Output inductor in power amps - pro and con - Page 4 - diyAudio
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Old 26th February 2013, 01:26 PM   #31
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It is reported more than once that application of output inductor may lead to higher distortion and less (!) stability with some amps. I remember that some John Linsley Hood amps disliked it and Alexander current feedback amp deliberately avoided using it because he noted that harmonics are more pronounced. Also, Rod Elliott's P3A with CFP output stage dislikes output inductor.
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Old 26th February 2013, 01:34 PM   #32
dadod is offline dadod  Croatia
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Quote:
Originally Posted by ivanlukic View Post
It is reported more than once that application of output inductor may lead to higher distortion and less (!) stability with some amps. I remember that some John Linsley Hood amps disliked it and Alexander current feedback amp deliberately avoided using it because he noted that harmonics are more pronounced. Also, Rod Elliott's P3A with CFP output stage dislikes output inductor.
Its not that JLH amp disliked output inductor, just JLH used output resistor instead and thus decremented a dumping factor.
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Old 26th February 2013, 01:50 PM   #33
Bonsai is offline Bonsai  Taiwan
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I think an output resistor is totally sub-optimal.
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Old 26th February 2013, 02:02 PM   #34
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There is an interesting paper by Ben Duncan: "White Paper: A New Zobel Network for Audio". It is published in 2009 and is (probably still) available at Russ Andrews Accessories site.
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Old 26th February 2013, 02:04 PM   #35
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Dadod,

JLH designed many amps. The amp that showed some instability with output inductor was one of his MOSFET amps.
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Old 26th February 2013, 02:05 PM   #36
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Quote:
Originally Posted by Bonsai View Post
I think an output resistor is totally sub-optimal.
JLH used 0.22R only and I like the sound of that amp.
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Old 26th February 2013, 02:11 PM   #37
dadod is offline dadod  Croatia
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Quote:
Originally Posted by ivanlukic View Post
Dadod,

JLH designed many amps. The amp that showed some instability with output inductor was one of his MOSFET amps.
Do you mean this amp? I never used it with output inductor so I can't tell.
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Old 26th February 2013, 02:13 PM   #38
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As a big believer in the devil is in the details I really enjoy threads like this, thanks.
I think that one needs to determine for themselves what additional circuitry if any is needed and why. In my case I became interested in trying to better understand why emitter followers can oscillate and found "Why Circuits Oscilliate Spuriously, by Dennis Feucht" to be a good read (in addition found that you can model negative impedances such as -R in LTSpice). In an example amplifier I am looking at, such oscillations can take place when there is a load capacitance between 50p and 4000p (when parasitic inductances are added especially to the output transistor base). At which point combinations of rc's and L's and base stoppers can be used to tame such oscillations (and remove resulting resonances). Dave, not 100% certain but having an R immediately on the emitter followed by the rc would be my first guess at the better solution.
I have seen mentioned numerous times that simulating a nearly full scale sinewave can bring on such oscillations, and that an even better approach is to add a small square wave pulse train to the sinewave. What I have found is that that changing the rise/fall times of the pulse can also make a big difference, especially if these times are made longer, closer to the equivalent bandwidth.

Bonsai, for the case described above I dont see any difference in the main loops phase margin, although there can a difference in gain margin especially if you run the ac sweep with various dc inputs (bias the amp output closer to a rail).

Thanks
-Antonio

Last edited by magnoman; 26th February 2013 at 02:25 PM. Reason: additions
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Old 26th February 2013, 02:45 PM   #39
dadod is offline dadod  Croatia
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My TT-amp simulated with a small square wave pulse train to the sinewave.
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Old 26th February 2013, 04:13 PM   #40
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Quote:
Originally Posted by Bob Cordell View Post
Hi davidsrsb,

I seem to recall higher values in the early 70's on several amps, perhaps as high as 4uH, but values declining over the years to 2uH or below. I used 0.5uH on my MOSFET power amplifier in 1983.

For me, less than 0.5uH of isolation gets scary and seems insufficiently effective.

Cheers,
Bob
1uH is 0.12 Ohms at 20 kHz so I suppose it is already low enough not to disturb the frequency response across the audio band. There is no point in trying to push for even lower values.
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