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Thread: What´s in output cap values ? Quality or quantity ?

  1. #1
    Join Date: Feb 2008

    Location: Canaries

    Posts: 193

    Default What´s in output cap values ? Quality or quantity ?

    Following Guy´s very kind suggestion I have just upped the value of the output caps in my souped up P10 phono stage from 1.0uF to 2.2uFto match current production models. The result is plain to hear : bass levels have increased and are much easier to follow so I´m chuffed. I took out the ex-Bat amp 1.0uF copper coloured caps I was using and fitted some used LCR Au Ca Pa 630v caps because I already had them. Any suggestions on which caps to buy to improve on these. I have some unused 4.7uF Auricaps but do not know if fitting these would be OTT because I don´t think I need any more bass, just better defined I guess although the LCRs haven´t burned in yet - apparently they do take a while ( Humble Homemade hi-fi Caps test dixit). My phono stage feeds a Sugden A21SE and World Designs WD25TEx closed box speakers.

  2. #2
    RothwellAudio Guest

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    The most important thing to know is how many volts the cap will be subjected to. I presume this is a valve phonostage so there's a good chance it will be at least 100V, probably more like 250V but could be higher than that. Anyway, you need to know so you know what your options are when it comes to looking at alternatives.
    Basically, you want as big a cap as possible/practical. Polypropylenes are good and are available with high voltage ratings but tend to get big and quite expensive above about 1uF. Polystyrenes are even better but aren't available in such large values at high voltages. Certain types of ceramics are also very good (despite what you may have heard) but also aren't available in large values. Polyesters are available in large values at high voltages but they do exhibit some distortion.

    Electrolytics are usually scoffed at and they do exhibit distortion but there is a solution - use a much bigger value than you would expect from simply doing a -3dB LF cut-off calculation. Use about 100 times more. Unfortunately that probably isn't an option in this case because if you need a 350V rating a 200uF electrolytic would be huge (and expensive, if available).

    Is there a PCB that these caps need to fit? If so, that reduces your options unless you want to mess about trying to fix the cap to the case somehow and use flying leads.

  3. #3
    Join Date: Feb 2008

    Location: Canaries

    Posts: 193

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    Thanks, yes there is a pcb in this case, but of my own making and space is not too much of a problem and even then it would be limited by the cap height standing vertical and yes, it is a valve stage. From what you say I think I’ll try thr Auricaps at 400v which are not physically too large, just 4.7uF @400v compared with 2.2uF@ 630v in the case of the LCRs.
    Last edited by Chris; 04-10-2017 at 16:25.

  4. #4
    Join Date: Mar 2010

    Location: Sheffield

    Posts: 2,898
    I'm Simon.

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    Output cap, phonostage, shouldn't see more than a few volts. Unless of course you can name a preamp that expects tens volts..
    Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m

  5. #5
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    Simon be careful posting comments like that. Whilst it is true the signal amplitude will only be a volt or so it's the DC blocking in a valve preamp that needs a large voltage rating.

    The output valve in a preamp can have anything up to around 300V on it, depending on the design, so the output cap will need a rating of at least this voltage.


  6. #6
    Join Date: Mar 2010

    Location: Sheffield

    Posts: 2,898
    I'm Simon.

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    Ah the joys of not being dc coupled.
    Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m

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