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FS-100 Output Wiring

24.27.41.140

The FS-100 transformer has six output wires that must be connected in various ways to select the desired 4-9-16 ohm output impedance. (This is the FS-100 model intended for use with a single-ended 211/845.) I want to add a few dB of NFB to the amplifiers using these transformers. However, if the feedback resistor is connected to the white output wire, the value of the resistor will need to be changed whenever the output impedance is switched. Does anyone know which output tap carries the highest constant output impedance (output voltage), regardless of how the wires are connected?

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Buy Chinese. Bury freedom.

Edits: 12/22/21

Follow Ups:

RE: FS-100 Output Wiring - Gordon Rankin 13:45:57 01/03/22 (1)
TK,

You can't think of it that way and you are correct. White wire would be the feedback point and yes along with the change in switching the feedback would have to change.

I did that with a 4 pole 3 throw switch back when this came out. I think I had the first pair of these. I used 3 poles to change the impedance and the final set for feedback.

Basically these are 3 sets of distinct outputs that when wired as you see it create parallel and series wiring so that all 3 sets are always in use and you don't have any flying leads.

Thanks,
Gordon
J. Gordon Rankin
- RE: FS-100 Output Wiring - Triode_Kingdom 18:22:10 01/05/22 (0)
  I'll probably just connect feedback to the hot end of the 4Ω winding and call it good. I'm not sure the specific method I'm using for switching can even accommodate another connection for switchable NFB. I originally used a 3P rotary switch, but I'm changing to the octal plug for more secure connectivity. An 11 pin plug and socket would probably work for switching NFB, but I don't think this approach is materially better than using the 4Ω tap throughout.
  I'm making multiple upgrades to these amps, enough to warrant tearing them down completely. Output switching is actually one of the smaller issues. I have also designed a simple auto-bias circuit to stabilize current drift in the A2 driver and will be making a few additional changes to the power supplies.
  
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  Buy Chinese. Bury freedom.
  
  Edits: 01/05/22 01/05/22
RE: FS-100 Output Wiring - Paul Joppa 18:28:08 12/23/21 (3)
I believe that black to brown is always 4 ohms.
- RE: FS-100 Output Wiring - Triode_Kingdom 21:06:44 12/23/21 (2)
  Thanks! Do you know if there's a common point among all the taps that would maintain a higher voltage than the 4Ω Brown over the three impedance configurations? Now that I think about it, this could probably be determined experimentally by making AC measurements with the transformer driven in each of the three configurations. I don't suppose you have access to a schematic of the secondary?
  
  Incidentally, I figured a way to use a single octal plug that can be rotated to three different positions to select impedance. Speaker wiring would be connected to a separate binding post. I also did this with the push-pull amp that I recently built. The FS-100 is more complex, and I spent a good deal of time figuring it out.
  
  Thanks again for your help with this!
  
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  Buy Chinese. Bury freedom.
  - RE: FS-100 Output Wiring - Paul Joppa 12:08:13 12/24/21 (1)
    Yes, I know the winding details. (I consulted with Mike when he was developing the parafeed version. It's surprisingly sophisticated...) As far as I know, the details are still proprietary, but effectively there are three secondary windings:
    black to brown(*) 2N turns
    blue to white(*) N turns
    green to yellow(*) N turns
    The (*) indicates the dot.
    Because of the way they are interleaved, you cannot wire it for more than one impedance at a time without increasing the leakage inductance. The octal plugs are a great idea!
    (I deduced the color scheme from the arrangements you posted; my information shows only the internal terminal numbers. So I may have the dots on the wrong ends, but the relative phasing is still certain.)
    
    Edits: 12/24/21
    - RE: FS-100 Output Wiring - Triode_Kingdom 19:48:04 12/24/21 (0)
      Many thanks for that info, Paul. Maybe I should just use the 4Ω winding after all. There's probably no downside to the lower impedance tap, I'm just accustomed to using the 8 or 16 ohm taps on OPTs for this purpose.
      
      Best wishes to you and your family for the holidays!
      
      --------------------------
      Buy Chinese. Bury freedom.

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