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Chip Amps Amplifiers based on integrated circuits

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Old 23rd August 2007, 06:13 AM   #11
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Some hysteresis trick with the ADC reference would have solved the problem.
Just a quick thought, how, I don't know.

Gajanan Phadte
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Old 23rd August 2007, 06:44 AM   #12
banana is offline banana  Hong Kong
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Hi Ampliton,

That's AP2 dual domain, but I didn't own it

I designed all PCBs and etched the discrete ADC board by myself. The audio main board (4-layer) and power supply board (double side) were made by sample house.

I don't have other commercial preamp to compare with right now. But I'll lend this preamp to my friends and hear their comment soon.
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Old 23rd August 2007, 07:48 AM   #13
MaxS is offline MaxS  France
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Hello Banana,

I like your idea !
I've started to think to a same kind of project but I've never start to do something. Is it enough if I ask you to get the schematic ?

Thanks Click the image to open in full size.
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Old 23rd August 2007, 05:58 PM   #14
banana is offline banana  Hong Kong
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Quote:
Originally posted by gmphadte
Some hysteresis trick with the ADC reference would have solved the problem.
Hi Gmphadte,

There is no access to the comparator front end inside the ADC chip, hysteresis can only be applied after the DC voltage is encoded. Some memory buffer plus MCU programming should do, but I've turned to the hardaway before figure out how...
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Old 23rd August 2007, 06:04 PM   #15
banana is offline banana  Hong Kong
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Hi MaxS,

Give me some time, let me tidy up the schematic into more readable form first.
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Old 24th August 2007, 04:32 AM   #16
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When it is a hysteresis problem, invariably the VU led meter ICs come to my mind as a simple, easy-to-lay solution. However 50 steps will pose some problems. Moreover the encoding will need some more ICs.
So much to get some hysteresis...anyhow, it is another solution

Gajanan Phadte
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Old 27th August 2007, 06:13 AM   #17
banana is offline banana  Hong Kong
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A little problem measuring intermodulation with AP2.
Again, every opamp combination measured the same. I tried direct connect feed, and confirmed it's equipment limitation.

Input signal 19KHz+20KHz, 3.6Vrms, AP2 analog out to analog in (XLR->XLR)
There's a strange 500Hz sideband I didn't expected, FFT plot below.
Click the image to open in full size.


Then I use the SPDIF output from AP2, connected to my AD1853 DAC as the analog generator.
DIY AD1853 and AP measurement
Now the sideband is gone, noise floor is lowered, and difference between OPA227 and AD797 is revealed.
Click the image to open in full size.
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Old 27th August 2007, 07:14 AM   #18
banana is offline banana  Hong Kong
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Casing finished.

Click the image to open in full size.

Click the image to open in full size.

I used the detachable type Neutirk XLR socket, which is quite expensive. Also there are very few choices of PCB mount RCA socket available. Sadly I can't afford those luxurious brand like WBT and Cardas.
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Old 27th August 2007, 03:38 PM   #19
alexw88 is offline alexw88  Hong Kong
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WOW, your DIY DAC performs better than AP2!
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Old 27th August 2007, 04:26 PM   #20
banana is offline banana  Hong Kong
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Only in the case of 19K+20K intermod
THD+N is not as good.
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