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dual split 729s

Discussion in 'd.i.y.' started by jpk, Jul 4, 2020.

  1. jpk

    jpk pfm Member

    I wanted to draw a starfish-style preamp but couldn't find schematic files, only some fragments here on PFM. Below a diagram of a dual+split railed 729 circuit:

    [​IMG]
    Hope this is not totally wrong: if yes please alert me! Also does anybody have a starfish PDF to share?
     
    Last edited: Jul 5, 2020
  2. DaveMc64

    DaveMc64 pfm Member

    Hi, There's an error on the first current source. The string of 22k resistors should connect to the collector of Q4, not the base of Q1.

    More fundamentally though, one of the reasons the Naim preamps as a whole are sensitive to power supply quality is that noise from that is injected into the signal via the 270K resistor at the start of the circuit. One of the ideas behind splitting the rails is to avoid this by referencing that point to ground but the way you have drawn it you'll be injecting the noise from both the +12v psu and the -12v psu into the same point.

    As an aside, the noise injection at that point in the standard Naim configuration isn't quite as bad as simulation might suggest because of the (hopefully) low output impedance from the previous stage.
     
  3. jpk

    jpk pfm Member

    Thanks, corrected the error. Regarding noise injection: I just removed it and hope thats fine:

    [​IMG]
     
    Last edited: Jul 5, 2020
  4. DaveMc64

    DaveMc64 pfm Member

    Should be ok. Apologies as you may already be aware of this but I'd point out that your input capacitor has no DC voltage across it so should really be non-polar rather than a Naim style tant. The point (collector of q6) before the output capacitor will be at approximately 2 diode drops below 0v so if you stick with a polar (e.g. tant) cap there it needs to be reversed polarity orientation compared to Naim normal.

    For the 47uF filtering capacitors, I'm not sure where the best place to terminate those are. As drawn, you run the risk of injecting the noise from the -12 supply into there. You may want to consider a noisy earth separate from the signal earth for termination of those caps and including the 0v lines of the psus. This noisy earth would then be referenced to the signal ground at one point only.

    Just some thoughts.
     
  5. jpk

    jpk pfm Member

    [​IMG]

    I applied your suggestions. Regarding the coupling caps: I wonder if I just could omit R1, R4, R19, C2, C10 (and the corresponding parts in the gain stage) especially in case I use a stepped attentuator instead of a volume pot as was suggested here...?
     
    Last edited: Jul 7, 2020
  6. DaveMc64

    DaveMc64 pfm Member

    You could conceivably remove C2 though it does provide protection from external DC sources. If you do remove it then you could delete either R1 or R4 but not both. C10 is required because of the DC offset of the Q6 emitter point. If you took it out you would get either rustles from the following pot or pops from the following attenuator every time you altered the volume.

    I think the attenuator in the link could apparently cope with DC on it's input so it probably had a cap built in. That may or may not be the case depending on your attenuator. I was referring to just a bare switched attenuator above.

    For the gain stage you're going to need the output capacitor again as the output will be a diode drop below 0v.

    Overall I'd probably keep all the caps in place. C2 and equivalent in gain stage just need to be non-polar.

    I don't know any details of the Starfish preamp as it was before my time. Did that have split rails as well? If so then those that have built it might be best placed to give more specific implementation info. I previously looked into the possibility of split-railing the Naim preamp (a la 552) but in the end stuck with single rail.
     
  7. jpk

    jpk pfm Member

    Thanks, I corrected the diagram in my last post and changed some values to what was suggested in various starfish threads.

    I received a starfish PDF from a helpful forum member: it has multiple symmetrical power sources to the circuit and looks almost identical. The gain stage I can take from there too. I will open a new starfish thread!
     
  8. chiily

    chiily PFM Special Builder

    PFM has not seen a Starfish thread in many a year :)
     
    JimmyB likes this.
  9. jpk

    jpk pfm Member

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