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Thread: PreAmp Cathode bias Capacitor

  1. #36
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    Well, you deserve an answer , why not?
    In this case the Led is both biasing (like a resistor) *and* bypassing, if you want to see it that way, because its internal impedance is *way* lower than the value of any sensible resistor you would use there.
    So, strictly as an analogy, it might be considered "full bypass" ... and flat at any audible frequency and then some.
    In fact, flat down to DC.

  2. #37
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by J M Fahey View Post
    ... and flat at any audible frequency and then some.
    In fact, flat down to DC.
    And that's the way Gary likes it.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  3. #38
    Senior Member Enzo's Avatar
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    REminds me of a young woman I knew in our nation's capital...
    Education is what you're left with after you have forgotten what you have learned.

  4. #39
    Supporting Member Chuck H's Avatar
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    Enzo... Your not suggesting that Gary smokes cigars!?!
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  5. #40
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    REminds me of a young woman I knew in our nation's capital...
    Dear Enzo.
    I *HOPE* , for your own good, that you refer to Mrs Wenzo by this.
    Or I guess youŽll have to cook your own breakfast and wash your own clothes for the next couple of months
    Not to mention being given the "cold look"

  6. #41
    Senior Member Enzo's Avatar
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    Well, I had 40 years of life behind me when I met the missus. She knows I wasn't sitting in a church pew.

    Besides, she knows "flat, down in DC" couldn't refer to her. Now if I had said "sizeable butt" then....
    Education is what you're left with after you have forgotten what you have learned.

  7. #42
    Supporting Member Chuck H's Avatar
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    Wenzo never reads the forum over hubby's shoulder I can see.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  8. #43
    ToneOholic! big_teee's Avatar
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    Recap!
    I did finally get some additional Caps to try.
    I tried a .68uf at Cathode of pin 3 of V2A.
    It didn't pass enough low end, but does give that tightened up marshall sound.
    I tried a 25uf bipolar, it was ok, but to much low end.
    I tried a 4.7uf cap, the only thing I could find in that value was a radial electrolytic.
    It didn't sound right.
    I ended up putting 2 of the large mojo djon .68uf Caps in parallel.(takes a lot of space)
    Still gave the marshall crunch but allowed more bass that just one .68uf did.
    Then I increased the PI coupling caps from .022 to .047uf.
    The .1uf were too much, so I stuck with the .047uf.
    I tried bypassing the 10k cathode resistor of the low input side of Pin 3 of V1.
    It was too much.
    I may try changing the 10k to a 8.2k later on.
    About to achieve all the gain one could hope for in a jcm800 2204, and still clean up and sound good.
    Thanks,
    Terry
    Chuck H likes this.
    Technicians Run the World, but Bankers, Lawyers, and Accountants, Take All The Credit!
    Keep Rockin! B_T
    Terry

  9. #44
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    When ever you reduce this cap the bottom end will be reduced and the amp will sound brighter as the frequency balance across the spectrum has been slanted more toward the top end.
    You want to do this (reduce that cap) in 2nd, 3rd, 4th etc. gain stages in a high gain preamp so as to limit the bass energy being passed to the next stage, else the sound becomes muddy.

    There is also an arguement about suppressing sub sonic intermodulation products - I don't know how valid that arguement is but it seems to make sense.

    Cheers,
    Ian
    CAUTION: Ian's posts (mine) are based on my own preferences which don't include Marshall Amps (horrible shrill nasty things).

  10. #45
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    Quote Originally Posted by J M Fahey View Post
    Well, you deserve an answer , why not?
    In this case the Led is both biasing (like a resistor) *and* bypassing, if you want to see it that way, because its internal impedance is *way* lower than the value of any sensible resistor you would use there.
    So, strictly as an analogy, it might be considered "full bypass" ... and flat at any audible frequency and then some.
    In fact, flat down to DC.
    because the a/c impedance of a led in this configuration is so low, any size or type of "merlin by-pass cap" has no effect.
    don't take my word for it. Try it yourself...

    -g
    ______________________________________
    Gary Moore
    Moore Amplifiication
    mooreamps@hotmail.com

  11. #46
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Gingertube View Post
    When ever you reduce this cap the bottom end will be reduced and the amp will sound brighter as the frequency balance across the spectrum has been slanted more toward the top end.
    You want to do this (reduce that cap) in 2nd, 3rd, 4th etc. gain stages in a high gain preamp so as to limit the bass energy being passed to the next stage, else the sound becomes muddy.

    There is also an arguement about suppressing sub sonic intermodulation products - I don't know how valid that arguement is but it seems to make sense.

    Cheers,
    Ian
    CAUTION: Ian's posts (mine) are based on my own preferences which don't include Marshall Amps (horrible shrill nasty things).
    "Balance" being the key word there.... depending on the impedance of the circuit being bypassed, using a bypass cap of 1,360uF will pass the mids, highs and extreme highs as well as the 1.36uF cap. But it also will pass the deep lows, while the 1.36uF will not pass those frequencies as well.
    Also, since Ian mentioned it...
    although used much more commonly in Radio speak, not audio talk, what are the "sub-sonic" intermodulation products of linear and non linear frequencies found in most guitar inputs? Tricky question.
    Bruce

    Mission Amps
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    www.missionamps.com
    303-955-2412

  12. #47
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    I can answer that one: many think that since the lowest frequency generated by a string is lightly over 80 Hz, there's no way of having any lower.
    And distortion always produces harmonics which are *higher* in frequency than the original one; in fact at least 2X or 3X above, so .... ?????
    True .... for single notes.
    But in chords you have many frequencies, and not only that, they are not wildly separate but often close to each other.
    Now when you hit a non linear stage (the most extreme example being clipping), you generate a whole new set of frequecies which were never there before: the sum frequency ... and the difference.
    And there you have a strong, modulating (by definition) frequency lower than any of the original ones.
    Maybe not in that same stage but ib a later one, that modulation of useful sound by a low frequency, more often than not harmonically unrelated (since it's neither a multiple nor a submultiple of any of the originals) can sound ugly, muddy, you name it.
    Keeping a tight control of lower frequencies minimizes that.
    One classic example is early distortion pedals, as, say, an MXR Dist+ or many similar.
    When playing single notes they provide sustain and bite, and are quite usable, but with chords they fall apart.
    Even worse is the Big Muff.
    The better behaved TubeScreamer already cuts frequencies below around 600Hz, go figure.
    JM2C

  13. #48
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    Low frequency roll-off is accomplished another way...

    -respectfully;
    -g

  14. #49
    Supporting Member Chuck H's Avatar
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    Your vague setups are failing to intrigue me anymore. Get to the subject. The only reason I'd even look into what you may have to say anymore is because you've responded to one of my subscribed posts. If all you have to say is 'I know somethig." or 'I know something you don't.' I'm tired of it. Old game. Come up with a new one or come clean. What are you bringing? Why are you posting? Are you participating or just amusing yourself?
    Last edited by Chuck H; 03-24-2012 at 03:18 PM. Reason: rough edges
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  15. #50
    ToneOholic! big_teee's Avatar
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    Quote Originally Posted by mooreamps View Post
    Low frequency roll-off is accomplished another way...

    -respectfully;
    -g
    I am the OP, long ago.
    It's been an interesting Thread, and one I personally have learned a lot from.
    When I started it, I didn't even know enough about bypass Caps, to even name the thread correctly.
    I'm like Chuck, this is supposed to be a place to learn, and share with others.
    If you have something to share about Bypass Caps, and high and low frequency, total and partial bypass?
    Please share.
    No petty Games, we are all Adults here.
    I prefer things that can be backed up by documentation like the Merlin Preamp Book.
    Thanks, and good luck,
    B_T
    Technicians Run the World, but Bankers, Lawyers, and Accountants, Take All The Credit!
    Keep Rockin! B_T
    Terry

  16. #51
    Gaz
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    No petty Games, we are all Adults here.
    I prefer things that can be backed up by documentation like the Merlin Preamp Book.
    Thanks, and good luck,
    This made me LOL. Clearly Big Teeee has not met Gary yet!

  17. #52
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Gaz View Post
    This made me LOL. Clearly Big Teeee has not met Gary yet!
    Or he has Not that Terry is the type to poke at someone
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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  19. #54
    Noodle of Reality Steve Conner's Avatar
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    Maybe that's how he got the sack from NASA?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  20. #55
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    Quote Originally Posted by Steve Conner View Post
    Maybe that's how he got the sack from NASA?

    ....and this is why I don't publish my schematics on the internet....

    -g

  21. #56
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    Quote Originally Posted by mooreamps View Post
    ....and this is why I don't publish my schematics on the internet....
    In the spirit of channeling Spock and Data, I mean this purely out of curiousity and with all due respect.

    Since the internet treats you so badly, why are you here?

    It doesn't seem to work well for you. You don't want to participate in ways that the rest of the internet seems to want to work. Although I haven't done any looking, I have read here that your participation was banned at other forums.

    What's positive enough about it to make it worth what you clearly regard as abuse?

  22. #57
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by R.G. View Post
    What's positive enough about it to make it worth what you clearly regard as abuse?
    Maybe Gary doesn't have a peer group elsewhere. Sort of like those Nat Geo programs where you see the outcast critter of one species or another that keeps hanging around the pack or herd in the periphery. Kind of sad really. Not being too sympathetic. There is a reason that animal isn't allowed to hang with the group.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  23. #58
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    I really do wish that Gary could learn to deal on a reasonable basis with the greater "us". A while back I had this idea that he could learn to do that. And it really seemed to be working, because his last several posts were not belligerent, even if a little short on actual participation. I look up his posts here whenever I drop by, just to see how he's doing.

    Maybe it's a temporary setback. I keep thinking he has something to contribute, but it doesn't seem to show up. Sad, really. Can't be any fun for him, in the normal sense of things.

  24. #59
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    I suspect his idea of normal and yours are not the same.

  25. #60
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    Quote Originally Posted by mooreamps View Post
    ....and this is why I don't publish my schematics on the internet....

    -g
    could not care less about your schematic.

    that thread was about something you claimed to have invented, tested, and verified blowing up in something you built.

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