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Old 12th January 2013, 03:51 AM   #11
qusp is offline qusp  Australia
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Originally Posted by abraxalito View Post
You're not doing yourself a favour by following up a NFB-free stage with a common-or-garden opamp!
yes, I found that somewhat ironic too
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Old 12th January 2013, 04:04 AM   #12
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Originally Posted by abraxalito View Post
You're right that the DC output impedance will be the output impedance of the current mirrors in the chip, however its audio so the AC output impedance is the one to consider and that will be determined by the shunt capacitor. You're not doing yourself a favour by following up a NFB-free stage with a common-or-garden opamp!
Yes I am at the moment but at least it's not trying to do I/V which I think has the more impact on if it's an opamp or not.
But later I will try something discrete I have here ready made (attached) straight off the TZ pin if I know it's not going to load down the TZ output.
Let me know what you think.
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Old 12th January 2013, 04:09 AM   #13
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You're right that the I/V stage is the most critical to get linear, so using a normal opamp there is indeed the worst possible place for one. I have no experience of using FETs so I'll leave appraisal of your circuit to others with more relevant expertise. A FET buffer does seem to me to be the right way to go but I'd want to apply at least some passive filtering with a series inductor first.

PS forgot to mention - if you don't load down the TZ node then there's too much gain, you can't just delete the 1k5 resistor, it'll just clip the rails.
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Old 12th January 2013, 06:56 AM   #14
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Quote:
Originally Posted by abraxalito View Post
You're right that the I/V stage is the most critical to get linear, so using a normal opamp there is indeed the worst possible place for one. I have no experience of using FETs so I'll leave appraisal of your circuit to others with more relevant expertise. A FET buffer does seem to me to be the right way to go but I'd want to apply at least some passive filtering with a series inductor first.

PS forgot to mention - if you don't load down the TZ node then there's too much gain, you can't just delete the 1k5 resistor, it'll just clip the rails.
Thanks for that abraxalito, I wondered if I had to keep the resistor to ground on TZ I use a 2.7k with a 470pf across it that should cut the highs -3db @125khz.
So then I'll come off TZ to the discrete buffer, should I take off anything on the input of the buffer to make it easier for the I/V to drive?
Also what do you suggest as filtering on the buffer input? I'm in your hands.
BTW I think it was John Curl who said that the buffer circuit I posted was a good sound design.
Cheers George
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Old 12th January 2013, 07:08 AM   #15
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Well the way I'd go would be to work at a lower signal level rather than a higher one - so reduce that 2k7 down to something at least 10X lower. This is because its hard to get high enough series impedance with inductors coming from such high impedance as 2k7. Inductors always have shunt capacitance which takes over at HF. If your source impedance is low enough then you can start using ferrite beads as inductors which are small, cheap and don't need winding These days I'm using long strings of such beads (more than 10 in a row) to get small, cheap inductors - I've recommended a TDK one further up this thread. Each bead needs a shunt resistor - say 100-200R. Experiment with how it sounds as you add more and more beads, you might find this fun, I did
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Old 12th January 2013, 07:15 AM   #16
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When you say use a 100-200ohm shunt resistor with the ferrite beads, do you mean after the ferrite bead and the resistor to ground?? Would that not be impossible for the i/v to drive through?

Cheers George
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Old 12th January 2013, 07:18 AM   #17
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I mean parallel each bead with its own resistor of that value. So at low frequencies its the inductance which dominates the combined impedance, but as the frequency goes up the resistor takes over. I've had unexplained weird effects without including the resistor - even though the beads are lossy at much higher frequencies there seems to be something odd going on in intermediate freqs (say 1-10MHz).
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Old 12th January 2013, 07:26 AM   #18
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Now I'm with you, a 100-200ohms across the ferrite bead/s. That sounds simple enough, I may not even need much as the PCM1704 is supposely not very noisey.

Cheers George
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Old 12th January 2013, 07:31 AM   #19
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Of course once you're resigned to working with signal levels around the 100mV range then what's the point of doing any I/V with active components? Just put the 50-100R resistor directly on the output of the PCM1704 and do the first stage filtering passively. Then once the signal is securely band-limited add a gain stage.
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Old 12th January 2013, 07:42 AM   #20
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I'm going around in circles, I might as well just love the beautifull sound I have now, with the AD844 output buffer and OPA134 LP filter.
Cheers George
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