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Old 22nd May 2011, 10:15 PM   #681
roger57 is offline roger57  Canada
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I have discovered on the schematic that AGND (Pin 5) and DGND (Pin 14) are tied together...
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Old 22nd May 2011, 11:26 PM   #682
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Quote:
Originally Posted by roger57 View Post
Hi,

Right now - I am using just 50 ohm resistors to ground to see if I can just get a voltage sine wave. Although it will be small, I should be able to see it. However, I have measured DC voltages at the AOL and AOR pins and something odd - they are both at -103mV. That does not seem correct, I thought these pins must be at 0V in order to work correctly. Why would I get this voltage at those pins?
Gary
Gary, I am inclined to think that your testing procedure is the problem.

The circuit simply requires that you stuff the board and fire it up.

Other than a component failure, and given you have satisfied yourself that there are both a) correct voltages on the chip AND b) correct i2s signals, you have only one option - the output stage.

...which in your case is a bit of an experiment (that I confess I don't understand!), so if you don't own a proper output stage to try it with, and you want to knock together something both competent and cheap, might I suggest EC designs very simple 2SK170 based design, which as I remember doesn't even require power, and is very simple and cheap to build.

Personally, I think you'll discover that you do have sound after all. I really can't recommend Oliver's Tube-I-Zator board enough for a totally superb output stage. It is the classic lampizator design, and I love it with 6n2p tubes.
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Old 23rd May 2011, 12:39 AM   #683
roger57 is offline roger57  Canada
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Hi,

Thanks for your help!
I've verified all parts on the TDA1541A PCB, so no issues there.

The TDA1541A is a current output (source) DAC with a full scale range of +/-4mA. The purpose of the I/V resistor is to convert the current into a voltage of (with a 60 ohm resistor) +/- 240mV. If I'm using a 1KHz sine wave file, it should result in the above described sine wave across the resistor, which should be clearly visible on my scope. The purpose of the Tube-I-Zator, EC Designs or any other amplification stage is to bring that small voltage signal up to drive the input of your preamp or amp. So right now, the output stage is irrelevant.

If this is incorrect, someone please correct me. Otherwise, I still have a problem I cannot identify. On 2 PCBs, nonetheless. I'm beginning to think it's in the USB to I2S Teralink, but the signals all look OK. Clearly a mystery...

Gary
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Old 23rd May 2011, 05:30 AM   #684
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Gary,

so lets start at the beginning.

Did you have a PC or MAC ?
What SW did you use for the 1KHz test signal?
Please make some pictures from the isolator, the X2 and he backside of the DAC pcb.

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Old 23rd May 2011, 06:11 AM   #685
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Hi Gary,

TDA1541 output has +/- 2mA swing with -2 ma offset, not +/-4ma as you wrote.
So with 60 Ohm and normalised 1kHz signal you should have AC signal which should be between -240 mV and 0 V (with negative 120mv DC offset on digital zero).

If you want to test Teralink's chip for sure, easiest way is to put the TERALINK's optical or RCA SPDIF output back to PC's SPDIF input and check with sofware do you really have that 1kHz signal.


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Originally Posted by roger57 View Post

If this is incorrect, someone please correct me. Otherwise, I still have a problem I cannot identify. On 2 PCBs, nonetheless. I'm beginning to think it's in the USB to I2S Teralink, but the signals all look OK. Clearly a mystery...

Gary
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Old 23rd May 2011, 07:07 AM   #686
roger57 is offline roger57  Canada
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I connected up the Twister Pear SPDIF to I2S converter - my shop CD player has SPDIF output. I used a test tone on a CD, the same as the USB test. I also ran the signal through Oliver's hi-speed buffer. I now get an analog signal, finally, although very noisy
I confirmed the digital signals entering the DAC were the same levels as the USB.

So now the issue must be the Teralink. I will have to understand why this is not working as it should, perhaps something odd with the timing relationship. I hope it is not broke internally, but it appears it might be.

Noizas, can you please elaborate on the TDA1541A "offset"? Are you saying there should be 120mV of DC at the AOL and AOR pins?

Thank you all who have helped.

Gary
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Old 23rd May 2011, 08:37 AM   #687
noizas is offline noizas  Lithuania
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Originally Posted by roger57 View Post

Noizas, can you please elaborate on the TDA1541A "offset"? Are you saying there should be 120mV of DC at the AOL and AOR pins?

Thank you all who have helped.

Gary
Yes, if you connect ONLY 60 Ohm resistance on AOL (or AOR) output to ground, here should be negative voltage offset about 120mV (0,002A*60 Ohm).

Saulius
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Old 23rd May 2011, 08:51 AM   #688
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Gary, when testing the digital input, test for (K)Hz rather than volts. It's the only way to know you're actually getting the signal.

Oh, and the "correct" I/V resister values are 56 Ohm per channel for a single dac and 27 Ohm per channel for a 2x parallel dac.

These values are the highest safe values before distortion occurs, and recommended.
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Old 23rd May 2011, 10:51 AM   #689
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Exclamation Transformer for Pedja Rogics DDNF I/V Stage

AE-Europe in the Netherlands were so kind and calculated a normal EI-Core
and a High-End C-Core version for a custom made transformer.

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Old 23rd May 2011, 03:04 PM   #690
roger57 is offline roger57  Canada
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Noizas,

Thanks for the reply.This static DC voltage a result of...what? In addition to the signal, I also observed this at -103mV.
Is it not necessary to null this before sending into the amplification stage (tube or transistor)?

Gary


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Yes, if you connect ONLY 60 Ohm resistance on AOL (or AOR) output to ground, here should be negative voltage offset about 120mV (0,002A*60 Ohm).

Saulius
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