information needed about low power amp topology

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Ami
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#31 Re: information needed about low power amp topology

Post by Ami »

shane wrote: Mon Nov 26, 2018 4:29 pm Not to mention the likes of Emerson and Wakeman who manage to satisfy thousands with an electric organ.







Be careful what you unleash, Amy.
I was just kidding really, I much prefer the oboe :oops:
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#32 Simple adaptive/sliding bias amplifier.... finalized.

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OK, I was so convinced that I'd found the best solution to the amplifiers requirements that I emailed Guru Norman the circuit.

Here's what he said:
....well done on the diagram Ami, that's very neat, however, I must point out a couple of mistakes.

Firstly, for your current application this will be a trivial point, but in a more powerful version it could cause damage to the output stage, the 0.1microfarad decoupling capacitor at the wiper of the bias potentiometer should be connected to the positive supply, not the zero volt side. It was a good thought to include it in the design, but placed as it was would cause a heavy current to flow in the output transistor as the capacitor charges up at switch on.

Secondly, and a more serious design flaw, is the speaker being returned to the zero volt of the supply.

As you gave the working voltage of the output coupling capacitor a value of three times that of the supply (470 microfarads 10 volts), I'm sure you grasped the notion that including a choke load for the output transistor means that the output node where the transistor connects with the choke nominally holds a potential of zero volts, and will be driven positive by the transistor and negative by the choke. You will therefore realize that the capacitor will spend half its life reverse biased, something it will reward you with by eventually shorting out and blowing itself apart.

You need to reverse the polarity of that capacitor and return the speaker to the positive supply to prevent this happening....
Every day is a school day....

Here's the revised circuit....
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Nick
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#33 Re: information needed about low power amp topology

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You will therefore realize that the capacitor will spend half its life reverse biased
But only by AC, DC conditions will always be +ve biased by the DCR of the choke. As the cap will be a short circuit to AC it will never have any AC voltage across it.

Just a minor point to be picky, its better where it is now, but I just wanted to argue about the justification.
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#34 Re: information needed about low power amp topology

Post by shane »

Ami wrote: Mon Nov 26, 2018 9:40 pm
shane wrote: Mon Nov 26, 2018 4:29 pm Not to mention the likes of Emerson and Wakeman who manage to satisfy thousands with an electric organ.







Be careful what you unleash, Amy.
I was just kidding really, I much prefer the oboe :oops:
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The world looks so different after learning science. For example, trees are made of air, primarily. When they are burned, they go back to air, and in their flaming heat is released the flaming heat of the Sun which was bound in to convert air into tree.
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#35 Re: information needed about low power amp topology

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Nick wrote: Mon Nov 26, 2018 10:28 pm
You will therefore realize that the capacitor will spend half its life reverse biased
But only by AC, DC conditions will always be +ve biased by the DCR of the choke. As the cap will be a short circuit to AC it will never have any AC voltage across it.

Just a minor point to be picky, its better where it is now, but I just wanted to argue about the justification.
Image

Yes I'm sorry but I read that and thought it was nonsense! There is a standing DC charge on the capacitor, roughly equal to the supply Voltage*, which will hardly change so at no time will it be reversed.

HTH

( * cap is connected at pos end to the supply via the speaker, and neg end is connected to ground via the choke. Therefore it only needs to be rated for the supply Voltage, plus a safety margin. )
 
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#36 Re: information needed about low power amp topology

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Mike H wrote: Tue Nov 27, 2018 12:41 pm
Nick wrote: Mon Nov 26, 2018 10:28 pm
You will therefore realize that the capacitor will spend half its life reverse biased
But only by AC, DC conditions will always be +ve biased by the DCR of the choke. As the cap will be a short circuit to AC it will never have any AC voltage across it.

Just a minor point to be picky, its better where it is now, but I just wanted to argue about the justification.
Image

Yes I'm sorry but I read that and thought it was nonsense! There is a standing DC charge on the capacitor, roughly equal to the supply Voltage*, which will hardly change so at no time will it be reversed.

HTH

( * cap is connected at pos end to the supply via the speaker, and neg end is connected to ground via the choke. Therefore it only needs to be rated for the supply Voltage, plus a safety margin. )
It was this circuit

Image

So the top of the choke can (and will) swing -ve (below ground) and so take the +ve side of the cap -ve WRT its -ve side, but it will only ever do that with AC when the cap will not see any voltage across it and just take the loudspeaker -ve with it.
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#37 Re: information needed about low power amp topology

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AH!!! :oops:

I shall chastise myself with birch twigs directly...
 
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#38 more on that capacitor!

Post by Ami »

I must admit that I'm still a little unsure about the magic that the inductor performs in this circuit, and intend to ask to use the schools oscilloscope to look at what's going on either side of the output capacitor. I have my dads old oscilloscope, but it has its own capacitor issues, so, I can't use that at the mo.

I sent a link to Norman who expanded on his comments.
.... electrolytic capacitors are pretty poor things to have in the signal path, I don't like them there, but sometimes they are a necessary evil. For polarised types, without a charge of the correct polarity, and not within a range of from say 10% up to rated voltage they have degraded performance, regards capacitance value, ripple current performance, temperature stability etc, and if reverse biased they will break down and may pass DC which results in heating expansion of the electrolyte and possibly a loud bang. Passing pure AC through a polarised electrolytic will end in its demise.

Yes, Nick made comments regards the fact that a capacitor is a virtual short circuit to AC, and that is quite correct, (that in the context of your amplifier is the capacitor handling ripple current), however, I maintain that your original circuit would have virtually no standing polarising charge and the capacitor wouldn't lead a very happy life. In your revisions however, the capacitor is always charged with the correct polarity and will operate reliably. Also in these choke loaded single ended amplifiers the capacitor's working voltage needs to be twice the supply voltage, your example may swing to negative 2 volts for instance.
Comments?

Oh, and if anyone can point me to an easy method of accurate RMS power measurement, please.

I'm sorry if I'm flogging a dead horse here but my getting these things clear will help me get the write up straight. Thanks.

Talking of flogging.
.
Mike H wrote: Tue Nov 27, 2018 3:42 pm AH!!! :oops:

I shall chastise myself with birch twigs directly...
Birch twigs eh? Old school flagellation! In the right season I don't think you can better a fresh handful of spring nettles. :twisted:

Time for bed.
Luv,
Ami
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#39 Re: information needed about low power amp topology

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I maintain that your original circuit would have virtually no standing polarising charge and the capacitor wouldn't lead a very happy life
Yes, I agree, but after you have finished moving those goalposts, this is what I was questioning:
You will therefore realize that the capacitor will spend half its life reverse biased,
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#40 Re: more on that capacitor!

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Ami wrote: Wed Nov 28, 2018 12:42 am I must admit that I'm still a little unsure about the magic that the inductor performs in this circuit, and intend to ask to use the schools oscilloscope to look at what's going on either side of the output capacitor. I have my dads old oscilloscope, but it has its own capacitor issues, so, I can't use that at the mo.
Be good if you can. Should be revealing. :D
 
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#41 Re: information needed about low power amp topology

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the magic that the inductor performs in this circuit
Ideally that would start in the physics department, but I don't know whats covered when in the modern syllabus.
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#42 Re: information needed about low power amp topology

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Nick wrote: Wed Nov 28, 2018 8:52 am
I maintain that your original circuit would have virtually no standing polarising charge and the capacitor wouldn't lead a very happy life
Yes, I agree, but after you have finished moving those goalposts, this is what I was questioning:
You will therefore realize that the capacitor will spend half its life reverse biased,

That was confusing me too, so I asked again, making myself unpopular with Norman, (shame). He wrote....
....In the conjunction that an electrolytic capacitor has no polarising charge, AC will alternately reverse bias the component.

Also, In the conjunction that the source of that AC is of low impedance, and the load is also of low impedance, so that appreciable current flows, as in an audio power amplifier output, then the component can be damaged.

Whether the output capacitor in the "Version 2" of your little amplifier would ever take harm is doubtful, it was just bad practice to arrange the circuit that way, I was just trying to put it to you as politely as possible. If you are still not clear on this, I believe you may be better instructed by asking your teacher.

Also, I don't believe that I "moved any goalposts"

Any one who wishes to discuss it further can contact me by pm at the DIY audio forum, user name Free grid. Please post this message.

Thank you.
Nick wrote:
Ami wrote:the magic that the inductor performs in this circuit
Ideally that would start in the physics department, but I don't know whats covered when in the modern syllabus.
I will do more reading up on the inductor, teach just says that he didn't ask me to include it in the design and its a subject of a more advance unit for study later, thou he offered that it is like an electrical spring that stores energy and tries to preserve it when released, I feel I must be a thickco, because I can grasp that, but can't see how it relates.

I'm sorry for being annoying, I won't be bothering anyone about this again :(

All the best,
Luv,
Ami.
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#43 Re: information needed about low power amp topology

Post by Mike H »

You are not annoying.

Start with this thought - when a current flows through an inductor, it creates a magnetic field. When the current is removed, the magnetic field collapses, which induces a current in the wire in the opposite direction. And therein lies the magic .. :D

So a bit like a spring, yes, it pushes back when let go.
 
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#44 Re: information needed about low power amp topology

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Mike H wrote: Wed Nov 28, 2018 11:29 pm You are not annoying.

Start with this thought - when a current flows through an inductor, it creates a magnetic field. When the current is removed, the magnetic field collapses, which induces a current in the wire in the opposite direction. And therein lies the magic .. :D

So a bit like a spring, yes, it pushes back when let go.
Not just let go, when the current is first applied it pushes back against the applied current. In the same way that a varying voltage creates a current through a capacitor, a varying current through a inductor creates a voltage. In the case of the cap a positive voltage creates a positive current, in the case of the inductor a positive current creates a negative voltage.
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#45 Re: information needed about low power amp topology

Post by Mike H »

Did anyone spot the deliberate mistake? When the field collapses it's the Voltage that's reversed, not the current. :D

Only thought of that this morning.

^ What he said, inductive reactance.
 
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