OPENAMP1 - MM phono preamp open project

Hello,

I simulated the circuit and noted, that the deviation from the ideal RIAA is 4.5%. Precision starts at less than 1%, for me at least.

The dusty NE5534A actually is a little bit less noisy than the OPA627 with the Shure Cartridge.

Greetings,
Udo
 

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I've added some photo's. I've substituted the ceramic caps with polyprop (my supplier couldn't supply me with decent ceramics). These are mounted on the underside. I've also added polyprop caps on C4, C5, C104 and C105.

I was forced to remove the electrolytic caps of 10uF, and the current caps are therefore mounted on top of the board instead of fully through (they do make full contact, no cold joints).

I've temporarily swapped the OPA627 for the NE5534, because of stocking issues. I've tried to keep the BOM as stock as possible, as only the C4... etc. are a conscious choice. I haven't measured any parasitic capacitances thus far.

I've grounded the standard chassis via the ground plane of the PSU to the wall. Adding another line from X1-3 to this ground plane doesn't improve the hum. Turntable was earthed elsewhere.

The pop made me think of a DC-transient, maybe caused by faulty servos.
 

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I've temporarily swapped the OPA627 for the NE5534, because of stocking issues.
I've grounded the standard chassis via the ground plane of the PSU to the wall.

The pop made me think of a DC-transient, maybe caused by faulty servos.

1) NE5534 must not be used, because it is not unity gain stable. It will oscillate. Try anything with unity gain stability - OPA134 will be a good candidate.

2) There must be no external ground loop. The board should be placed into metal case, connected with metal case in one point (I use input RCA point), but the case should not be connected with wall PE protective ground. The preamp should be in class II with only 2-wire power cord. Signal ground loop must be avoided.

3) Initial thump is possible, turn on phono preamp as first and power amp as last. But first please fix (1) - the 5534 issue.

Summary - there must be no audible hum in case you use proper opamps and avoid all ground loops. Capacitor types do not affect the hum.
Power supply transformer of the preamp should not be close to the board.

Edit: please check the photo and a terminal to connect optional turntable grounding wire - if it exists.
 

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Last edited:
I will add to that: If I hear a "pop" when switching that indicates a DC offset somewhere, so your first step (after conforming to the BOM) should be to measure for DC on the outputs of the OpenAmp1. If there is DC there, then you have made a mistake somewhere.

I built mine to PMA's specs and have none of the problems you have mentioned.
 
Thanks Pavel! I thought the NE5534 was unity gain stable, but it seems that only the NE5532 is. Guess I mixed them up.

Both my multimeters are faulty, but it seems that there isn't any DC coming from the output (according to my least faulty meter at least below 0.5mV). Thanks for the advice anyway :).

I seem to have gotten the hum out of the amp, but due to current lack of a decent chassis, it now picks up faint radio signals (it's grounded on an old radio receiver/amplifier).
 
Well, I finally got around to finishing the preamp. Finally able to listen to my records again!

Turns out the housekeeper broke my turntable needle, so I can't hear anything from the left side -_-

Well, it sounds pretty much noiseless when hooked up to an old iPod, and I'm using the OPA134, so there's still more room for improvement :)

Thanks for all the help and the design Pavel!