Mod/upgrade NAD C 541i

[klitgt]

I made som mods on a friend's NADC542 with great results:
Lower grain
More resolution
Better transparency
Cleaner sound
Better bass
(quote: Tentlabs.com)

BUT, there has come an adverse effect, that I did not notice in my own setup:
With power on the CDP but not playing a CD, and volume turned to MAX on the preamp, a noisy, crackling hiss comes from the tweeters in my friends setup . This did not happen before the mod, it was totally silent.

The mods are:
Coupling caps from the DAC to the opamps
C375 parallel with C377/C376 parallel with C378
replaced with cupperfoil paper-in-oil caps. They are quite large and are suspended in foam to avoid that they touch any components on the PCB.
The M302A crystal and the ceramics C328-339 removed and a Tentlabs XO2 clock is applied with a dedicated PSU (transformer, rectifier, smoothing elcap in parallel with a small 100nF MKT).
Muting transistors Q301,302,303,304 removed.

I do not understand why the sound has improved so much but at the same time this noise has been added to the system?

Well... what can I have done wrong ?? What can the noise be, RF pollution, digital noise picked up by the large PIO caps??

Appreciate some ideas to get rid of the noise

[mlloyd1]

might have been useful to apply your mods one at a time to assess impact. then, the cause would be clearly apparent and a little easier to "correctly" address.

is it too late or too painful to do that?

good luck,
mlloyd1

[Extreme_Boky]

BUT, there has come an adverse effect that I did not notice in my own setup:
With power on the CDP but not playing a CD, and volume turned to MAX on the preamp, a noisy, crackling hiss comes from the tweeters in my friend’s setup. This did not happen before the mod, it was totally silent.

2 things:

1. Re-install the muting transistors. Is the problem gone? If YES, consider relays - shorting the output to ground. Or, leave the muting transistors installed.
2. Try recording a wav file, but without any signal (silence) - like white noise. Transfer to CD-R and play it in the CD player. Is the crackling hiss / noise still there? If YES, it could be "something else" you did, or even a bad original design (yes, yes, I know - it's NAD!!). You see, the problem is noticeable only without muting transistors - you can not be 100% positive if the crackling noise problem was there BEFORE you started the mods, because the sound being played / reproduced could've masked the problem....

Extreme_Boky

[klitgt]

Originally posted by mlloyd1
might have been useful to apply your mods one at a time to assess impact. then, the cause would be clearly apparent and a little easier to "correctly" address.

is it too late or too painful to do that?

good luck,
mlloyd1

Probably the only way to identify exactly what caused the problem, it is possible. I never burn any bridges behind me!
To Extreme_Boky I can say that it is 100% sure that the bad sound was NOT there before the mods, whether there is a CD in the tray or not.
thank you for helping!

[klitgt]

Originally posted by Extreme_Boky


2 things:

1. Re-install the muting transistors. Is the problem gone? If YES, consider relays - shorting the output to ground. Or, leave the muting transistors installed.
2. Try recording a wav file, but without any signal (silence) - like white noise. Transfer to CD-R and play it in the CD player. Is the crackling hiss / noise still there? If YES, it could be "something else" you did, or even a bad original design (yes, yes, I know - it's NAD!!). You see, the problem is noticeable only without muting transistors - you can not be 100% positive if the crackling noise problem was there BEFORE you started the mods, because the sound being played / reproduced could've masked the problem....

Extreme_Boky

In another thread is is recommended to bypass the transistors at the output (muting transistors??)

Tweak report NAD 510

How would this be recommendable if it causes problems?
I did remove (not bypass) Q301-304 shown in attachment.

What is the purpose and the function of these transistors?
Why could their absence cause the noise?
And once more to avoid misunderstandings: The noise is there if I turn up the volume to max. at the preamp without playing a CD.
Before the mods I could turn up the volume to max. without any noise, only a weak hiss if I place my ear 1 inch from the tweeters.

[Extreme_Boky]

Well, sometimes if you remove the muting transistors the noise can creep in. Muting transistors are there to mute (short the output to ground) when the CD player is in stop or in pause mode. They also mute the output, more or less - depending on design, during forward and reverse search. By removing them, you are leaving analog stage output always "active", even when in stop mode. This wasn't the case before the mods were done - the analog stage output used to be shorted to ground if the player was in stop mode.

The crackling noise problem usually means something else (other than analog stage) might be wrong with the CD player. It could be even impedance mismatch between analog out and pre-amp analog in impedances + high capacitance interconnect cables. You can trace the problem backwards if you follow the crackling noise backwards, starting at RCA's, with a CRO (have one? - you ought to get yourself one if you are serious about mods!).

First step would be to re-install the muting transistors. See if that fixes the problem. People suggest their removal, but usually warn of the popping side-effects during track search, or reading the TOC, loading the disk, during turning the unit ON-OFF and similar... These side-effects are minor and should disappear after a week or 2 (joke, lame one though!)

Good luck

Extreme_Boky

[klitgt]

Originally posted by Extreme_Boky
Well, sometimes if you remove the muting transistors the noise can creep in. Muting transistors are there to mute (short the output to ground) when the CD player is in stop or in pause mode. They also mute the output, more or less - depending on design, during forward and reverse search. By removing them, you are leaving analog stage output always "active", even when in stop mode. This wasn't the case before the mods were done - the analog stage output used to be shorted to ground if the player was in stop mode.

The crackling noise problem usually means something else (other than analog stage) might be wrong with the CD player. It could be even impedance mismatch between analog out and pre-amp analog in impedances + high capacitance interconnect cables. You can trace the problem backwards if you follow the crackling noise backwards, starting at RCA's, with a CRO (have one? - you ought to get yourself one if you are serious about mods!).

First step would be to re-install the muting transistors. See if that fixes the problem. People suggest their removal, but usually warn of the popping side-effects during track search, or reading the TOC, loading the disk, during turning the unit ON-OFF and similar... These side-effects are minor and should disappear after a week or 2 (joke, lame one though!)

Good luck

Extreme_Boky

Thank you, now Í understand the suggestions given. In the business I am working in, a CRO means a clinical research officer.
What does it mean here?

[Extreme_Boky]

Cathode Ray Oscilloscope

Cheers,

Extreme_Boky

[sangram]

I have also removed the muting transistors from my 521i. In addition there are two 150 ohm resistors in the signal path for the muting operation. I removed those as well and there is no problem.

I have taken the output directly from the opamp pins without an output coupling cap (the paralleled set of caps). No adverse effects and only 3 mV offset so I'm assuming I'm doing OK.

Haven't touched the caps between DAC and opamps.

[Franz G]

In addition there are two 150 ohm resistors in the signal path for the muting operation.

This resistors dont have something to do with the muting circuit.

The resistors are there to avoid problems with driving capacitive loads.

It is recommended to use at least one 100R resistor.

Franz