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Old 14th April 2008, 03:18 PM   #71
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Default then i will

rest my case and keep in minde that your testing gear or knowledge about this is limited ....

no probs my friend if sounds good to your ears is perfect for me also ........

but on the other hand i have never seen a vas stage created with darligton any way .... so all of these is totally academic .....plus if we keep in mind that the miller cap is actually conected from the base of the driver transistor to the collector of the output transistor of the all darligton .....

so i am not familiar with this topology also .....
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Old 14th April 2008, 03:44 PM   #72
Nordic is offline Nordic  South Africa
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I was thinking about the question of which should come first the cap or the resistor....

My first response was, it should not matter... then it started haunting me...


Imagine the resistor is connected to ground side... the side the capacitor sees of this resisor is not at 0V DC, it is at whatever the voltage over that resistor would be at x frequency...

With the cap to ground (its internal resistance hopefully much smaller than the resitor) has a closer to 0 potential at the junction point....

I doubt it is audiable, and I certainly don't have the gear to test the validity of this conclusion, I am likely missing something very obvious....

A question for you sakis....

I think it was on one of your boards I once saw a screw on bracket for attatching boards to heatsink... I think you even had it in eagle... if it was you.. care to give me the details...
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Old 14th April 2008, 04:44 PM   #73
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Default soory nordic

it seems that i dont remember what are you talking about ....email me
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Old 1st March 2012, 09:56 PM   #74
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What would be the correct order to put these elements at the output of a power amplifier?

-output capacitor (for cheap speaker protection)
-output switching relay (headphones out/speaker out)
-Zobel 10R / 0.1u
-Zobel L/R
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Old 2nd March 2012, 09:32 AM   #75
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by macplauder View Post
What would be the correct order to put these elements at the output of a power amplifier?
fourth -output capacitor (for cheap speaker protection)
third (or second) -output switching relay (headphones out/speaker out)
first -Zobel 10R / 0.1u
second (or third) -Zobel L/R

fifth - add a resistor to charge and discharge the DC blocking capacitor

The combination of 1st & 2nd is the output Thiele Network.
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Old 10th May 2014, 07:46 PM   #76
AndrewT is online now AndrewT  Scotland
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Bonsai
Zobel Or No Zobel ?
Is your opinion stated in 2008 still the same some 6 years on?
Quote:
always use a Zobel network and an output series inductor/resistor network on an amplifier.
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Old 11th May 2014, 10:44 AM   #77
AndrewT is online now AndrewT  Scotland
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GK is not here to reply.
Can anyone else explain GK's statement
Quote:
Though I think in many cases an RC Zobel is included just out of habit and is not necessary in many cases – particularly so when included in the misguided belief that a high frequency load is generally necessary to keep an amplifiers open loop gain and phase margin under control.
particularly this part:
"misguided belief that a high frequency load is generally necessary to keep an amplifiers open loop gain and phase margin under control."


I thought that the R+C part was specifically there to provide a resistive load at high frequency.
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Old 11th May 2014, 04:32 PM   #78
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I think it would depend on what GK was driving at; the habitual use of a Zobel rather than experimental proof in each example, or that he believes that an HF load has nothing to do with the control of OLG and PM.

Douglas Self concludes with more certainty, based on his own experimental evidence from the CFP blameless design which is in all APAD handbook editions I've read:

Quote:
"There seems no doubt that a Zobel network is required with any load that is even mildly inductive."

The principle reason given being that amplifier HF instability is induced by interaction with the loudspeaker's inductance.

The load of the RC network on the amplifier is not great, being around 12 mA current @ 20kHz with 1V output. For comparison, an 8R resistive load draws 125 mA at all frequencies.
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Old 11th May 2014, 05:05 PM   #79
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For interest, here's a reply by Bob Cordell, addressed generally to that comment and others in this thread by GK:

"I don't think I have ever built a solid state amplifier without a Zobel, and in my opinion only a fool would do so. Here is why. Even if the amp is stable on the bench with no load or a resistive load, you don't know what the complex nature of the load might be when speaker cables and a speaker are connected. The presence of the Zobel network largely guarantees the absence of a high-Q resonance on the output node of the amplifier.

Cheers,
Bob "
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