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Old 10th November 2011, 07:41 PM   #1
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Default lf412 for phono preamp- thoughts?

Hi all,
I've been playing with a phono preamp I put together.
- DIY Phono Preamp : Recording Magazine -
It uses a single op-amp and is for mm cartridge- sumiko Pearl.
I made some modifications that keeps it from clipping. I changed the 470k resistor to a 1.2k. This lowered the gain and keeps it from clipping even at higher volumes. I then changed the 47u to an 82u wich seemed to improve the noise. I'm also using a +/- 12V supply.
It is going into a Marantz 2226b vintage receiver.
The good news is it beats out the built in Preamp.

My question is what are your opinions on opamps.
I'm using a lf412 which has the balance between smoothness and detail.
I've tried-

OP249- harsh but detailed and good imaging

LF353- pretty good

NE5532- detailed...still a little harsh

tl-082 decent- I know I'm not supposed to like this chip...but in this circuit it sounds pretty good.

So now I have an NTE889- replacement for a tl062.

I'm looking at trying the TL072

Any thoughts?
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Old 10th November 2011, 08:03 PM   #2
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Location: Haarlem, the Netherlands
Quote:
Originally Posted by rcollege View Post
I changed the 470k resistor to a 1.2k. This lowered the gain and keeps it from clipping even at higher volumes.
I hope that you mean that you changed the 470 ohm resistor to 1.2 kohm, because if you really lowered the 470 kohm to 1.2 kohm, you have no RIAA equalisation anymore.

The input voltage noise of an LF412 is a bit high for a phono preamp, same holds for TL08X, but if it doesn't bother you it doesn't bother you.
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Old 10th November 2011, 09:58 PM   #3
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Oops...you are correct, it was a 470 ohm to a 1.2k.

I also noticed a difference changing the 1k power resistors to 120 ohm...just for kicks seeing as I am using a +/-12V. More power to the chip...which seems to change the sound...I'm not sure how "safe" it is. I'll do some math to find out what resistor will get me closer to the +/-15v.
I also changed the power caps? to 1200u to clean up the barely audible buzz when no music is played.
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Old 13th November 2011, 10:28 AM   #4
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Location: Haarlem, the Netherlands
Quote:
Originally Posted by rcollege View Post
Oops...you are correct, it was a 470 ohm to a 1.2k.

I also noticed a difference changing the 1k power resistors to 120 ohm...just for kicks seeing as I am using a +/-12V. More power to the chip...which seems to change the sound...I'm not sure how "safe" it is. I'll do some math to find out what resistor will get me closer to the +/-15v.
I also changed the power caps? to 1200u to clean up the barely audible buzz when no music is played.
I don't understand what you mean. If the voltage at nodes A and B is +15 V and -15 V, respectively, then you will get +/- 15 V on the op-amp when you use zero resistors. If it is +12 V and -12 V, you will never get higher than +12 V and -12 V.

The larger the RC product of R1 and C1 and R2 and C2, the better you filter off any ripple on the supply. Ripple on the supply might be the cause of the buzz, but it could also just be magnetic coupling from the supply transformer if the transformer is within a metre (3.28 feet) or so from the inputs.
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Old 14th November 2011, 07:37 AM   #5
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Quote:
Originally Posted by MarcelvdG View Post
I don't understand what you mean. If the voltage at nodes A and B is +15 V and -15 V, respectively, then you will get +/- 15 V on the op-amp when you use zero resistors. If it is +12 V and -12 V, you will never get higher than +12 V and -12 V.
.
My Apologies for not being clear...
I meant that +/-12 using 120ohms is closer to the +/-15 than using the 1kohms listed in the schematic.
The schematic shows +/-15V going into 1Kohms before reaching the chip.
I have a +/- 12V. So I changed the resistors from 1Kohm to 120ohm.
I know that I'll never reach more than +/-12V.

My question is do I need any resistors? the chips I use is +/-18V, so I wouldn't think I need resistors...but it makes the voltage low enough to use an LED to see that the power's on.
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Old 14th November 2011, 07:12 PM   #6
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This time I don't understand the remark about an LED.

Anyway, I expect that the one and only purpose of R1, C1, R2 and C2 is to filter off hum and noise from the supply. If you have a sufficiently clean supply, you won't need R1 and R2 at all. It is still advisable to put capacitors close to the op-amps as local decoupling, to prevent oscillations due to wire inductance.

The simplest way to find out if your supply is clean enough is to simply try with zero and non-zero resistors and check if you hear any difference in hum or noise.
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