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.

Hi, I have the Quad 988 speakers and I am also considering changing out the input capacitor and resistor. I find the upper extension to be somewhat lacking in comparison to my Magnepans, which was a real surprise. I am using Cary 805 Anniversary Amps. I have tried super tweeters, but they never worked well. I did pull the bottom panel on the 988's and measured the input resistor at 1.6 ohms (it has a rating of 1R5 at 5W), I was unable to measure the capacitor in the circuit because of the bypass resistor. The bipolar electrolytic is rated under 100v, so I would assume a film capacitor with a 250v rating should do fine. I was planning on using a Duelund 1R5 resistor, but wanted to get some feedback on using the Obbligato film capacitors (4x50uf) vs the Mudorf MKP (2x100uf), and then bypass with a 3.3 or 4.7uF Mundorf silver/oil. I have used the Obbligato and Mundorf silver/oil in my Magnepans with great success. I was wondering where you put the new capacitors and if you removed the old resistor/capacitor from the circuit board, or just bypassed them? Any follow-ups would be on your listening experience would be appreciated.

Note: Working on Quad speakers is inherently very dangerous due to their high voltages, although you can do these mods at your own risk (and could result in death), I recommend that you send your speakers to someone that has experience in working with these speakers. This way you get to enjoy the music.

It took me a awhile to figure out a capacitor configuration to replace the stock 220uF 50V electrolytic, and the low voltage signal resistors. I have used Mundorf caps, and have been very pleased with their price/performance ratio. The MCap dimensions given at Mundorf's web site are incorrect as they listed a length of 66 mm for their 56uF-100uF caps, the 100uF are 72 mm. The stack height is an issue (66 mm max length) and the diameters are limiting factors of what will fit in the Quad base. Most other high end caps just won't fit and I am not sure of the benefit with such a large capacitor value. The MCap 100uF would not fit, unless you use the 250V version, per Mundorf, they say their 400V version sounds better then the 250V. I finally settled on the MCap (400V) 3 x 68uF + 15uF and a MCap Silver Oil 2.7uF, this configuration just fits stacked vertically in front of the step up transformer on the side opposite the power transformer. This also keeps all the signal wiring away from the AC. I used some 4 mm acrylic sheet, drilled/milled and glued the capacitors on one side, I hardwired everything with Cardas wire and it bolts to two of the studs that hold the upper electrostatic panel. Thin circuit boards may have been easier. With the MCap Silver Oil cap., one could go up to 3.9uF given it is the same dimension as the 2.2 or 2.7, I just went for the one that was available. MCap's have very tight tolerances, so both banks of caps measured at 221uF, they had unmeasurable ESR & DA. I replaced the stock 3.3 wire wound resistors with Caddock MP821 3.3 ohm resistors on the main board, they fit perfectly in the stock holes when you bolt two to the same heat sink (back to back and I needed to offset the heat sink I used). They are rated at 20W each as opposed to the stock 7W. I attached them first to the modified heat sink and then mounted them to the circuit board. I used a Duelund 1.5 ohm 10W resistor at the input, seems to smooth out the sound and costs only slightly more than the Caddock. I replaced the stock 1.5uF cap 250V cap. with a 400v 1.5uF Mcap bypassed with a ERO1837 10nF. Removed the stock wiring and pulled the PTC (FS2) . I left the stock electrolytic/1.5 ohm resister on the board (no really reason to pull them with the PTC/input wiring removed). Replaced the brass binding posts with some decent Cardas CCBP S posts. I wired the Duelund resistor (from the + binding post) and capacitor bank directly to the IN1B terminal (they are long). The negative binding post was connected directly to the OUT1 terminal (moved the bottom plate ground lead to the chassis ground circuit). This made for very short direct signal runs, O resistance in the runs and was easier to wire.

In the stock configuration, I did find some grounding issues (increased resistance between the chassis/HV transformer grounds and the circuit board ground/- terminal), this was because ground connections where screwed together. So I soldered the black step up transformer ground taps to the ground tab screwed in at the power transformer and continued the same wire and soldered it directly to the board OUT2 ground plane with 14g silver Teflon wire. The resistance in the ground circuit dropped to 0 ohms. Before this the step up transformer ground taps were grounded mechanically at the power transformer and the screw was not very tight. As mentioned the ground for the metal base cover was soldered to the ground tab that connects to the step up transformer near the new capacitor bank.

I lined the complete base cavity and cover plate with thin sound deading material, this significantly reduced the cavity resonance. I changed out all the 8 mounting washers that bolt the electrostatic speaker panel to the base with larger/thicker steel washers, and added lock washers to the bolts that did not have a ground tabs attached. This made a significant difference in the speaker flex between the base and the panel. Ideally I should have probably used washers with an even larger area or a small fender washer. The stock washers are no larger than the nut and were dug into the plastic (i.e. loose, especially those without a lock washer).

Fired it all up last night and frankly my wife and I were floored at the difference. I have had the 988's for several years, initially listened to them infrequently and then they were in storage for several years. I never was really satisfied with the sound, great mids, but lacking at the extremes. I replaced the amps, replaced the preamp, tried different wiring and even tried supper tweeters to no avail. Never could figure out what I was missing until now. Well, with the mods they are completely different speakers. Much better base and articulation, vocals sound like vocals, better air at the upper end and improved balance across the music spectrum. Since they only have a few hours on them, I look forward to them getting better as they break in. I am using Cary 805 anniversary amps that are good for 50W, I also have a Mac275 that I want to try. The changes made a significant difference, but I cannot really say the contribution of each change. Great addition for the 988/989 speakers, not sure of the space in the 2805/2905 Quads but the mods bring these speakers to whole new level.

Agree with you, replacement of the Quad ESL speaker 220uF electrolytic capacitor probably accounts for the majority of the improvement. Probably replacing the electrolytic cap/resistor and eliminating the PTC gives the most bang for the buck and the simplest to install. You would not need to pull up the circuit board, just remove the stock wiring and snip out the PTC. I am surprised that even in the latest Quad 2805 and 2905 speakers that the electrolytic is not at a minimum bypassed with a decent film capacitor given their increased price. But then I have had Magnepan speakers through the years, and they are no better in the crossover department. Hardwiring the ground system and using a modified star configuration is easy to do and improves the safety.

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.

Hi Tsushima1, if you need some photos of the quad 988/989 mods/info, I can email photos of the mods I installed. I would leave the stock electrolytic cap/resistor in place and clip out the PTC that connects them on the PC board to the wire going to the step-up transformer, then reroute the speaker input wires. You need to wire in a separate bank of film capacitors/input resistor that are ideally mounted in the base (an not on the PC board).

Let me know how you want the information to be sent or posted, I have provided other forum members with more detailed information. I am not aware that the information can be uploaded to this forum. Newer Quad models after the 988/989 present more of a challenge because of their interior space, and have had more quality build issues.