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Old 27th August 2012, 03:38 AM   #1
mbeards is offline mbeards  United States
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Default Tuning Negative feedback

Hi everyone,

So im wrapping up a build on a PP KT88 project and plan to power it up in a day or two.

My question is how do I best set the negative feedback when looking at the output on a oscilloscope?

My transformer manufacture said that I will need less with these particular transformers. I assume that is because the secondary impedance is less, in terms of what the speaker sees, than other units. How do I know what level of NFB is best? Also I assume that the less I can get away with the better.

Thank you
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Old 27th August 2012, 09:17 AM   #2
DF96 is offline DF96  England
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NFB should be part of the design process. The best amount of NFB is the correct amount for that circuit; less or more could degrade things.

I suppose you could set NFB with a scope by looking for ringing on a square wave and reducing NFB until the ringing disappears. Not a good method, as you can't distinguish between too much NFB and too little compensation.

If your transformer manufacturer actually told you what you think he told you then he was probably talking nonsense. The ratio between output source impedance and intended load impedance is set firstly by the output topology (e.g. pentode, UL, triode) and output valves, not the transformer. All the transformer can do is raise or lower both impedances, but the ratio is not set by the transformer.

To be blunt, you are asking the wrong question. If you are building from an existing circuit, then assume that the designer knew what he was doing and use whatever NFB he used. If it is your own design then you need to design in the level of NFB you require, then adjust the compensation (if necessary) to keep it stable.
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Old 27th August 2012, 09:54 AM   #3
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Manufacturer may stated necessity of smaller feedback factor because of relatively large leakage inductance or low resonant frequency.

You probably should follow original schematic first and check it for stability without feedback first. If amp oscillates without feedback, you need to stabilize output stage first with zobel across anode <-> g2.

Then, repeat this procedure with feedback on.
If its unstable, try to lover feedback to 14dB or so.

PS. When posting question, please attach schematic and technical data sheet for components (in this case for transformer). We can't look inside someone's head for missing or incomplete info.
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Old 27th August 2012, 10:24 AM   #4
DF96 is offline DF96  England
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Yes, 'need less feedback' could be code for 'cannot cope with more feedback'. Generally, the better the transformer the more feedback which can be safely applied - although this does not mean that more feedback will be required, just that it is possible.
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Old 28th August 2012, 07:52 AM   #5
multi is offline multi  Australia
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The M-O valve company claims that 14Db feedback is adequate. {GEC}
i have noticed a lot of current designers are only using 6db. and claim there amps sound better.
Unless you have very good out put transformers best, to keep feedback low you certainly would not want to use the amount shown in the circuit unless you are using the exact same output transformers.
Phil
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Old 28th August 2012, 08:55 AM   #6
palmas is offline palmas  Portugal
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Quote:
Originally Posted by DF96 View Post
Yes, 'need less feedback' could be code for 'cannot cope with more feedback'. Generally, the better the transformer the more feedback which can be safely applied - although this does not mean that more feedback will be required, just that it is possible.
phse shift is a bitch!
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Old 28th August 2012, 09:25 AM   #7
oshifis is offline oshifis  Hungary
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Location: Budapest, Hungary
Quote:
Originally Posted by mbeards View Post
My question is how do I best set the negative feedback when looking at the output on a oscilloscope?
Using a dual-channel oscilloscope, display the input and output signal of the amplifier without feedback. Use an audio signal generator as input. Find the frequencies where the output is opposite phase to the input, that is 180 degrees phase shift. You can apply as much NFB that the closed-loop gain is < 1 at this frequency. Otherwise it will oscillate.
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