Salvaged Phono Preamp - Begginner Help - diyAudio
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Old 30th August 2014, 10:33 AM   #1
bopfrog is offline bopfrog  Germany
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Default Salvaged Phono Preamp - Begginner Help

Hi everybody - first post! Love this forum.

I just pulled a little independent pcb out of an Onkyo TX-7520 amplifier that I pretty well have identified as the preamp for the phono input. I would like to try to use this as is and gradually modify it to suit my purposes. The board sports various capacitors, diodes, and resistors complementing a 4560DX opamp a lot like this one:

http://www.ti.com/lit/ds/symlink/rc4560.pdf

Where I am having problems is deciphering the pins that socketed to the main pcb. Google has failed to supply meaning for what I can only assume are antiquated or non-standard abbreviations. From top to bottom it reads so:

L0
E
L1
+B
E
-B
R1
E
R0

where I am assuming that L and R are the channels (most likely IO and not 1,0) and that B is dc current. What I cant figure is what E could be - ground?. Any help with that or corrections would be much appreciated.

Also, as I said, the little board seems to be an independent unit, BUT, there was a capacitor on the main board across +B and -B. Any clues as to if I need to include this in my circuit or what it is for would be a big help. Unfortunately I am too green behind the ears to identify the capacitor. Its a little, brown, round pill marked 2232 and a symbol I am not familiar with: Cm 85 (like a big C eating a little m).

Thanks a lot.
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Old 30th August 2014, 11:27 AM   #2
AndrewT is offline AndrewT  Scotland
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Guessing I'd say:
LO = left out
E = earth (probably Left Signal Return)
LI = Left in
+B = +ve supply
E = Power Ground
-B = -ve supply
RI = Right in
E = Earth (probably Right Signal Return)
RO = Right out

A twisted pair for LI & Left return
A twisted pair for RI and Right Return

A twisted triplet for +ve, 0V & -ve

A twisted pair for LO and Left Return
A twisted pair for RO and Right Return.

Notice that the "groups" keep like signals together (if my interpretation is correct). This indicates good design expertise.
BUT BEFORE you connect power, check the trace from +B to pin 7 of the opamp and check the -B to pin 4 of the opamp.
Find any additional capacitors on either of these two traces.
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Last edited by AndrewT; 30th August 2014 at 11:33 AM.
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Old 30th August 2014, 11:30 AM   #3
AndrewT is offline AndrewT  Scotland
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2232 could be 22n for the 223. But that seems far to small for local decoupling.
Can't decode the last "2"
The "pill" shape could be a tantalum, or a very fat ceramic.Cm 85 could be manufacturer and cap type.

Can you posts pics of the PCB from both sides with good close up detail. macro and daylight gives good pics.
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Old 30th August 2014, 01:13 PM   #4
bopfrog is offline bopfrog  Germany
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Thanks for the help AndrewT, means a lot.

Sure thing. Sorry about the lousy pics though. If they are not detailed enough I can give it another go. Wasnt sure which pcb you meant, so I included both.

After you ran it down, it occurred to me that that layout IS pretty obvious from a design standpoint. Let me posit a guess as well. You see that big controller chip on the main pcb above the capacitor that I pictured? Could the capacitor have something to do with the power being routed through that switch? (LC7821)

LC7821 pdf, LC7821 description, LC7821 datasheets, LC7821 view ::: ALLDATASHEET :::

Two views of the front of the preamp
Click the image to open in full size.

Click the image to open in full size.
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Old 30th August 2014, 02:00 PM   #5
AndrewT is offline AndrewT  Scotland
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Is that the correct link to a pic?
Datasheet?

attach a pic, or many, to your post.
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Old 30th August 2014, 02:05 PM   #6
bopfrog is offline bopfrog  Germany
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The back of that board
Click the image to open in full size.

The component in question is front and center
Click the image to open in full size.


Click the image to open in full size.
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Old 30th August 2014, 02:18 PM   #7
Mooly is offline Mooly  United Kingdom
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223Z

22nf (0.022uf). Z is the tolerance, +80% -20% I believe.
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Old 31st August 2014, 02:09 AM   #8
bopfrog is offline bopfrog  Germany
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Thanks Mooly,

I think I can move forward with my project.

Cheers guys.
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