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Parallel preamps mixing at LTP help

Discussion in 'Glowing Bottle Tube Amp Forum' started by fraggy0117, Mar 15, 2021.

  1. fraggy0117

    fraggy0117 TDPRI Member

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    Hey All,

    I've got a Hammond organ chassis to Matchless Spitfire amp that has an extra tube socket I'm itching to fill. I'd like to experiment with two parallel channels and need a place to mix them. Some online searching led me to the method of using the LTP but that gives me a few followup questions on the implementation.

    So,
    1. How exactly do I attach the second channel to the LTP
    2. How do I make sure the two channels aren't phased in a way that it cancels each other out.

    Using this schematic, and looking at this post on Aikenamps, I think I attach the second channel at the place indicated in the attached file (red lines) with the circled capacitor acting as the coupling capacitor. Meaning I can go straight from the channel volume pot to that spot indicated. However, I'm confused why it wouldn't just short the entire channel to ground, so I feel I must have missed something.
    Original Matchless Input LTP schematic.png
    Matchless second channel Input LTP schematic.jpg

    As for the problem of phasing, am I correct that I would need two gain stages in order to invert the second channel in such a way that it would be in the "proper" phase for this method of channel blending. So the matchless channel would use the parallel 12AX7, but the second channel would use two triodes in series (and I'd put the tonestack between the first and second triode). Would that work?
     
  2. Wally

    Wally Telefied Ad Free Member

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    Okay...I’ll open up the discussion and risk correction....design is not my forte.
    You are going to need some mixing resistors to isolate the two channels. Also, you are mixing at the wrong point, I think. Your insertion for both channels...following the mixing resistors....would be on that top coupling cap....right where your existing Matchless channel is inserted into the PI.
    There is no phase issue since each channel has only one gain stage.
    Is there a tail resistor in the PI input circuit...or am I misunderstanding that area?
     
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  3. fraggy0117

    fraggy0117 TDPRI Member

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    Thank you, Wally.

    The two mixing resistors to that top coupling cap would work, but I'm trying to find a different way to mix the channels that would cause less interaction.

    On Aiken, he has a section that seems to suggest I can add another channel at the same point as an NFB resistor:

    aikenampsscreengrab.png

    "It should be noted that there are actually three inputs used in this type of phase splitter. The first input is the obvious one, the left side of C1. The second input (the lower end of C2) is useful as a feedback input, a reverb or effects return input, or as a second channel input. In the circuit shown above, the second input is used as a feedback return input, taking the signal off the junction of the feedback divider." Emphasis mine.

    In his example, the R7 resistor doesn't connect directly to ground due to the presence control. As the Spitfire doesn't, I'm unsure how to implement what Aiken is talking about.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    Thanks for that...learning would be for me here, too. Aiken does not go forward with an explanation of how to implement this second channel input directly into the grid of that second triode, does he?
    We need a designer to step in and help us. Would one want to utilize the same capacitance at C2 as one uses at C1? I might think so. pin the Spitfire p, there is no NFB loop and therefore no Presence control.
    A4F77DE7-09FE-4EB7-B926-13E43ECD88F7.png

    Would one simply interrupt that junction between the 47k resistor going to ground and the bottom .01mfd cap, maintain the grounding of that 47K resistor, and insert the second channel through that .01mfd cap into the control grid of the second triode? we need robrob, printer2, bendyha, or someone with some greater experience than I have to confirm this. I do not recall ever having seen this done, and I have never had a problem with channel interaction with a two channel amp with mixing resistors feeding the above type of LTP arrangement. Willing to learn here.....
    How do we give one of those call outs???

    @robrob, @printer2, @Lowerleftcoast, @Bendyha...
     
  5. Ten Over

    Ten Over Tele-Holic

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    Look at the Matchless DC30.
     
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  6. fraggy0117

    fraggy0117 TDPRI Member

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    Thank you - the DC30 solves the LTP question, and is exactly what Wally was suggesting.

    Now the phase issue: if I have two parallel-triodes tube in channel one, and 2 triodes running in series in channel 2, will that eliminate any phase issues at the LTP?
     
  7. gusfinley

    gusfinley Tele-Afflicted

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    I tried the Aiken "Third input" point and it was plagued with strange filtering issues. I could never get both channels to sound good. One always sounded better than the other.

    As for LTP, your "inputs" are differential, so if you send them in at the same phase they will mix out of phase. In the first figure shown above, there is only one input, so the "other" "unused" input is grounded. If you don't ground it there you will get very little signal and A LOT of humm. If you are using it as an input, you do not connect it to ground.

    In-phase mixing can be accomplished prior to the phase inverter using mixing resistors. See the JTM 45 for such an implementation.

    You are correct that using two series triodes will give you an out-of-phase signal that can be mixed by the LTP. You could also use an "anode follower" if you want to keep the gain the same.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Ten Over, my apologies for not giving you a shout out. My memory is not very good sometimes, and there is a long list of folk here
    With greater knowledge and ability than I. You are certainly one of them.
     
  9. Ten Over

    Ten Over Tele-Holic

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    Yes.

    The LTP amplifies the difference between the two inputs, so they call it a differential amplifier. Usually the second input is AC grounded through a capacitor so that the input at the first input is the difference between the two inputs. If you put identical, in-phase signals on both inputs, then there would be no difference to amplify and you wouldn't get any output. This is called common mode rejection.
     
  10. Ten Over

    Ten Over Tele-Holic

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    It's no skin off my nose. I'm only here to help people (and to ruffle some feathers every once in a while). Being an expert seems like it would entail some sort of responsibility and I'm not sure that I'm mature enough for that yet.

    The more I learn about any given subject, the more I realize how little I know about that subject. This pretty much keeps me from claiming to be knowledgable about anything.
     
    Wally likes this.
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