Quad ESL63 input cap question

[Le Basseur]

Not belonging in a tube-oriented forum,but,still,a theoretical issue:
A good friend of mine have a pair of Quad ESL63 loudspeakers in great shape except that at the input,the former owner "tweaked" the R15/C25 tandem.The resistor looks OK (non-inductive type) but the cap is something of an inferior quality and my buddy wants it replaced by all means.
Please take a look at the Quad ESL63 schemo here:
http://www.geocities.com/quad_esl63/...atic/esl63.jpg
My question is about the working voltage of the C25 cap:is really necessary to use a min.100V-or-higher in this circuit?
I'm asking this because I'm unable to find an "audiophile grade" cap in that capacitance/voltage range as a suitable replacement.According to other Quad tweakers,the capacitance value has to be somewhere around 150 mF but this value looks impossible to find amongst audiophile brands since the voltage is "in-between".
Could I use a,say,non-polarized 150...168 mF@25V or @63V without risking anything?
(not to mention that a physically-smaller cap would be kindly welcomed as in the loudspeaker's base isn't too much space).
TIA!
Le Basseur

[Steve Conner]

As far as I know, C25 (and R1, R2 and R15) are just there to adapt the ESL63's transformer drive arrangement to solid-state amps. R15 is too small for C25 to have any significant high-pass filtering effect.

You normally can't connect a solid-state power amp directly to a transformer, because transformers have a very low DC resistance, and many solid-state amps have a good few hundred millivolts of DC offset, so high DC currents would flow. The above mentioned components help with that. The resistors may damp self-resonances in the transformers too. Finally, they may help to save your amp when that famously brutal triac protection circuit kicks in...

So I would say: No, C25 does not need a 100V rating, since it's unlikely to see more than a few volts across itself in service. If you are using the speakers with a tube amp, or a solid-state unit with a particularly low DC offset, you can maybe even short C25 out altogether.

PS: I find it quite funny that you think it's worth upgrading to non-inductive resistors in a loudspeaker that has two transformers and 12 inductors in the signal path.

[Le Basseur]

Hey Steve,
Thanks for the detailed info!

I find it quite funny that you think it's worth upgrading to non-inductive resistors in a loudspeaker that has two transformers and 12 inductors in the signal path.

I find it funny too! Actually,this issue doesn't belong to me (the non-inductive resistor was ALREADY installed there when I opened the Quad's base...I'm supposed to upgrade only the cap) and I'm constantly struggling to explain and motivate to my friend (as to any other thin-documented hi-ender) the futility of such approaches.In most cases,I get a raised eyebrow if not some more extreme reactions...hifists become weird when get pertinent explanations.... ..but I think you already know that.
Thanks a lot!
Regards,
Le Basseur