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OT inside a feedback loop

Posted by Gingertube (A) on April 4, 2007 at 20:50:28

In Reply to: I disagree with this point. posted by Allen Wright on April 4, 2007 at 01:20:30:

As well as the tube amp hobby I design servo amps for stabilsed platforms in aircraft and laser scanners in my day job.

In any feedback system there is a very usefull "Rule of Thumb" for best performance.
For stability (without special compensating techniques) the loop gain can only be as high as the ratio of the two most dominant poles. In a tube amp with global feedback one (well 2 actually) poles are set by the Output Transformer leakage inductance and interwinding capacitances. You then want to introduce a dominant pole = to that pole divided by the loop gain (say 20 for a typical example). That means for common garden variety Output Transformers you would be wanting to introduce a pole at 1.5 to 2 KHz. Imagine what that would sound like !!!!.

So unless using a Toroidal Output Tranny with bandwidth to 250kHz, whenever you apply global feedback, you Will always be fiddling with special compensating techniques. These techniques (from a first principals point of view) seek to modify (reduce)the high frequency gain (amplitude) whilst attempting to leave the phase response intact. This basic point is largely missed or not understood and 98% of "stabilising circuits" I've seen are done poorly. It is also true that to a certain degree (not completely) reducing high frequency gain whilst leaving phase response intact are mutually exclusive requirements.

Do the math - that zobel network across the anode load resistor in the first stage you see as a typical stabilising technique should have a resistor value of 1/10th the anode load - how many times have I seen a value of 1/2 or 1/3 used - a sure sign that someone doesn't know what they are doing and are proceeding by trial and error - mostly the latter.

Aside: HF response will be nearly 6dB down when Zobel Xc + R = the load resistor BUT the 45 degrees phase shift (usually associated with the -3dB point in a simple low pass) will be when Zobel Xc = the Zobel R
Thats why you want the Zobel R = to 1/10th the load resistor (certainly never more than 1/5th)- to maximise the difference in frequency between the amplitude -3dB point and the 45 degrees phase shift point.

Having said all the (sorry - I pushed my own button)I'm stongly with Allen on this - any way to avoid global feedback and hence avoid inherently compromised stabilsing techniques should be embraced. As he says reducing the effective rp of the output stage itself helps at both ends of the frequency spectrum - it lowers the low frequency roll off (6dB/octave) associated with rp and Lp (OT Primary Inductance) and raises the High Frequency roll offs associated with rp and leakage inductance and interwinding capacitances (12dB/octave total) Hence I use triode mode or ultralinear with balanced shunt feedback from the output tube anodes. If you still have to introduce global feedback to control Zout then the rp reduction will have made this simpler by extending the frequencies at which problem phase shifts occur in the output tranny.

I know he is not an Ultralinear fan. From the above you can see why that might be. Some OT phase shifts are involved in the feedback to the screens. The same arguement would apply to cathode feedback schemes.

Cheers,
Ian

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