OTL Asylum

RE: Futterman H-3 restoration frequency response

198.144.203.111

rogerh113

In Reply to: RE: Futterman H-3 restoration frequency response posted by 33dot33rpm on March 07, 2024 at 16:58:18

Thanks for posting the response curves. Most surprising to me is the roll off at 8kHz. Wonder what is causing that? The signal caps should all be presenting less impedance the higher the frequency.

Follow Ups:

RE: Futterman H-3 restoration frequency response - 33dot33rpm 08:05:29 03/08/24 (2)
IMO the roll off is minor at less than 1db. It's the same for both channels so I think it's in the circuit design. What's more interesting is the square wave response. There is practically no ringing but rolled off shoulder. The cause is most likely the compensating network of C6-R12 & C8-13. I would try reducing the value of C6/C8 to mitigate the roll off and maybe live with a little ringing. When you get your H-3 running and you find the sound too warn, try disconnecting the network or drop C6/C8 to 0.047uf. That should brighten the tone. If you look the the H-3aa diagram that network has been deleted.
- RE: Futterman H-3 restoration frequency response - 33dot33rpm 08:12:58 03/08/24 (1)
  Oops I mean to say rounded shoulder not rolled off. That network may also be responsible for the 7k ramp down of the frequency response! If I find time this weekend I'll try disconnecting the network and report back.
  - RE: Futterman H-3 restoration frequency response - rogerh113 06:55:03 03/09/24 (0)
    I agree that they are likely related. You need all the speed you can get to climb the leading edge of the square wave and then flatten off cleanly.
    
    Probably more representative of musical transitions or blended frequencies of multiple instruments and voices. There is a lot more demand with those transitions than a couple of sine waves.

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