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Old 27th May 2010, 10:29 AM   #11
SY is offline SY  United States
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What makes these amps "current mode?" The feedback looks like conventional voltage feedback, unless I'm missing something.
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Old 27th May 2010, 11:00 AM   #12
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Hi,

Thanks once again for the helpful suggestions. I've done some more measuring, and indeed, the two input Jfets are nowhere near matched. I tweaked the value of LR8 just to get the amplifier in a working state.

I'll have a look at the thread that Homemodder suggested, and try to purchase some matched Jfets for this.
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Old 27th May 2010, 11:27 AM   #13
timpert is offline timpert  Netherlands
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Quote:
What makes these amps "current mode?"
I think it's a current feedback design, because the current to charge/discharge LC2 is provided via the two current mirrors by LT1 and LT2, and so by the signal on the inverting input (the common point of the source resistors of the jfets). However, the source resistors are somewhat high in value, so the feedback network's impedance into the inverting input will only have a limited influence on the location of the dominant pole once its impedance gets lowish (as it is now). It will however make the amplifier slower once its impedance gets significantly above 60 Ohms.
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Old 27th May 2010, 01:52 PM   #14
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Quote:
Originally Posted by sakis View Post
reminds of terrible elektor designs that are simulation products and mostly based on perfect parts that actually dont exist ....

the construction /time/effort /cost/ consumed to pick up the proper parts might be twice as much than the cost of amplifier it shelf .... Titan i thing the miracle was called ...

i will go though with amv8 and see where is this 2 volt originate from ...

It will work quite happily but with reduced performance without matched parts provided you trim the offset the way I discribed.

Although this amp vas is different to what I use I wouldnt call it a simulation product, Im not 100 percent sure but this type design has been in use for some 30 years. Is it worth time, effort, cost, my answer is absolutely yes, designed and built properly it simply sounds better than any Self type amp and much better than a P3 amp, its simply in a diffirent legue.
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Old 27th May 2010, 02:28 PM   #15
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Quote:
Originally Posted by timpert View Post
I think it's a current feedback design, because the current to charge/discharge LC2 is provided via the two current mirrors by LT1 and LT2, and so by the signal on the inverting input (the common point of the source resistors of the jfets). However, the source resistors are somewhat high in value, so the feedback network's impedance into the inverting input will only have a limited influence on the location of the dominant pole once its impedance gets lowish (as it is now). It will however make the amplifier slower once its impedance gets significantly above 60 Ohms.
I guess I'm a bit thick. It still looks like the feedback signal is derived by dividing down the output voltage and applying that to the inverting input. LC2 is only 45pF so would appear to be compensation. So it still looks to me like a conventional constant-voltage amplifier with voltage feedback.
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Old 27th May 2010, 02:47 PM   #16
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Quote:
the feedback signal is derived by dividing down the output voltage and applying that to the inverting input
A current feedback amplifier has that too. Only, the impedance provided at the - input determines how fast the amplifier will settle because the current into the inverting input is passed onto Cdom. Low impedance = fast settling but reduced phase margin. So with low gain you provide a high(ish) impedance to get the amp stable, while at high gain, the impedance of the feedback network can be lowered because the increased gain margin allows you to do so. As a result, a high gain configuration can be as fast as a low gain config. Voltage feedback amps cannot do this.

Last edited by timpert; 27th May 2010 at 02:54 PM.
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Old 27th May 2010, 03:16 PM   #17
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OK, you'll have to go a little slower for me. I get that when they say "current mode," they don't mean "constant current" amplification in the Mills & Hawksford sense. But looking at the feedback, the capacitor is across a 47k resistor. That's an f3 of 80kHz- I can't imagine that, in most of the audio band, there's any significant effect of the cap, and the feedback looks like plain vanilla voltage feedback. The cap appears to me to be a way of slowing down the NPN half to match the PNP half.

Sorry to be a pain, but I'm trying to understand this.
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Old 27th May 2010, 03:41 PM   #18
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Yeah, but C0 effectively shorts the bases of the NPN and PNP drivers for any highish frequency. So LC2 is essentially parallel to both. If it really is compensation, then the designer is relying in the ESL of C0. Bad practice.

The design is a little strange (didn't look at the values), but the fact remains that the current into the - input matters, its impedance is pretty low and effectively in parallel with RF2. The current mirror is set to a ratio of about 4.8, so the current injected into the - input will be sent through LR12, LR14 (which are in parallel for this current path) and LC2 multiplied by 4.8. I guess that is whay they mean by "current mode". Altering the impedance of the feedback network would probably show a bit of shifting of the pole by LC2, but I don't know how much and currently I'm too lazy to grab a calculator.
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Old 27th May 2010, 04:08 PM   #19
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Quote:
Originally Posted by SY View Post
Sorry to be a pain, but I'm trying to understand this.
Have a squint at figure 1 in this AD appnote - the amp here is a variant of the Alexander Current Feedback amp : ADI AN-211. The main difference is here we have a complementary pair of JFETs acting as the input buffer in lieu of an opamp.

Mark Alexander also explains how the output offset arises and shows how to fix it with a servo amp.
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Old 27th May 2010, 04:31 PM   #20
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Merci!
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