Quad ESL Cap mods

Hi Sugarspice, the RC does not limit the frequency range to the speaker, a characteristic of a dipole loudspeaker is that below the cancellation frequency, determined by the size of the speaker, the output will decrease with 6dB/octave. The series resistor of 1.5 Ohm compensates this effect that by increasing the Q factor. In parallel is a 220uF capacitor for bypassing the mid/high frequencies. So it would not warrant an external crossover/filter unless you were using a subwoofer. Removing the RC in the speaker would present the amp with a dead short if the protection circuit shut down the speaker. So I believe this is best dealt with at the speaker level, especially given the significant improvement with the changing out the parts as noted.

It's been almost a year since I applied a duct-tape fix to the 989 by using Nichicon bipolars (I thought they'd be better than the orig. Bendix) bypassed with a film cap.

The fix only marginally improved the threshold before protection kicks in and the sound was best described as phasey.

Finally I got around to getting a big Solen (for convenience in mounting and cost) and rewired the input filter with 12 gauge wires, replaced the 3 Ohm resistors with heatsinked Caddocks, and replaced that nasty 1.5uF cap with a Solen (again slightly compromising quality for the sake of space). I also bypassed the FS2 which doesnt make any sense unless I m using monster amps. I did keep the triac clamp circuit in place for safety and protection of the panels.

The result is stunning, I can play it at dynamic-speaker volumes with absolutely no distortion or stress, and the sound is much more transparent and coherent.

If I ever get around to making the mounting I might eventually go with paralleled Mundorf or VALAB 47uF caps but for now it's a huge step above what it was ever capable of.

Thanks again to all contributors esp mksj2.

Sorry to be tardy - I just found this thread.

I started modding my 2805 and 2905 nearly two years ago. First I messed about with caps and resistors - metallized poly all sounded the same; the big difference came with using more film and foil. When I got up to 70uF film and foil plus 150uF metallized, the sound was dramatically improved. Duelund resistors were better than Mills WW.

The big breakthrough came with changing the step-up transformers to the Vanderveen design manufactured by Plitron. I used the 75:1. To my ears, the best sound came with using NO signal shaping other than an input resistor (as Vanderveen suggests). Best resistor in my setup was obtained by using nichrome wire as speaker cables - I have a monoblock behind each speaker, so short wires are feasible. Each of the two wires is a bit more than one ohm.

Best improvement of all was making my own low power monoblocks with low voltage rails. With this setup, it is physically impossible for the amps to punch a harmful spike into the speakers - so I could safely eliminate all of the protection circuitry. Sound is quite magical.

I hope that you are experienced with ELSs, because they can kill the unwary in several ways. Please be careful. All the best!

Hi Terry9, did you end up doubling up the Duelund resistors to match the orig. power rating? My experience with Duelunds are outside of Quad mods but were you able to fine 20W+ Duelunds for the job or was your mod only for the input R and C before the split-tranny?

Hello Johnsonwu. I doubled the Duelunds to get the 20W.