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Building the ultimate NOS DAC using TDA1541A
Building the ultimate NOS DAC using TDA1541A
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Old 16th April 2019, 04:37 PM   #7241
koldby is offline koldby  Denmark
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I am still quite puzzled by the way this circuit works. To make it easier to adjust the bias resistors . I first tried to adjust each dac, with the data in disconnected, to 0 V dc, with no connection to the other dac, but that was way off.
If I now measure the DC at the outputs, after the adjustments, and with the two 1 KOhms between the two dac's, the positive dac is @ - 157 mV and the negative is @ +161 mV.

I would , initially , have thought, that both should be @ 0 VDC....

What does the simulation say about this?
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Old 17th April 2019, 08:27 AM   #7242
koldby is offline koldby  Denmark
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I am going to the hospital for some hand-surgery today, so nothin much happens here.
But.
I was thinking , that my next step would be to see if the same results can be achieved, whit using two dacs from the sam TDA1541A. This would be easier to implement.
Next speculation was , what will happen, if I still use two 150 Ohm but connect the with a 10 Ohm trimmer and take the signal out on the wiper and inject the bias at this point also. When I just used 75 Ohms and shorted the two outputs, the od-order harmonics were all gone, so perhaps this will work?
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Old 17th April 2019, 10:33 AM   #7243
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Hi Koldby,

Wish you an easy surgery and a fast recovery!
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Old 17th April 2019, 11:18 AM   #7244
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by xaled View Post
Hi Koldby,

Wish you an easy surgery and a fast recovery!
Easy. Just repeat:

"I am the light. I am the divine vital energy that cures all wounds and heals all injuries."

But I am sure you will be thinking about the circuit during operation!

Best wishes.
M.
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Old 17th April 2019, 02:20 PM   #7245
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by koldby View Post
Hi Hans.
What a coincidence. I have just finished some trimming and measurements, and sat down to post it, when I saw your post
I am not sure I see what you want to do with two in parallel. Could you elaborate that a little or perhaps a schematic?

What I have done is this.
I put trimmers in both bias and I/V resistor in all 4 dac's (2 tda1541A) .
I initially adjusted all the I/V to excatly 150 Ohms and all the bias resistors to 2mA (3,15 K @ 6,3V).

The result was not very good. A lot of even and uneven harmonics. Then I adjusted the bias resistors (long time, they interact) and by this I could trim the uneven harmonics to -100dB. Then I trimmed the I/V. This reduces the even harmonics. Again interacting , not with the other dac but with the bias setting, and that makes perfect sense. I only had a 100 ohm trimmer (in series with a 100 Ohm resistor) and this is way to coarse. But I managed to get to the result in the picture. Pretty impressive right?
This is a -6 dB signal. It clips at 0dB.
Fantastic that you've got it working, very impressive.
With 0dB, the signals on both Dac outputs should look like in the image below.
There you can very well see how precise the tuning has to be done.


Hans
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Old 17th April 2019, 02:25 PM   #7246
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by koldby View Post
I am still quite puzzled by the way this circuit works. To make it easier to adjust the bias resistors . I first tried to adjust each dac, with the data in disconnected, to 0 V dc, with no connection to the other dac, but that was way off.
If I now measure the DC at the outputs, after the adjustments, and with the two 1 KOhms between the two dac's, the positive dac is @ - 157 mV and the negative is @ +161 mV.

I would , initially , have thought, that both should be @ 0 VDC....

What does the simulation say about this?
In my Sim with no signal applied, one side is 250mv and the other side -250mV.
So I guess your resistors have a lower value.


Hans
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Old 17th April 2019, 02:28 PM   #7247
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by koldby View Post
I am going to the hospital for some hand-surgery today, so nothin much happens here.
But.
I was thinking , that my next step would be to see if the same results can be achieved, whit using two dacs from the sam TDA1541A. This would be easier to implement.
Next speculation was , what will happen, if I still use two 150 Ohm but connect the with a 10 Ohm trimmer and take the signal out on the wiper and inject the bias at this point also. When I just used 75 Ohms and shorted the two outputs, the od-order harmonics were all gone, so perhaps this will work?
I would be surprised, because this problem has a complete different cause, but nevertheless, trying cost no money.

Succes with your hand surgery.


Hans
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Old 17th April 2019, 03:47 PM   #7248
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by koldby View Post
Hi Hans.

This is a -6 dB signal. It clips at 0dB.
Koldby,
since you tested at -6dB, this is how the signals from both Dac's and at the summed output should look at -6dB and just for completeness.


Hans
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Old 18th April 2019, 02:56 PM   #7249
koldby is offline koldby  Denmark
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Just changed the circuit so I now use two dac's from the same TDA1541A and I can adjust it to the same excellent performance. Apparently it is not the time difference, but the level difference that result in elevated even order harmonics. This is good news, as it makes it easier to make a good PCB and I think the circuit will maintain the good performance without readjusting all the time because of temperature drift.
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Old 18th April 2019, 03:46 PM   #7250
koldby is offline koldby  Denmark
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And now even better news!!!

I tried what I suggested earlier:
100 Ohm I/V resistor from each dac to ground. A 20 Ohm multiturn trimmer between the two dac output where the wiper is connected to a bias trimmer pot to +6,3V. And the output from the same wiper.

The pic shows a spectrum from a 98% FS signal after trimming bias and the 20 Ohm trimmer. If I raise the input to 99 or 100 % some strange sidebands comes up. Only about -80 dB but it dosent look as clean as this pic. These sidebands are not related harmonic to the main signal

Bye the way THANKS for all the good wishes
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