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Old 31st October 2002, 06:26 PM   #1
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Question Why 7805 is better than LM1085?

Over the past months, I have been tweaking my Rotel RCD991 extensively. Doing all the usual stuffs like, SMD caps on the power pins, OSCON caps, inductors on the power lines, passive I/V conversion, chassis damping etc ...

However, when I replace the two 7805s in the CD player with LM1085s, the sound become worst. In summary:

7805: more body, especially in the mid range and overall warmer.

LM1085: More details but sounds a bit harsh, the mid range is quite thin and overall doesn't sound very good.

The output of the regulators were loaded with a Panasonic FA cap (22uF) and a ceramic cap.

So, what's happening here? I am lost.
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Old 31st October 2002, 06:35 PM   #2
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Dumb question: when you put a scope on the PS lines and play music, what do you see?
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Old 1st November 2002, 01:41 AM   #3
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SY:

I only have a 20MHz scope, so I only see perfect flat line on the output of the LM1085 or 7805.
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Old 1st November 2002, 01:44 AM   #4
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SY:

some more info, but there is a small noise, a few mV, on the power pins of the ICs.

The noise used to be quite a bit bigger (~20-200mV), but after extensive SMD caps and inductors tweaks, the noise is down to an average 2-5mV on most ICs' power pin.
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Old 2nd November 2002, 12:54 PM   #5
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I also found out that the newer 7805's really aren't bad anymore.
Could it be that the regulatordesign has changed ??
Faster semiconductors or whatever. I use ST 7805's ( made in Morocco ! ) and Onsemi 7805's and they are simply OK.
Decoupling is still very important.
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Old 2nd November 2002, 02:22 PM   #6
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Well, DC is DC. If all you see with a scope is DC, that's all you've got. If you're talking about 2 mV of noise on the power rails ( and I assume here you actually mean random, uncorrelated noise), and you've got a circuit with even minimal PSR, I'd sweat the more important stuff and stop trying to convince myself that there's something magical and mysterious going on.

Doing a quick and dirty calculation, let's assume 2 V max output of your CD. Let's assume 40dB PSRR at the top of the audio band(which is pretty mediocre). That means that the noise at your output would be 20 microvolts. That's about 100dB S/N at 20 kHz. As you go lower into the audio range, the PSRR will increase substantially- you'll probably have 100 dB or more PSRR at 1 kHz, even with old opamps. With 160 dB S/N at midband due to power supply noise, you're doing pretty well.
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Old 2nd November 2002, 02:35 PM   #7
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If DC is DC why do different regulators sound different ?
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Old 2nd November 2002, 02:59 PM   #8
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Lightbulb Regulators

Hi Patwen,
I had a similar experience when trying the TDA1543. The Philips DAC did not like the Jung-like regulators I am using for analog supply in my DAC. Audionote is also using these in the Audionote1.1.& 1.2.
When connected to a LT1086-5 (fixed +5V regulator from Linear Technology) sound was fine but when connected to the Jung like regulator sound was "uneasy" not relaxed, fatigueing.
I wonder how the AD1865N-K reacts to this. Something not tried yet. It seems a matter of not harmonizing those two elements i.e. regulator and chip powered from it. I made a post on AA on this issue. Will see if I can find it back.
http://db.audioasylum.com/cgi/m.pl?f...or&r=&session=

Well that may be interesting but I meant this post:
http://db.audioasylum.com/cgi/m.pl?f...or&r=&session=

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Old 2nd November 2002, 03:48 PM   #9
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You don't always see just DC on the lines. That's why I like to test my regulators with test tones and with music playing. Along, of course, with the usual standard tests of banging the output rails, feeding signals to the output to check dynamic source Z, feeding signals to the input to check dynamic line rejection, etc, etc.

If all you see is stable DC under those conditions, then any perceived differences are illusory. There ain't no such thing as magic. DC is DC.
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Old 2nd November 2002, 04:13 PM   #10
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Default Output cap

the output caps should be:

"The output of the regulators were loaded with a Panasonic FA cap (2200uF) and a ceramic cap."

Will replacing the ceramic cap with an OSCON (100uF) helps?
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