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Coupling cap question - 2 caps separated by a resistor?

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  • #1

    Coupling cap question - 2 caps separated by a resistor?

    Just out of curiosity, I have a question about the coupling caps in my Heathkit amp project.

    Linked below is the schematic I'm working from. Notice how after V1a there's a .1 mfd cap then a resistor and another .1 mfd cap on the other side of the resistor. Same thing after V1b, except it's the vol control that sits between two caps.

    I've never seen this sort of arrangement for coupling caps before. What is the theory behind this, what does it do? Could the .1 caps, for example, be thought of as one .05 cap in terms of bandwidth?

    http://www.visi.com/~sstolle/heath/aa-23-mod-002.JPG

    Thanks for any insight guys!
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  • #2
    Well, the volume control is at least grounded at one end, so it makes some obvious sense.
    The other arrangement (C-R-C in series) doesn't ... quite, except that it's a 220k/1M voltage divider and a high-pass filter (0.1uF/1M) in one step.

    I'm pretty sure you're safe lumping the two caps together - they're in series after all.

    Making that HP filter a 0.05u/1M.

    Hope this helps!

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    • #3
      I'd have to agree, I don't see the point of the two caps in the first circuit. All it takes one to block the DC and act as a filter.

      The second circuit is a little easier. The first cap C6 blocks the DC at the plate of V1B from the volume pot. The cap C7 after the pot passes the signal on to the following grid, but prevents the pot from affecting the DC grid resistance at that grid. So when the bolume pot is at zero, the signal is grounded off, but the tube grid still has a 1 meg from ground. I don't get the impression it is there for response shaping.

      Interesting that the first plate load resistor R7 is labelled low noise.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        This being from the "golden" age, maybe Heath had a bunch of leaky .1u caps hanging about?

        That DC coupling for the phase inverter is a neat trick - I saw that on a Sano amp schematic (wonder where they got it?), been meaning to try it out.

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        • #5
          The direct coupled PI driver is not really new. For examples in guitar amps, look at most any of the late 1960s era Sunn amps like the SOnara, SOlarus, Colliseum, etc.

          Schematic Heaven has a number of them online.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            That is weird. All I can think is that Heathkit had a surplus of .1's and not enough .047's.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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            • #7
              Well, I'll tell you that the first section with the .1 caps is a result of me editing the original schematic to remove the RIAA equalization circuit. V1a used to be the gain stage for the mag phono input. All other inputs in the amp went directly to V1b. Noob mistake on my part. I'm guessing the resistor and second cap could have come out too

              However, the info I really appreciate is the explanation of the two caps and the vol control. I want to duplicate that setup after V1a to give gain controls for the first two stages. Does the grounded pot effectively seperate the caps so they don't act as one .022 cap?

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