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Old 14th February 2018, 05:53 AM   #581
ppy is offline ppy  Russian Federation
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Quote:
Originally Posted by mmerrill99 View Post
That seems odd? Any thoughts on what could be going on?
I'll try to find a problem. I do not understand the cause of these residual noises after loud signals. They are like murmur or chirping sparrows.
Theoretically, the scheme for differential FF looks more beautiful than on AHCT595 registers.
If you have any ideas, I'll be happy to listen.
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Old 14th February 2018, 09:59 AM   #582
mmerrill99 is offline mmerrill99  United States
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Quote:
Originally Posted by ppy View Post
I'll try to find a problem. I do not understand the cause of these residual noises after loud signals. They are like murmur or chirping sparrows.
Theoretically, the scheme for differential FF looks more beautiful than on AHCT595 registers.
If you have any ideas, I'll be happy to listen.
Could it have something to do with propagation delay differences (i.e settling time) between the differential FFs & the 595 FF you were using before?

What exact chips were you using in both configurations - got links to their datasheets?
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Old 14th February 2018, 11:06 AM   #583
ppy is offline ppy  Russian Federation
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v2.5-2.6 74AHCT595 http://www.ti.com/lit/ds/symlink/sn74ahct595.pdf
v3.0 74VHC175 https://toshiba.semicon-storage.com/...e=TC74VHC175FT
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Old 15th February 2018, 09:01 AM   #584
mmerrill99 is offline mmerrill99  United States
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Nothing obvious in the datasheets.

Are you sure you are still using a moving average filter with the Quad FF?
DSD input ->1D which outputs 1Q & 1Qbar
1Q connected to 2D which outputs 2Q & 2Qbar
2Q to 3D with 3Q & 3Qbar out
3Q to 4D -> 4Q & 4Qbar
4Q connected to 1D of next Quad Diff FF & and so on
Each Q & Qbar connected to series R for balanced output into transformer

Last edited by mmerrill99; 15th February 2018 at 09:04 AM.
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Old 15th February 2018, 09:07 AM   #585
ppy is offline ppy  Russian Federation
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Quote:
Originally Posted by mmerrill99 View Post
Nothing obvious in the datasheets.

Are you sure you are still using a moving average filter with the Quad FF?
DSD input ->1D which outputs 1Q & 1Qbar
1Q connected to 2D which outputs 2Q & 2Qbar and so on
Each Q & Qbar connected to series R for balanced output into transformer
Yes, exactly as you wrote.

http://www.diyaudio.com/forums/attac...1&d=1518689107
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File Type: pdf dsc3.pdf (115.7 KB, 113 views)
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Old 15th February 2018, 09:20 AM   #586
mmerrill99 is offline mmerrill99  United States
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There's one possibility - increased differential noise.
It's hardly common mode noise as this would largely be removed by the transformers on the outputs

AFAI see & I raised this question before - the possibility that connecting the Q outputs to both serial resistor & the next data input pin introduces some difference in Q Vs Qbar output - the result being extra differential noise introduced?

When doing differential output with 595s each output is driving into the same load

So maybe using moving average filter with differential FFs upsets the differential balance of the outputs?
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Old 15th February 2018, 10:54 AM   #587
zenelectro is offline zenelectro  Australia
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Quote:
Originally Posted by ppy View Post
I assembled the DSC layout on the differential FF 74VHC175.
Works badly. I again hear noise (murmur) as in the original non-differential DSC1 scheme. The overall noise level also deteriorated compared to v2.5 and v2.6.
Wow, that was fast. Good work PPY!

VHC175 looks pretty similar to AHCT595 WRT prop delay so that doesn't appear to be culprit.

Looking at FFT results there is dominant H2 which is pointing to a lack of symmetry or balance.

Driving same chips (595) with opposite signals appears to have better symmetry and hence cancellation of distortion.

As far as the noise is concerned is there any difference between VHC and AHCT besides different manufacturers version of the same thing?

Are the resistor networks same, ie; 32 x 8k?

A surprising result but nevertheless there is something to learn here.

T
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Old 15th February 2018, 04:13 PM   #588
mmerrill99 is offline mmerrill99  United States
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One way of overcoming this issue & testing if it's really an imbalance in differential outputs is to disconnect the Qs from the following D inputs & instead use 595s in front of the quad FFs feeding their outputs into the D inputs of the quad FFs. That way there should be no imbalance between Q & Qbar outputs but it will introduce an extra bit of propagation delay in th e595s - doubt it will matter, though as these are low nS delays.
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Old 15th February 2018, 08:24 PM   #589
mmerrill99 is offline mmerrill99  United States
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Just looking at your pcb & I first thought you had the wrong pinouts for the 175 FF but I think I see what you did & I'm wondering if it might have a bearing on the imbalance between Q & Qbar outs?

Correct me if I'm wrong but you seem to have no output resistor connected to Q1 output - instead you connected it to D2 (under the chip?) & the output resistor is connected to D2. This is the same scheme used throughout. I wonder if this is one additional source of imbalance between Q Vs Qbar outputs?
Maybe others can answer the implications of current being sourced/sunk at the Q pins but the pcb trace having the D input pins as spurs off this trace.

Alternatively, when you think about the design - connecting D inputs to Q outputs means that every pulse at D input will result in the pulse also being present on the connected Q output & causing a voltage spike through the connected resistor. Could this be the real source of the extra distortion? Normally it wouldn't matter when using FFs as a shift registers but when we have resistors connected to the Q outputs, it does, no?

Last edited by mmerrill99; 15th February 2018 at 08:26 PM.
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Old 16th February 2018, 09:25 AM   #590
mmerrill99 is offline mmerrill99  United States
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Scrub that last thought - I wasn't thinking clearly - the pulse originates at Q1 & connects to D2 not the other way around - Doh!
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