Quad ESL Cap mods

The Mundorf is rated at 250V, which should be more than enough.

HI
I have Auricaps and Mundorf PP caps in my tube amps (not the same application, I know). I prefer Mundorf Supreme to Auricaps IMS.
Mundorf caps are metalized polypropilene.
Munforf silver and oil is a significant step forward in transparency as compared to M. Supreme or M. tube cao, but it is only economically feasible as a bypass cap.
Many high-end speaker manufacturers are switching to Mundorf caps for their top model's crossovers: BW 800D, Magico, ZU Definition and some others I forget.
Any cap smaller than 220uF will attenuate deep bass response.

Regarding the paraleel resistor, find out its value and power rating (usually written in the transistor body).
Mills MR12 is widely well regarded as a neutral, non inductive resistor up to 12 Watts per resistor. You can use several in paralell to reach the correct ohmic value and power handling. E. G. ten 80 ohms 12W resistors in paralell will add up to 8 ohms and 120 watts.
Dale makes aluminum-clad non-inductive resistors in 20W and 50W ratings.
DO NOT install the resistor flush with the cap(s), because it will get very hot on music peaks and may damage the caps.
Any PP or MKP cap will certainly sound much better than an electrolytic cap, but if you can avoid the cheapest ones, it will be a wise investment, given the Quad ESL's transparency and midrange naturalness.
Good luck

I hope this helps

I have upgraded the capacitor/resistor with 9-22uF Mudorf Supreme capacitors and a 1.5 ohm Duelund resistor on each 2905 speaker.

Here are my impressions at 100 hours:

Much more coherent, space around instruments sounds more "right". Timbres and brustling sounds from musical instruments are less smeared/obscured. For instance I can now hear Stan Getz taking breaths between notes on "Getz/Gilberto". Glare is reduced to the extent that some recordings which were bordering on unlistenable are back on my playlist. The 60 Hz bump in the low end has been tamed. My big concern that the highs would get edgy or sterile did not happen. Overall changes to the frequency response I would characterize as a slightly less warm/heavy balance which I prefer. The midbass is less round and smeared so a lot of new information is heard in this range. Textures in the mid to upper bass are easily discerned and transients on drums and piano are much more realistic. The low frequency rolloff seems to be about the same but I will probably still try an additional 22uF to evaluate the effects. I also going to try a 4.7uF Mundorf silver/oil on the capacitor bank and will report back on bypassing with other capacitors. Possible suggestions would be appreciated. I will probably also try a 1.35 ohm resistor.

I did make one disturbing discovery during the upgrade. One stock capacitor tested at 216uF and the other was 255uF. This would partly explain why there was such a big improvement in imaging and bass resolution. Both resistors measured 1.6 Ohms. Concerning the cost, I purchased the caps and resistor with a nice 20 percent discount that Partsconnexion was offering, so $1300 was a lot easier to swallow. Would I do it again? At this point, it's a no brainer especially considering how badly matched the stock caps were. I'd say, on the major characteristics I hoped to improve, all were easily met with the exception of the leaner bass which I would rate a draw although its seems to be improving every day with further break-in.

I appreciate all the information from other agon members. I will issue an update after the break-in is complete.

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.

Hi Mksj
By all means remove the original resistors and capacitors if you perform this mod.
Also, make sure you buy the right wattage resistor. I suspect that a lot of power flows through the resistor at high volume level.