April 16, 2010
Fixing TDL RTL3 transmission line speakers
Above: On the left are my custom made transmission line speakers, with the TDL RTL3s on the right and a Diva Acoustics stand mount on top.
British speaker manufacturer TDL made a few popular speakers in the 90s that retailed in Australia for around $1100. Many considered them better than the alternatives at the price. Recently I fixed up a pair with blown tweeters (common problem with this speaker), and had a chance to open up the box to see what's inside, run them actively and compare to my own TL speakers and a pair of modest Diva speakers. How do they compare to the other two speakers? Are they really transmission line speakers? Read on!
I'm recently fixed up some TDL RTL3 transmission line speakers with blown tweeters. It's a classic case of pushing speakers too hard with an amp that doesn't have a lot of juice. It's not hard to see what went wrong.
On the binding posts you can see warped plastic suggesting a heat issue. Opening it up I could see that a 10W resistor in the woofer network had heated up and it was sitting up against the plastic part of the binding posts. The solder had come loose - it got hot enough to soften the solder! Yes, this speaker has seen it's share of late night parties cranked up to within an inch of it's life and then some! Apparently it's common for this speaker to suffer this fate. A Google search reveals many others have searched before for a replacement tweeter.
I believe part of the reason is that the tweeter wasn't padded. A 3db pad would have doubled the power handling, given a more natural voicing and possibly saved the tweeter from a thermal meltdown. Read more about Lpads for tweeters >
Both tweeters were blown. On the outside they looked fine, but the voice coil wire has melted. Finding a replacement would have been an easy fix, and I thought I'd have a chance considering it's a Vifa. Sadly no success - I could only find another Vifa 3/4" D19. This isn't the same as the original tweeter, which is a Vifa D20TD. The two are very similar, but the original is a silk dome while the D19 is a poly dome and at the time of writing, you can still buy them even though it's a discontinued tweeter.
The Diva stand mounts shown in the photo were purchased second hand as a cheap way of sourcing the tweeter, which is the Vifa D19.
In the end, I managed to buy the tweeter from Brian Maddern at Decibel hifi. Brian was very helpful and makes regular purchases from Parts Express. If you are buying just a pair of cheap tweeters like this, PE have a minimum order. Brian was happy to add the tweeter to his order and ship it to me for a great price. Brian is a good source for things that aren't easy to find. In the past I also purchased some cable and "cable pants" that I couldn't find elsewhere locally. What are cable pants? They are a moulded rubber sock that allows you to make neat speaker cables when using multi core cable.
Cable pants and speaker cables at Decibel >
I managed to get this in another speaker going cheaper than getting a pair of tweeters - Diva speakers shown bunyip listed for a friend. These speakers were surprisingly good and very listenable. The vifa mids had balsa wood placed on the cones very much like a cheaper version of the slitted Scan Speak Revelator midwoofers.
Part 1 - replacing the tweeters with second hand units
I modified the baffle cutout for the new tweeter, inserted it in to the crossover. The new tweeter has a slightly bigger magnet, so the inside cutout needs to be filed back.
Unfortunately, something was clearly wrong. The sound was muffled in one speaker and I discovered that one tweeter was not working well. This can be seen in a quick measurement:
Frequency response: Blue - full range, black - tweeter only, red - faulty tweeter only
Due to less than ideal measurement conditions, only the treble response data is valid. The black line and red lines show the two tweeters with identical input. As you can see, the red shows a loss in efficiency. The tweeter dome had been pushed in, and while I was able to get it back out with the vacuum cleaner, it's no longer useful. The level is 12 db less above the crossover point! This is where I went to Brian at Decibel to source a new tweeter.
Part 2 - new tweeter
When I installed the new tweeter, everything changed. I evaluated by ear with familiar reference tracks and felt that the sound was now as it should be. The owner was very happy with the result. I did consider and discuss the possiblity of reworking the crossover, but in the end we both decided that it wasn't necessary. I simply added a 3 db L-pad.
A word of caution
Generally, it's not possible to simply put in a new tweeter without changing the crossover. In this case, it worked out fine. As a general rule I suggest not to try it unless you are prepared to either re-design the crossover or risk getting a bad result and throwing away the driver. At the very least, compare the data sheets.
Is it a true TL speaker?
The interior of the box turned out to be a surprise. Typically a TL speaker would have a long folded tube so that the entire box is actually a tapered tube. In this case there was simply a slot port that has a taper. It seems to tune the speaker in the midbass range, but it is certainly not a transmission line. Instead it's a vented box with a slot port that hasn been filled with dacron. This gives the bass a different quality and the stuffing makes the midrange more like a true TL - neutral and fairly open.
Comparing the three speakers
How do the TDL speakers sound? Quite good for their price range. I believe the treble without the pad is a bit tizzy, but with a simple pad this improves. I found the sound was very similar to my TL speakers and at times you could mistake one for the other. The midbass driver is polycone and has a similar sonic signature. Due to the stuffing, it achieves a midrange not normally found at this price, however, compared to my TLs there is a certain graininess that draws some attention and stands in the way of the illusion of performers in the room.
In the bass, my TLs are clearly in another class. This is where it becomes clear that the TDLs are sadly not what they were marketed to be - a true TL speaker. We also can't expect too much of the cheaper drivers used, which are part of what holds it back. There is about an octave less of extension. While mine get down to around 23 Hz in room, these roll off in the midbass around 50 Hz. I have some room mode peaks in that range, so when these speakers are run without any bass EQ they really do sound quite boomy. If they are combined with high quality subwoofers that are well integrated, then they do get quite close to the sound of mine and there is only the slight midrange grain issue. That's pretty good performance when you consider that the TLs are equivalent to AU $5k speakers.
The Diva speakers need the subs to compare, since they have a sealed box rolling off at 80 Hz. They are quite comparable to the TDLs and their RRP is about half. Being a sealed box speaker filled with dacron they also have a nice neutral midrange. The midbass driver is at the same approximate price and quality, however it's been modified with balsa reinforcement on the cone. It's a similar technique to Scan Speak Revelator drivers which are high end drivers costing a great deal more. If I had to pick a winner I'd say the Divas have a slight edge in the midrange and unlike the TDLs I could not pick any specific fault. They aren't equal to my TLs either, but they come closer than you might guess.
It could be that in a more rigorous comparison the difference would become more clear, but I compared in a fairly casual way to satisfy my curiosity. Both of these speakers challenged my ideas about how well budget speakers can perform. The only problem is that they both require decent subwoofers - neither of them have finesse, fidelity and capability in the bass. If you have read the rest of my blog you will realise that I don't think much of small speakers as bass drivers.
Do you want a real TL speaker?
Look elsewhere. There is nothing special about the bass of the TDLs. I've built a much better TL and they will suit the purists who prefer 2 channel without subs. If you want a real TL then DIY is the best bet. Sadly you can't always trust the marketing guys. If you are in Australia, consider a TL6 kit at The Loudspeaker kit (yes, that is their actual name). TL6 kit > That is one way around the driver availability issue.
Posted by Paul Spencer