Tubes Asylum

RE: OK...

45.64.242.72

SuperB

In Reply to: RE: OK... posted by Ralph on January 03, 2025 at 13:46:44

I go to passive autoformer type linestage. Thanks for your help 😃

Follow Ups:

The problem with a passive transformer volume control - Ralph 08:51:29 01/21/25 (4)
All transformers transform impedance which is how they have their name.

The input to an amplifier is a fixed impedance. The source is also a fixed impedance.

If the transformer is not correctly loaded at its output it will do one of two things. If the load is insufficient (the impedance too high) the transformer will 'ring' or 'overshoot' which is distortion. If the load impedance is too low the transformer will roll off highs. There is an exact load impedance where neither is happening called 'critical damping'.

A transformer volume control relies on the output Voltage changing depending on which tap is selected on the transformer. I think you can see now what the problem is: for each tap, the load on the source changes since the ratio between it and the amplifier is changed. In addition, to make the change and prevent ringing, an output load resistor has to be selected as well. Amplifiers have different input impedances which complicates this. About the only way you can make this happen is to have a transformer that steps down so low resistance values of loading resistors can be used (but this may mean that you won't get enough drive Voltage for the amp). So if an amplifier with a 10K input impedance is to be driven, the load resistors might be 1000 Ohms and less so the amplifier load is insignificant.

If these issues are not addressed, its possible the TVC might color the sound quite a lot.
- RE: The problem with a passive transformer volume control - Tre' 09:40:41 01/26/25 (3)
  I have had my AVCs on a scope and have not noticed any of this.
  
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  - RE: The problem with a passive transformer volume control - Ralph 10:26:04 01/26/25 (2)
    In that case its likely the issues I mentioned were addressed.
    
    You used a square wave for measurement?
    - You're right - Tre' 16:13:34 01/27/25 (1)
      I was testing it wrong. It does ring.
      
      Time to re-think. Thanks.
      
      Tre'
      
      Have Fun and Enjoy the Music
      "Still Working the Problem"
      - I think you want as low an output impedance as you can get - Ralph 08:58:26 01/28/25 (0)
        
        In Reply to: RE: You're right posted by Tre' on January 27, 2025 at 16:13:34
        
        The problem driving single-ended cable is a lack of standards. So the sound of cables is all over the map and the sound changes from one system to another.
        
        You can minimize that with a low output impedance which in turn is driving a low input impedance at the amp.
        
        That means either a large coupling cap, which IMO is inelegant, an output transformer or direct-coupling. Now you know why we did a direct-coupled output in our preamps.
        
        David Berning made a single-ended preamp with a direct-coupled output decades ago called the TF10. If you can find one used it might make a nice platform to work with.
        
        Plan B: put the volume controls inside your amp(s) and remote control them. That eliminates a good number of the problems with passive volume controls.

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