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Old 5th April 2012, 08:37 PM   #111
jcx is offline jcx  United States
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driven guard rings, planes can bootstrap away much of the pcb dielectric stray C - someone even patented the concept for audio amplifiers

but again to optimise the VAS, CCS to the degree PCB parasitics are limiting means you have to have spent lots of effort on the VAS load - the output stage - for its much larger device parasitics, reflected load current to not be the orders of magnitude larger limiting terms
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Old 5th April 2012, 08:41 PM   #112
bcarso is offline bcarso  United States
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driven guard rings, planes can bootstrap away much of the pcb dielectric stray C - someone even patented the concept for audio amplifiers
Astonishing (that such could be patented).
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Old 5th April 2012, 09:09 PM   #113
jcx is offline jcx  United States
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if it were still active I wouldn't pay any attention to it as legal threat "obvious to a practitioner of the art" and all

in fact I didn't let that patent stop me: Spice simulation

Last edited by jcx; 5th April 2012 at 09:13 PM.
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Old 6th April 2012, 05:54 AM   #114
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Does the fact that the impedance of these CCS designs changes with frequency, and often becomes lower in the higher areas of the audio band mean that the response of the VAS will change also? Instead of going for the maximum impedance, shouldn't we be more concerned with a constant impedance across the audio band even if that means the absolute impedance is less?
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Old 6th April 2012, 10:05 AM   #115
AndrewT is offline AndrewT  Scotland
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If the VAS can drive the varying impedance then it's not a problem.
It's when the impedance forces the VAS to misbehave that you are asking too much.
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Old 6th April 2012, 03:59 PM   #116
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If the VAS can drive the varying impedance then it's not a problem.
It's when the impedance forces the VAS to misbehave that you are asking too much.
Gain in a VAS is a function of the load, so the gain changes with changes in the load. So, I guess what I'm saying is the bandwidth of maximum gain for a VAS is a function of the bandwidth of the CSS load. I suppose if there is enough gain over a broad enough bandwidth for the applied feedback, then this isn't a problem. It would be a problem for very high gain circuits I think because the bandwidth would be reduced.
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Old 6th April 2012, 07:45 PM   #117
tvrgeek is offline tvrgeek  United States
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Does the fact that the impedance of these CCS designs changes with frequency, and often becomes lower in the higher areas of the audio band mean that the response of the VAS will change also? Instead of going for the maximum impedance, shouldn't we be more concerned with a constant impedance across the audio band even if that means the absolute impedance is less?
That is a major question. I suspect the linearity to be more important which is why I am trying to find any FETs that are fast enough to be constant over the audio band even though they are not as stiff as a standard 2N5551 and red LED. Remember the "vast" differences between types of CCSs are trivial compared to just a resistor, and there is still a large camp who believe the resistor sounds better. The only way to tell is to build them.

The overt response of the VAS or IPS would not change. It is the smaller differences that are not clear what we are measuring. I suspect that the FBP topology is so widely used in VAS where the BJT/led is widely used in the IPS is a hint. In my DH-120 project, the single FET IPS actually modeled better when I looked at multiple levels and multible frequencies to for the FFT, but only with a very fast unobtainable FET.
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Old 6th April 2012, 08:28 PM   #118
bcarso is offline bcarso  United States
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Linearity is a misnomer. Constancy is the term applicable, in this case constancy with frequency. If it were somehow key then a resistor would be best.

In fact what is frequently done is that the output node of the so-called VAS is loaded anyway with one or two resistors to common. This provides a limit to the low-frequency gain which some believe they can hear the effects of. It also makes the overall amplifier's response to load impedance more consistent, that is, such that the loop gain at low frequencies through midband stays roughly constant regardless of load.

Putzeys argues that the concern about increasing open-loop gain at low frequencies is not a big deal, and shows the settling behavior for a step signal for two cases in his "The F-Word" article in Linear Audio. But people disagree, and the notion that more loop gain entails more canine tail-chasing is a prevalent one.
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Old 6th April 2012, 08:36 PM   #119
tvrgeek is offline tvrgeek  United States
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So, here are my results and test circuit for the Central Semi 3.5 mA CRD (CLD). I think it's pretty good, with constant current fading at about 20kHz and beyond. The voltage source VG1 is set for an internal impedance of 1k.
If you look at what this is, you will see it is exactly the single FET, self biased with a resistor. I have not imported their model. I hope they picked a very fast FET. If so, it could be a winner. Looking at the data sheet does not give me a lot of hope.
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Old 6th April 2012, 08:46 PM   #120
tvrgeek is offline tvrgeek  United States
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Originally Posted by bcarso View Post
Linearity is a misnomer. Constancy is the term applicable, in this case constancy with frequency. If it were somehow key then a resistor would be best.

In fact what is frequently done is that the output node of the so-called VAS is loaded anyway with one or two resistors to common. This provides a limit to the low-frequency gain which some believe they can hear the effects of. It also makes the overall amplifier's response to load impedance more consistent, that is, such that the loop gain at low frequencies through midband stays roughly constant regardless of load.

Putzeys argues that the concern about increasing open-loop gain at low frequencies is not a big deal, and shows the settling behavior for a step signal for two cases in his "The F-Word" article in Linear Audio. But people disagree, and the notion that more loop gain entails more canine tail-chasing is a prevalent one.

Syntax. I would consider something that is consistent over frequency to be linear. We do both mean the same thing. I have not found the exact references to Leach where he states " ccs measure good but sound bad". For all I know, it is totally out of context. Now, given the huge difference between just a resistor and the least stiff, but most "consistent" ( self biased FET) , compared to the most stiff ( JFET cascode), which sounds better? That folks is the question at hand. Build and listen. I would bet the answer may depend on the quality of the power supply which is something not included in the models.
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