Posted by Roy on January 29, 2005 at 12:33:29:
When I first installed my A3 speakers I had problems getting the frequency response correct and my room was partly responsible for this. The speaker back plate offers many options to fine tune the response but whatever setting I tried I could not fill a dip in the response around the 200Hz region. You will notice that in the tweaks section on SLOG there is an article by Kevin Covi that discusses changes he made to his back plates because he also felt the need to tune his speaker response in this region. Kevin’s crossover changes helped a little to fill the dip in the response that I had but it still wasn't enough so I then contacted Sound Lab and asked Roger West if he could make suggestions that would help solve the dip in response that I had. Roger suggested that I try lowering the crossover point between the Toroid and the Large E core base transformer even more than I had done with Kevin’s tweaks, there were times when I was using capacitors summing more that 100uF in the crossover. This saga went on for months and months but I was unable to get the perfect response from my speakers so I contacted Roger West again to see if he could offer another solution, this was when the breakthrough came. Roger passed on information to me about the mixer and suggested a directional change that I could make to solve the dip in response I had. I can make no claim for this modification, we have Roger West to thank for this.
A few weeks later I installed the new mixers, I then fine tuned my crossover and midrange inductor to give the flattest in room response. I was gob smacked when I played some music, what a change this made to the sound in this most vital part of the frequency spectrum. The sound was truly lifelike and natural and as you would expect, vocals are simply superb.
At this point I contacted Kevin, he had done a lot of work previously on the crossover and I knew he would be interested in this modification. Kevin has the speaker Computer Spice Modeled and he is able to predict changes very accurately in the response when component changes are made. Never the less we tested combinations of components for many months, it was a monumental task. Over this period I took an extraordinary quantity of measurements to produce impedance and response curves, I would happily never do that again. On the subject of impedance the lowest impedance at the crossover region is just over 4ohms.
I have included a link that shows the speaker response I took out doors on a quiet evening before and after the mixer change.
Also included in the picture are comparisons I took of midrange inductors between 1mH and 10mH. As you can see a value between 1mH and 2mH works very well.
Thanks to Kevin’s Computer Spice Modeling, refinement of the crossover values was easy for him and he came up with the following optimal values.
Shunt resistor 8 ohms
Crossover capacitor 36uF
Midrange inductor 1.5mH
Lew Markoff also has the new mixer and crossover values installed. Lew has M1 speakers and Kevin and I have A3’s. The changes work very well for all of us even though our rooms and speakers are different. I am delighted to be able to pass our findings on to everyone, we are all blown away by the improvement that these changes have made and I hope that other Sound Lab owners will be as impressed as we are.
I passed on the mixer and crossover details to Sound Lab and Roger West has offered to produce the new mixer as a factory replaceable item, so you will need to return your back plates to Sound Lab so that they can swap over your mixers if you want to try this new modification.
Sound Lab are also willing to change all the crossover components over for you at the same time as the mixer.
If you wish to supply higher quality components for the crossover that’s fine but the size of them must not be so large that they cannot be fitted to the back plate in a safe manner.
DISCLAIMER - SAFETY FIRST
The secondary side of the audio transformers where the mixer is located have very high voltages and an electric shock from this part of the back plate assembly could be FATAL. Only qualified personnel should carry out this work. Do not investigate this part of your back plates, there is nothing to see anyway as the mixer is potted with epoxy resin and you may damage the wiring or connections in this area by doing so. Simply send your backplate to Sound Lab and they will do the work for you.
DO NOT ATTEMPT THIS WORK YOURSELF.
THIS IS STRICTLY A FACTORY REPLACEABLE ITEM.
ONLY QUALIFIED PERSONNEL SHOULD DO THIS WORK.
Please don’t ask me to disclose any information about the mixer, the parts are not available off the shelf, they have to be specially manufactured.
I am sorry for not coming out with this modification earlier but safety in implementing it has been of paramount importance, hence the delay.
Roy
