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Old 30th June 2013, 01:24 PM   #1
Arno Pf is offline Arno Pf  Netherlands
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Default PCM63 resistor Value

After several prototypes I am wondering what the best path would be.

Load the PCM63 with a 10 Ohm resistor and add 10X extra amplification behind it?

Or...stick with the max 100 Ohm and leave the amplification behind in 10x lower?

I know the data/measurements for values above 100Ohms, but is there also data (or blind listening tests) for values below 100 Ohm?
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Old 1st July 2013, 01:07 PM   #2
Arno Pf is offline Arno Pf  Netherlands
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69 people looking...so better put some answers here

Well, with the LL1636 attached (no secundary load), I varied the Prim resistor values at the DAC side and measured the peak-voltage at the DAC-output with a scope.

It appears that the DAC current source has it's stable region with a load of 15...50 Ohms
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File Type: jpg PCM63RloadDataFennix62013.jpg (23.7 KB, 199 views)
File Type: jpg PCM63RloadGraphFennix62013.jpg (18.6 KB, 197 views)
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Old 2nd July 2013, 12:10 PM   #3
Arno Pf is offline Arno Pf  Netherlands
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20Hz Different loads, signal behind the I/V transformer. Sine signal distorts at high Rdac (68 Ohm).

This is either caused by the current source of the DAC, not having enough internal voltage to provide sufficient current....or the higher voltage of the input I/V transformer when using the higher Rload causes the transformer to saturate (?No?). Could be verified with a CD having 20Hz signal at 0dB, -10dB, -20dB, -30dB etc...

The 68 Ohm load causes a clear deformation of the signal:
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File Type: jpg IMG_2847 Rdac 68 Ohm.jpg (47.4 KB, 182 views)
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Old 2nd July 2013, 12:12 PM   #4
Arno Pf is offline Arno Pf  Netherlands
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Next with other loads where 15 Ohm and 22 Ohm show hardly any problem and here THD should be measured to see the impact.
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File Type: jpg IMG_2848 Rdac 37 Ohm.jpg (45.2 KB, 178 views)
File Type: jpg IMG_2849 Rdac 22 Ohm.jpg (43.8 KB, 176 views)
File Type: jpg IMG_2850 Rdac 15 Ohm.jpg (47.2 KB, 19 views)
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Old 6th July 2013, 03:04 AM   #5
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Quote:
Originally Posted by Arno Pf View Post
20Hz Different loads, signal behind the I/V transformer. Sine signal distorts at high Rdac (68 Ohm).

This is either caused by the current source of the DAC, not having enough internal voltage to provide sufficient current....or the higher voltage of the input I/V transformer when using the higher Rload causes the transformer to saturate (?No?). Could be verified with a CD having 20Hz signal at 0dB, -10dB, -20dB, -30dB etc...

The 68 Ohm load causes a clear deformation of the signal:
try the same measurements and test, only this time leave pin 5 floating....

Boky
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Old 6th July 2013, 04:17 AM   #6
Arno Pf is offline Arno Pf  Netherlands
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Quote:
Originally Posted by Extreme_Boky View Post
try the same measurements and test, only this time leave pin 5 floating....

Boky
Did that, even THD was the same..
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Old 6th July 2013, 06:41 AM   #7
Zoran is offline Zoran  Serbia
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You are measuring transformer rather than dac here
transformer is reactive component with 20zHz You exploring more trafo against the
dac which is in this set up a generator.
.
If You are willing please try to measure same bur without trafo?
.
I am interested because I am almost to finish my classic dac with 4 x PCM63PK per channel

.
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Old 6th July 2013, 07:06 AM   #8
Zoran is offline Zoran  Serbia
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BTW BPO pin can be left NC
or connected to ground or via some current source of 2mA to -V.
In this case the Iout swings from 0 to 4mA
Giving the only "negative" voltage @ Riv phase shift of 180deg.
While BPO is not consuming the 2mA and like in original topology it goes from -2mA to 2mA? This leeds out that @ Riv will have phase of 0deg when Iout have negative mark,
to 180deg when it is positive.
.
I am not sure what will I do?
probably I will usee this value like some Vref for the external IV circuit?
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Old 6th July 2013, 07:24 AM   #9
Nrik is offline Nrik  Denmark
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How large would the input impedance of a typical opamp current-to-voltage converter be?
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Old 6th July 2013, 08:58 AM   #10
Nrik is offline Nrik  Denmark
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Quote:
Originally Posted by Arno Pf View Post
20Hz Different loads, signal behind the I/V transformer. Sine signal distorts at high Rdac (68 Ohm).

This is either caused by the current source of the DAC, not having enough internal voltage to provide sufficient current....or the higher voltage of the input I/V transformer when using the higher Rload causes the transformer to saturate (?No?).
Did you connect the primary between pin 6 and ground, or between 6 and 4? Or thorugh a capacitor?
If you don't take care of the DC-offset then that will saturate your transformer.
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