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Old 21st February 2013, 12:16 AM   #241
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I don't think you'd "hear" the difference of that filter--it's effect at audio freq would be minimal.

What it's probably doing is changing the loading of the AD844 or the increase in high freq is affecting something down chain. Something strange and technical no doubt! I'll sim it up for ya later tonight!

Gotta go play with me' boy!

Cheers,
Jeff


PS Filters are weirdass, what freq is your 3rd order at? It could be interacting with that 100k ...

Last edited by AudioLapDance; 21st February 2013 at 12:21 AM.
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Old 21st February 2013, 12:35 AM   #242
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Thanks Jeff, but I have my doubts whether siming it will show anything, as I think it's a stability issue only under musical dynamic conditions. As it looks perfectly stable on the crow with all test waves either way. I believe the 570pf is sending it into slight ringing, as audibly it 's giving the opposite to what you would expect, sweeter without the cap. And I forgot to mention with the 560pf capthe DC offset absolutly stable to within .5mV Without the cap it can wander just a touch more, still very good.

Cheers George
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Old 21st February 2013, 02:26 AM   #243
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PCM1704: current output DAC, dual mono, single ended, 1.2 mA full scale current, 0V reference voltage. I'm not sure how much current offset it has but I left it as 2mA and cancelled it with the external current source (IS1 2mA).

AD844 iv PCM1704 tina circuit.png

2.7k with 560pf and 1pf
AD844 iv PCM1704 freq resp cap nocap.png

Actually I take it back, maybe this would affect high freqs ...
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Old 21st February 2013, 03:03 AM   #244
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300Hz square wave
AD844 iv PCM1704 300hz square cap nocap.png
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Old 21st February 2013, 03:11 AM   #245
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Quote:
Originally Posted by AudioLapDance View Post
PCM1704: current output DAC, dual mono, single ended, 1.2 mA full scale current, 0V reference voltage. I'm not sure how much current offset it has but I left it as 2mA and cancelled it with the external current source (IS1 2mA).


2.7k with 560pf and 1pf
Attachment 331768

Actually I take it back, maybe this would affect high freqs ...
Thanks Jeff.
I take it
The brown in both graphs is 2.7k no cap
The red in the top graph is 2.7k with 1pf
The green in the lower graph is 2.7k with 560pf

I also noted that your taking the output from pin 6 and not pin 5?

Cheers George
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Old 21st February 2013, 03:16 AM   #246
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Sorry I understand now ones gain and the other is pahase angle.

Cheers George
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Old 21st February 2013, 03:23 AM   #247
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So yes with the 560pf cap I hear a tad more forwardness in the highs, which gives a change in phase angle from 5k> and it's this that I maybe hearing as a peaked up top end, not the hf response droop.
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Last edited by georgehifi; 21st February 2013 at 03:52 AM.
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Old 21st February 2013, 03:55 AM   #248
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Can anyone hear a 3 degree phase difference between 5khz to 20khz? And does it manifest itself as being a tad brighter?

Cheers George
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Old 21st February 2013, 04:09 AM   #249
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Hey George,

I wanted to give some 'pay back' for all the work you've done in the thread! Since you're not into Tina, feel free to request any sim.

Speaking of sim, all I wanted to show was the basic freq changes. I'm a bit leery of trying to connect even something that seems so basic as higher roll off = more highs
If this was basic filters maybe .... but this is a wacked out IV
Again, I don't understand enough about the common base configuration to know if the cap is required ...

What we do know is at least what the sim is telling us:

A lot more hi freq gets to the next stage with no cap ...


And there is some change to audio freq (I'll zoom in for you)

AD844 iv PCM1704 freq resp cap nocap 10Hz to 100kHz.png

Seems minimal but you should know whats going on ...

Last edited by AudioLapDance; 21st February 2013 at 04:23 AM.
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Old 21st February 2013, 04:32 AM   #250
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Here's pin 5:

AD844 iv PCM1704 tina circuit pin 5.png

And freq:

AD844 iv PCM1704 freq resp cap nocap pin 5.png

And close up 10Hz to 100kHz:

AD844 iv PCM1704 freq resp cap nocap 10Hz to 100kHz pin 5.png

Last edited by AudioLapDance; 21st February 2013 at 04:35 AM.
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