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Old 17th January 2013, 02:13 PM   #101
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Quote:
Originally Posted by georgehifi View Post
No Dirk, just how it/they sounded, and I did look on the scope for occilations and how the square waves looked and noise, and measure offsets and such to make sure nothing was going on that shouldn't have been, but no distortion measurements were done.
They (all feedback opamps) sound bad in comparison to the "feedback less AD844". Just just they all have a different type of bad and it comes down to which you don't dislike the most, I have found after listening to the AD844.

Cheers George
Well, my ears and my brain are different than yours so subjective evaluation isn't of much value to me because what sounds good to you may not sound good to me. I believe you when you say that what you are hearing is better but obviously I can't evaluate that myself without going through the very significant trouble and expense of designing and making an appropriate circuit. I may not be capable of hearing the difference either, so a higher performance circuit may be lost on me anyway.
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Old 17th January 2013, 02:32 PM   #102
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Originally Posted by georgehifi View Post
I gave it here
Using the AD844 as an I/V

Cheers George
Ok, I modeled this circuit in Tina using Analog's spice model for the AD844. Distortion is low, but not screaming low. Noise is high. S/N is less than 110dB @ 1kHz. Bandwidth rolls off @ 20kHz. Gain is high @ 68dB or so. It's not suitable for the PCM1794a because the output of that DAC is too high. It's ok for the PCM1704 @ 1.2mA.

I don't see any evidence so far that shows this is better than the normal feedback IV converter. That's not to say it doesn't sound better to you.
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Old 17th January 2013, 05:23 PM   #103
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I'm so impressed with the LM6181 that I'm going to buy 4 of them for my IV converter. Noise is about the same as the LME49710, but distortion is even lower in an IV converter application, even at high frequencies. It needs feedback to function so it can't be used in place of the AD844 circuit on this thread.
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Old 17th January 2013, 07:12 PM   #104
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Hi Dirk you modeled it the way I used it in post 62?
Can you do the same for Peja's way as well in post 63?
I would particually like to know what the input impedance differences are of both ways is from 5hz to a few mhz. Not interested in disortion or noise if you want to keep it simple, as I cannot detect any of these over normal opamp based I/V's.

Thanks George
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Old 17th January 2013, 07:34 PM   #105
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Originally Posted by georgehifi View Post
Hi Dirk you modeled it the way I used it in post 62?
Can you do the same for Peja's way as well in post 63?
I would particually like to know what the input impedance differences are of both ways is from 5hz to a few mhz. Not interested in disortion or noise if you want to keep it simple, as I cannot detect any of these over normal opamp based I/V's.

Thanks George
Yeah, I modeled the one in post 62. I'll get to the other later.

I'm really in love with the LM6181 now. I partner it with the BUF634 and use a 200 ohm feedback resistor.
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Old 17th January 2013, 08:50 PM   #106
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It would be interesting to know the reasons why Ayre stopped utilizing the AD844 in the output stage of one of their earlier CD players, I don't recall the model. I believe they used the chip in the manner Pedja Rogic does, open loop with the transimpedance node connected to ground via a resistor.
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Old 17th January 2013, 09:00 PM   #107
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The LM6181 to me (which isn't much) looks good also on paper, but it doesn't have the magic TZ (pin 5) that the AD844 has that allows one to use it without feedback.
I feel this is one of the reasons the AD844 sounds so good. It's as though feedback in an I/V stage is a big no no, as it can get corrupted with all the HF glitches and stuff comming out of the dac chips. Please correct me if I'm wrong all you I/V aficionados that know better than I do? Because I'm going more on what's sounding better, rather than what's measuring better.

Cheers George
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Old 17th January 2013, 09:31 PM   #108
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Quote:
Originally Posted by dirkwright View Post
Ok, I modeled this circuit in Tina using Analog's spice model for the AD844. Bandwidth rolls off @ 20kHz.
If feel Tina is not giving the correct info here as I, look on the scope at the TZ point and the output buffer point of a 1khz test square wave I have on cd going into the AD844 from the PCM1704.
And the square wave was pin point square (no rounding at all) on the leading and trailing edge, it's as though it's bandwidth was at least out to 100khz, it did have a very small Gibbs ringing on both corners but that is natural, the sine wave sweep that I have goes from 5hzh to 20khz, it's dead flat all the way. I will take pics next time it's on the bench and show them.

Cheers George
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Old 17th January 2013, 10:05 PM   #109
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It's all a bit above me some of you may find it interesting.

A quote from John Curl on the designer of the AD844 Barrie Gilbert
"This is what makes Barrie Gilbert's analysis significant and useful. It shows a potenial distortion that is NOT removed by global feedback"

And here is Barrie Gilbert's paper called "Are Op Amps Really Linear"
http://www.linearaudio.nl/Documents/...y%20Linear.pdf

Cheers George
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Old 17th January 2013, 10:12 PM   #110
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Originally Posted by georgehifi View Post
The LM6181 to me (which isn't much) looks good also on paper, but it doesn't have the magic TZ (pin 5) that the AD844 has that allows one to use it without feedback.
I feel this is one of the reasons the AD844 sounds so good. It's as though feedback in an I/V stage is a big no no, as it can get corrupted with all the HF glitches and stuff comming out of the dac chips. Please correct me if I'm wrong all you I/V aficionados that know better than I do? Because I'm going more on what's sounding better, rather than what's measuring better.

Cheers George
The phase shift for the AD844 circuit is minimal over a huge bandwidth. That may have something to do with it.
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