Output inductor in power amps - pro and con - Page 3 - diyAudio
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Old 26th February 2013, 07:19 AM   #21
dadod is online now dadod  Croatia
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Originally Posted by magnoman View Post
To clarify, the voltage on the attached plot was Vout (as defined in your schematic) at the Amps output (before any rc's or inductors), so dividing it by I1 wont give you the Amps impedance.

Thanks
-Antonio
Antonio, I thing I simulated the amp output impedance correctly, look here: http://www.cs.toronto.edu/~relu/audi...pice-zout.html
Damir
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Old 26th February 2013, 07:32 AM   #22
dadod is online now dadod  Croatia
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Originally Posted by Bob Cordell View Post
Hi Dave,

Yes, the inductor thing and where to put the Zobel can be a point of controversey and maybe even some art.

I always recommend an inductor, even if it is fairly small. Amplifiers with unknown capacitive loads (or, effectively unterminated transmission line cables) can sometimes oscillate in brief bursts that may not be seen, but which may affect the sound. Without some decoupling, as provided by the inductor, it is difficult to get an amplifier to be unconditionally stable for nearly arbitrary loads, especially given the wide variation in operating points of the output stages as they go through an infinite variety of combinations of current and voltage. Even 1uH shunted by about 2 ohms can make a big difference.

The Zobel is very important to the stability of the usual EF output stage, and should be very close to the output stage transistors, with a well-controlled local return to signal ground. It wants to provide good resistive damping to very high frequencies, especially with fast output transistors. I even believe in what I call distributed Zobel networks in the output stage, when a plurality of small Zobels effectively in parallel is physically distributed across the output stage. Then the individual zobels can be smaller and less inductive. Then there is no need for big-wattage, non-inductive power resistors for the Zobel.

I also prefer to have a Zobel on the speaker side of the inductor, and located at or near the amplifier output terminals. We thus end up with a sort of pi output network. This can help the EMI situation and sometimes to some extent the possible speaker cable mis-termination concern. The key to recognizing the possible value of this is to realize that there is often some wiring inductance in the signal path from the output stage to the speaker terminals. Doug may regard this as over-kill, citing the damping effect already there as a result of the resistor across the inductor. The Zobel need not necessarily be big.

Also bear in mind that star grounding can make the return ground path at the speaker terminals be very different at high frequencies than the signal ground present at the output stage. A Zobel at each end mitigates any effects of this difference at high frequencies, damping the line to BOTH grounds.

You can tell that I like Zobels. They tend to help things behave at those high frequencies where there is some magic and layout sensitivity.

Finally, if you really want to be sure, put a Zobel at the speaker - at the far end of the speaker cable. I am not the first to advocate this, and I think some speaker cable manufacturers build this in. This helps make sure that the far end of the speaker cable is at least somewhat terminated at the far end (loudspeakers can look inductive at HF, even when there is a resistive L-pad in the tweeter circuit). While it is certainly true that speaker cables of reasonable length do not act like transmission lines at audio frequencies (and certainly need not be properly terminated at audio frequencies), the transmission line effects that can begin to emerge in the 1 MHz region can in some cases act to de-stabilize an amplifier.

I suspect that there are many amplifiers out there that may sound different merely because they are not unconditionally stable, and they may be issuing subtle parasitic bursts of oscillation in different ways. Similarly, different speaker cables connected to a marginally-stable amplifier may cause the sound to be different by altering the way in which the amplifier is mis-behaving.

Cheers,
Bob
Thanks Bob for your clarifications.
I have your book and it helped me to understand many things a lot better. What I missed was some more LTspice simulations and explanation how to do it. I am learning each day something new. I am retired telecommunication engineer and electronic is just my hobby.
BR Damir
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Old 26th February 2013, 09:41 AM   #23
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Quote:
Originally Posted by Bob Cordell View Post
... I even believe in what I call distributed Zobel networks in the output stage, when a plurality of small Zobels effectively in parallel is physically distributed across the output stage. Then the individual zobels can be smaller and less inductive...

I also prefer to have a Zobel on the speaker side of the inductor, and located at or near the amplifier output terminals. We thus end up with a sort of pi output network. This can help the EMI situation...
Your explanation in the book made sense to me and I planned to do this.
But it is nice to have confirmation, so thanks to Antonio for the data to quantify the benefits of an outboard capacitor to reduce RFI.
I am somewhat cautious about RFI because I plan to use so-called "phantom zero" compensation which will increase RF return ratio.
A question - I have multiple BJTs in the OPS and it seems reasonable to attach the distributed Zobels directly to the emitters rather than downstream after the emitter resistors. Does that make sense to you?

Best wishes
David
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Old 26th February 2013, 09:55 AM   #24
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Originally Posted by davidsrsb View Post
It's odd that the output inductor has always been about 1uH, from the days of 2n3055 onwards. You would expect the value to have dropped to 100nH with modern transistors and FETs. Too high a value will be above its self resonance at problem frequencies.

When you talk about RFI... These days the sources are mostly above 1GHz
The inductor is outside the loop, so simplistically it is determined by the speaker impedance and acceptable roll-off, that have not altered, rather than the transistor Ft.
But your point about the self resonance issue makes me think. Do you have any data?

Best wishes
David.


OK. a quick check of typical 1uf inductor has a SR frequency of about 160Mhz. Not a problem I think.

Last edited by Dave Zan; 26th February 2013 at 10:10 AM.
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Old 26th February 2013, 10:06 AM   #25
jcx is offline jcx  United States
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external fr considerations set the max value - Self uses up to 6 uH

as for the min - it wouldn't make sense to bother with a added part not much more than the amp internal wiring parasitic L
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Old 26th February 2013, 10:39 AM   #26
dadod is online now dadod  Croatia
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Originally Posted by Dave Zan View Post
But it is nice to have confirmation, so thanks to Antonio for the data to quantify the benefits of an outboard capacitor to reduce RFI.

Best wishes
David
David, I think you've got it wrongly, look the post #7 again.
Damir
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Old 26th February 2013, 11:25 AM   #27
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Originally Posted by davidsrsb View Post
It's odd that the output inductor has always been about 1uH, from the days of 2n3055 onwards. You would expect the value to have dropped to 100nH with modern transistors and FETs. Too high a value will be above its self resonance at problem frequencies.

When you talk about RF ingress, back in the 70s that meant amateur HF and CB. These days the sources are mostly above 1GHz
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
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Old 26th February 2013, 12:34 PM   #28
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Originally Posted by dadod View Post
David... look the post #7 Damir
Yes. I noticed that Antonio's post #15 seems to be inconsistent with your #7.
If I understand Antonio's chart correctly then the outboard capacitor circuit results in a much lower input into the feedback network. This is makes sense to me.
I don't understand your result where a low value shunt added across a source makes no difference. I suspect there is some issue, perhaps with how the cable source is handled, but I have not yet checked and did not want to comment prematurely.

Best wishes
David

Is current source a reasonable model?
I have no RF model experience. How should cable capacitance and inductance be handled?

Last edited by Dave Zan; 26th February 2013 at 12:53 PM.
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Old 26th February 2013, 12:46 PM   #29
Bonsai is offline Bonsai  Taiwan
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I am using 0.6 to 1 uH. I design so that I have plenty of phase margin- typically more than 60 degrees. I am absolutely with Bob on this one. Good engineering practice demands the use of an output L and at least an in- board Zobel.
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Old 26th February 2013, 01:05 PM   #30
dadod is online now dadod  Croatia
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Quote:
Originally Posted by Dave Zan View Post
Yes. I noticed that Antonio's post #15 seems to be inconsistent with your #7.
If I understand Antonio's chart correctly then the outboard capacitor circuit results in a much lower input into the feedback network. This is makes sense to me.
I don't understand your result where a low value shunt added across a source makes no difference. I suspect there is some issue, perhaps with how the cable source is handled, but I have not yet checked and did not want to comment prematurely.

Best wishes
David
Antonio simulation shows output voltage before an inductor(.ac sim) only. Certainly it shows an influence of the cap connected after an inductor influence, but the same influence shows on the voltage after an inductor. I was trying to show the difference between output voltage(or in my case, impedance) after and before a inductor, actually a speaker line garbage suppression by showing V(out)/I(I1)/V(vout)/I(I1) in dB. Could be that my result are not good enough, I did not include any cabel resistance/capacitance, but I think it was not importand.
Sorry but it not easy for me to express my self in English technically correct.
BR Damir
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