Tube DIY Asylum

RE: Yes, with DHTs hum can be an issue

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mwhouston

In Reply to: RE: Yes, with DHTs hum can be an issue posted by Tre' on February 16, 2025 at 20:39:48

It maybe an earth configuration issue. Ive used my design HT double Pi filtration in wellover 100 builds. Power amps and preamps. None have hums.
| retro-thermionic |

Follow Ups:

RE: Yes, with DHTs hum can be an issue - Tre' 21:37:40 02/16/25 (5)
I see the preamp PS is earth grounded. Is the power amp earth grounded?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
- RE: Yes, with DHTs hum can be an issue - FlaCharlie 07:45:40 02/17/25 (4)
  "I like your 22k//68K idea."
  Yeah, if you need to place the gain reduction solution on the preamp I figured that would be a good compromise.
  On the other hand, if you could put it on the amp end of the interconnect that would seem to be even better.
  If I'm understanding this correctly, that would lower the output impedance of the preamp and result in a higher -3db rolloff point. Right??
  Either a simple two resistor voltage divider or a volume pot could be mounted on the amp itself, at the input. This would be great if the preamp was always used with the same amp. But this seems like an unlikely scenario.
  A more versatile solution would be to build the voltage divider into an interconnect which would be used exclusively with this preamp. Or any other pre that's either too noisy or has too much gain.
  One question. If the voltage divider was put on the amp end of the interconnect would it be better to use much larger resistance values than 68k in series with 22k to ground? Wouldn't the 22k to ground be in parallel with the amp's existing input impedance (let's assume 100k)? Or would the 68k, 22k and 100k all be in parallel? Either way that would lower the input impedance significantly (to ~15k) and make the amp harder to drive.
  It that's the case than you could use a combo of 2.2 meg in series and a 750k to ground to get about the same voltage/noise reduction. Or would this create other issues??
  Obviously, if the hum is the result of a ground issue then you could eliminate the voltage divider as long as the increased gain is not an issue.
  
  Edits: 02/17/25
  - RE: Yes, with DHTs hum can be an issue - mwhouston 16:42:17 02/19/25 (0)
    All good ideas.
    | retro-thermionic |
  - Those are all good ideas but... - Tre' 09:06:57 02/17/25 (2)
    He could just fix the hum problem that started all of this and then he wouldn't need the attenuation at all. Impedance problem solved.
    
    Tre'
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    - RE: Those are all good ideas but... - mwhouston 13:50:02 02/20/25 (1)
      I tried for months but to no avail. This allows me to enjoy something I have spent a huge amount of time on.
      | retro-thermionic |
      - "I tried for months but to no avail." - Tre' 18:16:22 02/20/25 (0)
        
        In Reply to: RE: Those are all good ideas but... posted by mwhouston on February 20, 2025 at 13:50:02
        
        I hate when that happens.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"

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