As a follow up to this Q&A with Garth Murray from Theophany Loudspeakers, I received some additional information about his thoughts on crossover design and component quality.
Witchdoctor – I’ve had some questions on our forums about crossover design and the importance of high quality components and wiring etc. I recall you saying that you’re a fan of a minimalist crossover and using the natural roll-off of the drivers where you can. Has that changed? Would you mind giving me a brief overview of your thoughts on this topic?
Garth Murray – This is a very interesting point and one that we have really done a lot of work on in the last six months. I will give you a run down of what we did. I need to however fill you on the process. Many of our old drivers required no crossovers, they were called full frequency drivers. The problem with this is in two specific areas. The way to get a natural smooth 12db per octave roll of is to have a quite steep inductance curve. This is very successful however does mean that efficiency is sacrificed a complaint that some people have had about our speakers. The only way to change this is to flatten the inductance curve. This however means that there is then a need to filter the signal so that the driver is not getting used outside its intended audio spectrum. This is mostly done with capacitors, inductors and resistors. We felt that there was a need to improve efficiency so we decided experiment.
Firstly we model the crossover based on the drivers we had built. We used LEAP which gave us several options. Firstly you can have 1st order through to 4th order crossovers. The main difference being the steepness of the SPL curve as they drop off. However within each section there are many different options. By this I mean that while you might use a 2nd order crossover which has a 12db per octave drop there are many ways to do it. Butterworth, Linkwitz Riley, Bessel as well as some other changes within each of these options. For each speaker we modeled five or six options and then played with them. Some crossovers gave a flat response but because of phase shift and Transient changes sounded lifeless. We found several we liked that gave good results on the test bed but also provided good listening results which in my opinion is more important.
When we had done this we decided to replicate each of the crossovers to three different standards. In the first we used air core inductors and polypropylene capacitors for the tweeter and with iron core inductors for both woofer stages (both the Psuche and Psallo are three way) as well as ceramic capacitors and some other cheaper options. We also made one with the air core inductors and polypropylene capacitors in the tweeter and mid woofer stage and cheaper caps and inductors in the woofer stage where it is not so critical.
We also made a pair that were entirely air core inductors, polypropylene capacitors and even wire wound resistors. This last being up to twelve times more expensive than the first. When we did our testing we found that the difference between the three was enough to justify building the better one so we went with the most expensive.
As far as components went, we tried all kinds and as usual it came down to the law of diminishing returns. If we used Solen capacitors, there was a very tiny improvement but the cost was significantly more expensive, especially for the woofer. It was so much more expensive that it would have meant the price of our speaker would increase at retail by up to fifty percent.
We did however find some remarkably good capacitors and air core inductors (we have them wound specifically for us) that were at least 90% as good as some of the very expensive ones. We tried about six different varieties before settling on the ones we have used. We are very pleased with their performance, however I am always looking for ways to improve our speakers so will continue to test drivers, crossover components and cabinet designs from all over the world. Because of the numbers we are now producing, we do have a little more freedom than previously. I have found an amazing new world of technology and passion that we have been able to tap into while remaining a niche, New Zealand product.