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

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    Testing Global feedback stability and question about practical componsation for outputstage

    I started doing some square wave testing on my output stage to for any instability, measure open loop gain, etc., and see if it's really worth having any global feedback anyway. I have a function generator on my phone which I used initially for the square wave source, and monitored the output at the load.
    With the feedback engaged, I snapped a pic of the waveform. It was looking ugly and seemed to have some nasty ringing in the loop. Here is the image:
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    Then I though I should check the signal coming out of my phone to double check. This was the output:
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    That is entirely useless, and quite pathetic. So, I wired up a quick 1458 square wave oscillator with a buffered output and was much happier. Here is the circuit (schematic, build, and output):
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    not too bad, eh?

    Here is a schematic of the output stage and feedback loop.

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    I'm going to run out of attachment room so I'm going to post scope traces in the next one....

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    One of the reasons I wanted to test this was I was getting some shrill high frequency dominating the sound with the presence control at max settings.
    Starting with the GFB engaged, I tested the output with the presence turned all the way down and here is the shot:
    Click image for larger version. 

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    Here is the presence at full:
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    Here is the output with no global feedback:
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    As an aside, I'm becoming really underwhelmed with the resolution on my scope and wonder if I should be doing this on an analog scope.
    But my questions are; first, since this is a guitar amp, what kind of realistic phase compensation do I need to concern myself with? I was thinking of adding the 8k2/200pF circuit shown in parenthesis on my schematic.
    Second, what kind of linearity should I be looking for? What can be inferred from the wave forms above?
    Last, any thoughts on taming the HF in the presence circuit? I think more useful would be some kind of actual small bandpass filter to shut actual mid frequencies for an active mid boost.

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    Square wave testing is great for a for an instant overall picture or what is going on but it's hard to get useful numbers out of it. Therefore you should consider running a sine wave frequency sweep through it. Say 100hz to 100Khz.

    One thing that jumps out at me from the iPhone test, is that ringing is due to a sharp low pass filter. It might be the one after the DAC in which case it's going t be ringing at something like 20Khz. The point it that it's very high (ultra sonic for me!) frequency and you probably don't what that coming out of your power amp anyway, I'm thinking intermodulation and nasty high harmonics. The sweep will tell you how much and where. The issue shows up as the tilt on the square wave test.

    The traditional way of looking at the loop gain to design the feedback network is open the loop and to drive the feedback node (5.6k resistor) from a signal generator and then look at the response where the loop was broken. It's gets a little tricky as you need to choose source and load impedance so the loop doesn't know it;s been opened. It's even trickier if there is DC feedback. There are other ways too. To really make it fly you need phase info too and now the test gear s getting expensive. At this point it subject turns into a book so I suggest you search using suitable terms such as "open loop testing". I think the guys at Cleverscope have a nice white paper on it IIRC.

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    You have to watch the open loop gain with EL84s. Did you measure it? What is the OL gain and gain after feedback? Try 1K or 470 in series with the Presence cap.

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    Tube PA gain varies with load impedance. So (open loop) gain and phase response will be different at a real speaker load. Preferably stability testing should be done with a load that emulates speaker impedance at high frequencies up to the 100kHz range, where instabilities often show.

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    Quote Originally Posted by SoulFetish View Post
    ... I was thinking of adding the 8k2/200pF circuit shown in parenthesis on my schematic.
    Second, what kind of linearity should I be looking for? What can be inferred from the wave forms above?
    Last, any thoughts on taming the HF in the presence circuit? I think more useful would be some kind of actual small bandpass filter to shut actual mid frequencies for an active mid boost.
    Rather than the 8k/200pf you show - you might try bypassing the 120K NFB resistor with a small cap - try something between 500pf and 1000pf. That provides additional NFB at very high freq's - likely above what you can hear from the spkr, while still allowing the presence control to work in the audible range. Besides removing highs you can't hear, it reduces the stage gain at these HFs and may help stabilize the amp, if it has a stability problem.

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    Quote Originally Posted by uneumann View Post
    Rather than the 8k/200pf you show - you might try bypassing the 120K NFB resistor with a small cap - try something between 500pf and 1000pf. That provides additional NFB at very high freq's - likely above what you can hear from the spkr, while still allowing the presence control to work in the audible range. Besides removing highs you can't hear, it reduces the stage gain at these HFs and may help stabilize the amp, if it has a stability problem.
    Yeah, I think this is called lead (as opposed to lag) compensation. And original proposal seems to work mainly on one half-cycle.
    Don't forget the option of a Zobel/Boucherot network across the OT primary.

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    From the scope shots it appears that there is hardly any increase in gain when disconnecting NFB. Is this real or did you change/adjust input level?

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    Sorry for the delay in my responses. I've been laid up, fighting a cold for the better part of the last few days.

    Quote Originally Posted by loudthud View Post
    You have to watch the open loop gain with EL84s. Did you measure it? What is the OL gain and gain after feedback?
    I did measure it... at least attempt to measure it. I was getting what looked like conflicting measurements. On my multimeter which claims true RMS, I was getting a figure which seemed quite a bit off from looking at the p-p sine wave on my scope crudely dividing Vpeak/1.414.
    So, I'm going to measure it again, and confirm it using another true rms meter. I'll probably do that tonight.

    Quote Originally Posted by loudthud View Post
    Try 1K or 470 in series with the Presence cap.
    Thanks, I'll try that.

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    Quote Originally Posted by Helmholtz View Post
    From the scope shots it appears that there is hardly any increase in gain when disconnecting NFB. Is this real or did you change/adjust input level?
    right, that was by design. Looking at the schematic, I put in a potentiometer at the NFB switch to equalize the gain so I can get a more accurate feel for any differences in touch dynamics, sound, etc.
    The pot will ultimately be replaced by a fixed voltage divider.

    If helpful, I can remove the attenuation and show traces of true open loop gain and closed loop gain.

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    Quote Originally Posted by uneumann View Post
    Rather than the 8k/200pf you show - you might try bypassing the 120K NFB resistor with a small cap - try something between 500pf and 1000pf. That provides additional NFB at very high freq's - likely above what you can hear from the spkr, while still allowing the presence control to work in the audible range. Besides removing highs you can't hear, it reduces the stage gain at these HFs and may help stabilize the amp, if it has a stability problem.
    I was thinking about this as an option as well. I believe this is a more common approach. I wasn't sure about what values to try. I tried bypassing it with a 100pF and 200pF, but saw no visible change in the leading edge of the square wave. I'll experiment with the values you suggested.




    Quote Originally Posted by Helmholtz View Post
    Yeah, I think this is called lead (as opposed to lag) compensation. And original proposal seems to work mainly on one half-cycle.
    Don't forget the option of a Zobel/Boucherot network across the OT primary.
    I was wondering if slugging the dominant pole may be the way to go. I got this idea reading Turner's Audio site http://www.turneraudio.com.au/ (or it may have been Morgan Jones). In any case, the suggestion was to connect this network from plate to ground to compensate from the HF phase shift created by the capacitance to ground in the loop. But, the half cycle observation is a good one. I believe in his example feedback was inserted in a 12AX7 stage feeding a cathodyne phase inverter. In my case, I would probably have to add it to the non-inverting plate as well.
    I considered the Zoble network across the OT primary, but didn't know how to implement it with the MOV over voltage protection I have from each plate to the CT.

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    Quote Originally Posted by nickb View Post
    Square wave testing is great for a for an instant overall picture or what is going on but it's hard to get useful numbers out of it. Therefore you should consider running a sine wave frequency sweep through it. Say 100hz to 100Khz.
    I don't thing I'll have any problem at 100Hz, but I'm wondering if Ill run into slew limiting distortion at 100kHz with my humble 1458. Oh, wait, you said sine wave. I can find something to do that.

    Quote Originally Posted by nickb View Post
    The traditional way of looking at the loop gain to design the feedback network is open the loop and to drive the feedback node (5.6k resistor) from a signal generator and then look at the response where the loop was broken. It's gets a little tricky as you need to choose source and load impedance so the loop doesn't know it;s been opened. It's even trickier if there is DC feedback. There are other ways too. To really make it fly you need phase info too and now the test gear s getting expensive. At this point it subject turns into a book so I suggest you search using suitable terms such as "open loop testing". I think the guys at Cleverscope have a nice white paper on it IIRC.
    I don't know If I understand what you mean by driving the 5k6 node with the signal. I'll read up on it though..
    Do you see any problem from where I'm inserting the signal currently? (marked on the schematic)

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    It can be difficult for others to help if the water is muddy for starters.

    Can you find a nice sinewave signal, such as make up a wav or mp3 file and burn it on a cd or add to your phone or a USB memory player or ipod. Just 100-200Hz is fine as your meter may have that bandwidth (have you looked at your meter specs?) and if there is no hum loop (ie. a battery powered player) then it could be connected to the 'input' (before the 4n7 and to ground). That needs to be at a level that doesn't get near output clipping with or without feedback.

    Can you disconnect the tone shaping presence network, and do you have a 10-20W power resistor of about 6-10 ohm to connect to the output? Is 'com' taken directly to the ground symbol of the Marshally LTP ?

    What is the bandwidth of your Rigol and are you using a 10:1 probe that is compensated ok?

    You should then be able to compare output signal level on meter and scope when feedback is in and out, and then calculate feedback drop in dB.

    That may give us a better idea when the 1-2kHz square wave is applied to the input (as per the sine source connection) and you show us a scope plot.

    If you don't have a sinewave generator, or a 192kHz sampling soundcard or USB interface of some kind, then its a bit difficult to do a frequency sweep and see if there are any quirky peaks, or what you base frequency response is with and without feedback. If you got that far, then you could look further at low and high frequency performance issues.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by trobbins View Post
    It can be difficult for others to help if the water is muddy for starters.
    okay, lets see if I can clear things up


    Can you find a nice sinewave signal, such as make up a wav or mp3 file and burn it on a cd or add to your phone or a USB memory player or ipod. Just 100-200Hz is fine as your meter may have that bandwidth (have you looked at your meter specs?) and if there is no hum loop (ie. a battery powered player) then it could be connected to the 'input' (before the 4n7 and to ground). That needs to be at a level that doesn't get near output clipping with or without feedback.
    I can get an actual signal generator, but I've been using a signal generator on my phone (battery powered, and isolated). For sine waves, it seems adequate, but as show above, useless for square waves. I can set and adjust the output amplitude to either clip the input, or not. But before the 4n7 cap is a DC potential of the preceding stage's plate voltage and there's no way I'm putting that across the output of my phone. I can buffer it and capacitively couple it if that is where to inject the signal.

    Can you disconnect the tone shaping presence network, and do you have a 10-20W power resistor of about 6-10 ohm to connect to the output? Is 'com' taken directly to the ground symbol of the Marshally LTP ?
    It is a bit more "Marshally" than "Fendery" no doubt.
    Here is my home shop bench equipment I'm working with:
    20A variac with digital rms readout.
    Incandescent current limiter (various values)
    4Ω/300W, 8Ω/300W, and 16Ω300W resistive loads.
    100MHz rigol
    10:1 probe compensated to the FFT calibration port at the front of the scope
    2 DMMs both true RMS


    What is the bandwidth of your Rigol and are you using a 10:1 probe that is compensated ok?
    see above

    You should then be able to compare output signal level on meter and scope when feedback is in and out, and then calculate feedback drop in dB.
    That may give us a better idea when the 1-2kHz square wave is applied to the input (as per the sine source connection) and you show us a scope plot.
    yes, I will measure the open loop gain/closed loop gain, and post it. I would have done it last night, but it was 4˚F outside and my garage was too damn cold

    If you don't have a sinewave generator, or a 192kHz sampling soundcard or USB interface of some kind, then its a bit difficult to do a frequency sweep and see if there are any quirky peaks, or what you base frequency response is with and without feedback. If you got that far, then you could look further at low and high frequency performance issues.
    If I need to, I can bring it into work where I have access to more equipment.
    cool, thanks for your questions and recommendation.

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    Quote Originally Posted by SoulFetish View Post
    One of the reasons I wanted to test this was I was getting some shrill high frequency dominating the sound with the presence control at max settings.
    Starting with the GFB engaged, I tested the output with the presence turned all the way down and here is the shot:
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    Here is the presence at full:
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    Here is the output with no global feedback:
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    .
    As the square wave test shows no ringing or overshoot, there will be no pronounced high frequency peak in the response. (Both square wave and frequency response will be different into a speaker, though).

    Also the effect of the presence control as well as the level difference without NFB are rather small (this correlates of course).

    If you just need to reduce some shrillness you might consider a "cut circuit" as used in AC30s, either with a trimmer or in place of the presence control.

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    Senior Member trobbins's Avatar
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    I'd recommend disconnecting the previous stage, as your test signal can then be applied in the normal manner (ie. to ground), and not applied as a voltage across a resistor within the feedback loop.

    Have you checked the frequency response for your DMMs, as a cross-check for measuring sine and square wave rms levels (many DMMs roll off above mains ac or 1kHz type frequencies) against your scope (which probably calculates rms quite well).

    You need a remote turn-on feature for a few valve amps in your garage - as a preheat

    Measuring the total output stage gain with resistive load and no feedback is the easier way to check for unsuspecting gain peaks at very low and very high frequencies, as they may not be controlled by the feedback.

    Setting up an X-Y plot of input and output voltages is also a very easy way to see when phase shift starts to kick in, and when phase shift exceeds 90deg and gets close to or exceeds 180deg, and align that with gain response. If you can get that data, then you can identify how stable the response is in a formal phase/gain margin manner.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by trobbins View Post
    You need a remote turn-on feature for a few valve amps in your garage - as a preheat
    Ha! ain't that the truth

    I'd recommend disconnecting the previous stage, as your test signal can then be applied in the normal manner (ie. to ground), and not applied as a voltage across a resistor within the feedback loop.
    okay, sounds good. This is the approach I'll take.


    Have you checked the frequency response for your DMMs, as a cross-check for measuring sine and square wave rms levels (many DMMs roll off above mains ac or 1kHz type frequencies) against your scope (which probably calculates rms quite well).
    I just tracked down the specs for both DMMs I'm using. I'll post them here, if anyone is interested in them.
    Extech EX205T:
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    Aneng AN8001:
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    Interestingly enough I think the Aneng uses a AD536A chip. I took a quick look through the datasheet and it seems like they're barely tapping into that chips capability.
    https://www.analog.com/media/en/tech...ets/AD536A.pdf
    https://www.analog.com/en/products/ad536a.html


    Setting up an X-Y plot of input and output voltages is also a very easy way to see when phase shift starts to kick in, and when phase shift exceeds 90deg and gets close to or exceeds 180deg, and align that with gain response. If you can get that data, then you can identify how stable the response is in a formal phase/gain margin manner.
    I...uh.. only have one probe at the moment.
    I bought some surplus Pamona insulated panel mount BNC connectors to install in my load box so I can run a direct BNC cable and monitor output.
    I should probably wire a 10:1 voltage divider to attenuate the signal at the BNC jack then... 1MΩ:100k fine?

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    Senior Member trobbins's Avatar
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    Yeh I have two AN8009 - great up to 1kHz sine.

    Your scope will be interesting to see what rms accuracy can be achieved, but it should happily get to above the range you need to be aware of (eg. a few hundred kHz). It appears you need to use the FFT screen, and probably have to play with averaging and windows - there may be some relevant forums/posts (eg. over at eevblog and testgear) or you could start one if you were keen, as it is obviously a tool worth using to its best advantage.

    I'd suggest just getting a few clone 10:1 standard scope probes off ebay - they may indicate the BNC input capacitance range they can cope with, to compare with your scope spec. They may not get you 50MHz bandwidth, but that's not the point, as they are cheap and would be fine to a MHz. I have a few, and a 100:1 for use with soundcard inputs. Trying to make your own probe may not easily yield acceptable bandwidth or flatness, and cause some frustration, and make it difficult for others to believe your plots unless you can show how you calibrated it.

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    Senior Member SoulFetish's Avatar
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    Status update:

    I was able to get back to this project tonight. So I set up my bench to measure the open loop and closed loop gain, run some other tests, and share the results. Sounds simple enough, right?

    trobbins, I followed your instructions for the testing proceedure – I lifted the tone/presence connection; disconnected the previous stage; using an isolated(battery powered) Velleman function generator, I connected the leads to the 4n7 capacitor and to the grounded side of the 5k6 resistor in the tail.
    I set the test signal for a 1kHz sine wave, at an amplitude of 200mV RMS and confirmed the voltage with both DMMs.
    I disconnected the global feedback so I could measure the open loop gain. I connected my scope at the 8Ω output to observe the signal, and did the same with my multimeter to measure the RMS voltage. This was pretty straight forward. I got a nice clean looking sine wave at the load measuring 4.61VRMS for a gain of ≈23 (27.25dB). Easy peasy.
    Then I connected the negative feedback to measure the closed loop gain and here's where it all went to hell....
    At first I didn't suspect anything. I powered down between tests. When I was ready to power up, I set my scope on auto and was checking my meter for a voltage reading and got a measurement of around 447mV at the output. I was like 'What the hell is that.. down something like -20dB??'
    That didn't seem right, so I looked at the sine wave on my scope and could see what looked like quite a bit of noise on the signal and noticed that my scope was set to 10s/div. I set my meter to Hz to double check and it was measuring around 40kHz! When I set my scope to 500s/div, there was way to much high frequency content to make out anything resembling a 1kHz sine wave.
    So,... uh. what do I do now?
    did I misunderstand your setup instructions?

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    Last edited by SoulFetish; 02-01-2019 at 07:16 AM.
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    Senior Member trobbins's Avatar
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    At least you made a good start

    I'd suggest you dumb down the feedback level by say inserting 1-2Megohm in series with the 120k. Switching that feedback in/out should then hopefully only show a small change in clean output voltage - if the voltage goes down a bit then good - if it goes up then that's positive feedback.

    Perhaps without any feeback connected, it is a good idea to confirm the amp shows relatively clean, lowish distortion sinewave as input voltage varies, and then output stage clipping starts to show at some nominal output voltage and load resistance, and then the clipping gets more towards a square wave with higher input.

    If you can vary frequency, then sweeping across the spectrum (well below clipping level) to find when output voltage halves at each end of the spectrum is a good initial measurement to. If you had a spectrum analyser that would then identify if the noise floor was flat, and not showing any spurious frequencies starting up unexpectedly as you raised signal level to clipping.

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    Quote Originally Posted by trobbins View Post
    At least you made a good start

    I'd suggest you dumb down the feedback level by say inserting 1-2Megohm in series with the 120k. Switching that feedback in/out should then hopefully only show a small change in clean output voltage - if the voltage goes down a bit then good - if it goes up then that's positive feedback.

    Perhaps without any feeback connected, it is a good idea to confirm the amp shows relatively clean, lowish distortion sinewave as input voltage varies, and then output stage clipping starts to show at some nominal output voltage and load resistance, and then the clipping gets more towards a square wave with higher input.

    If you can vary frequency, then sweeping across the spectrum (well below clipping level) to find when output voltage halves at each end of the spectrum is a good initial measurement to. If you had a spectrum analyser that would then identify if the noise floor was flat, and not showing any spurious frequencies starting up unexpectedly as you raised signal level to clipping.
    A couple of things I've been thinking about over the last hour or so:
    I'm wondering if I left the feedback vulnerable to oscillations and instability in carelessly making physical changes to accommodate the test setup. I had jumpers and shorting leads, coupled with meter probes spilling out of the small area where this circuit is located.
    The other thing is now that I have a pretty good idea what the open loop gain is, can I determine the feedback ratio from the 5k6 /120k resistors? Will this give me a "in a perfect world" idea of what to expect for a closed loop gain?

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    There is a lot of text-book circuit analysis to identify and wade through to preempt what feedback dB would occur. I recommend doing a few steps in feedback resistance to get some measured dB drop levels for starters.

    Unless your layout and wiring is very tidy and adequately avoiding signals from parasitically coupling to circuitry it shouldn't, then there is some doubt as to how clean your signal is being amplified (before even feedback is considered).

    Perhaps an easy path is to use square wave response to identify the onset of instability when choosing a certain dB level of feedback and a bad load like resistance and parallel capacitance - there are a few good magazine articles and websites describing how to confirm unconditional stability - well worth finding and reading, as they will give you confidence if you want to make the effort of testing.

    In the long run, the feedback level may end up determining the amp gain structure for your input level and clipping/cranking.

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    Quote Originally Posted by trobbins View Post

    Unless your layout and wiring is very tidy and adequately avoiding signals from parasitically coupling to circuitry it shouldn't, then there is some doubt as to how clean your signal is being amplified (before even feedback is considered).
    some considerable thought went into the physical layout, it more or less still looks like this:



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    Quote Originally Posted by SoulFetish View Post
    The other thing is now that I have a pretty good idea what the open loop gain is, can I determine the feedback ratio from the 5k6 /120k resistors? Will this give me a "in a perfect world" idea of what to expect for a closed loop gain?
    Feedback ratio is 5.6/(120+5.6) = 0.045 (i.e. the 120k to 5k6 potential divider ratio)
    Open loop gain is 23
    Which would make the closed loop gain 11.3

    But you are not going to be able to measure it with all that oscillation going on.

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    Dave shorthanded the calculations. Perhaps the easiest reference to look at for your circuit is http://www.aikenamps.com/index.php/d...ative-feedback.

    Can you confirm you connected an 8 ohm output load resistor (instead of a speaker) to the 8 ohm tapping to make your open-loop gain measurement - as that wasn't clarified in your earlier post.

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    Quote Originally Posted by trobbins View Post
    Dave shorthanded the calculations. Perhaps the easiest reference to look at for your circuit is http://www.aikenamps.com/index.php/d...ative-feedback.

    Can you confirm you connected an 8 ohm output load resistor (instead of a speaker) to the 8 ohm tapping to make your open-loop gain measurement - as that wasn't clarified in your earlier post.
    As already mentioned, the open-loop gain depends on load impedance (see Aiken link "The Effect of Changes in Load Impedance"), so using a load resistor is not representative for a speaker load.

    In other words: There isn't much sense in measuring a tube amp's open loop frequency response into a fixed load resistor.

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    Using a fixed resistance loading is the best way to take benchmark measurements, if you were keen enough to make measurements. The measurement being discussed was a single mid-band measurement for the purpose of identifying what nominal level of feedback was being applied (ie. 6dB if the open-loop gain measurement was with an appropriate resistive load).

    Another measurement set is to look at the open-loop frequency response, mainly to identify if there are unexpected gain peaks and phase responses at the ends of the audio bandwidth. That type of measurement can be more onerous to perform if a suitable test setup is not available, but can be very simple and cheap to make nowadays with software tools. Doing that measurement with anything but a fixed resistor load makes it difficult to appreciate why unexpected peaks are occurring, as equivalent circuit models are used to interpret response quirks and that analysis is typically based on fixed resistance loading, especially where an output transformer is included in the signal path.

    Although any typical speaker part has a non-linear response, that doesn't mean the amplifier, or the speaker unit doesn't have linearising networks in place (especially for mid-band to high=band frequencies), or they couldn't be added.

    One aspect of assessing if an amp is effectively unconditionally stable is to not use a fixed load resistance, but rather replace it with effectively worst-case speaker loading examples (eg. capacitance only, and no load). If stable response is observed, then its highly likely that any speaker load can be connected without concern.

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    Quote Originally Posted by trobbins View Post

    Can you confirm you connected an 8 ohm output load resistor (instead of a speaker) to the 8 ohm tapping to make your open-loop gain measurement - as that wasn't clarified in your earlier post.
    Yes i can confirm that

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    Although any typical speaker part has a non-linear response, that doesn't mean the amplifier, or the speaker unit doesn't have linearising networks in place (especially for mid-band to high=band frequencies), or they couldn't be added.
    I wasn't speaking about speakers' frequency responses but of frequency dependent speaker impedance. (Guitar speaker don't usually have "linearising networks" in place.)
    An 8 Ohm guitar speaker has an impedance of over 30 Ohm @ 10kHz and over 100 Ohm @ 100kHz. (Above around 400Hz a speaker's phase is inductive.)
    Consequently open loop high frequency gain with a speaker load will be significantly higher than at a resistor. Remember that the open loop output impedance of tube amps is in the 100 Ohm range.

    Testing tube amps' stability with capacitive or purely resistive loads is unrealistic and will not reveal problems with a real (inductive) speaker load at high frequencies.

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    Quote Originally Posted by trobbins View Post
    Dave shorthanded the calculations.
    I didn't want to post too much maths.
    To calculate the closed loop gain (G) I used the classic equation for closed the loop gain in a feedback amplifier i.e. G=A/(1+AB)
    Where A is the open loop gain and B is the feedback factor.

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    Quote Originally Posted by Dave H View Post
    I didn't want to post too much maths.
    To calculate the closed loop gain (G) I used the classic equation for closed the loop gain in a feedback amplifier i.e. G=A/(1+AB)
    Where A is the open loop gain and B is the feedback factor.
    After I posted wondering if I could calculate the closed loop gain, I remembered the Aiken white paper on his site. I used that info as a guide when first designing the amp. But at the time the phase inverter was designed for a 12AY7 anyway and had a different gain altogether. So whatever I settled on as a good feedback factor at the time would be mute for this version.
    I changed the LTP to a 12AX7 because I wanted a higher input sensitivity for this stage. For a 12AX7, I needed to rebias the stage and change the tail values (lowering the available voltage across the tail resistor) to allow for the same output voltage swing as the 12AY7. The trade off was at the expense of sacrificing some balance in the inverting and non-inverting gain.
    C'est la vie.

    (why was I telling you this again??....) Oh yeah. Aiken's site.
    Anyway, I checked it out briefly late yesterday and using the simplified equations in summary:
    Acl (neglecting the 8Ω Ro) = A / (1 + A* Ri / (Ri + Rf))
    I came up with a similar answer as you did.

    For me, the difficult thing in trying to accurately calculate this on paper has always been finding some definitive resource for understanding and determining pentode gain in the output stage. I have a "better" understanding of this now, thanks to conversations on this forum. But starting out, I didn't understand why the hell all the values for mu seemed to disappear in all the pentode datasheets, when it seemed like kind of a big deal for triodes.
    Of course, I was able to understand this much better from talking to folks who could answer this in this thread here:
    https://music-electronics-forum.com/...cathode+bypass

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    Finally!!
    I was able to solve the HF oscillation and stabilize the global feedback. I was almost gave up on being able to incorporate global feedback at all. I feared the output transformer I installed had would not allow me effectively stabilize the HF phase shifts enough to prevent ultrasonic oscillation.
    There were a couple of areas in the circuit where I suspected might be contributing to the oscillations and wanted to try some suggestions for common fixes.
    The first thing I did was remove any shunt capacitance or compensating components from the feedback circuit so I could isolate each area and start troubleshoot. The first thing I wanted to look into was whether the capacitance in the MOVs across the primary were problematic. According to the data sheet, each MOV was contributing 300pF of capacitance, across the primary. trobbins, I know you like to string series combinations of smaller diameter MOVs to lower the added capacitance. However, in this particular case, 300pF is pretty much the exact capacitance I need for a reactance of 2560Ω @ 100MHz to create a Zobel network for my primary load impedance.
    So after I added the necessary resistance, I powered it back up and tested the the output with feedback and little change I could see and the problem persisted. A little discouraged, I moved on and tried Malcom's suggestion of bypassing the feedback resistor with a 560pF build-out capacitor. After that didn't change anything, I increased the value to 1200pF and still saw no improvement, so I removed them altogether.
    There was one technique I wanted to try, although I was not optimistic at that point. It was something I learned from reading Jones, Turner, and Max Robinson on the topic of amplifier compensation. Jones calls it "slugging the dominant pole" and it involves a shunt RC network on the plate of the input tube in the feedback loop. Turner writes: "V1 anode to 0V, R = 1/10 of total ac and dc RL in parallel, C should have its reactance in ohms = 1/10 V1 RL at 100kHz".
    I my case this worked out to be 200pF and 8k2. Helmholtz correctly made the observation that I would need to in a LTP input stage, both plates would need to be fitted with this for it to work properly. So, I powered down to install the added components and powered back on and the sine wave was clean as a whistle. I mean, it was too clean. All that noise was gone. I checked twice to make sure the feedback resistor was still connected.
    Now I get to actually really test the output, and run some square wave analysis....
    But I want to put this into a speaker and just play some guitar for a couple of hours.

    Oh, one other thing I noticed;
    I began to see a roll off in measured RMS voltage running frequency sweeps, decreasing sharply around 3kHz to 5kHz. But I was looking at my scope and there was no real noticeable change in amplitude on my scope. I'm thinking this might be read errors because I've exceeded the bandwidth of my meters. So do you guys think it is worth building a bench meter for measuring true RMS, using Analog Devices's AD8436 chip?
    Anyone want to help me design a PCB for the project
    AD8436.pdf
    AN-1341.pdf

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    The OPT is the dominant cause of the high frequency gain and phase shift that is going to cause instability when feedback is applied. No matter how poor the transformer response is, global feedback can always be added to achieve a stable amp - its just how you go about it.

    Testing the open-loop response is one way to appreciate how an amp's forward path gain-phase changes, which would then allow a compensation scheme to be designed and added, and then final testing with feedback applied used to confirm that all is ok.

    You have gone straight to adding in a compensation scheme, in the form of a shelf-network, which is pretty much a standard approach for the last 70 years for feedback amps so will be described in many articles. The shelf network starts attenuating gain at about 8kHz with your component values, and the gain levels off at a low value at about 100kHz. At some frequency in between, the total loop gain will drop through 1, and hopefully the phase shift won't get too close to 180deg in that vicinity. Generally that means your level of feedback has to fall within the audio range (ie. distortion increases for signal frequencies above a few kHz) and the phase shift of the step network added to the phase shift from the OPT doesn't cause poor transient response in the vicinity of unity loop gain.

    Higher frequency compensation networks, such as a shunt capacitor across the feedback resistor, or parasitic capacitance from Miller effects or from MOVs, are typically only influential at quite high frequencies and would only be used if needed to subtly tweak the phase margin (eg. to flatten high frequency performance and reduce square wave twiddles). These types of compensation are typically never meant to be the main means to achieve feedback stability.

    I think some amps with LTP's have used a plate-to-plate compensation network (as per your original 200pF part, or as a shelf style with a series resistor) to provide a simpler network than separate plate shelf networks.

    Many multimeters have a bandwidth only up to about 1kHz. Only lab style meters typically have higher bandwidths, so yes that is the value of a scope and especially for X-Y display of input ans output signals to show up gain and phase change.

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    Quote Originally Posted by trobbins View Post
    Testing the open-loop response is one way to appreciate how an amp's forward path gain-phase changes, which would then allow a compensation scheme to be designed and added, and then final testing with feedback applied used to confirm that all is ok.
    I can see how this would make sense. I suppose I was anxious to start throwing "solutions" at the problem to make it stop, rather than test and observer gain and phase so I know where the problem starts to occur in the upper bandwidth. Without knowing any of that, all I really know is 50-100kHz was being a real problem for me.
    Plus, I'm not going to lie, I was just kind of hoping this would all just work out. I've never done this kind of testing, or compensation design. In order to do it properly, I fear I'm probably going to have to step by step do something like this:
    http://www.angelfire.com/electronic/...pensation.html
    and when you've never done it, that just really seems like a lot. A lot of plotting and boding... and then some more.
    But the XY mode for checking phase is pretty cool. It's a little less intimidating that I imagined.

    You have gone straight to adding in a compensation scheme, in the form of a shelf-network, which is pretty much a standard approach for the last 70 years for feedback amps so will be described in many articles. The shelf network starts attenuating gain at about 8kHz with your component values, and the gain levels off at a low value at about 100kHz. At some frequency in between, the total loop gain will drop through 1, and hopefully the phase shift won't get too close to 180deg in that vicinity. Generally that means your level of feedback has to fall within the audio range (ie. distortion increases for signal frequencies above a few kHz) and the phase shift of the step network added to the phase shift from the OPT doesn't cause poor transient response in the vicinity of unity loop gain.
    I'll have to get back to it, then.. but it will have to wait til tomorrow. I'm going to bed

    peace.

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