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Parallel blocking caps - tone considerations?

Discussion in 'Amp Tech Center' started by jhundt, Aug 1, 2015.

  1. jhundt

    jhundt Doctor of Teleocity

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    In another post here, member 'mcabe' talked about changing the 500pf first-stage blocking cap in a Vox Top Boost preamp for a larger value, such as 1200pf.

    http://www.tdpri.com/forum/amp-tech-center/573075-vox-top-boost-circuit.html

    I am interested in this idea. I have a home-built version of an AC30, with the 500pf cap. I also have an unused slide switch available. I am considering running a 750pf cap to this switch, and using the switch to enable/disable this cap in parallel with the existing 500pf. This should, in theory, give me a choice of 500pf or 1250pf.

    That's theory. I wonder how the tone may - or may NOT - be affected by parallel caps at this point in the chain. Anyone have any experience, knowledge, or advice to share here?

    And does anyone know if using different types of caps within the network will cause any problems? I expect to use silver-mica... but I might try whatever I can dig out of my parts box if there is no reason not to.
     
  2. alnicopu

    alnicopu Friend of Leo's

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    Stick it in there and see how it sounds. I have a pair of jumper leads I use for stuff like that. They have a little spring loaded hook on each end. No soldering needed till you get it like you want it. You can switch out different cap types/values in about 5 seconds.
     
  3. jhundt

    jhundt Doctor of Teleocity

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    thanks for the suggestion; I will probably do exactly that. But I have a lot of other projects stacked up, and it's a pain getting the chassis out of the box I built for it, so I thought I would cast about for as much info as I can get before I get around to opening that thing up.
     
  4. bparnell57

    bparnell57 Poster Extraordinaire

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    Some of the best hifi and industrial amplifiers in the world used parallel caps. Not only can small caps in parallel with filter caps eliminate fizzy noise from the power supply, but parallel capacitors of varying type for coupling caps can enhance high end clarity while also functioning in a perfectly fine way. In guitar amps it could even give unique tone shaping qualities, such as the harshness and rawness of high voltage ceramics mixed with the transparency of film caps. It's basically imperceivable, but high frequency radio amplifiers meant for broad ranges of frequencies often put mica and ceramic caps in parallel for this reason. Doubt it'll do anything at guitar frequencies.
     
  5. alnicopu

    alnicopu Friend of Leo's

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    Impreceivble is a good word on caps in an instrument amp. I have a friend who's a builder and used to own a music store in Gainsville. GA. He had a guy come in wanting an amp built with only carbon comp resistors, silver mica tone caps and sprague electrolytics, etc. The guy was adamant he could tell the difference. Just for fun he built 2 identical amps. (He pre builds some and just sold them off the floor). Same spec transformers but different makes and one with the little cheap Radio Shack green caps and ceramics through out and some off brand electrolytics. The other with the components the guy specified. All he did was make sure all values were within spec. He had the guy try both and pick the one he liked best without telling him what he used. Neither could tell any difference. He told the guy what he did and the guy got pissed off (probably more embarrassed than anything) and walk out without buying either. True story.

    Lots of cognitive dissonance (basically voodoo) in components.
     
  6. mcabe

    mcabe Tele-Meister

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    I actually used a 3-way switch: 500pf alone, 500pf and 2.2nf in parallel, or 2.2nf in series with a 1nf cap both parallel to the 500 pf cap. In theory it gives 500pf, 1.2nf or 2.7nf capacitance. I have not compared this arrangement to actually switching between 3 distinct caps, but the effect is as expected: the bass response increases with higher capacitance.

    Curious if any advantages to distinct caps as well.
     
  7. mcabe

    mcabe Tele-Meister

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    I think I saw a schematic with different types of caps in parallel. Mica parallel to ceramic maybe? Perhaps it was a dumble or a trainwreck. I am skeptical that it would make a big difference, but I'll try to look that up later.


    Edit: It was the Trainwreck Express that had a parallel poly/ceramic coupling capacitor. So it's been done on at least one pretty highly regarded amp.
     
    Last edited: Aug 4, 2015
  8. PurpleStrat

    PurpleStrat Tele-Meister

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    I think it all has to do with the size of the circuit. Modding some 5f2a Tweed Princeton amps I can tell a diffrence in cap brands and such but in bigger amps not so much.
     
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