of speakers. I used to write to him about replacement parts for the HLII's, and that was his standard answer.

The problem with changing tweeters is the crossover would likely need some adjustment, and that might well be near impossible. The old Harbeth's were made with thin wall cabinets (and I think they still are) and incorporated this into the crossover design.
Of course you could just use the replacement tweeter and just live with it.

Just curious, do you have a pair of Made in England HLII's or a pair that were made in Seattle for a short time in 1979/80 by Eakin Audio?
These were the real cats meow in 1980.

Steve
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Old 11th October 2009, 11:57 PM   #3
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Quote:
Originally Posted by oranda View Post

HD12x9 D25
I may well have a pair of those...

dave
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Old 13th October 2009, 06:55 AM   #4
oranda is offline oranda  United States
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Hi Steve,

Thanks for the reply. Very good point about the cabinet design and its incorporation into the crossover. Yes, they still use a similar design in their current speakers. So I guess that pretty much eliminates any ideas I had about updating the crossover or getting full integration of a new tweeter. That said, do you have any thoughts about replacing the tweeter with one that best matches the fs and sensitivity of the original vs. going the more common route of using the modern Audax TW025A0?

As for contacting Alan Shaw, I have tried once sometime ago with no reply. I peruse the Harbeth forum from time to time, and while he is usually quite good at responding to questions about new models, he's told owners of older Harbeth models that the specifications of legacy Harbeth speakers are no longer available. I found this exceedingly hard to believe, so I searched for cached Harbeth.co.uk pages and did manage to find an old press release as well as a page with some specs for the original HLs. Clearly, there is no effort being made to support the old product lines, which would really bother me if I owned a new pair of mega-$ Harbeths.

The speakers I have say "Assembled in the USA" on them. I always found that a bit puzzling, but less so now that you have pointed out that some MkIIs were made in Seattle. Any significance to having the speakers made in the US?

If you don't mind, since you own these, what are your overall opinions? Personally, I keep them around because I like how natural they sound in the mids. Maybe you've found something better in this respect? Also, they work well with solid state amps, probably due in part to the use of an auto-former on the high frequency side of the crossover.

Thanks for the input! If you happen to find the tweeters, PM me.

-Tony

Quote:
Originally Posted by stvnharr View Post
Hello,
I've had a pair of these speakers for 30 years. They are pretty well retired from use now. Might have a spare pair of original tweeters for you. BUT, I'm not sure where they might be, or if they've been thrown out. They may be in storage at my brother's house, where I will be visiting around Thanksgiving. They may or may not be any better than what you now have.

As for getting T/S parameters of the drivers, well, have you written Alan Shaw? He most likely will just tell you the speakers are too old to be any good, and just try to sell you a new pair of speakers. I used to write to him about replacement parts for the HLII's, and that was his standard answer.

The problem with changing tweeters is the crossover would likely need some adjustment, and that might well be near impossible. The old Harbeth's were made with thin wall cabinets (and I think they still are) and incorporated this into the crossover design.
Of course you could just use the replacement tweeter and just live with it.

Just curious, do you have a pair of Made in England HLII's or a pair that were made in Seattle for a short time in 1979/80 by Eakin Audio?
These were the real cats meow in 1980.

Steve
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Old 13th October 2009, 06:59 AM   #5
oranda is offline oranda  United States
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Quote:
Originally Posted by planet10 View Post
I may well have a pair of those...

dave
Hi Dave, would you mind sending me a PM with the details on these? Many thanks for the reply!

-Tony
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