Any good TDA1541A DAC kit? - Page 22 - diyAudio
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Old 20th January 2012, 04:44 AM   #211
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by abraxalito View Post
Why would you believe what I say when you don't believe what ADI says? I'd only be quoting from that - that DS and the related parts using the same architecture plus ADI's design support materials are my only source.

Even if you didn't read the explanations the fact that the attenuation vs control voltage plots are smooth without step jumps might tell you something which undermines your FET switch opinion.
I did read the explanation.

I find it opens up more questions than it answers. They clearly show a ladder attenuator. There has to be some form of switching somehow to use this.

As for the smooth curve, if they use (for arguments sake) an amplifier block with adjustable Gm (say over a 7dB range which is easy to do with good linearity) and then a resistor ladder with 7dB steps they could achieve the precise result shown.

Ciao T
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Old 20th January 2012, 04:57 AM   #212
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Quote:
Originally Posted by ThorstenL View Post
As for the smooth curve, if they use (for arguments sake) an amplifier block with adjustable Gm (say over a 7dB range which is easy to do with good linearity) and then a resistor ladder with 7dB steps they could achieve the precise result shown.
And where would the claimed 'FET switches' be if (for arguments sake) this were indeed the arrangement they were using?

<edit> This might help : http://www.analog.com/library/analog.../xAmp/xAmp.pdf
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Old 20th January 2012, 06:31 AM   #213
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Hi,

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Originally Posted by abraxalito View Post
Interesting topology. I can see how this would mitigate against some of the problems of classic VCA topologies, but it is in essence pretty much a take on standard VCA's. It also explains the relatively poor linearity.

I think I'll pass on this circuit style for high quality audio though. I'd rather actually have the Fet switches...

Ciao T
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Old 20th January 2012, 11:27 AM   #214
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Originally Posted by ThorstenL View Post
Why?

According to you (post 186) 12V/uS slew rate is all that is needed. Maybe you should try a TL072 instead?

Ciao T
Because that affirmation was for TDA1541 in 4x mode (topic of the thread is TDA1541). Faster OpAmp (more than 24V/us) might be needed for PCM61 in 8x mode - I have both of them so I can tell the difference.

Last edited by SoNic_real_one; 20th January 2012 at 11:30 AM.
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Old 20th January 2012, 11:47 AM   #215
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Do the rising and falling edges of 1 LSB of a DAC output steepen when you drive the DAC with higher clock frequencies?
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Old 20th January 2012, 01:11 PM   #216
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Not to my knowledge, no. But if you're playing the same content, just upsampled, then the step sizes will get smaller.
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Old 20th January 2012, 02:36 PM   #217
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Hi,

Quote:
Originally Posted by SoNic_real_one View Post
Because that affirmation was for TDA1541 in 4x mode (topic of the thread is TDA1541). Faster OpAmp (more than 24V/us) might be needed for PCM61 in 8x mode - I have both of them so I can tell the difference.
Just looking at the initial steepness of the step current step into an I/V resistor (easily done by feeding a full scale squarewave into the DAC), I would suggest that for a TDA1541A an absolute minimum of around 30V/uS are needed to keep up with the current step, however this would still mean that we are constantly at or near the slew-rate limit of the Op-Amp even for a 1LSB step.

So it would seem at least sensible to allow for some extra slew rate to make sure we are well away from the slew rate limit, as even signals that do not cause the circuit to actually "slew-limit" may increase distortion appreciably. Based on this little set of notes I would think that the OPA627 might just about suffice.

HOWEVER, in practice, with a TDA1541A at 4 * OS I did find it subjectively beneficial to use a OPA637 instead, with a capacitor from -in to ground to set sufficient noise gain for the OPA637 to be stable. This capacitor also had a subjectively beneficial effect for the OPA627 and I would strongly recommend applying it regardless which Op-Amp is used, unless a CFB Op-Amp is used.

Ciao T
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Old 20th January 2012, 03:00 PM   #218
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Hi,

Quote:
Originally Posted by SoNic_real_one View Post
And, even if I still hear distortions well, I trust measurements more.
Quite many years back I build an Amplifier based on a schematic in a magazine. It had phantastically low measured distortion. Yet I kept hearing distortion , lots of it. I put it on a distortion meter, which had the meter not even twitch. I gave up using this amp. Others had the same problem, much was written about this issue.

In the end this distortion was acknowledged to exist in the design, even though it could not be measured. Essentially it was a specific case of TIM/SID.

So, I do rather trust my ears if they tell me something is distorted, even if the meters and measurements tell me it is not...

Ciao T
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Old 21st January 2012, 01:26 AM   #219
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So... YOUR measurement tool/method was flawed. That does not mean that ALL the tools/methods are flawed. Or that the measurements are useless.

However, in the subject at hand, you are basically trading SNR (using a compliant resistor for I/V you have a low signal) to avoid an eventual slew-rate induced distortion.
Small capacitors on inputs of fast OpApms (and on feedback) limit that slew-distortion well and they are not so much of a load to induce distortions.

Last edited by SoNic_real_one; 21st January 2012 at 01:54 AM.
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Old 21st January 2012, 01:55 AM   #220
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Quote:
Originally Posted by SoNic_real_one View Post
So... YOUR measurement tool/method was flawed. That does not mean that ALL the tools/methods are flawed. Or that the measurements are useless.

However, in the subject at hand, you are basically trading SNR (using a compliant resistor for I/V you have a low signal) to avoid an eventual slew-rate induced distortion.
Small capacitors on fast OpApms limit that distortion well and they are not so much of a load to induce distortions.
Have you ever owned a well setup TDA1541A DAC ?
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