Preamp Output Capacitor: Mundorf Supreme vs. Supreme Silver Oil

I forgot to add that IMO part of the issue with bypass caps has to do with the size of the coupling cap you are bypassing. Generally speaking I've not liked what larger coupling caps do as they impose more coloration. But in some cases they have to be used- such as at the output of many tube preamps and also driving output tubes in a power amp. For this reason I've avoided coupling caps in these specific areas (instead I employ direct-coupling) as this allows me to use much smaller values overall. I can easily see resorting to bypass caps in the case of larger caps- 1uf and above in particular.

Experience and try for yourself.  This is the only way to know for you .

@grannyring  A lot of people are of this opinion. I've just found that the more transparent the system is, the more you hear that paralleled caps  don't have the desired effect. But I'm sure neither of us want to argue about it- in fact I suspect my viewpoint on this topic could easily be unpopular.

People tend to think that since we make some pretty esoteric products like our fully balanced triode zero feedback OTLs, that we might be doing more listening than measurement or something like that. But to get to where we are takes engineering- and that requires math. If I can't make it work on paper, I have trouble with the idea that it will work in practice. I don't like it when things work and I don't have a good explanation- I remember the first time I heard a power cord make a difference and it really bothered me until I sorted out why. I'd love to be proven wrong on this capacitor thing but so far that hasn't worked out- we certainly *have* tried it and more than once with exactly the same results, 22 years apart. It doesn't work out on paper either, so I've maintained this position for a very long time.


One area that might be a variable is output transformers. They have capacitive effects of their own (both inter-winding and to ground) so it may be that in a transformer-coupled amp there is a benefit as one is compensating for a transformer. I've not worked that out as I try to stay away from transformers if I can.

Could someone explain why adding a 0.01uF capacitor in parallel/bypass with a 2.2uF capacitor improves sound quality?

@kalali   It doesn't! Any capacitor has a certain 'speed' defined by a  combination of its dielectric constant and series resistance. This varies from cap to cap (inductance plays a role too; this is caused by the fact that the cap is rolled in a spiral fashion) and even from value to value of caps of similar construction.

BTW this is quite measurable (notwithstanding the known effects of the specifications on paper :) ) and one way to do it is to measure bandwidth. Some caps roll off the signal a lot quicker than others. The bigger the capacitor, the more profoundly its affected by these aspects.


Since smaller caps tend to be faster than larger caps, essentially a phase shift is introduced by the inclusion of a smaller cap in parallel with a larger cap. In a power supply this is advantageous, but as a coupling cap in a nutshell it smears the signal (and being real, the signal is messed with by the cap regardless). The more transparent your setup, the easier it is to hear this!


If this is done in a circuit that employs loop feedback, the phase shift introduced can interfere with the phase margin of the circuit; YIKES! this could lead to oscillation.


So as a result, unless you are compensating for a deficiency elsewhere in the circuit or system, the best approach is to give it your best shot and simply install the best capacitor you can and keep it as low a value as is practical.


This is also why increasing the value of a coupling cap (for example to extend bass response) isn't always a good idea. Circuits behave according to math  and the use of coupling caps is a really good example.

With tube gear where the input impedance is typically 100K or more, a 0.68uf coupling cap is fine.

The smaller value you can get away with, the better the cap itself will sound (and no need for a bypass). That is why we have a direct-coupled output in our preamps, so that the significant cap in the circuit is only 0.1uf.

I never tried Teflon capacitors. Teflon isolated wire sound good but less natural compared cotton isolated wires.

Teflon *insulated* wire has to be either silver or silver plated. Its more likely that the wire is the problem rather than the insulation.

Yeah, I had them in the pre for a couple months of daily listening... would still love to try the cutf and odams.

The Cutf is really the only part we've been playing for some years. My comments are based on them. We installed the ODAMs in one of our preamps (UV-1) and they were quite nice.

In my pre, with a pretty large cap (2uf) right on the output, going to the vcaps was like turning on a fluoresce light.

Did you give them adequate time to break in? We've got plenty of feedback that says that takes a while.


On paper, Teflon has the best characteristics of any dielectric (although at come frequencies polystyrene can be a little better, but polystyrene is all but extinct). So it should be no surprise that the Teflon also sounds better- lower dielectric constant, equivalent series resistance an so on.

they're more tonally similar than different (whereas V-Cap TFTFs and Mundorfs will definitely both shift the tone in different directions).  

We've used the V-Caps a lot and never run into anything like that FWIW.

Yes, actually the original reason I started modifying the capacitors was because the miflexs were right on the edge of being too small for my amplifier's input impedance.

Be careful- just increasing capacitor values can get you into trouble. The output section probably the least, but in many cases, the designer is juggling the inductance that a larger capacitor has against the capacitance- and has arrived at a value that has the best compromise between bass bandwidth and coloration. Bass bandwidth good, coloration bad :)

(This is why we have direct coupled outputs on our preamps- this problem is avoided.)
There is a timing constant involved (which sounds like you might already know) the formula is F = 1/R x C x 2Pi
Its a bit inconvenient as stated, but if you replace 1,000,000 for the 1, then R will be in ohms, C will be in microfarads and F will be in Hz. Ideally you want the -3dB point (which is what this formula gives you) to be no more than 2Hz- allowing the preamp to then have no phase shift at 20Hz which will give you neutral bass. R in this case is the input impedance of the amplifier.