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Old 30th August 2003, 06:51 PM   #1
Zombie is offline Zombie  Sweden
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Default Phono stage and caps - change?

Hello!
I have a phono RIIA circuit in my Quad 44 preamp. There are two tantalium caps 2,2uF (C310/311) and 10uF (C304/305).

What is the function of those caps and can I remove any of them to improve the sound or replace them with smth better?

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Cheers
Tom
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Old 30th August 2003, 08:30 PM   #2
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Hello Tom,
I have modified the RIAA in my QUAD 34.
You should not remove the 10uF (C304/305), as they are
in the feedback path, and provides full feedback on DC-signals
while letting the AC-signals pass to GND.
However you can replace them with better types.

The 2.2uF (C310/C311) prevents DC to pass to the next amplifier stage. I have short circuited these in my preamp, as the DC out of the OpAmps was well below 1 mV.

Meassure the DC voltage on R318/319, and post it here
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Old 30th August 2003, 09:15 PM   #3
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Hello Jan!
Thanx for the input, I've shorted C310/311 out and the sound improved. DC voltage was below 1mV.
As for the 10uF, dous it matter if I put in larger ones, like 100uF?
What other modifications did you do to the phono amp? I've put OPA627 instead of the old opamps.
Don't know if I should take out the 47pF compensation caps...

I've put OPA627/637 in the radio cards, I've got "pops" when I turn the volume and sometimes pops when I change the source. It also hums occassionally, and dissapears when I press different source button. What could that be?
Cheers,
Tom
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Old 30th August 2003, 09:30 PM   #4
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Zombie

You will get better results if you short the 10uF caps as in that position any cap screws up the sound and replace the horrid tantals at output with anything reasonable, ie BG-N, polystyrene (if you can make room), etc.
The 47p are not necessary and i suspect the little inductors at the input are best removed.
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Old 30th August 2003, 09:38 PM   #5
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Ok, but short out the 10uF? Not the 2,2uF?
Have you removed your inductors?
tom
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Old 30th August 2003, 09:43 PM   #6
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Hi again,
I also experinced a major improvement in sound, mostly by removing the two DC-caps (C310/311).
You can remove the 47pF caps if you are using OPA627 as it has internal compensation, and no external connections for such caps.
However pin 1 is used for offset (you do not need that).
If you are still using the original TDA Opamps you coyld try to short circuit the 47pF caps, but watch for frequency responce and oscillation.

Others:
Just replace all caps to newer and better ones. There are so many suggestions in this forum, but I still like the "old" 1% polystyrene caps.

Your questions regarding your Radio Card are more tricky...
Did the Pops show up immd. after you have changed to OPA627?
Or is the pops scratch from the volume control??
Your 44 has logic selector circuits using CD4066 analog switches.
Normally they last for ever!!!!!!!!!!!!
Try tapping or cooling some of the components in the radio and selector circuit next time to see if you have a bad solder....
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Old 30th August 2003, 09:57 PM   #7
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Hello!
Yes, when I changed them to OPA134, it disappeared. It's only one of the cards that create it, it seems.
When it is connected to a cable the problems lessen...some kind of groud fault maybe...
Tom
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Old 31st August 2003, 07:11 AM   #8
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It could sound like a Ground problem...
Check it out by tapping components and wiring in and around the faulty circuit. You may get lucky in locating the problem
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Old 31st August 2003, 07:15 AM   #9
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Quote:
When it is connected to a cable the problems lessen...
What cable are you refering too???
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Old 31st August 2003, 07:48 AM   #10
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Good morning!
Sorry, I referred to a connection cable, in this case a DIN that comes from an FM2 tuner...
Cheers,
Tom
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