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Thread: Testing Global feedback stability and question about practical componsation for outputstage

  1. #36
    Senior Member trobbins's Avatar
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    Pleased there was no lasting visual damage - just goes to show the variety of hazards on the bench.

    Even the Williamson amp showed about 45 deg phase shift at circa 40kHz, which keeps increasing to about 120 deg at circa 70kHz, and then a dip/rise as the first main resonance is passed through, and finally approaching 180 deg at circa 150-200kHz.

    That early phase shift is from the output stage plate resistance interaction with the output transformer's leakage inductance and shunt capacitance and reflected output loading.

    I'd guess you will find some form of resonant twiddle occurring around 40kHz and perhaps 180 deg phase shift circa 80-100kHz, although there could be further resonant twiddles showing up depending on the output transformer.

    It can be fun watching the XY plot rotate and morph its shape as frequency is both increased, and reduced. Keeping the output level not too high may help minimise the influence of output stage distortion on waveform shape, although the input then gets smaller and perhaps noisier. You also have to swap gain steps as output level falls at bandwidth extremes, to keep a sort of balanced X and Y excursion waveform on the screen. I recall Partridge showed some X-Y images at low frequencies to show just how much distortion and morphing was happening in an output transformer at low frequency, and hence how to mitigate it - and that was back in the 1930-40's.

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    ...and reflected output loading
    This is where speaker inductance/impedance gets in with a speaker load.

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  3. #38
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by trobbins View Post
    You also have to swap gain steps as output level falls at bandwidth extremes, to keep a sort of balanced X and Y excursion waveform on the screen. I recall Partridge showed some X-Y images at low frequencies to show just how much distortion and morphing was happening in an output transformer at low frequency, and hence how to mitigate it - and that was back in the 1930-40's.
    trobbins, I'm not sure how you mean swap gain steps. Could you elaborate?


    I swapped the output transformer and installed the one I originally bought for this project. The one I'm using now is an Edcor CXPP30-10K(see link for specs):
    CXPP30-10K

    So, I got back to testing the Open Loop Gain and phase response again. This time I used my Rigol Digital Scope. The Rigol was a good choice to get Amplitude measurements of the input and output signals. But it was near useless in XY mode.
    Even so, I took some measurements and screenshots.
    I'm not sure if this is correct-- but from what I could see looking at the lissajous patterns, it seemed that the phase shifted 45 at ~13kHz, 23kHz, and 33kHz. But I'm not sure if that's correct? It certainly doesn't seem like it should shift 180 that quickly. I'm going to try again on the 465, but I was able to trace the signal amplitude at 1kHz, 20kHz, and 33kHz.
    Edcor's specs state that -1dB extends to 20kHz, and it looks like the -3dB corner frequency is right around 33kHz.

    here is the input and output showing the open loop gain at 1kHz
    Click image for larger version. 

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    Here is the output at 20kHz
    Click image for larger version. 

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    and here is the input and output traces at 33kHz
    Click image for larger version. 

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    edit: I should have stated that the open loop gain at 1kHz is 23.12 (27.27dB)

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    Last edited by SoulFetish; 07-15-2019 at 02:24 AM.
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    Edcor's specs state that -1dB extends to 20kHz, and it looks like the -3dB corner frequency is right around 33kHz.
    With such limited bandwidth also the scope will produce some frequency dependent phase shift between its channels.

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    Last edited by Helmholtz; 07-15-2019 at 01:23 AM.
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  5. #40
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    Quote Originally Posted by Helmholtz View Post
    With such limited bandwidth also the scope will produce some frequency dependent phase shift between its channels.
    Still, I'm not sure.. I think I'm doing something wrong. The output maintains a pretty steady gain approaching 20kHz. So, if there was there was a 45 phase shift at 13kHz, wouldn't that be where I would measure a -3dB drop in voltage?

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    One other thing – I'm using Max Robinson's paper on Amplifier Compensation (found here:Design Stable Feedback Loops). He indicates that the first breakpoint on his Newcomb D10 amplifier was at 9.5kHz where the phase shifted 135.
    Why is he counting down from 135-90-45?

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  7. #42
    Senior Member trobbins's Avatar
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    Max implies (I didn't have time to read everything so he may explain it somewhere) that the amp has in inverted output phase (ie. -180deg) for mid-band response, so that the output can be connected back to the feedback junction to provide negative feedback (not positive in-phase feedback).

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  8. #43
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    I would recommend the aim is to display an X-Y plot, where output signal is say Y axis and input signal is X-axis, as that form of display allows the phase difference to be much more easily discerned than using a display format where the two signals are on separate displays (or even the same dual display) and where X-axis is time. One of the photos on Max's webpage you link to has an X-Y plot on the screen. Crowhurst shows some XY shapes in his Hi-Fidelity circuit design book.

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  9. #44
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    Quote Originally Posted by trobbins View Post
    I would recommend the aim is to display an X-Y plot, where output signal is say Y axis and input signal is X-axis, as that form of display allows the phase difference to be much more easily discerned than using a display format where the two signals are on separate displays (or even the same dual display) and where X-axis is time.
    Right. That's how I've been trying to check the phase shift. But I've never done it before. So when I get results I'm not expecting, it makes me question whether or not I'm doing it correctly.
    I've been putting the output on the X axis, but I'll switch it up and see what I get.


    One of the photos on Max's webpage you link to has an X-Y plot on the screen. Crowhurst shows some XY shapes in his Hi-Fidelity circuit design book.
    One of the things which had me worried, was the diagonal line on the scope which is supposed to show a straight line with 0 phase shift had an "S" curve. I figured this indicated an operational problem in the amp, but in the Crowhurst plots you mentioned, this can be caused by the tube characteristic. See the red box below:
    Click image for larger version. 

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    okay, there's only so many times I can get the same results before I have to admit I'm in denial and the bandwidth and phase response of my amplifier is total shit.
    Using my venerable 465, I got a pretty clear trace of my first break point at ~10kHz with a phase shift of 45. It's tough to argue with this -



    2 different output transformers produced similar results. I couldn't get a clean image of 90, but it seemed to confirm that there was a phase shift of 135 around 31/32kHz.
    What can I take from these results? Is there something in the design I should have done differently with the loop containing 3 stages? Should I care?

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  11. #46
    Senior Member trobbins's Avatar
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    Sorry to drag you in to the quagmire. The X-Y plots can get pretty bizarre, especially when distortion from different sources (output stage valves, and especially output stage transformer) impart their own quirkiness.

    Can you confirm you have disconnected all the high-freq roll-off circuits that are not of influence - the schematic in your first post shows a 200pF cap shunting the PI output, and you need to short out the grid-stoppers as they are likely an influence with miller capacitance. Also the dominant phase shift is likely to be from the plate resistance of the EL84 (which could be quite high in pentode mode) and the shunt capacitance of each half primary winding (ie. remove the MOV's or add capacitance to see how much of an influence that is).

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  12. #47
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    Quote Originally Posted by trobbins View Post
    Sorry to drag you in to the quagmire. The X-Y plots can get pretty bizarre, especially when distortion from different sources (output stage valves, and especially output stage transformer) impart their own quirkiness.
    No worries, Tim. it's a little slower going trying to learn something this way, so I appreciate the help.

    Can you confirm you have disconnected all the high-freq roll-off circuits that are not of influence - the schematic in your first post shows a 200pF cap shunting the PI output, and you need to short out the grid-stoppers as they are likely an influence with miller capacitance. Also the dominant phase shift is likely to be from the plate resistance of the EL84 (which could be quite high in pentode mode) and the shunt capacitance of each half primary winding (ie. remove the MOV's or add capacitance to see how much of an influence that is).
    I disconnected all the compensating components, as well as the MOVs.
    Here is a schematic of the current test setup:
    Click image for larger version. 

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    I was wondering about the large value grid-stoppers after testing it last night. I'll short those out and re-test.

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  13. #48
    Senior Member trobbins's Avatar
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    I have a VOX AC15 (sort of) clone on the bench at the moment, so looked up an STC 6BQ5 report for plate resistance Ra - it is stated as 38kohm for pentode and 1.5kohm for triode mode, so another test if you were keen would be to temporarily connect the EL84 screens to the anode (say through a 100 ohm stopper) and check the frequency response/phase shift using triode mode, as that may confirm that your phase shift gets pushed out by about a decade (assuming a simple RC filter response, although the actual output stage dominant filter would also include leakage inductance).

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  14. #49
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    After shorting the large grid stoppers, the first breakpoint in the open loop response moved up to around 15kHz with a shift in 45deg.

    My next question is: Now what? do I close the loop and retest as is with all compensation removed? Or do I choose a suitable frequency to bring down the first breakpoint and choose the lag compensation?

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  15. #50
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    Perhaps try and get a few more measurements points in open-loop first - see if you can get a gain value for 90deg, 135deg, and then see how your brain copes when trying to record values if the phase starts getting less then more, and the gain also goes through ups and downs. This is where an automated Bode plotting test setup makes life so much easier.

    Then yes, try and set up for some minor feedback like 3 or 6dB, and repeat the X-Y tests to see if gain can reduce below 0dB whilst phase doesn't go much below say 150 deg. And choose a simple compensation scheme and some nominal values and repeat. If you have some graph paper, you can try and follow some of the old articles that go through response plotting, and feedback compensation - that may see painful, but it does push you to realise what others have done decades ago. You may not really know if you are at a good spot without further trials, or just do some basic stability tests to give some confidence.

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