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Old 13th November 2012, 10:00 PM   #621
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I asume MiiB that for the PCM1704 you are saying this (attached red circled) has to be change to 2kohm and 870pf as Serg says?

Cheers George
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Old 13th November 2012, 10:07 PM   #622
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Sorry, just done the calculations, and yes both values of cap and resistance are -3db at 90-95khz.
Thanks George
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Old 13th November 2012, 11:17 PM   #623
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Quote:
Originally Posted by smms73 View Post
I didn´t know that you leave sonics i have to check Canalis.

The difference in this implementation is that normaly you see DC servos connected to input of the amplifiers and the noise of the servo opamp will be injected in the main amplifier, so you need to use a audio grade opamp for low noise like opa627 , but in this case the servo is not in the audio path (as Ken Newton has explain in post#561) so you just need to use a opamp with low input offset , the noise is not important (like for exemple max4238 less than 1€ and much beter in dc precision than audio opamps).
This is more economical (opa627 cost more than 20€) and better sonically as the servo is not in the audio path.
It doesn't matter where you connect the servo, it is -always- in the audio
path. Over the years this discussion has come up repeatedly.

The servo has to adjust the -audio- OP DC level and do so by integrating
the audio itself. Every servo works this way, you can connect them wherever
you like but if they are to work, they are in the audio path.

As such they definitely are audible and can often sound worse than a good
coupling cap. I know people who have in fact gone to extreme lengths to
make the servo truly out of the circuit, one worked on heat feedback and
another was a sampling system which made small adjustments at pre defined
time intervals, otherwise being off.

Just socket the opamp and try swapping it - you might get a surpise

Quote:
Originally Posted by georgehifi View Post
Sorry, just done the calculations, and yes both values of cap and resistance are -3db at 90-95khz.
Thanks George
George,

The optimum value of the LPF cap will probably be system / DAC dependent.
You might even get away without one, if the I-V is stable as such. The
1704 has low OOB noise.

WRT the 2k OP resistor, I would first try this I-V without an OP buffer.
It may well sound better 'in the raw' so to speak.

The issue with OP buffers is you will be hard pressed to find a design that
preserves the linearity of this I-V's super low distortion.

It will almost certainly require some sort of bootstrapping.
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Old 13th November 2012, 11:27 PM   #624
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Quote:
Originally Posted by zenelectro View Post



George,


WRT the 2k OP resistor, I would first try this I-V without an OP buffer.
It may well sound better 'in the raw' so to speak.

The issue with OP buffers is you will be hard pressed to find a design that
preserves the linearity of this I-V's super low distortion.

It will almost certainly require some sort of bootstrapping.
Hi Zen, I need the lower output impedance of the buffer, (BTW what is the output impedance with and without buffer)? as this needs to push into the Lightspeed (10k'ish) together with whatever the poweramp may be, total can come down to 7k'ish. I also love the idea of direct coupling, hopefully with dc servo. When all is finalized
Cheers George
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Old 14th November 2012, 06:43 AM   #625
zinsula is offline zinsula  Switzerland
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Originally Posted by MiiB View Post
[...]As the PCM1704 and also the TDA1541 operates +/- No counter-current is needed at the input.
Careful here. TDA1541 has 2mA offset and no ± operation.
Page 6 here.
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Old 14th November 2012, 07:03 AM   #626
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Zen, at the beginning I was reluctant in using a servo, because I am very aware of the problems that conventional servo has . Then I fought of adding a second pole to filter the AUDIBLE noises and distortions produced by the servo integrator , and this work very well, I have lost a lot of time trying to come up with a good solution for this , and I think that I have found it. But if someone with your experience thinks that this particular servo may have an impact in sound , others with less knowledge in analog design may as well feel uncomfortable , so I will add a optional trimmer in the PCB for manual regulation of DC offset.
I have made a lot of tests with this servo, and I can assure you that it has a minimum impact in audio band.
Please take a better look at this.
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Old 14th November 2012, 07:27 AM   #627
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Thank for the correction zinsula ,tda1541a is one that I will try for sure , as it served me well for over 15 years.
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Old 14th November 2012, 07:53 AM   #628
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Quote:
Originally Posted by georgehifi View Post
Hi Zen, I need the lower output impedance of the buffer, (BTW what is the output impedance with and without buffer)? as this needs to push into the Lightspeed (10k'ish) together with whatever the poweramp may be, total can come down to 7k'ish. I also love the idea of direct coupling, hopefully with dc servo. When all is finalized
Cheers George
You dont need a buffer just use a 2k5 resistor at the output, this resistor in parallel with the 10k of your atenuator will have the efective 2k out impedance that is require for 2 volts rms out.
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Old 14th November 2012, 08:25 AM   #629
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When you use the Light speed attenuator you sometimes see quite low impedance's hence my suggestion for a Buffer... If I was to make a product.. I would for sure include a buffer... I have often used the simple N-JFeT buffer, with no sonic penalty.

Regarding the servo I totally agree with Sergio.. this way of implementing the Servo has no sonic penalty. This type of servo simply adjust the driving currents on a DC level, and not like most servos by inserting a DC offset in the signal path.

Thanks for the correction on the 1541...This has slipped my mind. sorry.
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Old 14th November 2012, 08:49 AM   #630
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MiiB there is no problem with low impedance at the output of tagus , you can even shunt the output to ground , there will be no damage, there is a good idea to use a attenuator at the output of tagus and then if you want , use a buffer.
Later i will post some ideas for buffers, and the method to include them in the DC servo loop.
I am looking for dual potentiometer with 1k or less for audio use, if someone know any, please let me know.
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