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In Reply to: RE: Tube preamplifier output coupling capacitor install posted by Duster on November 27, 2019 at 06:44:05
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
Follow Ups:
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.
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