Power supply re-cap on B&K Ref 50

I'm not 100% certain the caps are the issue, however given the age (approx 17 yrs) I'm considering replacing them anyway.

With my amp on, there's barely any noise from the tweeters, or mids for that matter with my ear right up against them. When I turn the Ref 50 on, there's a pretty loud hiss from the speakers, easily audible from 2-3 feet away. I want to say theres even a faint buzz in the mids as well, but you really have to get close to make it out.

I've done the receptacle merry go round, trying to look for ground loops, but no cure.

I want to get a scope on the unit this weekend when I get back from vacation, and look at what I can. Not exactly sure what I need to check but I'm going to give it a valiant effort.... B)

The caps in the power supply are three 3900uf 25V, and one 1000uf 50V. I understand the average ps value is close to 4900uf but not sure where the others come into play...maybe on the lower voltage taps?.....that's where my pay grade taps out...lol.

I haven't pulled the caps yet, but here's what I'm working with, maybe they stand out to someone here.





The power supply leads I did measure...if anyone needs to know.


White: Mains neutral
Yellow: 121.8 VAC
Orange: 220.6 VAC
Red: 240.4 VAC


Purple: 2.71 VAC.......... Blue: 24.65 VAC
Black: 185.3 mVAC......Gray: 16.22 VAC
Purple: 2.73 VAC...........Gray: 16.22 VAC
Brown: 4.68 VAC...........Blue: 24.68 VAC
Brown: 4.68 VAC

I'm not sure what's readily available in these values,or what caps are best for power supplies. There are a few Nichicons, Panasonic and I believe an Elna.

This is what I came up with from Mouser.

3900uf caps:



1000uf cap:



There are also ten small 100uf caps in the circuit but not sure to how critical they are.


Never really dove into diagnosing a power supply circuit, all input and criticism appreciated.


  • corticocortico Posts: 433
    F1nut wrote: »
    A loud hissing issue is not related to a ground loop.

    Have you looked to see if there are Mundorf M-Lytics in the values required?
    I would recommend the M-lyrics as well... that’s what I ll be using from now on, 148f1dcvp6jm.jpeg

    "If it measures good and sounds bad, -- it is bad. If it sounds good and measures bad, -- you've measured the wrong thing." Daniel Von Recklinghausen
  • gmcmangmcman Posts: 1,318
    I can get a 1000uf Mlytic, but not the 3900...unless they do special orders, they list a 3300uf then a 4700uf. What made me investigate a ground loop was the very, very faint buzz in the mids, so faint you need to try to hear it, not your normal ground loop buzz through.

    There has always been an increase in hiss from the tweeters when this preamp is turned on, not a lot, but it seems much more noticeable now.
  • corticocortico Posts: 433
    Grounding issues noise is very easy to detect. ground loops It usually come out as a low freq hum, and poor shielding has a slightly higher freq buzz.

    try to inspect cabling spect cabling in outside the unit, clean control contacts, etc... sometimes improves the situation.

    Hiss and crackling can be more complicated to diagnose...
    "If it measures good and sounds bad, -- it is bad. If it sounds good and measures bad, -- you've measured the wrong thing." Daniel Von Recklinghausen
  • gmcmangmcman Posts: 1,318
    Thanks, I did check out HiFi Collective, they had a plethora of Mundorf caps, but I was only able to find the 1000uf AG Mlytic......I did get one ordered so I can at least get a start. I have a few Nichicons and Panasonic caps on the way also, inexpensive enough to have on hand.

    The grounding points seem clean, I will check the solder joints as well. Pretty much all the connections appear like new, no oxidation or residue.


  • gmcmangmcman Posts: 1,318
    edited April 2018
    Progress so far, I'm not well versed in using a scope but there was no magic smoke.. ;)

    This is the comparison between the 120V mains input and the 120V tap of the secondary winding, as it enters the circuit board.


    I ran a 1 kHz sine wave from the iPad into the CD input, and measured the output on each of the front main outputs.

    I don't know how to interpret the output trace in terms of distortion, but this is what I have.

    Blue is the input to the preamp, yellow is the front main output.

    Left front output:


    Right front output:

  • gmcmangmcman Posts: 1,318
    edited April 2018
    I haven't put the scope on the outputs yet, but I did get the caps replaced. Not sure the cap replacement will affect what I'm viewing on the scope but at least they are fresh.

    I was hoping for a set of Mundorf M-Lytic caps, however I was only able to find the 1000uf M-Lytic, as well as the Panasonic FC and an Elna Silmic 2.

    These are the OE caps:

    The 3 Panasonic FC caps (3900uf), and the 1000uf cap, not sure of make or model the 1000uf.


    The Elna compared to the OE, same values:


    The new Panasonics were slightly taller than the old ones.

    I wanted to try the Mundorf, I still might later for comparison, but it had a larger footprint and may need to be mounted from the bottom. I went with the 3 Panasonics and the 1 Elna.


    I powered it up and I noticed the hiss was still there, maybe slightly less but definitely didn't go away. I performed a quick listening test and I will say it sounded more articulate, not by a large margin but the quiet passages had slightly more detail and the midrange sounded cleaner but again, by small margins. I was used to hearing a slight, muted midrange, not muddy or distorted, but not overly detailed. Now it seems more articulate and could be the noise floor being lowered but can't say for sure...I'll give it a few weeks.

    Now the fuse.

    I installed this about 5 days after the recap.

    I assume this fuse is likely original, either case, not much different than any standard fuse..along side the Synergistic Blue.


    I verified the polarity and played a few familiar recordings. Right away, I could clearly hear the "cleanliness" for lack of better terms, and detail in the upper midrange and treble. The sound isn't forward or bright, but it's like a void was filled. The REF 50 still sounds like the slightly warm-natured pre it is, but there seemed to be that absence of something in and around the lower to mid treble. The blue fuse seems to allow that detail to come through, again, could be a drop in the noise floor.

    With the replacement of the power supply caps and the blue fuse, the REF 50 sounds noticeably improved and more detailed. I will continue to compare notes in the next few weeks.

  • tonybtonyb Posts: 30,391
    Cool man, glad to see you breathed some new life into a worthy piece. Enjoy
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  • gmcmangmcman Posts: 1,318
    Thank you sir.
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