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Old 28th February 2008, 03:10 PM   #1611
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Hi,

....that's why I have asked Mr. Pass for the purpose of the caps.... :-)

I would not be surprised when there is nothing really new implemented and I keep my mind: the only secret is a tricky bias circuit.
Mr. Pass said that his resistors are choosen for double duty. Perhaps it is possible that the gate - supply resistors have two jobs:
One duty is to source current to the VBC in stand by mode and the other is to get more voltage swing. I try to understand the behave of such a resistor in parallel to a device working in common source mode.
There has to be a way to get more swing (and perhaps more music swing, too :-) )

....am waiting for my boards......hoping to get them next week.....
My hands are burning! :-)

Dirk
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Old 28th February 2008, 03:53 PM   #1612
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Quote:
Originally posted by noisefree
Hi,

....that's why I have asked Mr. Pass for the purpose of the caps.... :-)

I would not be surprised when there is nothing really new implemented and I keep my mind: the only secret is a tricky bias circuit.
Mr. Pass said that his resistors are choosen for double duty. Perhaps it is possible that the gate - supply resistors have two jobs:
One duty is to source current to the VBC in stand by mode and the other is to get more voltage swing. I try to understand the behave of such a resistor in parallel to a device working in common source mode.
There has to be a way to get more swing (and perhaps more music swing, too :-) )

And Nelson was kind enough to answer

I too noticed the "double duty" phrase but this could mean any number of things. My guess would be for standby and for loading the FE for better DC stability.

The "tricky bias circuit" is the interesting bit. To get as much as a 20% increase in power, this has to be doing something significant. I don't really see why the bias circuit should restrict the power in the first place, the "swing" is limited by the need to swing the gate above the PS voltage by ~Vgs of the output devices. I'm obviously missing something...

Ian.
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Old 28th February 2008, 06:33 PM   #1613
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Quote:
Originally posted by Ian Macmillan
Let me start with some caveats. I donít pretend to be any kind
of expert on the subject of bootstrapping.
Based on the description that followed, I would say you understand
all there is to know. One item, and that is where I have seen
arguments about the detrimental effects of "regenerated
distortion" which would refer to the fact that the bootstrap
voltage contains the output signal and its distortion, thus
reinforcing that distortion at the output of the VAS. I don't
think this is a real problem in intelligently designed circuits,
but you will want to use sufficient resistance in the circuit
to avoid it.
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Old 28th February 2008, 06:48 PM   #1614
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Quote:
Originally posted by Ian Macmillan
I too noticed the "double duty" phrase but this could mean any number of things. My guess would be for standby and for loading the FE for better DC stability.

The "tricky bias circuit" is the interesting bit. To get as much as a 20% increase in power, this has to be doing something significant. I don't really see why the bias circuit should restrict the power in the first place, the "swing" is limited by the need to swing the gate above the PS voltage by ~Vgs of the output devices. I'm obviously missing something...
It's really pretty simple. By driving the center point of the bias
voltage, the front end can effectively swing about 4 more volts
on each peak. The resistors from each supply function to provide
current to the bias voltage generator and load the front
end with a known resistance.

On a circuit like this, that 4 extra volts can be the difference
between 100 watts and 144 watts.

Also, keep in mind that the output devices want some extra
volts left over between Drain and Source on peaks, something
on the order of 5 volts or so.
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Old 28th February 2008, 10:28 PM   #1615
gl is offline gl  United States
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Pillar of smoke by day and fire by night - and in the morning.........breadcrumbs all over the ground!

Graeme
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Old 29th February 2008, 08:06 AM   #1616
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Thank you very much Nelson - more help than I had any right to expect. I'll start working up an implementation, one without resorting to AC coupling if I can.

Ian.
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Old 29th February 2008, 12:38 PM   #1617
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Yes, very, very likeable and noble from our master giving so many hints!

Many thanks!!!!!!

OK, all VBCs were wrong.....
New try......

Perhaps I am wrong again, but think there has to be a voltage divider in parallel to the voltage reference where each of these two resistors is bypassed with an elcap. In the center of this bypassed divider the front end (neg and pos output connected together) will drive it. Without the divider there is no trimming possibility for differential and absolute offset.
When the series resistor idea should work in this new approach, than there could be switchable resistors in parallel or in series to the known 10k/3W, to allow sourceing two different currents through the bias stage.

Regards
Dirk
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Old 29th February 2008, 04:19 PM   #1618
gl is offline gl  United States
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Nelson has said 1) that the two 220uf caps are across the bias circuit and 2) made reference to the advantages of driving the center point of the bias voltage.

Where have we seen this before? I would suggest the F4.

Graeme
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Old 29th February 2008, 04:29 PM   #1619
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Quote:
Originally posted by gl
Nelson has said 1) that the two 220uf caps are across the bias circuit and 2) made reference to the advantages of driving the center point of the bias voltage.

Where have we seen this before? I would suggest the F4.

Graeme
Nelson also said that the bias circuit is not the same as the F4. Of course it depends on whether you interpret this as just a little different or something more radical. I would guess it is a derivation of the F4 circuit but I doubt that it uses AC coupling of FE to output stage as otherwise the feedback cannot correct for any DC releative offset.

Ian.
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Old 29th February 2008, 04:35 PM   #1620
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Quote:
Originally posted by noisefree
Perhaps I am wrong again, but think there has to be a voltage divider in parallel to the voltage reference where each of these two resistors is bypassed with an elcap. In the center of this bypassed divider the front end (neg and pos output connected together) will drive it. Without the divider there is no trimming possibility for differential and absolute offset.
When the series resistor idea should work in this new approach, than there could be switchable resistors in parallel or in series to the known 10k/3W, to allow sourceing two different currents through the bias stage.

I feel you may be on the right track with the bypassed voltage divider (which would also place the caps across the bias circuit to answer Graame's comment). I'm less convinced by the suggestion of needing to add additional trimming. Come to think of it, do we know that the FE is shutdown in standby or was this an assumption? I'm thinking that it may make more sense to lower the bias by shunting the bias control resistor and leaving the FE powered on to deal with relative DC offset. I suppose one might also have to short the input or similar to ensure the amplifier wouldn't pass any signal in this condition.

Ian.
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