reparing my mid-fi speakers

Long before the arrival of the wide spread use of internet I invested in some good hi-fi equipment. I didn't have paid TV and I only watch PBS in those days so listening to music in front of the hi-fi was a frequent home entertainment. Among these durable goods is a pair of Dohlquist DQ20i loudspeakers. It was quite well regarded in the era of high fidelity audio but often dismissed as mid-fi by the well heeled enthusiasts.
Coming home from my Asia trip I noticed the left channel was making terrible scratching sound.

The Sony receiver and the speakers are all very old - they meet my notion of durable goods. I have to determine which is the cause. I just swapped the speaker wires to the two speakers and sure enough the scratching sound stays with the left speaker. It is the left speaker and not a problem with the receiver. With the help of a flash light I could see the deteriorated foam surround of the woofer behind the protective grill.

removed the front grill and the woofer

 

 nearly all the surround has turned to dust

More than 10 years ago one of the woofer failed like this and I had the surround replaced by a local speaker shop. This time around I decide to try a pair of new 12" woofers instead of rebuilding the original woofer.
The original woofer is the most peculiar as the designer chose to use a 12" frame with a 10" cone. Apparently it was done for sonic clarity reason. The Dolhquist DQ20 earns high regard as a very natural sounding speaker. While it cannot delivery deep or exaggerated heavy bass this is not a crippling weakness for people looking for accurate music reproduction. It's biggest strength is reproduction of excellent mid range - or better yet from mid bass to mid high with excellent clarity as long as you don't try to play it too loud.
the woofer uses very humble treated paper cone with foam surround

removing the old woofer requires a few tools as there is flexible caulking around the speaker basket to prevent air leak (of the enclosed un-ported acoustic chamber design).

new 12" woofer with polypropylene cone and rubber surround on the left

the original woofer's very budget sensitive construction

I am amazed how well it sound despite the cheap look; however I often notice the muddier sound when the air humidity is high due to the paper cone construction

 2" voice coil vs 1.25" of the old

it is important to determine if the new speaker has the same phase as the old - I use a 1.5V battery to find out and they are consistent; both cones thrust forward when the positive pole of the battery is connected to the positive terminal.

my local speaker shop recommend this new woofer over a more expensive one; it has a freer movement so it can reproduce lower bass

The mounting holes of the new woofer do not line up with the old on the cabinet so I have to drill new holes 45 degrees from the old ones. That required some careful work to center the woofer to the opening of the cabinet.
I have long consider discarding the speaker grill as they make the speaker too over bearing in a modest space. The main barrier to doing this is I have to be willing to forego the ability to sell them. Also for some the look of a naked speaker may considered as unacceptable eye sore. Not me as I like the all business look that reveals the unusual "cabinet" design.
none of the 3 drivers shares the enclosure

you can see the appearance with and without the speaker grill - I feel with the grill it look too overbearing

As it turned out the tweeter on the right side speaker has failed - it is amazing how bad the speaker sounds when a part of sonic spectrum is missing
it is made by very well respected Scanspeak - a Danish high end tweeter manufacturer; it is a 50 year old design and the equivalent part is still available for around $85 USD

 it is the most unusual looking doomed tweeter

while waiting for the replacement to arrive I put in a tweeter I saved It is too big to fit the cavity as a temporary stand-in until the new replacement arrives

the blankets on the floor is to experiment "what if" I dampen the relative hard acoustics of the tiled floor room

 So how does the new woofers sound? Before replacing both woofer I left one original one on one side to compare. I did a series of auditioning with different music genres. Some of the best recording are from long ago with analog masters. It is amazing how good some recording done over 50 years ago still sound today when one listen to them in a good listening environment.
I was very surprised despite the use of 10" cone the original woofer is hardly pale in bass capability compared to the 12" new driver with polypropylene cone and rubber surround. I am also pleased that the new woofer sound very comparable to the old. It has been 2 weeks now that I listened to the new woofers extensively and so far I found no flaw.

I was expecting the new 12" woofers to improve the low bass (below circa 50 Hz) over the original 10". If there is an improvement it is hardly noticeable. This may due to the stiffer rubber surrounds on the 12" woofers. The local speaker show owner told me the stiffness will lessen over time and with use. One thing to keep in mind too is the rather airtight bass cabinet, especially with the caulking around the speaker opening. This is what they called air-suspension design. It is analogous to a automobile shock absorber with a well-damped setting which limits the travel - the displacements of the speaker cone in this case.

I wanted to explore "what if" scenario with the addition of a subwoofer. To this I use the subwoofer from my AV setup. It has a 12" driver which is bigger than most people have.
the subwoofer (left most) added

Since the Dolhquists already have a rather larger woofer I knew I have to careful with the set up of the subwoofer:
1) phase alignment - this means the placement of the subwoofer as well as the phase setting
2) crossover frequency of the subwoofer - I set it to only cut in below circa 50 Hz
3) amplitude balance - it is all too easy for one to exaggerate the lower registers by cranking up the gain
Again I listened to the new setup with a good selection of different music genres. There was no noticeable improvement I was looking for. Sure, I could increase the gain but in doing so it threw the spectral balance off and the music loses euphony. With more auditioning it is clear even with sensible gain setting the addition of the subwoofer actually reduces the bass clarity. The sound is muddier and the transients is less crisp. It is not a surprise as some destructive as well as constructive phase cancellation are expect to occur. From this experiment I decide against adding a subwoofer, at least for the time being. I think a subwoofer that may work is one with a larger driver - like a 15" for example. I am quite happy with the Holhquist even after 20 years.

In the era now we often listen to music in very compromised environments (in automobile and with headphones) it is refreshing to sit in front of a good home audio once in a while. What's wrong with mobile audio? Poor dynamic range and very compromised sound stage. What's wrong with headphones? Bastardized sound stage and often too poor dynamic range in noisy ambient.

Update Apr 6:
I am writing this update while listening to and recording Metropolitan Opera's das rheingold on the FM radio, postponing my grocery shopping.

The replacement tweeter shipment was delayed because for inexplicable reason. I call them and only then that they realize the shipment did not go out. Two weeks later I finally received it. As these speakers are so old the replacement tweeter do not have the same part number. It is also very likely that the OEM speaker part number was set up for Dohlquist anyway. According to the manufacturer it should be a equivalent part...

The new tweeter does not come with the felt cover on the front. I just peel off the one from the old and transplanted to the new tweeter. I did not have my hope up on doing this, thinking either the felt fabric would fall apart, or the adhesive would be so dried up making the removal impossible. To my amazement the adhesive hardly deteriorated. The felt fabric peels off the old tweeter easily and the mastic is as good as new after almost 2 decades!
old tweeter with felt cover on left; new tweeter without felt cover on right

 installing it is just a breeze; just pop it in and secure with 3 screws and solder the leads

 as good as new

After doing a bit of research on the web I was at peace with discarding the speaker grill. There is not much market for any big speakers these days so resale value is not a consideration. Dismentling the grill assembly for disposal took quite a bit of work. In the process I saw how much of the manufacturing cost of the speakers went into the front grill. The Dohlquist DQ20 speaker was no cheap mass produced consumer product in it's days.

The speakers sound just fine without the aid of a subwoofer. In fact the subwoofer ends up interfering with the bass they produce.