Posted by Roy on October 28, 2003 at 13:51:35:
Steven has raised the issue below of correct bias adjustment. Here is my take on bias adjustment and how this whole issue and its associated problems may be improved upon.
Setting the bias is good fun for the tweekers amongst us. I am pretty sure though that most users would prefer to set it once and come back to check it perhaps every 1-6 months depending on your climatic conditions. I have found the stability of the bias once set to be very good whether adjusting for the older insulation or the new insulation that requires a higher bias potential. If the bias does need frequent adjustment to account for changing humidity then play safe and set it a little lower, the difference in performance isn’t so great.
I personally think that the best approach is to reduce the bias between listening sessions. There is no point in setting the bias to give the maximum efficiency and then to only listen for 10% of the time. If I leave the panel on a low bias setting overnight and I turn it up to a correct level just prior to listening the speaker is ready to play almost immediately. The coating on a SL membrane is very, very conductive, the complete opposite to say, a Quad panel. The Soundlab membrane charges up fast, especially if it has been set at a low idle level of bias previously. I know that I am not the only user who does this now.
Brands such as Audiostatic have automatic bias reduction. This means that if the drive to the speaker is not present for say 5 minutes the bias is automatically reduced (that brand may shut off the bias completely). The benefits are two fold, less dust build up/attraction and a reduction in the serious problem of insulation breakdown.
I did ask Roger West about automatic bias reduction last year, he told me that this had been considered already and may be incorporated in speakers some time in the future. When this is likely to happen is anybody’s guess. Would a dealer be good enough to find out if this is still planned for and if it is, what the likely time scale for its implementation will be.
If this is not likely to happen in the near future, say 6-12 months then I would like to incorporate it now. Electronic design skills are not my strongpoint so would one of you like to design a circuit for this. One important design consideration should be that it does not form part of crossover at all, It needs to SENCE the presence of drive from the amp. There should be no direct electrical connection other than its ability to switch two potentiometers, One for RUN and one for IDLE.
The idle pot could be changed to a fixed value resistor later.
Roy