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OPENAMP1 - MM phono preamp open project
OPENAMP1 - MM phono preamp open project
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Old 8th November 2014, 04:47 AM   #161
mlloyd1 is offline mlloyd1  United States
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OPENAMP1 - MM phono preamp open project
i agree that openamp sounds very good. i use mine with an old borbely sulzer regulator.
folks looking for a nice mm preamp will not be disappointed.

mlloyd1
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Old 28th January 2015, 10:43 AM   #162
udok is offline udok  Austria
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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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Old 8th December 2015, 02:47 PM   #163
tER299 is offline tER299  Netherlands
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Maybe an odd question, but the THD is measured at 1 Volt. Does the THD increase at 2 Volts of output?
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Old 8th December 2015, 05:21 PM   #164
PMA is offline PMA  Europe
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No, it makes almost no difference.
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Old 16th December 2015, 09:06 PM   #165
tER299 is offline tER299  Netherlands
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Ah, that's nice to know.

I've completed the build, but when I turn it on I hear a rather loud pop, as well as a bit off buzz that isn't there when other amps are attached. Is this a servo problem? Or just bad grounding?
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Old 16th December 2015, 09:16 PM   #166
PMA is offline PMA  Europe
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This is difficult to guess at long distance. Normally the preamp is very quiet. I would guess ground loop .... or another mistake. Will you post a photo of the whole arrangement?
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Old 16th December 2015, 11:20 PM   #167
tER299 is offline tER299  Netherlands
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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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Old 17th December 2015, 05:27 AM   #168
PMA is offline PMA  Europe
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Quote:
Originally Posted by tER299 View Post

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 by PMA; 17th December 2015 at 05:34 AM.
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Old 17th December 2015, 06:08 AM   #169
nezbleu is offline nezbleu  Canada
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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.
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Old 18th December 2015, 12:40 AM   #170
tER299 is offline tER299  Netherlands
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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).
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