Quad ESL Cap mods

Hi, Johnsonwu. My Quad ESL 988 speakers measure around 2.8 ohms across the speaker terminals. The Quad ESL 988, 989 and the Quad ESL 2805, 2905 use the same circuit. If your resistance measures 0 and then goes to infinity, it probably is just seeing the capacitor and you have cooked (read open) the resistor that parallels the 220uF input capacitor. In either case they should not measure 0, this would suggest that your other resistor may be shorted. The protection circuitry measures the noise radiation that occurs with the onset of ionization. This triggers the triac clamping circuit to prevent arcing. The circuit operates by limiting the input, and when that fails, by short-circuiting the input. When the triac T1 shorts the amplifier see's just the 1.5 ohm resistor in the input filter. This leads me to believe that at least in one speaker the resistor is now open (resistance goes from 0 to infinity). The R15 input resistor on the Quad schematic is 1.5 ohms 5w, so it probably cannot handle a sustained overload. My understanding is that the combination of 220uF capacitor and 1.5 ohm resistor is a form of input filter, most of the mid and high frequencies pass through the capacitor, as the frequency goes lower it attenuates the signal and more voltage (LF signal) is passing through the resistor. If the resistor is open you would probably have no base. My other concern is that you may have damaged the triac if you are reading 0 ohms in the other speaker (it is not seeing the resistance of the transformers and 3.3 ohm resistors).

As far as capacitors, I have used Solen Caps in the past, but they are just OK. I have used the Mundorf's and also Obbligato films in my speaker crossovers and have been very happy with them. Obbligato's are inexpensive, but do not fit in the Quad. The 220uF Solen cap would cost about $60 and the 3x68uf + 15uf Mundorf MKP is about $100, with either one I would bypass with a high quality cap. So I do not think there is a big savings going with the Solen (not sure it would fit). Mundorf MKP series also has a 250V version of these caps that is a little less expensive (and smaller) than the 400V series, but were talking maybe 10% less cost. I tend to like the Mundorf Silver Foil Oil for a crossover bypasses, as the best bang for the buck without going into the stratosphere (V-Cap or Duelund). I also have heard that for crossovers caps, a smaller bank of equal sized capacitors sound better than one large capacitor. Ideally serial crossover capacitors should be 10uF or smaller, but the 3x68uF is the smallest I could go inside the Quad base. Bypass caps should be 1-2% of the total value. This is important if the capacitor is in series, if it is in parallel (i.e. across the - to +, or shunt) then size or number doesn't matter, use a single cap.

Regarding the resistors in speakers, most of the information I came by recommended the Duelund as the best sounding and then the Caddock MP820 / MP821 series. The stock Quad resistors look like they cost 5 cents. By increasing the wattage of the stock resistors, you would have a larger overload margin. The Duelund 1.5 ohm across the input capacitor is rated at 10 watts (stock is 5 watt and the Duelund can take much higher short term voltage because it is a chunk of carbon not wire), The Caddock resistors are films and are rated at 20 watts with a proper heat sink vs the stock of 7 watts. Some people use Mills resistors, but I think in this application they are a step down in sound reproduction.

I got most of my parts from Part ConneXion when they were running there 20% sale. If you are not adventurous, I would suggest that you consider sending your speakers to Kent McCollum at Electrostatic Solutions. I had spoken to Kent about this mod for the 988/989 series speakers and the 2805/2905. I believe he was going to fabricate an upgrade circuit board/kit with Mundorf capacitors. He has quite a bit of history on the different web forums, is very knowledgeable about Quad speakers and does great work.

Thanks Mksj2 a lot for the detailed and insightful response.

Guys - If this input cap and resistor are simply acting as a high pass filter then why deal with the problem at the speaker end? Wouldn't it be better at the line level stage? This way the power amps wouldn't waste valuable resources trying to reproduce the most energy demanding information only for it to be disposed of? Of course to do it digitally before the DAC of any system would be optimum. Another consideration is that many of the valve amps you guys are using will already use coupling caps between stages to prevent DC passing - why not chage the value here and do away with the second one? Far more efficient and one less cap in any audio chain has to be a winning solution....

Sugarspice, I think that protection at the speaker is safer.
Many well-regarded amps thump when powered up or down, not to speak of the absent-minded user that turns the amp on, then the preamp.

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.