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Old 13th December 2012, 01:31 AM   #41
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Joachim Gerhard View Post
Here is the second stage of my phono RIAA where you can see the principle. Ignore the name of the Opamp and the inbuilt EQ in the feedback loop.
Joachim , I think the output buffer is the principal cause of the good results that your circuit achieve. Last week I have made a headphone amplifier with the same type of buffer , with opa134 , and I really, really love it .
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Old 13th December 2012, 01:39 AM   #42
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Sure, just a joke. Dirk can take it.
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Old 13th December 2012, 01:41 AM   #43
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Quote:
Originally Posted by dirkwright View Post
Don't get too excited about this circuit. It's actually worse than a "normal" IV converter using opamps, either in terms of noise or distortion. I believe what you would want if you do this is the lowest possible output impedance for the opamp which is why I added the buffer.
Patrick, I must agree with dirk. Is not a good idea taking the output from the supply pins , I know some amplifiers that do that , like the Alexanders amplifier , but in that case the opamp is inside the feedback loop , and that is not the case with the circuit that you like. Tomorrow I will post a circuit that may serve you better.
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Old 13th December 2012, 01:46 AM   #44
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Joachim I was talking about your phono riaa.
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Old 13th December 2012, 02:11 AM   #45
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Ok, i am a bit tired, sorry.
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Old 13th December 2012, 05:22 PM   #46
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I would say that this one deserves the prize as the most simple active IV converter ever. One chip and one resistor! (I am joking)

Anyway, THD = 0.002% for full output, drops to 0.0007% @ half (4mA input). Noise is low.

So, not too bad.
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Old 15th December 2012, 04:26 AM   #47
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I thought I got the OPA627 sounding as good as an opamp could for an I/V stage, nulling out the dc offset from the PCM1704 to it seemed to do a lot of good for them, don't ask me why, all I can say is that it seems the OPA627 as an opamp I/V doesn't like to see dc offset from the dac chip comming into it.
Then yesterday I got my sample LME49710 TO-99 tin can package, I thought no the specs are not as good for an I/V as OPA627 but I jerry riged them into the 8 pin dil sockets and low and behold they are today better than the 627's, cannot fault them, they make the OPA627 sound a bit restricted and not as open top to bottom.
They seem to be more transparent and extended through the top end, the bottom end is punchier yet seems to go lower and tighter and the mids are nice and relaxed and more followable for want of a better word.
I can't wait to get a discrete I/V, one that has cred, hopefully Sergio's once it's sorted and available.
But it seems discrete I/V's are still in there infancy design wise, then there are the prima donas who claim to have the holy grail of discrete I/V's but no one get's to hear them, they have to take their word on it. Are they scared to let them out? or are they waiting for the the offer of big bucks from manufactures to buy their designs even though unproven and unheard?. And never to be disected and appraised by those who can on these forums.
I'm glad I'm not up to designing one, all I can do watch from the sidelines and see the circus go round and round on this and other forums, thank you Serg for presenting yours and getting input from all the others who can, that can only be a good thing to make a design complete.

Cheers George

Cheers George
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Old 15th December 2012, 03:12 PM   #48
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you may find they get tiring in the long-run, I did (the LME49710 to99) somethinga bit too flat about them, not neutral flat, just kinda lifeless flat. Great in regulators though.

then again I dont place quite as much ability on a single opamp swap to make extreme flavor differences in circuit if applied well these days as I did back then and I have more skill in applying them, so maybe it was more application. I do tend to prefer jfet opamps though, except for lme49990, which is rather special, but perhaps not the best for IV

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Old 15th December 2012, 07:31 PM   #49
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I think your right qusp, I listened last night till midnight, and kept saying to myself how good is this bass, how good is the dynamics and micro detail everywhere, how extended/detailed are the highs, why am I not enjoying myself, maybe I'm not in the mood.
I'll swap back to the OPA627 later today or tomorrow to see what gives. Just in case I was not in the mood to enjoy

Cheers George
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Old 21st December 2012, 11:25 PM   #50
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I have to say that all opamps for I/V seem to be a comprimise, I prefer for the long term the OPA627 but I just wish it had the bass power and tightness and dynamic swing of the LME49710. A proper discrete I/V could be the only way to go, but then this seems to be a black art in itself to design right, there doesn't appear to be set guidlines on how these are to be made either.
I'm over it, I just hope that Sergio (SMMS73) has the holy grail, can't wait to try it out once he get a kit going, I'll be in like Flyn (Errol) to try it out.

Cheers George
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