dac I/V convertion with very low distortion - Page 53 - diyAudio
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Old 30th October 2012, 01:29 AM   #521
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Quote:
Originally Posted by georgehifi View Post
Thanks Ken, so this is not an ideal circuit for a PCM1704 then? as to get 2v out, the loading for the dac would be too high at 2.35kohms? When the ideal should be much lower, around 10-100ohms. Is this correct? still learning about I/V's.

Cheers George
George,

This circuit will work extremely well for PCM1704 with a few simple mods to
'bend' it towards lower current / higher OP impedance.

I have built virtually same circuit with 10k OP Z (load) and the simulated
results were somewhere around 0.0001% at 20kHz depending on the
particular design. There are many variations to the basic theme.

It will need a servo but that is also not too difficult.

cheers

Terry
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Old 30th October 2012, 01:36 AM   #522
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Thanks Terry, I also dug this up with a bit of searching, I think the author has a good rep, the name rings a bell. M.O.J. Hawksford.

Cheers George
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Old 30th October 2012, 09:36 AM   #523
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Quote:
Originally Posted by zinsula View Post
If you prefer BJT's, you could also try the C4S current source (Camille Cascode Constant Current Source)
The output transistors already keep the voltage variations of the current sources at minimum, so there is no need for cascoding the current sources, only a basic ccs is needed. Thanks for the image.
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Old 30th October 2012, 11:30 AM   #524
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looking for low noise reference voltage, have found this.

http://www.diyaudio.com/forums/parts...ts-leds-zener-
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Old 30th October 2012, 11:41 AM   #525
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test with 3 different output configuration. this was made with 1kohm of input impedance to bring up distortion.
first is normal x connection, second is with drivers collector connected to OUT, third is drivers collectors connected to ground.

with high input inpedance the distortion will not be visible in the first 2 images.

This proves that low distortion can be achieve without the x connection.
Attached Images
File Type: png output.png (23.8 KB, 271 views)
File Type: png dist_to_x_1k.PNG (50.2 KB, 262 views)
File Type: png dist_to_out_1k.PNG (49.9 KB, 240 views)
File Type: png dist_to_gnd_1k.PNG (49.9 KB, 228 views)
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Last edited by smms73; 30th October 2012 at 11:46 AM.
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Old 30th October 2012, 11:57 AM   #526
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Quote:
Originally Posted by Ken Newton View Post
Sergio, very cool. Thanks, for posting the schematic. I'll offer further comment after I've had opportunity to model the circuit in my simulator. Specifically, I'm curious to examime how sensitive the circuit is to producing D.C. output offset, due to small amounts of parameter mismatch between the two complementary current sources.

The one i have in breadboard is with blue led (bad choice), the output voltage varies no more than +-20mv with temp. But I recommend a output capacitor. Its possible to use a dc-servo, but i dont think it is not necessary, it makes sense in a power amplifier to get rid of the bipolar capacitor in the feedback , but in this case i think that a polypropylene capacitor will be the best choice, but it is open to discussion
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Old 30th October 2012, 02:15 PM   #527
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Originally Posted by regal View Post
This is amazing. Can you show a photo of you setup? Did you have to go through hundreds of transistors find a low dc offset?

Very elagant looking circuit, really hope it pans out.
There was no transistor matching, the transistores was chosen at
random, and the resistors are 5%, the output dc offset is set by a 100 ohms trimmer in the upper currente source.
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Old 30th October 2012, 02:20 PM   #528
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Originally Posted by MiiB View Post
SMMS... do you balance the circuit by unbalancing the current sources, or do you inject a counter current at the input... or do you use a transformer to get rid of the current offset...??
I will use a current source of 6.2ma in the input to cancel the dc offset of dac as suggested by Calvin somewhere in this thread
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Old 30th October 2012, 02:26 PM   #529
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Thanks.... I think, a good solution... if you make a cascoded Jfet CCS. you have Tera-Ohm impedance for the first more than 100 KHz then dropping with the capacitance of the cascode Jfet. but never lower than a couple of hundred Kohms.
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Old 30th October 2012, 02:31 PM   #530
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Quote:
Originally Posted by zenelectro View Post
I just dug through my archives - there are a few, You have mail.

Thanks very much Terry, i will take a closer look.

Do you have any suggestion to improve the circuit ?
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