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In Reply to: RE: Tube preamplifier output coupling capacitor install posted by Palustris on November 27, 2019 at 08:37:02
A mating of both components is confirmed to involve a direct-coupled output stage into a direct-coupled input stage. Recommendations of output coupling capacitor values tend to range from 2.0uF to 10uF. What is your perspective about capacitor values to suggest for the purpose, especially as it might apply to the output stage of a tube preamplifier vs. the output stage of a SS component such as a DAC if there are different factors to consider, Palustris?
Follow Ups:
The coupling cap, whether it is located at the output of the source or the input of the receiver, along with the input impedance of the receiving device, form a 1st order capacitor/resistor high pass filter. A high pass filter circuit only passes signals above the selected cut-off point eliminating the low frequency signals from the waveform. That makes it fairly easy to determine the correct value for the coupling cap once we define the lowest frequency that we want to pass through the device using the formula fc = 1/(2πRC). Fc is the cutoff frequency, R= the input impedance and C= the capacitor value.
So, for example if the receiving device has an input impedance of 10k ohms and we select a 2uF capacitor, the cutoff frequency will be 7.95Hz (use the calculator below). With a 1st order filter the slope of the filter is 6dB and the output at the cutoff frequency will be 70.07% of the non attenuated signal. What this means is that the effects of the high pass filter will affect frequencies above the cutoff frequency. So for instance in this example, the filter will affect frequencies up to about 12Hz.
Since the purpose of the coupling capacitor is to block DC, is there any reason why a target Fc should even be considered, other than the higher cost of a large value film cap and perhaps a less transparent sonic signature vs. a smaller value film cap? Since the application I mentioned is a pro audio application involving a pragmatic approach, it seems the typically largest value recommended 10uF film cap would be a more versatile and straightforward option to consider if DC-blocking is the only purpose. That said, the power amplifier features 50k ohms input impedance, so a 2uF cap seems to be suitable for the application, but I certainly don't wish to risk potential roll-off at the bottom octave and subtle phase anomalies. I'm open to being talked-out of using a compact 10uF 250V film cap for the purpose in favor of a smaller value film cap that's a low-cost option, nothing expensive in this case. TIA
What Palustris said, unless the low frequency cut off is of no importance, which seems unlikely. Two other points: R in the equation is the input impedance in parallel with the output Z of the preamplifier, which is typically very high and therefore usually can be ignored. For a true net Z of 50k ohms, I'd use 2uF of the best quality film cap you can afford. There's no reason to go higher unless you either plan to use the preamp with other amplifiers or the output Z of the preamp is low enough to lower the net impedance significantly. But I am wondering what tube preamplifier has no pre-existing output coupling capacitor. You probably will find out .
The reason to use a cap as large as 2uf @ 50k, (1.59uf for 2 hz) is to limit most of the phase shift, likely audible and harmful to the sound quality at some point, to under 20hz, the approximate limit of human hearing. I'm pretty sure that's the whole reason for the calculation as any or almost any cap will block dc. Put a .1uf in the equation and you will get the bulk of the phase shift going up to 318 hz and still block dc. Did you measure the dc at the output? As others have stated if there is no or very little dc there is no need for a cap unless maybe a fault in the preamp would send dc out. That's above my pay grade as to the level of danger. I suspect that most of the reason for the stated range of like 2-10uf is because of the difference in input impedance in different equipment. A reason why there are larger than necessary caps used in outputs, to cover all bases. This is my understanding of the situation. If you are going to do it I suggest not using junk, at least use a middle of the road film cap. A of S EDIT: Well the whole reason for the calculation to be around like 2hz or very low isn't just to limit phase shift but also to limit the roll off in general of low frequencies.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Edits: 11/30/19
The manufacturer clearly states the tube preamplifier output does not feature a coupling capacitor, so don't use it with a direct-coupled power amplifier. I'm leaning towards using a Mundorf MCap Classic 2.2uF 250V polypropylene capacitor, since the particular application does not require anything more than a competent film capacitor for the purpose.
Perfect value for 50k. Mundorf will be fine if there's room.
Tony Gee's (HHH) ratings are pretty reliable, translate well from his crossover trials to coupling duty.
berate is 8 and benign is 9
I'll give the polypropylene Mundorf MCap Classic 2.2uF 250V a try, madisonears.
Tony Gee rates the Cornell Dublier 940c and the Wima MPK10 higher than the M Cap so since you don't have the M Caps lying around you might consider a cap that Tony Gee rates higher. Both are inexpensive.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
The formula (and calculator) is correct for the 3db down point, but it does not take into account phase effects, which typically extend up to ten times higher than the cut-off freq. So in the example you gave, the phase of low frequencies would be affected up to appx 80Hz. Every competent designer aims for 2Hz or lower cut-off freq for a DC input filter. Be sure to enter farads in the calculator, which is a million microfarads.
berate is 8 and benign is 9
Edits: 11/27/19
Aristarchus described the single-order filter completely right, and the relationship between the amplitude response and the phase response is defined. Where to put the -3dB point is a matter of conjecture and personal taste but to bring out the 'competent designer' line to suggest he is wrong is inaccurate and poor form.
13DoW
My response was to Palustris, not the other guy. Although the calculator Pal provided is accurate, the example he gave was not a good one and would result in poor low freq response. That's a bad design, and calling it out is in no way bad form.
Please learn how to use the forum before you start criticizing people. There was nothing at all wrong with my post, but everybody lost their mind over it. Jeez.
berate is 8 and benign is 9
" It turns out that, within very generous tolerances, humans are insensitive to phase shifts. Under carefully contrived circumstances, special signals auditioned in anechoic conditions, or through headphones, people have heard slight differences. However, even these limited results have failed to provide clear evidence of a 'preference' for a lack of phase shift. When auditioned in real rooms, these differences disappear.. ."
Dr. Floyd Toole
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
As to phase problems there seems to be some controversy about what you can hear or not. Some don't think it's necessary to go down to 2hz. In Chris's discussion he mentions that the larger the capacitor the more negative effect the capacitor has on the sound extraneous to phase effects , that there are trade offs. He makes fun of people obsessed with going down "too" far by calling them elephant ears. What would sound better may I ask, a .15cent electrolytic at 2hz or so fostered on the public or a copper foil at say 6hz and it's increase phase shift? Is it possible that the phase shift caused by a "too" large cap might actually correct for phase shift from some other place?? A of S
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Zat all you got? Lytic vs copper foil? Get real.Chris is a great salesman, knows his stuff. He clearly recommends 2 Hz cutoff. If you can't comprehend that, reread his blurb. He also charges a fortune for his capacitors, so he might be motivated to sell you smaller ones because no one can afford the larger values. There are many, many choices for caps that are the correct value and sound quite wonderful that do not cost as much as CuTF or other V-caps.
berate is 8 and benign is 9
Edits: 11/27/19 11/27/19
"Disclaimer: A 2Hz -3db point is a general guideline, based on our own experience, as well as the experience of many audiophiles, and modificaiton professionals who have shared their experiences with us. As with all things in high end audio, there is no magic bullet or perfect fix for every system and every application. We encourage you to experiment (with many different values and series of V-Caps, of course), listen for yourself, and reach your own conclusions." Chris VenHaus's "clear" recommendation of 2hz and nothing but 2hz?
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Chris VenHaus owner of VH Audio. As to getting "real" about the difference between a cheap or even a high quality electrolytic capacitor or a high quality say copper foil capacitor IN THE SIGNAL PATH I can't even begin to understand why you would make that an issue of any controversy. Perhaps there is some phase shift in your brain waves?? Above 2hz of course. Maybe you should shorten your name to Madears son.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Don't continue with personal insults. They achieve nothing. If you want to discuss audio topics like an adult without calling me names, I'll be happy to do so.
I know very well who Chris is; been dealing with him for years. I also know the difference between the electrical and sonic characteristics of electrolytic vs film/foil caps. There is a broad range of materials, construction, and quality between a cheap lytic and a copper foil cap, in a broad range of cost. That's why I thought your initial argument was absurd. It's not a choice between only those two extremes. And it is not worth compromising low freq response to save a few bucks on a coupling cap.
Competent designers aim for a 2 Hz or lower cutoff. There is no debate there, despite your silly comment about whether phase changes can be detected at low freq's. I'm not sure you know what phase means in that context.
berate is 8 and benign is 9
"Zat all you got? Lytic vs copper foil? Get real." That's not an insult? Did you not intimate a disparagement of Chris by implying he is giving out false info for profit?
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Aristarchus
This guy is repeatedly nasty and negative, spews veiled (or not so veiled) insults - and then tries (and fails dismally) to pretend that he has not engaged in any of the above.
Any contributions he may have are more than offset by this behavior.
n/t
berate is 8 and benign is 9
Thanks for the heads up Dave. I sure didn't see any need for that type of interaction especially since I was pretty much parroting and presenting a seemingly very well respected source of info and audio grade parts. Anyhow ,the OP has gotten his question answered, situation explained and knows what to do and hopefully others have found useful info also. A of S
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Thanks for the comprehensive info and link to the high pass filter calculator, Palustris.
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