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Old 25th April 2013, 02:11 PM   #761
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Quote:
Originally Posted by Joachim Gerhard View Post
I also think 120 Ohm is about optimum. Making it much smaller will amplify the noise of the mirror, not a big problem in power amps but here i think it is critical not to worsen the noise of the input stage. Making it bigger makes the output impedance of the mirror higher
and yes, it will eat up some dynamic range. The output impedance of this cascoded mirror is already sky high so i think we are fine.
yes, I fully agree with it.

I am trying to debug the distortion heard. The circuit seems not to produce any clipping. I took RIAA out and checked circuit with square wave.
There is no over/under-shoot.
Triangular wave shows proper sharp edges.

So I moved on and checked if there is a problem with the preamplifier although in SE mode with the CD it doesn't do it, I am making sure that in Balance both halves are working fine.
I also noticed that there is a strange wave form on the bench if I connect phono and line and it seems to be ground related, not sure though.
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Old 25th April 2013, 03:50 PM   #762
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one quick update:

Noise simulation shows a significant increase if noise performance if the GND connection from the RIAA is removed.
Therefore schematic goes back to GND connection on the RIAA.
Helpers will still be floating though.
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Old 25th April 2013, 03:57 PM   #763
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Audio ground is needed, sure.
Only PSU ground can be floating.
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Old 25th April 2013, 04:29 PM   #764
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Quote:
Originally Posted by Joachim Gerhard View Post
Audio ground is needed, sure.
Only PSU ground can be floating.
yes correct.

with a cartridge though you have a floating GND.
Ground reference is generated at the circuit with shunted load.
Am I wrong?
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Old 25th April 2013, 04:42 PM   #765
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One more comment: Optimizing the values on the RIAA i.e. doubling caps and halving resistors (roughly) and lowering degeneration resistors at the input stage, significantly reduced noise level of this circuit which now is getting magnificently low.

It probably needs a little more tweaking around and then I will repost the updated circuit.
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Old 25th April 2013, 04:48 PM   #766
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Very good.
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Old 25th April 2013, 04:51 PM   #767
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Yes, i think ground is needed at the input.
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Old 25th April 2013, 05:01 PM   #768
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Quote:
Originally Posted by Joachim Gerhard View Post
Yes, i think ground is needed at the input.
yes, but that is not what I was saying.
I was sayiong that signal coming from the Cartridge when used in balance mode only has two wires one goes to + side and the other to - side, which leaves a floating balance signal.
GND is generated at the board inside the circuit and referenced to the GND at the ouput so basically who is offering the stable audio GND is the next stage which is the preamplifier and not the input stage.

Am I saying something wrong?
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Old 25th April 2013, 06:21 PM   #769
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Since Paradise is certainly a reference noise wise, here is the comparison of paradise vs. masterpiece.

Noise floow of MasterPiece is pretty good. I wish it could be improved further though.
Anyhow during listening the noise contribute is pretty low and you can hear all sorts of subtle details.
Anyway for the kick below is the simulation report.
Attached Images
File Type: jpg noise comparison.JPG (102.0 KB, 111 views)
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Old 25th April 2013, 06:48 PM   #770
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A balanced stage always has a 3dB noise disadvantage.
The only way to lower the noise is more paralleling of input devices.
You already lowered the emitter resistors, good for less noise but also more distortion because of lower local current feedback. That is the reason we use quite a few input devices in parallel in the Paradise with quite high degeneration.
The " compound " is then low noise plus low distortion. this is one of the reasons i am not strictly against paralleling. Theoretically it makes sense. How it sounds ?
That is at least in part subjective.
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