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Old 25th October 2004, 05:53 PM   #1
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Default st c4558 opamp, help needed

I have a problem with a Philips universal player I bought a couple of months ago: when a heavily modulated disc is played -CD, dvd,etc- there is a lot of distortion and compression in the analog outputs. It´s easily audible and annoying, but depends of the relative loudness level at which each disc is recorded. So, quieter or less hot recordings sound a lot better than louder ones.
I opened the lid of the unit and detect the output section, based on a ST c4558 opamp. I have searched the web and the only thing close to that designation was ne4558 by Philips.
The THD is not quoted in the datasheet, but in the distortion vs. frequency graphic for a 1 volt output I saw that at 20 kHz and 30 Hz distortion reach 2%!!
Am I misinterpreting something about the data of this opamp, or it is really that crap?
I am planning a substitution of the stock opamp for a better one -opa2604 or similar-, but I don´t know if such modification could help.
Thank you very much in advance for your help.
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Old 25th October 2004, 05:57 PM   #2
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4558 opamps have been around a long time. pretty standard opamp....


Not sure of the specs, but i do know that the JRC brand of 4558 were specifically choosen for there distortion characteristics in many of the early 80's Guitar FX distortion pedals!!!


Any dual Opamp should be a direct swap NE5532 etc....



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Old 26th October 2004, 06:15 AM   #3
Enzo is offline Enzo  United States
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A subtle distinction if you will. The guitar FX chips were chosen for the resulting distortion when driven into it on purpose, not the everyday distortion experiienced under normal audio use.

Op amps are cheap, so why not try some new ones, but you may have some lack of linearity in the DAC circuits or even a soft power rail
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Old 26th October 2004, 04:08 PM   #4
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Default C4558

Zero: as you pointed, some eighties distortion boxes used the c4558. While searching on this subject on the web I noticed that... So maybe I have not a DVD player, but some sort of processor...
It´s incredible how those things are made: the fascia sports the brand ¨Philips¨, and a very cool look, but the innards don´t have a single chip of that manufacturer. It´s a generic ítem -just like some computers- designed by numbers.
I suppose it have a potential to sounding better than it´s sounding now, but...

Thank you for your advice.

Enzo:
The dac is a Wofson 8761, a very tiny chip. I don´t understand how such a complex device can be so little. The datasheets claim 24 bit resolution, a very optimistic proposition, I suppose, taking in account the S/N ratio and thd characteristics quoted.
For a soft power rail, what do you mean? A lack of voltage, a lack of current or a lack of both?
The PSU is a switching type one. I don´t know the way to make it more robust.
Maybe it´s time to make some measurements and some swap of capacitors in a first instance, and leave the opamp change for a second try.
Thanks for your suggestion.
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Old 27th October 2004, 06:36 AM   #5
hjelm is offline hjelm  Sweden
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I had a jrc4558 in a Pioneer cd player and it was ok, not great a change to a opa2134 revealed its waknesses. Anyway i do not think it is your opamp that is the problem if the distortion is that obvious. You say heavily modulated music causes problems my guess is either a powersupply which cannot cope, unlikely since it is only driving line level outputs (if you are using headphones it is another issue)? or that the rails are too close to the required output voltage, the 4558 seems to need vout<Vsupp-2 so if it is fed with +-5 it is not possible to get more than +-3 V. That and the fact that it is much more sensitive to powersupply noise if it is supplied with a low voltage makes it the first thing i would check.

Link to the 4558 opamp

As for changing caps in a switched powersupply, I wouldn't some are sensitive to the ESR of the cap and changing the cap could cause problems. Do you have the possibility to measure the supply rails, for ripple?
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Old 27th October 2004, 07:07 AM   #6
dayvel is offline dayvel  United States
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A lot of CD players these days have really hot outputs. Try padding down the output a bit to make sure you're not overloading something later in the chain before you operate on your CD player.
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Old 27th October 2004, 04:54 PM   #7
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Default c4558

Hi, dayvel. The dvd player is feeding a preamp whose volume control is before the active section -opa627/buf634-, so the overload would be dependent of the vc position. Even at minimum vc settings the distortion is there.
I have tried feeding directly a NAD tape deck and the distortion was there too, even at minimum input volume.
Besides that, I coincide with you in the hot output characteristics of a lot of cd players. There is really too much volume sometimes.
Thank you very much for your suggestion.

Hi, hjelm. I´ll try to take some measurements this weekend at the supply pins of the 4558 to see what happen. Of course the Philips player is at line level. It has not headphone output.

Surfing the threads yesterday I noticed that modifiying a switched ps is something beyond my skills and knowledge, so I´ll take your advice and leave it as it is.

I have noticed too that my player has a pseudo equalizer -you know, that sort of horrible gadget with settings labelled ¨rock¨, ¨soft¨, ¨classic¨, etc.-, that is managed via tv set and remote. When you select the hotter settings -¨rock¨, for example, with a lot of boom and tizz-, the distortion worsen to incredible levels. Maybe the distortion is born in the digital domain and hence beyond any cure. Who knows how those crappy things were made...
Maybe it´s time to go for a better player. But i´ll take some measurements and I´ll let you know. Thank you very much.
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Old 28th October 2004, 12:57 AM   #8
anatech is offline anatech  Canada
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Hi federico,
The are two other possibilities.
First, check the supply voltages to the op amps. They are happier at +/- 10V rather than +/- 5V, up to +/- 15V is okay.
Second, the output muting transistors could be beaking down, or there is a problem in the muting circuit. Remove the transistors temporarily to see if the problem goes away. If it does, replace the transistors with units with high reverse breakdown ratings. Normal transistors will not do. Otherwise, the bases may not be help negative enough. Troubleshooting time.
-Chris
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Old 28th October 2004, 07:14 AM   #9
hjelm is offline hjelm  Sweden
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If the equalizer causes clipping in the digital domain try to level it and perhaps lower the volume. Also test with different volume settings if the saturation disappears. It could also be clipping in the output stage where you could gain something by lowering the volume in the equalizer, you are probably tossing away bits if you do so do see if there is an optimum.

In short see if the problem persists if you have a flat equalizer and volume max. If so try lowering the volume.
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Old 2nd November 2004, 04:07 PM   #10
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Default dvd output

fine gentlemen:
In first instance, thank you very much for your helpful suggestions.
I have measured voltage at the psu pins of the 4558 opamp: +/- 12 volts, so I think it´s ok.
Regarding the equaliser, it´s always in ´flat´ position. I have run across all the tricks displayed in this thread, exception made of the muting transistors obliteration suggested by anatech. Maybe this weekend, with great caution, I´ll perform that trick to see what happens. Those components are incredibly tiny -the MSOP package, cuasi microscopic-, so I´ll have to be very careful to avoid problems.
Thank you everybody, again.
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