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Planning to use a tube preamplifier for a pro audio application, with a caveat that the SS power amplifier features a direct coupled input stage, whereas the tube preamplifier without output coupling capacitors is said to be incompatible with SS power amplifiers featuring direct coupled inputs. From what I gather, simply installing a film cap of appropriate value between the output signal wiring to the rca output jacks of the tube preamplifier would be a sufficient fix. Since the non-audiophile set-up is not a critical listening application, a pragmatic fix is all I seek. Insights and experiences are most welcome. TIA
Follow Ups:
There are some comments in this thread about how film caps sound better as coupling caps than electrolytics. Like many things, however, that isn't entirely true. A number of years ago I had a Threshold FET-10e phono preamp which was unusual in that each channel had two separate amplifying stages in parallel, from input to output. This wasn't mentioned in the Threshold literature nor did I ever see it mentioned in any reviews but it was pretty obvious from a visual examination. The outputs of the two separate gain stages were connected to a single 10uf Non-Polar electrolytic before going to the output jacks. When I saw that my precious phono signal was going through a crummy electrolytic, I immediately began to try different film caps instead since everyone knows they sound better. To shorten a long story, the different caps all sounded different but none of them sounded as good, overall, as the crummy electrolytic. (A Non-Polar electrolytic is simply two electrolytics in a series back-to-back arrangement in a single housing.)
The electrolytic Mundorf vs. polypropylene Mundorf caps are inexpensive enough to compare each of them for the low-cost pro audio application.
It should be great fun to hear how each will affect the presentation. I expect both will have distinctive strengths and weaknesses to evaluate.
https://www.partsconnexion.com/MUNDORF-71853.html
https://www.partsconnexion.com/MUNDORF-71332.html
While I agree that all electrolytics don't necessarily sound bad and of course haven't heard the vast majority of caps out there I do find it hard to believe that ultimately a film cap of some sort wouldn't beat an electrolytic. Anyhow you did the right thing, just like Duster is doing the right thing, finding out for yourself.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Well, consider this...Despite what you might have read in the popular press, all those bypass caps and power supply filter caps are truly in the signal path. Don't believe me? Do a simulation in LTSpice or a similar software tool and see where the current goes. Or, use a current probe with an oscilloscope or spectrum analyzer and measure it. This isn't some secret - the very first electrical engineering course at the collegiate level goes into depth studying mesh and nodal analysis. Check out the attached link.
Anyway, those caps I mentioned earlier are almost always electrolytic. There's exceptions, of course, but they are rare.
You can also get around that altogether with certain circuit topologies, but they are also rare.
So, there's better than a 99% likelihood that your audio signals are passing through electrolytic caps anyway.
Edits: 11/29/19
If true that just means we have to make amps with 30000uf or so copper foil capacitors. No biggie.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
An absolutely inexpensive solution.That's presuming copper foil capacitors actually do perform better in that application.
Update...
A quick check shows that a single rail of these caps would be about the size of a pirate's treasure chest and would cost about $800k. You can probably get it for half that price based on volume. But, the inductance of the needed wiring would not be exactly helpful.
There are better solutions I can think of.
Edits: 11/29/19
Me too, live music.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
For what it's worth, I've just bought a bunch of NOS Siemens MKL "Klangfilm" acetate capacitors on the bay; they cost peanuts and are supposed to sound fantastic (I sure hope it's true).
The ones I got are 10uF 25V with 1% tolerance (!) and are just 1 inch long and 0,5inch in diameter.
Most of the times you will find higher voltages and those are of course bigger.
I haven't got them yet so can't comment on the sound at this point. But if you are interested, I'll happily provide you the link to the sale.
For the record I paid 21euro including shipping for 5 capacitors.
Coupling Capacitor Calculator from VH Audio
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
A commercial pre-amp should already have a 0V DC output otherwise it would be unusable for the reason you state. If you have a voltmeter check for any DC bias on the output. As tube amplifier are usually single-ended they will already have an output coupling cap.
Regards
13DoW
What he said. ^^Plus, there is a UL or other safety regulation concern if the DC output is above 30 VDC, if I recall.
Generally, as some people have already pointed out, you want a pretty low frequency cut-off for the high pass filter introduced by adding a blocking cap. Ben Duncan has documented the reasoning in his books on high end audio amplifier design as well as the ones on live sound system design. The basic consideration is phase response. This might or might be germane to your installation. (Bass guitar is probably more of a concern than a flute would be.)
The compromise is that higher value caps are physically larger and often have higher distortion properties than lower value caps exhibit. This is especially true of electrolytic type caps.
There's information on this subject here:
https://linearaudio.nl/cyril-batemans-capacitor-sound-articles
It's not the end-all final determination on the subject for caps employed in distortion test gear or audio gear with ambitions of ultra low distortion. But, I don't think that's the application here. Mundorf ECaps might do a great job here and are pretty small.
Edits: 11/28/19
Thanks for sharing your insight and the link, CG. The Mundorf ECap Plain 2.2uF 70V should do the trick. It was very unexpected to consider an electrolytic capacitor for the task, but I think you understand the nature of the application, since the gestalt of pro audio stage monitors is a different animal than that of a home audio or recording studio environment. The price is well-below the target cost-level, so if the product does not provide what I seek, I'll try the polypropylene Mundorf 2.2uF 250V MCap Classic instead. Looking forward to hearing how an electrolytic capacitor performs for a line-level signal application. Seems to be an odd choice, but I reckon it's a worthwhile experiment.Cheers, Duster
Edits: 11/29/19
I'm not going to advocate one over the other. In regard to electrolytic capacitors, the key thing is to use bipolar caps pretty much everywhere outside of the power supply for audio applications, as Bateman explains.
Personally, I've tried highly rated caps that I thought sounded like crap. But, obviously enough customers like them for the manufacturer to make them.
One thing you need to keep in mind is that almost all audio manufacturers are driven to varying degrees by the BOM cost and ease of manufacturing. So, polystyrene foil caps are often avoided. They're somewhat hard to source and don't do well in post soldering PCB washing. Yet, they have stupid low distortion and low microphonics. On the other hand, NP0 ceramic caps are almost as good as polystyrene in regard to distortion and are robust in manufacturing. Pretty inexpensive, too. But, they have a bad rap because they are ceramic and get lumped in with other types of ceramic caps that truly are wretched with regard to performance.
Some companies also choose to use "boutique" caps, which may or may not be genuinely good. But, they add a certain level of, ahh, panache that might help with the marketing. In some ways, driving up the price in a way that you can advertise isn't the worst business move.
In any case, good luck. It shouldn't be hard to find a good solution for your application.
It's not just the "best" polystyrene and foil type caps are avoided, plenty of times the even decent polyprop non foil are avoided for the cheapest mylar along with 2 cent resistors vs. a .15 cent resistor.Not even a .15 cent resistor is a couple or even one spot. I wouldn't have so much contempt for many manufacturers if they didn't lie so much and have to save every penny in every spot.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Well...
Virtually all manufacturers in every market are governed by financial considerations. That shouldn't be a surprise. Probably whoever you work for does the same. In fact, I'd wager that you base a lot of your own purchasing choices based on the financial aspects. That's just the world we live in.
As for lying, I'm with you!
My money is on the film M cap 2.2. I'll give 10 to 1 odds. 100 to 1 if the test is blind. 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
I'll order both and compare the two. Should be a very revealing evaluation, Aristarchus.
The only true test regardless of me being a wise guy. Don't make me look like I have egg on my face. Ultimately I think it unlikely that any electrolytic couldn't be beat by a film of some sort.
"The party told you to reject the evidence of your eyes and ears. It was their final, most essential command." George Orwell-1984
Mundorf indicates the Ecap is suitable for signal applications. That's intriguing enough to capture my imagination.
I'm going to order a pair of Mundorf MCap Classic 2.2uF 250V polypropylene capacitors as a safe bet, and a pair of Mundorf ECap Plain 2.2uF 70V electrolytic capacitors for comparison. Should be great fun to evaluate for the pro audio application.
I've used this Mcap at the output of my CD player. My take on the sonics is that it's warm but detailed if that makes sense? Very pleased, especially at the price.
Most tube preamps are single ended with output coupling capacitors, or some with output transformer. So check your preamp. It's likely you don't have to add a capacitor.
Very few audio devices can tolerate DC on the input, so many devices have a small cap at the input to block any DC. However, it also seems that it would be difficult to design a tube stage (amplification or buffer) that doesn't require some form of coupling to the next stage; either capacitor or transformer. So, I would suspect that a "tube preamplifier without output coupling capacitors" will be a rare beast unless it has a transformer output; but a transformer will block DC also. So, I would recommend that you test your prospective preamp for any DC on the output. If you detect DC, the film cap can go at the output of the preamp or the input of the amp right at the RCA jack.
Edits: 11/27/19
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?
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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