Global NFB Baxandall circuit?

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Mr. Lime
Posts: 123
Joined: Sun Mar 05, 2017 12:57 am

Global NFB Baxandall circuit?

Post by Mr. Lime »

Hi,

I was looking at all different kinds of tone controls and came up with the idea to put a Baxandall treble & bass circuit instead of the local feedback loop of a triode, in the global feedback of the poweramp.
This should replace the common presence and resonance controls but I am really not sure how the dimensions have to look like, especially if I'd take the feedback from the 8Ohm tap of the OT after a bootstrapped cathodyne PI..

There is pretty much impedance matching and scaling going on I guess so maybe someone here can give some input which values could work somehow?

Aiken has some calculations concerning the global feedback loop: http://www.aikenamps.com/index.php/desi ... e-feedback

Maybe the impedance matching is easier with a cathode follower between the recovery stage and the tone circuit?
Is there a chance to get this to work properly?
Baxandall Cathodyne.png
Thanks!
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roberto
Posts: 1841
Joined: Tue Nov 27, 2007 4:45 pm
Location: Italy

Re: Global NFB Baxandall circuit?

Post by roberto »

Based on my experience, it works way better if you equalize with a dedicated stage just before the PI.
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Global NFB Baxandall circuit?

Post by pdf64 »

Give it a go by all means but bear in mind that presence and resonance are boost controls, they progressively remove NFB in their respective bands.
To put a 'cut' function in there means a fair bit more (frequency selective) negative feedback.
As there's a transformer in the loop (with its own phase shifts), introducing more phase shift and NFB in the loop may reduce the system's margin of stability (less gain doesn't necessarily equate to more stability).
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