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Old 7th January 2013, 07:40 AM   #771
qusp is offline qusp  Australia
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In contrast, the SwiftCurrent circuit performs this I/V conversion with Wadia’s patented current-conveyor integrated circuit (IC) that eliminates feedback. This bipolar IC consists of an innovative connection of Wilson current mirrors, an emitter degeneration compensation scheme to optimize transient response and stability, and a novel current mirror arrangement to enhance output impedance."
hmm as usual the claims of no feedback are smoke and...mirrors. conveniently deeming degeneration not part of the feedback network.

it still sounds very interesting Jan, so keen to see your schematics.
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Old 7th January 2013, 05:49 PM   #772
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Well I don't know, how can you learn anything with your mind already firmly made up

But here goes.

jan
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Old 7th January 2013, 06:26 PM   #773
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degeneration is feedback, thats all i'm saying, 'no feedback' is a marketing term. that doesnt mean it cant be an interesting circuit all the same

the thing is full of local feedback/degeneration, its pretty darn cool though

it has no global negative feedback, I just wish that was the term people used, but they almost never do.

thanks for the scheme BTW, another one filed for later.

Last edited by qusp; 7th January 2013 at 06:33 PM.
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Old 8th January 2013, 12:21 AM   #774
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Quote:
Originally Posted by jan.didden View Post
Well I don't know, how can you learn anything with your mind already firmly made up

But here goes.

jan
WRT Wadia I-V, It's a pretty complex circuit that doesn't really bring a huge
amount of linearity to the table. It won't come close to the circuit in this
thread.

With careful design, a lot simpler [open loop I-V's can do aswell or better
than the Wadia.

It's also arguable that it is a true open loop design as there is + feedback
used to iron out IP BJT non linear emitter resistance.
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Old 8th January 2013, 02:48 AM   #775
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I found an article from wireless world here.
Index of /Wireless-World-Stuff

PA630 looks not so complex circuit, but just a CC1 with buffer.
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Old 8th January 2013, 07:17 AM   #776
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BTW I have had very good results with the AD844, another CCII with buffer. In contrast to the PA630 it is a balanced circuit instead of single ended, although that only manifests itself in somewhat better DC characteristics.

I've used the AD844 in CCII role (open loop) in power amp feedback/error correction circuits with excellent results.

jan
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Old 8th January 2013, 07:31 AM   #777
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Funny you should bring up the AD844 Jan.
I found this circuit of Pedja Rodic's (attached) and used it as is (NO FEEDBACK) and it worked a treat, best opamp based I/V so far by a long way, it blew the OPA627 completely away I though it would never be beaten except for a discrete I/V of some sort. (And I’ve tried just about every opamp out there)
I used it without feedback as per the diagram and was very very happy. Used with TZ as he has it 1.5K with 1000pf to ground, I think this gives a 1st order rolloff –3db @100khz.
Does the 1.5k resistor have a bearing on what input resistance the dac (PCM1704 in my case) sees?
Because I changed the resistor for 2.2k to get some more gain but this time with a 260pf cap –3db @ 270khz (still all nice and stable with a bit more gain) still very very good.
But I had the feeling it was puchier and more dynamic with the 1.5k resistor even though I had less gain. I feel that the input loading for the PCM1704 is better with the 1.5k than the 2.2k resistor or am I imagining things?.

Cheers and thanks for your input George
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Last edited by georgehifi; 8th January 2013 at 07:40 AM.
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Old 8th January 2013, 08:52 AM   #778
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again, how is that No Feedback? conveniently thinking of local and internal mechanisms as something other than feedback does not make it so. transistors inherently have feedback, it cannot be switched off no matter how you arrange them.
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Old 8th January 2013, 09:43 AM   #779
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Quote:
Originally Posted by qusp View Post
again, how is that No Feedback? conveniently thinking of local and internal mechanisms as something other than feedback does not make it so. transistors inherently have feedback, it cannot be switched off no matter how you arrange them.
QSP, FYI - and you -should- know this by now:

zero FB = no global or inter-stage FB. Usually means no local FB but in the
case of the Wadia CCT and this threads I-V there are small amounts of + FB.

Get used to it, that's the terminology that has evolved and we don't need
another 100 posts discussing the fact that an EF has 100% FB etc etc.

We -all- know what you are talking about, it's just an accepted terminology,
right or wrong.

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Old 8th January 2013, 09:49 AM   #780
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OK, no global feedback.
Whatever it's got it's got magic like there's no rev limiter, and I have no idea what the distortion is but it sounds distortionless, it presents my cd's in a totaly different way than I've ever heard them.
I would love to know what the input impedance is configured this way, but I suppose only Mr Gilbert the designer for ADI would know has he only knows the whole circuit.
All other opamps for I/V sound a little different tonally to each other, but this AD844 "with no global feeback" has made my whole cd collection a new listening experience.

Cheers George
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