Why electrolytics in power supply

Discussion in 'Tube Audio' started by jwrosenthal, Apr 10, 2018.

  1. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    Low ESR is something to look for in a PS cap, and yes, do you have room for even a 1 Hy choke with a low ohm rating (think Dynaco 70). That said, a somewhat practical option for "some" factory built stuff is the motor run oil or wax cap. They are rated for ACV, so the lower rated (smaller) 330vac ones are good for 600vdc easy. I'm not a big lytic fan either.
     
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  2. sKiZo

    sKiZo Hates received: 8641 Subscriber

    Now in your grocer's freezer case!

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  3. jwrosenthal

    jwrosenthal AK Subscriber Subscriber

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    I emailed Terry a few weeks ago but haven’t heard back and had been feeling adventurous and curious lately- although two MR71’s are not for the meak or shadetree I admit. It is unlike him to no respond to email- I hope he’s ok.

    My original question was simply that when I see many of the builds and upgrades online (and here) electrolytics seem to always go in the power supply and fancy caps in the Audio sections, so I wondered if there was an electrical rationale behind it.
     
  4. FlaCharlie

    FlaCharlie Active Member

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    Originally, it was due to space issues. I'm not sure if they even had film caps available in typical power supply values. Today, film caps are smaller but still not as small as similar value electrolytics so space is still a factor but in some cases it's doable.

    I don't know much about tuners but my impression is that cap replacement wound not require a high level of expertise. It shouldn't really be any different than replacing caps in an amp. Alignment is another matter entirely. It requires specialized equipment and expertise that most hobbyists don't have. At least that was what I was told by a retired tech years ago. He had the expertise but since he had sold most of his equipment he wouldn't touch a tuner.
     
  5. soundmotor

    soundmotor super modified Subscriber

    Better caps (film) in signal path and "lesser" caps (electrolytic) in the power supply is the usual reason. That has changed a lot over the years andBut having a low-impedance power supply is a good thing too and that is what film caps offer (their inherent low ESR, better DF, etc.). As noted all through the above, the storage density is not great with films so to get enough the caps will be many or huge, sometimes both. Low-impedance allows the device to draw more easily from the power supply under load. This is definitely an issue for amplifiers. A tuner though? Not sure I'd bother as any gains may be at the margins and inaudible. It's your MR71, mod away but stay away from any caps in the RF sections. The majority are ceramic discs and were chosen for long-term stability. That is very important in analog RF tuning circuits. Looking at the underside, I see 2 caps I'd feel competent to swap and maybe a few others but the rest I'd leave be unless they were out of spec.

    /$0.02 & likely fraught with errors


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  6. Lavane

    Lavane AK Subscriber Subscriber

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    I just got a pair of 24uf Solen caps for a crossover. Your picture is a good comparison between the original lytic and the new film.
     
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  7. ConradH

    ConradH Addicted Member

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    Actually, ESR is irrelevant for line frequency power supplies as reactance dominates in most cases. The original question brings up how to use less capacitance. The answer, of course, is high frequency switching supplies. Though they too usually/often use electrolytics, film caps are much more practical for filtering switchers than line frequency supplies. Of course there's the debate over whether switchers are something you want in your analog audio equipment. I've never been a fan, but have to admit there is some excellent equipment that uses them to good advantage.
     
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  8. whoaru99

    whoaru99 Epic Member

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    In hindsight, I mispoke in the reply I posted (in the quote above). The calculator does max out at 15 years, but my application indicated 8.9 years (but that is 8.9 years 24/7/365). Read on...

    The electros I used are 105C, 2000 hours rated, Nichicon LGN series.

    I used 60C / 140F as the estimated operating temp and 400mA ripple, which is roughly 50% higher than predicted by simulation.

    The Nichicon life estimation calculator spits out 77978 hours. No, that's not a typo; 77978 hours for "2000 hour" capacitors. That clearly illustrates how big of an effect temperature and ripple have on electrolytic capacitor life.
     
  9. danrclem

    danrclem AK Subscriber Subscriber

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    Is there anyplace that electrolytic and other caps absolutely cannot be interchanged?
     
  10. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    Many vintage tube equipment are "voiced" with electrolytic caps and changing them out would change the sound. From a perspective of the collector, he/she would prefer all original everything. In other words there can be a synergy of parts that would be upset by changing to different types components. For example the EICO HF81 is a PP el84 amp of which thousands and thousands were made. The circuit is also very standard. Yet, people insist that for whatever reason that particular amp sounds better than most of its competition. Dynaco ST35 also comes to mind.
     
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  11. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Certain low-dropout regulator ICs will oscillate unless electrolytics within a specified ESR range are used on the output side. Some of those ICs have been selected for use as DC heater voltage regulators, e.g. LM2940.
     
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  12. gadget73

    gadget73 junk junkie Subscriber

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    This is really the answer. Full wave setups run at double line frequency, so 100 or 120 hz depending on where you live. Its still not high enough that ESR is really a problem for power supply filters in tube gear. Switching supplies can be a different story entirely, but most of us don't have switchers in our tube equipment.

    That said you're not going to hurt it by using film in the supply as long as you can afford and fit them. If you're concerned about switching hash bleeding through from solid state diodes, bypassing the electrolytic with a film cap will sort that out quite nicely for very little cost in $$ or space.
     
  13. ConradH

    ConradH Addicted Member

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    Everything has caveats! When I say caps for PS use are non-critical, that assumes the amp and other circuits have effective local bypassing.

    Audio people are not typically RF people, so they don't think in those terms. IMO, when somebody hears an improvement by adding films across PS electrolytics, they're either victims of expectation bias, or they've reduced an RF problem from the diodes. IMO again, RF problems are best taken care of as close to the source as possible, so a small cap across each diode is my preferred solution. Some try to do a proper snubber pair, but I've found a simple 0.1 uF usually does the trick.
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    I actually own exactly one piece of equipment from "back in the day" that came with factory-fitted bypass caps on the rectifiers. Its the bias supply in my Sherwood S-7000. The voltage doubler did not get bypass caps.

    RF is a weird demon indeed. I've spent some time playing with it so I have a little bit of understanding but there is still a lot of voodoo that I don't get.
     

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